The fourth phase of the Air Quality Model Evaluation International Initiative (AQMEII) focuses on deposition processes. Since 2008, AQMEII has analysed a number of critical aspects of regional-scale air quality models that require international collaboration and evaluation. Each of the previous three phases have resulted in multi-paper “special issues” in the peer-reviewed literature (two in Atmospheric Environment and one in ACP). AQMEII leverages the long-term experience in model development and application of the two largest communities of such models, namely the North American community and the European one. These two communities have been engaged in AQMEII model evaluation activities that tackled issues such as operational and probabilistic evaluations (phase 1), online model coupling between air quality and meteorology (phase 2), intercontinental transport of air pollutants (phase 3, ACP SI “Global and regional assessment of intercontinental transport of air pollution: results from HTAP, AQMEII and MICS”, 41 papers), and now dry and wet deposition processes (phase 4). The subject tackled in phase 4 is central to air quality modelling at any scale, and it has never been examined in the systematic and the detailed manner currently underway in AQMEII4. Given the complexity of deposition processes and the variety of approaches adopted by different groups, we opted for an analysis that comprises the individual deposition modules and the full regional-scale models that used those modules as part of a more complex computational system. As customary, for AQMEII two regional modelling domains are considered, a European and a North American one, and simulations are underway for 2 representative years for the processes of interest in the two continents. The process-oriented character of the activity is defined already in the selection of the simulation year. For the North American domain, the air quality in 2010 and 2016 will be simulated to allow comparisons across emissions changes, while for Europe 2009 and 2010 are the designated years to allow comparisons across meteorologically different years. The community presently participating is composed of 15 regional modelling teams and six deposition modules. Subgroups of regional-scale models may be using the same modelling system with a variety of parameterization implementations, or use original models developed in-house. A key feature of the AQMEII multi-model exercise is the submission of all model output on a common grid and at specified observation stations, to a common database at JRC in Ispra, Italy, where a common set of analysis tools which participants can use is available. The variety of the tools used makes possible the comparison of all models with measurements and of specific models with largely used community systems. It also makes it possible to work within the sub-community to exercise different features or modelling options of the community models. Whilst for regional-scale models the evaluation will be based on data on a wide range of pollutants in gas and particle form gathered from standard monitoring networks and soundings in the two continents, in the case of point models, ozone will be the focus, and data have been gathered from sites where high-quality instrumentation has been deployed. Analysis of the ozone dry deposition modules driven by observed site-specific meteorological and biophysical data (i.e., the point models) will focus on process-oriented model evaluation and intercomparison. As mentioned the central topic of the issue is the deposition processes, but differently from the past a deeper level of analyses has been added in AQMEII4. That pertains to the addition of diagnostic analysis to each of the participating models. These diagnostics are allowing participants to compare the parameterizations for deposition pathways across all participating systems in an unprecedented level of detail, with comparisons between individual conductance and resistances being carried out for the first time. The diagnostics have been mapped to a common set of land-use types, in order to allow a common analysis of the different deposition parameterizations for a given land-use type. The AQMEII4 activity goes beyond the comparison of the deposited quantities as has been customary so far and will allow the gathering of insight into the reason for model differences and the efficacy of some schemes compared to others through the comparison with measurements. For the first time the diversity with which the participating models (both point models and the full regional-scale models) describe the underlying surface on which the deposition occurs will be evaluated. This is therefore not a mere model evaluation exercise but one that aims to tackle in a multi-model fashion the most difficult of the four pillars of model evaluation (Dennis et al., 2010), i.e. diagnostic evaluation. The special issue will therefore include papers on process-oriented point model comparisons against deposition supersite data, detailed process-oriented model inter-comparisons of the regional-scale output, and the potential application of model results for ecosystem assessments. An introductory paper (already in preparation) will be the first submission of the special issue, which will describe activity set-up and rationale – an important and crucial preparatory phase which will be referenced by the individual papers that follow.

With the launch of the novel Aeolus wind profiling mission, a wealth of direct wind and atmospheric backscatter profile observations has become available across the globe, revealing new information on atmospheric dynamics and optical properties to the benefit of a number of applications. These applications include, for example, improvements in numerical weather prediction (NWP), general circulation models (GCMs) and their included parameterized processes, such as land and ocean drag, convection, stratosphere–troposphere interaction and atmospheric waves, air quality and air pollution transport, etc. In this special issue, new data products and their validation as well as studies using the Aeolus data for application in meteorology, air quality, and climate studies are discussed. Topics vary from observation interpretation to GCM or NWP model diagnostics, parameterization, and data assimilation experiments. This is the first special issue on scientific analysis of Aeolus mission data, including inter-comparisons with different collocated wind profiling, aerosol and cloud in situ and remote-sensing measurements, and atmospheric models. Contributions concerning Aeolus data calibration, processing, and tools for data access and exploitation are also provided.

Rapid population growth and urbanisation worldwide accelerate eco-environmental/socio-economic stress as well as adverse climatic and health impacts on urban dwellers. Atmospheric modelling research has largely been performed on a horizontal grid spacing of kilometres or larger due to a lack of understanding of the local-scale phenomena, appropriate parameterisations, and adequate modelling tools, and computer resources. Urban/local street-level air pollution, climate change, and their impacts on population exposure and human health have increasingly received attentions by both researchers and policymakers around the world. Several state-of-the-science models have been recently developed for urban/local street-level air pollution modelling, including the street-network model, the Model of Urban Network of Intersecting Canyons and Highways (MUNICH) that incorporates detailed representations of gas-phase chemistry and secondary aerosol formation pathways, and the Street-in-Grid model (SinG) that dynamically combines a 3-D Eulerian chemical-transport model (CTM), Polair3D, with MUNICH. There have been increasing numbers of developers and users for MUNICH, SinG, and other similar coupled 3-D CTMs and urban canyon models for street-level air pollution modelling worldwide. Recognising the urgent need for scientific advancement, pollution/exposure assessment, policymaking, and public health protection at urban/local-street scales, we launch this special issue to advance scientific understanding of local-scale atmospheric phenomena and promote research and discussion on state-of-the-science urban/local-street-level air quality model development and application for complex interactions among urban air pollution, climate, and health. The special issue is open for all submissions which address the following themes:

• 3-D Street-in-Grid (SinG) model development and application;
• urban canyon/network model development and its incorporation into 3-D CTMs;
• urban/street-level air quality modelling in support of human exposure assessment;
• impact of urban traffic emissions on air quality and human health at a street level.

The absorption of energy by water vapour keeps the average temperature on Earth at a level that makes life possible. However, the average temperature increased in recent decades due to increased greenhouse gas emissions. This anthropogenic increase is amplified by the water vapour feedback. Water vapour is further an important component of the water cycle and indirectly of the energy cycle. The role of water vapour in these cycles in a changing climate needs to be fully understood and quantified. Thus, high-quality observations of water vapour and the characterisation and validation of associated uncertainties are of high importance for climate applications. Topics covered around atmospheric water vapour include, among others, retrieval development, data records, uncertainty characterisation and validation, inter-comparisons, climate applications, and model validation.

This special issue originates from the GEWEX Water Vapor Assessment (G-VAP, http://gewex-vap.org) and is open for all submissions within scope, not just from G-VAP.

In this special issue papers resulting from two major combined field campaigns shall be aggregated: (i) the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD), and (ii) the Physical feedbacks of Arctic boundary layer, Sea ice, Cloud and AerosoL (PASCAL). These two concurrent campaigns took place in the vicinity of Svalbard in May and June 2017. They were designed to study processes important for explaining Arctic amplification, and, in particular, for investigating the role of microphysical and dynamical properties of Arctic low- and mid-level, mixed-phase clouds, and their interactions with atmospheric radiation and aerosol particles. Ground-based, ship-borne, tethered balloon, aircraft, and satellite observations have been combined. The research vessel (RV) Polarstern, an ice floe camp (erected close to the icebreaker) including an instrumented tethered balloon, and the two research aircraft, Polar 5 and Polar 6, were jointly operated. Polar 5 served as a mobile remote sensing observatory looking at the clouds from above, whereas Polar 6 operated as a flying in situ measurement laboratory mostly sampling inside the clouds. The permanent ground station of Ny-Ålesund observed the clouds from below, applying similar but upward-looking remote sensing equipment as Polar 5. Some of the flights were performed underneath respective satellite tracks. In this special issue we compile a number of papers reporting about the results of the observations conducted during ACLOUD/PASCAL within the framework of the (AC)³ project (http://www.ac3-tr.de/).

In the vast low-nutrient—low-chlorophyll (LNLC) ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be 1 order of magnitude greater over short timescales. Recently, significant experimental, field and modeling work allowed us to better link atmospheric deposition with nutrient availability, ocean productivity, carbon cycling and marine emissions. By improving our understanding on how atmospheric inputs are impacting biological activity and the carbon balance in oligotrophic environments, their representation in biogeochemical models at different timescales and space scales will be better assessed. In particular, processes may be affected by ongoing environmental changes in the ocean such as a decrease in pH and an increase in temperature.

This special issue aims at gathering contributions from both experimental and modeling approaches showing how natural or anthropogenic inputs from the atmosphere can impact biogeochemical and ecological processes in the present and future oligotrophic ocean. It will present the results obtained during the PEACETIME (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea) cruise conducted in 2017 in the Mediterranean Sea, but the special issue is open to any submissions provided that the subject is consistent with the objectives defined above.

The Chemistry and Aerosol Mediterranean Experiment (ChArMEx) special issue will be simultaneously presented in the ACP and AMT journals. It aims at gathering experimental and modelling contributions to the field of atmospheric chemistry in the Mediterranean region and its impacts on regional air quality and climate, both in the recent past, present and future decades. It addresses natural and anthropogenic emissions of tropospheric reactive species, source apportionment, chemical transformations, transport processes, atmospheric deposition, aerosol optical properties and interactions with radiation, water vapour and clouds, variability and trends, and future conditions following climate change and increasing anthropogenic pressure.

In order to improve our current knowledge on the budgets of the two most important anthropogenic greenhouse gases, CO₂ and CH₄, a co-ordinated measurement campaign in the central European region was successfully carried out in May-June 2018 with the German research aircraft HALO and two smaller Cessna aircraft as its main experimental platforms. The goal of CoMet is to combine a suite of state-of-the-art airborne active (lidar) and passive remote sensors (spectrometer) with in situ instruments to provide regional-scale data about greenhouse gases (GHGs) which are urgently required for their accurate modelling.

During the intensive observation period, HALO research flights were performed along extended latitudinal transects to capture GHG gradients, over known regions of strong emissions, and over the ground-based remote-sensing sites of the Total Carbon Column Observing Network (TCCON). While HALO provides a larger-scale picture, the two Cessna aircraft concentrated on a region of prime interest: the Upper Silesian Coal Basin in Poland, which, due to hard coal mining activities, is one of the hot spots of anthropogenic methane emissions in Europe. Here, a variety of ground-based instruments additionally supported the CoMet mission: FTIR spectrometers, wind lidars, mobile vans, and small drones provided valuable information to quantify CH₄ emissions from coal mining activities. A model infrastructure (regional inverse modelling, chemistry-climate modelling with regional refinement) has been employed in forecast mode to optimize flight strategies and is now used to hindcast the campaign period to evaluate GHG fluxes and transport. The CoMet mission is also part of validation activities for existing passive remote sounding GHG satellites (GOSAT, Sentinel-5P, and OCO-2) and preparation of the first active CH₄ satellite mission MERLIN. For that reason, the CHARM-F lidar developed at DLR as an airborne MERLIN demonstrator was one of the key instruments. In addition, CoMet is intended to investigate methodologies for the synergistic combination for GHG measurements using lidar and passive remote sensing.

Sand and dust storms (SDSs) are extreme meteorological phenomena that generate significant amounts of airborne mineral dust particles. SDSs play a significant role in different aspects of weather, climate and atmospheric chemistry. In addition, SDSs represent a severe hazard for life, health, property, the environment and the economy, which is aligned with several Sustainable Developed Goal (SDG) targets established by the United Nations (UN). Understanding, managing and mitigating SDS risks and effects requires fundamental and cross-disciplinary knowledge and filling current knowledge gaps on the dust cycle. Recent developments in Earth observation have led to big advances in the observational capabilities of desert dust particles. Research products show the possibility of exploring the wide dimensional range of desert dust particles by integrating different observational techniques on the ground, while satellite observations are currently providing aerosol description with finer and finer spatiotemporal resolution. Experts on dust observations and modelling participating in the EU-funded COST Action InDust (www.cost-indust.eu, CA16202) have identified critical scientific gaps in our current knowledge of dust aerosol measurements, modelling, processes and effects and have also identified a critical mass of scientists working towards addressing them. Such gaps include at least issues such as - the need for improvement of surface-based dust aerosol measurements (in situ, sun photometric, lasers), including the use of new technologies, improved algorithms and combined approaches; - the improvement of the global coverage on aerosol dust observation through improvement of satellite-based retrievals; - advancements in dust aerosol modelling (sources, mechanisms, deposition); - the presentation of intensive campaign results with a focus on dust aerosols; - dust mineralogy; - the assessment of the dust particle size distribution with an emphasis on giant particles; - enhancements regarding dust aerosol effects on different communities (e.g. solar energy, aviation, health, climate, air quality, agriculture, ocean life). This special issue focuses on addressing the above-mentioned gaps through individual and combined efforts from at least the COST InDust community (0000108:0000100 scientists from 52 countries). A list of papers to be submitted has been created including results related to SDS-WAS, ACTRIS/EARLINET activities, long-term measurement analysis of measured and modelled data, dust retrieval algorithm and techniques, dust effects, and foreseen results of the ESA Aeolus Cal/Val activity “ASKOS” (Cabo Verde, June - July, 2020).

The EARLINET ACP Special Issue aims to collect high-quality aerosol studies employing advanced lidar remote sensing techniques as developed within EARLINET, the European Aerosol Research Lidar Network. EARLINET was established in 2000 with the main goal of providing a comprehensive, quantitative, and statistically significant database for aerosol distribution on a continental scale. The 5-year EARLINET-ASOS (Advanced Sustainable Observation System) EC Project (2006–2011) has strongly contributed to optimizing the operation of the network. EARLINET is now a key component of the ACTRIS (Aerosols, Clouds and Trace gases Research InfraStructure Network) research infrastructure project that aims to integrate European ground-based stations equipped with advanced atmospheric probing instrumentation for aerosols, clouds and short-lived gas-phase species. The 15-year EARLINET database of aerosol profiles over Europe will be utilized in this Special Issue to reveal the network’s contribution to atmospheric and climate research.

The project Effect of Megacities on the Transport and Transformation of Pollutants on the Regional and Global Scales, EMeRGe, was created to experimentally investigate the transport and transformation of air plumes coming from major population centres, MPCs. It comprises a set of unique airborne measurement campaigns carried out in two parts of the world having MPCs of very different characteristics and emissions. The two measurement campaigns using the HALO research aircraft (www.halo-spp.de) were conducted in Europe in summer 2017 and in East Asia during the intermonsoon period in March-April 2018. The initial scientific team of EMeRGe, comprising scientists from four German universities and four research centres in 2016 (www.iup.uni-bremen.de/emerge/), was extended within the EMeRGe international membership, incorporating more than 50 research partners from 16 different countries. The combined and synergetic set of information obtained from airborne and ground-based data, satellite products, and their comparison with models enables the patterns of atmospheric composition and meteorological parameters observed during the campaigns to be constrained and thereby our understanding of the emissions, their transport, and physico-chemical transformation of the outflow from MPC emissions to be tested and assessed. The EMeRGe special issue aims to publish the scientific results achieved by the project EMeRGe. This comprises contributions of the partners of the national and international EMeRGe scientific community. It solicits all relevant submissions addressing the EMeRGe science objectives.

The Hydrological cycle in the Mediterranean Experiment (HyMeX, https://www.hymex.org/) programme is a 10-year concerted effort at the international level started in 2010 with aims to advance the understanding of the water cycle, and with emphases on the predictability and evolution of high-impact weather events, as well as on evaluating social vulnerability to these extreme events. The special issue is jointly organized between the Atmospheric Chemistry and Physics, Hydrology and Earth System Sciences, Ocean Science, Natural Hazards and Earth System Sciences, Atmospheric Measurement Techniques, and Geoscientific Model Development journals. It aims at gathering contributions to the areas of understanding, modelling, and predicting at various timescales and spatial scales of the Mediterranean water cycle and its related extreme events, including cyclones, heavy precipitation, flash floods and impacts, drought and water resources, strong winds, and dense water formation. The special issue is not limited to studies conducted within HyMeX: any multiscale or multidisciplinary approaches related to the Mediterranean water cycle are encouraged.

The version 8 spectral data of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), released in January 2019, are supposed to be the final set. Besides the ORM reprocessed version 8 level-2 MIPAS data distributed by ESA, a research processor has been run at IMK and IAA to generate temperature and trace gas concentrations. The IMK–IAA data product includes a larger list of atmospheric constituents and covers a wider altitude range in the case of the special observation modes than the operational data. The processing is based on independent algorithms. This special issue collects articles which 1) document the retrieval strategies used to derive these distributions along with uncertainty estimates, 2) report validation studies, and 3) use the data to answer current questions in atmospheric research.

Ice nucleation is a particularly hot topic at the moment in atmospheric sciences, with profound implications for the hydrological cycle and climate. However, the detailed links between the atmospheric aerosol chemical and physical character, ice-nucleating particles (INPs), and the ice nucleation processes in different temperature regimes remain open (e.g. Hoose and Möhler, 2012, Knopf et al., 2018). We performed a comprehensive field study in Hyytiälä, Finland, during 2018 (HyICE campaign) with a clear focus on characterizing the INP concentrations in different temperature ranges with a suite of ice nucleation counters (online and offline). The work was complemented with thorough measurement of physical (size distribution 1 nm–10 um, bioaerosol concentrations) and chemical characteristics (high-resolution AMS) of the atmospheric aerosols from nanometre scale to super-micron sizes done at the Station for Measuring Ecosystem – Atmosphere Relations II (SMEAR II) site. The location provides an excellent facility to study INPs as we can compare our data to the 20-year data set already available at the site. The observation period started in February, with the most intensive INP measurements in March–May 2018. We operated a suite of online ice nucleation counters (SPIN, PINE, PINCII) and performed filter sampling at the ground level, above the canopy, and onboard a research aircraft. We continue the filter sampling providing understanding of the seasonality of INP concentrations in the boreal environment. In SMEAR II we have also the weather radar and lidar capacity to address the cloud microphysics within the clouds. These activities enable us 1) to perform an intercomparison between the ice nucleation chambers, 2) to assess the vertical variability of INPs inside the boundary layer and in the free atmosphere, 3) to connect the observed INP concentrations to physical and chemical parameters and aerosol source apportionment and 4) seasonal variability of the INPs in the boreal context, 5) to connect the ground-based INP measurements to the cloud processes aloft, and 6) to develop parameterizations based on our observations and utilize them in models of various scales.

This special issue will include papers that guide the path forward in making reliable predictions of aerosol effects on the heat budget of the South East Asian subcontinent by improving our knowledge of aerosol processes and their influence on the Indian monsoon. Several recent major studies have aimed to provide a detailed determination of aerosol physical and chemical properties across India prior to and during the Indian monsoon, and results from these studies will be welcomed. Most recently the South West Asian Monsoon Interactions project used a combination of UK and Indian research aircraft and a network of ground-based measurement sites. The analysis of data will enable assessment of aerosol composition and mixing state, provide source characterization, and help quantify aerosol optical properties such as extinction, absorption, and single scattering albedo that drive regional climate. Results that use this detailed characterization to test representations of aerosol properties in regional and global climate models will be included. The special issue will report on evaluations of and improvements in model representations of aerosol properties with data over India.

This special issue includes papers based on recent field campaigns, including US Department of Energy Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA), NASA North Atlantic Aerosols and Marine Ecosystems Study (NAAMES), and the North Atlantic Climate System Integrated Study (ACSIS), and long-term measurements from observatories (e.g., the Atmospheric Radiation Measurement Eastern North Atlantic site on Graciosa Island in the Azores). Presently the responses of marine low-cloud systems to changes in atmospheric greenhouse gases and aerosols are major sources of uncertainty that limit our ability to predict future climate. Major contributions to this uncertainty derive from poor understanding of the cloud responses to aerosol changes and the natural aerosol state that is being perturbed by anthropogenic emissions. One overarching scientific objective of these recent field campaigns is to understand key processes that drive the properties and interactions of aerosol and clouds under representative meteorological and cloud conditions. Another important focus is to validate and improve ground-based retrieval algorithms using comprehensive in situ aircraft measurements. This will lead to high-quality, long-term datasets from the routine ground-based remote sensing, which allow greater statistical reliability in the observed properties and relationships among aerosols, clouds, and precipitation than is possible with the aircraft measurements alone.

In this special issue papers resulting from the project “Marine biological production, organic aerosol particles and marine clouds: a Process Chain – MarParCloud” and closely related projects shall be aggregated. MarParCloud is a highly interdisciplinary project funded by the German Leibniz Society (https://www.leibniz-gemeinschaft.de/en/about-us/leibniz-competition/projekte-2016/funding-line-2/), which aims to improve our knowledge on the nature, formation, transfer and effects of organic matter (OM) in the marine environment. To this end the biological formation of OM in the ocean, its accumulation in the sea surface microlayer (SML), the export to marine aerosol particles and, finally, the importance of OM for the formation of cloud droplets and ice crystals are investigated. Within MarParCloud several field and tank studies were performed to investigate ambient seawater, the SML, aerosol particles and cloud water as well as water and air samples obtained during artificial bubble experiments. Closely related campaigns comprise the EU project MARSU (“Advanced analytical and mass spectrometric techniques to study ocean-atmosphere interactions”) and the MILAN study (“sea-surface MIcroLAyer at Night”), focusing on diel dynamics of the uppermost layer of the ocean, as well as the cruise EMB184 “Aerosol” performed with the R/V Elisabeth Mann Borgese. In addition to results from these projects and campaigns, this special issue is open for all submissions concerning the cycling of OM in the marine environment with potential atmospheric implications.

Satellite limb measurements have become an essential tool to remotely sense the chemical composition of the Earth's atmosphere with a particular focus on the stratosphere. They enable combining near global sampling on a daily basis with vertical profiling capability and thus overcome some of the limitations of nadir or occultation measurements. Although the number of satellite limb instruments is currently declining, several missions are still operational (e.g., OSIRIS, SMR, ACE-FTS, MAESTRO, OMPS-LP and SAGE III) and several new missions are planned or will be launched in the near future (e.g., MATS, ICON, ALTIUS and several mini satellites). In addition, the data sets of several past limb sounders, particularly the Envisat instruments SCIAMACHY, MIPAS and GOMOS are still used for many scientific studies and new data products are developed based on measurements with these instruments.

This combined ACP/AMT special issue is open for submissions dealing with all aspects related to limb measurements (including limb scatter, limb emission and occultation) in the optical, IR and microwave spectral regions. This includes instrument development and characterization, retrieval algorithm development, validation studies, as well as scientific applications based on the retrieved data sets. We note that radio occultation measurements are outside the scope of the special issue.

The purpose of this special issue is the compilation of modelling and observational studies in connection with five international field deployments (AEROCLO-sA, CLARIFY, LASIC, ORACLES, and NaFoLiCA) that focus on the interactions of natural and anthropogenic aerosols with radiation, clouds, and regional climate in the South Atlantic Ocean and the southern African region. These deployments, based in Namibia, Ascension Island, and São Tomé, took place between 2016 and 2018 and support a significant number of investigations extending beyond just the individual science teams. The airborne and ground-based observations, as well as the related satellite measurements and climate modelling studies, address all aspects of aerosol–cloud–climate interactions, including the link of aerosol properties to meteorological fields and dynamical processes that influence aerosol emission and transport. The projects also target the advancement of remote sensing of aerosols for complex scenes over land, sea, and clouds. The special issue will be open to all submissions, with complementary goals to the five mentioned deployments, so as to encourage the exchange of ideas from inside and outside the science teams of all projects.

The Pan-Eurasian Experiment (PEEX) (www.atm.helsinki.fi/peex) is a multi-disciplinary, multi-scale and multi-component research infrastructure and capacity building programme. The PEEX originated from a bottom–up approach by the science communities in 2012. The programme aims to resolve the major uncertainties in Earth system science and global sustainability issues concerning the Arctic and boreal Pan-Eurasian regions as well as China. The PEEX solves interlinked global grand challenges influencing human well-being and societies in northern Eurasia and China by establishing and maintaining long-term, coherent and coordinated research activities as well as continuous, comprehensive research and educational infrastructures. The scientific issues covered by PEEX include climate change, air quality, biodiversity loss, chemicalization, food supply, freshwater and use of natural resources by mining, industry, energy production and transport. Our approach is integrative and interdisciplinary, recognizing the important role of the Arctic and boreal ecosystems in the Earth system.

The programme has had an open special issue in Atmospheric Chemistry and Physics in 2015–2019/2020 and has published 55 papers in total. A major part of the published papers originated from the PEEX collaboration, including organization of the PEEX science conferences, special sessions in the international conferences and on INAR Univ. Helsinki (PEEX-HQ) international collaboration, especially with the air quality research groups from China. The papers contributed to the large-scale research questions addressed by the PEEX Science Plan (www.atm.helsinki.fi/peex/images/PEEX_Science_Plan.pdf). PEEX is currently creating an overview of the research carried out during the last 5 years and is re-visiting its research agenda and large-scale research questions. PEEX continues organizing scientific conferences and workshops on a regular basis and is connected to the Arctic research community by U-Arctic thematic network "Arctic - boreal hub" (www.uarctic.org/organization/thematic-networks/arctic-boreal-hub/) coordinated by INAR Univ.Helsinki. The applied new PEEX- Part II special issue in ACP will be open for all submissions within these frameworks.

The research foci in the MICS-Asia III study include the following aspects: development of reliable emission inventories in Asia; ability and uncertainty of current air quality models in Asia in predicting ozone, nitrogen oxides, VOC, sulfur oxides, PM_2.5, etc.; ability of online coupled models to simulate aerosol radiative forcing and ozone and aerosol–weather/climate interaction through aerosol–radiation–cloud feedback; and prediction of future air pollution and climate. The special issue is open to all publications that inform about these aspects and provide answers to the MICS-Asia III scientific questions.

• What is the ability of models to reproduce pollutant concentrations of photochemical smog under highly polluted conditions (regional haze) and to model a sharp PM_2.5 increase such as the episodes between January 2010 and March 2018?
• What are the uncertainties of process (physical and chemical processes) and model resolutions (horizontal and vertical) in air quality modelling? In particular, uncertainties in key boundary layer parameters, which have both direct and indirect impacts on modelling results by transporting meteorological fields and providing parameters in physical/chemical modules, respectively, need to be addressed.
• What are the air quality responses to specific emission perturbations in a common case?
• What are the impacts of anthropogenic emissions on radiation and climate change over East and Southeast Asia?
We especially welcome the publications related to MICS-Asia Phase III studies to inform the research community about the recent findings and progress of air quality and regional climate change studies over East and Southeast Asia. The findings from these publications will also help to better understand capability, uncertainty, and future direction within the current CTMs.

This ACP Special Issue seeks to report research advancements in the understanding of mercury sources, distribution pathways, levels, physicochemical characterization, measurement traceability, impacts on global ecosystems, and policy-relevant information presented at the ICMGP 2019 and being developed under the MercOx (Metrology of oxidized mercury) project and two ERA-PLANET initiatives – Integrative and Comprehensive Understanding on Polar Environments (iCURE) and Integrated Global Observing Systems for Persistent Pollutants (iGOSP). The Special Issue is open to all publications related to the scope of the Special Issue.

In recent years, there have been multiple community-level efforts to improve our scientific understanding of the possible effectiveness and effects of solar geoengineering, especially the Geoengineering Model Intercomparison Project (GeoMIP) and the Geoengineering Large Ensemble (GLENS). These projects have wide international participation among climate modeling centers and researchers, including developing country researchers participating in the Developing Country Impacts Modelling Analysis for SRM (DECIMALS) fund. By producing simulations available for any interested researcher, these two projects have been highly effective in advancing the state of knowledge of solar geoengineering.

Several of these projects are reaching critical stages. A new round of GeoMIP simulations has recently been released as part of CMIP6. The DECIMALS teams are maturing and are at the stage where they are ready to produce papers, with the possibility that the project will be extended. GLENS simulations are continuing to be analyzed and supplemented with simulations from additional research efforts. To capture the interrelated research amongst these projects, the special issue is set up jointly between Atmospheric Chemistry and Physics and Earth System Dynamics. In ACP, we invite papers that emphasize process-level understanding and stratospheric dynamics; in ESD, submissions on Earth system effects, feedbacks, and impacts (like agriculture or ecosystem responses) are invited.

With a wide variety of aerosol optical, physical, and chemical properties, China offers a test bed for the study of aerosol processes and techniques leading to a better understanding of aerosol properties. Aerosol concentrations in the southeast of China are periodically among the highest in the world, while at other times they are relatively low. The prevailing aerosol chemical composition (e.g., sulfate, nitrate, black carbon, brown carbon, particulate organic matter, sea salt) also changes significantly with the quick evolution of both the economy and society, along with the government’s strong environmental actions. The complex mechanisms of the secondary formation of aerosols and haze-fog interaction are still not well understood in this important region. In areas other than the southeast of China, topography determines the aerosol concentrations, which are consistently high. Examples are Sichuan Province or the Guanzhong Basin, i.e. large basins surrounded by high mountains. Furthermore, the large deserts in the west and north of China are strong sources of dust particles influencing the aerosol properties over a large part of the country. There are however also large areas with relatively low aerosol loading, in particular in mountainous areas and over the high plateaus such as Tibet. Aerosol chemical properties in the center and east of China still need to be clarified both geographically and temporally, e.g. distinguish spectrally absorbing components like mineral dust and brown carbon from remote sensing observations. Anthropogenic emissions of aerosols and precursor gases have a large influence on aerosol, in regards to both chemical and physical properties, and consequently optical properties, and their effects can be augmented by the transport of aerosol from elsewhere, such as carbonaceous particles from biomass burning aerosol and dust particles emitted from the deserts. The complex mechanisms and intrinsic relations of aerosol chemistry and physics, and their impacts on optical properties, need better interpretation based on comprehensive observation and analysis. Weather conditions have a large influence on the formation of haze, and haze episodes occur more and more frequently. The influence of large-scale weather systems such as the monsoon or the Siberian high may result in inter-annual variations in different regions of China. In particular the progress of the monsoon from south to north during spring and summer, and back in the autumn, results in strong variations in aerosol content.

In this special issue we focus on the application of remote sensing techniques using both ground-based and satellite measurements to study the temporal and spatial variations in aerosol properties over China on different scales. Results from campaigns in the Taklamakan Desert (Kashi), on the Tibetan Plateau, and other remote areas such as the NE of China are contrasted with those from densely populated and industrialized urban areas in the east of China. The special issue aims to bring together results from different projects such as Kashi, MOST, and regional air quality studies. Contributions are solicited on aerosol optical, physical, and chemical properties inferred from remote sensing observations and related optical techniques, and the spatial and temporal variations in these properties across China, with emphasis on processes leading to the observed differences, including the influence of meteorological conditions and large-scale weather systems.

The 2019 Raikoke eruption emitted an estimated 1.5±0.2 Tg of volcanic sulfur dioxide (SO₂) into the atmosphere, which makes it the largest volcanic SO₂ perturbation in the upper troposphere and lower stratosphere since the 2011 eruption of Nabro. The 2019 Raikoke eruption triggered numerous discussions among scientists involved in the SPARC Volcano Response (VolRes) initiative and provides an excellent opportunity to test our understanding of processes involved in the formation, growth and removal of volcanic aerosols in the atmosphere. Remote sensing measurements from geostationary and low-orbiting satellites with ultraviolet, visible, and infrared sensors (HIMAWARI-8, OMPS/NPP, OMI/Aura, TROPOMI/SP5, IASI/MeteopA-B-C, AIRS/Aqua, CALIOP/CALIPSO, SAGE III/ISS) provided a wealth of information on the SO₂ concentration, ash and sulfate aerosol particle optical properties from the initial injection through to the growth and removal of aerosols. This eruption therefore provides an excellent opportunity to inter-compare satellite observations and validate the different retrievals for SO₂ and volcanic aerosol properties from new satellite sensors such as TROPOMI. After the initial injection into altitudes up to 15–16 km, the plume reached 25–26 km altitude in late 2019, which corresponds to a fast vertical ascent that appears unusual compared to previous eruptions. The fast vertical ascent of the 2019 Raikoke plume may suggest an influence of smoke aerosol from large wildfires that took place in Canada and Siberia around the time of the eruption. The veil of aerosol particles from Raikoke 2019 has noticeably reduced the transparency of the stratosphere and detailed information and measurements on the aerosol properties will be useful to constrain atmospheric and climate model simulations.

This inter-journal special issue of AMT, ACP, and GMD serves as a thematic collection for manuscripts arising from the work of Horizon 2020 integrated infrastructure project Eurochamp-2020; cf. https://www.eurochamp.org/ . As Eurochamp-2020 has project parts dealing with new instrumentation, with atmospheric chemistry and physics studies as well as with modelling, contributions are expected for all three journals as part of the inter-journal special issue.

StratoClim is a collaborative research project funded by the European Commission 7th Framework programme. The main objective of the project is to improve the understanding of key processes in the upper troposphere and stratosphere (UTS) for more reliable projections of climate change and stratospheric ozone. StratoClim focuses on the understanding of the microphysical, chemical, and dynamical processes that determine the composition of the UTS, such as the formation, loss, and redistribution of aerosol, ozone, and water vapour; how these processes are affected by climate change; and how they feed back on global climate.

The issue will include papers coming from all sectors included in the project: in situ and sonde measurement deployment, satellite data products and analysis, mesoscale and transport modelling, and CCM experiments and analysis. We expect a special emphasis on the results from the high-altitude research aircraft campaign in the Asian monsoon area carried out in 2017 deploying an innovative and comprehensive payload. Due to the expected contribution of instrumental manuscripts, the issue will also include AMT submissions.

The Aerosol Chemistry Model Intercomparison Project (AerChemMIP) is endorsed by the Coupled-Model Intercomparison Project 6 (CMIP6) and is designed to quantify the climate and air-quality impacts of aerosols and chemically reactive gases. Specifically, the aim of AerChemMIP is to answer four scientific questions. 1. How have anthropogenic emissions contributed to global radiative forcing and affected regional climate over the historical period? 2. How might future policies (on climate, air quality, and land use) affect the abundances of NTCFs and their climate impacts? 3. How do uncertainties in historical NTCF emissions affect radiative forcing estimates? 4. How important are climate feedbacks to natural NTCF emissions, atmospheric composition, and radiative effects? This special issue is to be devoted to papers that analyse targeted simulations with CMIP6 climate models to address these questions as well as papers that evaluate these models against observations. It also includes papers from other CMIP6-related activities (e.g. RFMIP, DAMIP), containing contributions to the understanding of climate forcing and feedbacks involving aerosols, reactive gases, and associated biogeochemical cycles.

Volcanic eruptions are one of the major natural factors influencing climate variability at interannual to multidecadal timescales. However, simulating volcanically forced climate variability is a challenging task for climate models and one of the major uncertainties in near-term climate predictions. The Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP) is an endorsed contribution to the sixth phase of the Coupled Model Intercomparison Project. This multi-journal special issue on VolMIP aims at collecting relevant research results obtained within the VolMIP framework, and specifically concerning different aspects of the radiative and dynamical climatic response to volcanic forcing, detailed description of effects of different implementation of volcanic forcing in current climate models, aspects concerning the dynamical and chemical atmospheric response to volcanic aerosols simulated by global aerosol models, and comparison between reconstructed and simulated climate evolution after major eruptions. Articles in the special issue should contain the following statement: “This article is part of the multi-journal special issue “The Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP)”. It does not belong to a conference.”

The Modular Earth Submodel System (MESSy) is a multi-institutional project providing a strategy and the software for developing Earth System Models (ESMs) with highly flexible complexity.

The strategy follows a bottom-up approach, meaning that the various processes and diagnostic tools are implemented as so-called submodels, which are technically independent of each other and strictly separated from the underlying technical model infrastructure, such as memory management, input/output, flow-control, etc.

The MESSy software provides generalized interfaces for the standardized control and interconnection (coupling) of these submodels.

The present time-unlimited Special Issue hosts scientific and technical documentation and evaluation manuscripts concerned with the Modular Earth Submodel System and the models build upon it. Moreover, it comprises manuscripts about scientific applications involving these models.

The climate research community uses reanalyses widely to understand atmospheric processes and variability in the middle atmosphere, yet different reanalyses may give very different results for the same diagnostics. For example, the global energy budget and hydrological cycle, the Brewer–Dobson circulation, stratospheric vortex weakening and intensification events, and large-scale wave activity at the tropical tropopause are known to differ among reanalyses.

The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) is a coordinated activity to compare reanalysis data sets with respect to a variety of key diagnostics. The objectives of this project are

1. to understand the causes of differences among reanalyses;
2. to provide guidance on the appropriate usage of various reanalysis products in scientific studies;
3. to contribute to future improvements in the reanalysis products by establishing collaborative links between the reanalysis centres and the SPARC community.

The project focuses predominantly on differences among reanalyses (although studies that include operational analyses are welcome and studies comparing reanalyses with observations are encouraged), with an emphasis on diagnostics in the upper troposphere, stratosphere and mesosphere. This special issue serves to collect research with relevance to S-RIP in preparation for the publication of the S-RIP report in 2018. Although participation in S-RIP is not a prerequisite for submission to this special issue, authors contributing to this collection are encouraged to consider contributing to the preparation of the S-RIP report.

The atmospheric chemistry box modeling system CAABA/MECCA is open-source code published under the GNU General Public License (GPL). The system is based on the box model CAABA (Chemistry As A Boxmodel Application). The chemistry code is provided by the sub-model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere). Additional sub-models provide the code to calculate photolysis, trajectories, isotope tagging and other features.

Fire is an essential feature of terrestrial ecosystems and plays an important role in the Earth system. Fires are regulated by climate, vegetation characteristics, and human activity and also feedback to them in multiple ways. For example, fires can shape vegetation composition and structure; adjust land carbon, nutrient, water, and energy cycles; change atmospheric composition, chemistry, and physics; and affect air quality and human health. Elevated fire activity in 2019 across the arctic, Amazon, and other regions underscores the urgency of a quantitative understanding of fire as an Earth system process that interacts with vegetation, climate, and humans. The EGU2020 B3.17 session on the role of fire in the Earth system received the most abstracts in the Biogeosciences division this year and was highly attended during the live chat. The abstracts cover several aspects of interactions between fire and the biosphere, atmosphere, and humans across various temporal and spatial scales using modelling, field and laboratory observations, and remote sensing. Given that the topic is highly interdisciplinary and overarching, we propose an inter-journal (ESD/BG/ACP/GMD/NHESS) special issue for this session with ESD as the lead journal, and we also encourage the submission of topic-related studies outside the EGU fire session. The special issue will bring together new advances in understanding the feedbacks and interactions between fire and other components of the Earth system at all temporal and spatial scales using various methods, including (1) impacts of fire on weather, climate, and atmospheric chemistry; (2) interactions between fire, the biogeochemical cycle, vegetation composition and structure, and land water and energy budgets; (3) influence of humans on fire and vice versa; (4) fire characteristics (e.g., fire duration, emission factor, emission height, smoke transport); (5) spatial and temporal changes of fires in the past, present, and future; (6) fire products and models, their validation, and error/bias assessment; and (7) analytical tools designed to enhance situational awareness among fire practitioners and early warning systems, addressing specific needs of operational fire behaviour modelling.

The ISTP11 AMT special issue aims at collecting original papers from the 11th Edition of the International Symposium on Tropospheric Profiling (ISTP11) as well as additional contributions fitting well within the scope of tropospheric profiling. The main topics will cover new algorithms, instrumental techniques and synergy to retrieve atmospheric profiling of state parameters and consituents (clouds, aerosols and trace gazes), improved understanding of tropospheric processes thanks to innovative dataset of tropospheric profiles and the use of experimental data to improve numerical weather prediction (model evaluation, advances in modelling and data assimilation).

The last 50 years have enabled significant progress in our ability to describe and model both the oceanic and atmospheric environments of the Indian Ocean and their interactions with the sea floor and coastal regimes. However, our understanding of the major processes of the Indian Ocean is still far from complete and is rudimentary in many respects. This SI compiles various synthesis articles about the recent results of the 2nd International Indian Ocean Expedition Program (IIOE-2: https://iioe-2.incois.gov.in/). The synthesis articles will review and highlight results of physical, biogeochemical, ecological, geological, coastal and atmospheric studies from all parts of the Indian Ocean. Moreover, the SI will serve to illustrate some of the key questions that are relevant for future research on the grand challenges in the Indian Ocean system.

The Wave-driven ISentropic Exchange (WISE) mission is a collaborative research project to investigate transport and mixing processes in the upper troposphere and the lower stratosphere (UTLS) over the Atlantic. Headed by Forschungszentrum Jülich GmbH and the Johannes Gutenberg University of Mainz, 14 measurement flights were conducted with the high-altitude research aircraft HALO in September and October 2017 from Shannon, Ireland. Further partners providing instrumentation were the Karlsruhe Institute for Technology (KIT), the German Aerospace Center (DLR), the universities of Heidelberg, Frankfurt am Main, and Wuppertal, and the German National Metrology Institute. The scientific flights were supported by a team of about 90 persons.

The main objective of this project is to study the relation between chemical composition and the dynamical structure of the UTLS with a focus on the following topics: (i) interrelation of the tropopause inversion layer (TIL) and trace gas distribution, (ii) role of planetary wave breaking for water vapor transport into the extratropical lower stratosphere, (iii) role of halogenated substances for ozone and radiative forcing in the UTLS region, and (iv) occurrence and effects of sub-visual cirrus (SVC) in the lowermost stratosphere.

The special issue mainly includes papers from the airborne measurement campaign itself but is also open to other studies close to the topic. This includes both advanced scientific analyses of observational and measured data during the campaign as well as related climatological studies, but also manuscripts based on instrumental developments.

The Water Vapour Phase II (WAVAS II), a SPARC activity, started in 2008 (SPARC Newsletter No. 30 (2008) p. 16: SPARC Water Vapour Initiative, by C. Schiller et al.). The activity includes satellite assessment and in situ comparison components. This international activity encompasses:

1) Providing a quality assessment of upper tropospheric to lower mesospheric satellite records since the early 1990s
2) Providing, as far as possible, absolute validation against ground-truth instruments
3) Assessing inter-instrument biases, depending on altitude, location, and season
4) Assessing the representation of temporal variations on various scales
5) Including data records on isotopologues
6) Providing recommendations for usage of available data records and for future observation systems


The main objective of WAVAS II is to assess and extend our knowledge and understanding of measurements of the vertical distribution of water vapor in the upper troposphere and middle atmosphere (UT/MA), where water has small concentrations, but significant radiative impact. This is a follow-up of the SPARC WAVAS activity, whose report was published in 2000 (SPARC Report No. 2 (2000) Upper Tropospheric and Stratospheric Water Vapour. D. Kley, J.M. Russell III, and C. Philips (eds.). WCRP-113, WMO/TD – No. 1043). Information gained from this activity will improve our ability to estimate long-term changes with associated uncertainties in UT/MA water as well as make recommendations as to what data would be most valuable for model validation and how such data should be used.

Papers will be accepted for this special issue according to the following guidelines, independent if they originate from the WAVAS II activity or other activities.

Guidelines for submissions:

• Papers covering existing UT/MA satellite water vapour measurements;
• Papers discussing comparisons of UT/MA satellite measurements, including discussion of quantities derived from these measurements, such as seasonal cycles, estimates of transport, or estimates of drifts, trends and variability.
• Papers discussing merging of water vapour measurements will be considered, although this topic is not specifically part of the WAVAS-II activity.
• Model papers that incorporate the datasets discussed and the uncertainty estimates resulting from the WAVAS-II activity will also be considered for inclusion.

The prediction of the winter weather over complex terrain is quite challenging due to the highly variable nature of winds, visibility, and snowfall. As a World Meteorological Organization (WMO) World Weather Research Program (WWRP) Research Demonstration Project (RDP) and Forecast Demonstration Project (FDP), ICE-POP 2018 (International Collaborative Experiments for PyeongChang 2018 Olympic and Paralympic winter games) was held in the PyeongChang region from November 2017 to April 2018 with contributions from 29 agencies from 12 countries. The region was quite unique for observing winter weathers that are influenced by cold air and warm ocean interaction, sudden uplifting by steep terrains near the coast, and modulation by complex terrains. The main scientific goal was to understand the precipitation processes in this unique region during the cold season and to evaluate/improve forecasting from numerical models based on intensive observations. Dense observational networks of upper air observation (eight soundings, two wind profilers, shipborne sounding, and dropsonde), remote sensing (three X-Pol radars, one Ku/Ka-Pol radar and three S-Pol, one S-band, two C-band, and three Doppler lidars), microphysical observation (2DVD, MASC, PIP, Parsivel, MRR, POSS, Pluvio), and surface stations (64 stations) were implemented, in particular, to observe the evolution of precipitation along and across atmospheric flows. The field experiment and real-time forecast demonstration ended and the second phase of the experiment has started for better understanding of the microphysical processes, their better representation in the numerical modeling, and further improvement of winter weather prediction through various international collaborations.

The main purposes of the special issue are

1) to document the scientific findings on the winter weather during the forecast demonstration project

2) to share scientific knowledge on processes of winter weathers that have been investigated with unprecedented dense observational networks,

3) to share current status and improved knowledge of forecasting of winter weathers, and

4) to document new retrieval and quality control methods of the operational and advanced instruments.

The special issue will include all manuscripts related to observational data, products, NWP modeling, researches on observational instrumentation, process/mechanism study, reanalysis, integration of observation and numerical modeling, and prediction of the winter weathers.

The fourth phase of the Air Quality Model Evaluation International Initiative (AQMEII) focuses on deposition processes. Since 2008, AQMEII has analysed a number of critical aspects of regional-scale air quality models that require international collaboration and evaluation. Each of the previous three phases have resulted in multi-paper “special issues” in the peer-reviewed literature (two in Atmospheric Environment and one in ACP). AQMEII leverages the long-term experience in model development and application of the two largest communities of such models, namely the North American community and the European one. These two communities have been engaged in AQMEII model evaluation activities that tackled issues such as operational and probabilistic evaluations (phase 1), online model coupling between air quality and meteorology (phase 2), intercontinental transport of air pollutants (phase 3, ACP SI “Global and regional assessment of intercontinental transport of air pollution: results from HTAP, AQMEII and MICS”, 41 papers), and now dry and wet deposition processes (phase 4). The subject tackled in phase 4 is central to air quality modelling at any scale, and it has never been examined in the systematic and the detailed manner currently underway in AQMEII4. Given the complexity of deposition processes and the variety of approaches adopted by different groups, we opted for an analysis that comprises the individual deposition modules and the full regional-scale models that used those modules as part of a more complex computational system. As customary, for AQMEII two regional modelling domains are considered, a European and a North American one, and simulations are underway for 2 representative years for the processes of interest in the two continents. The process-oriented character of the activity is defined already in the selection of the simulation year. For the North American domain, the air quality in 2010 and 2016 will be simulated to allow comparisons across emissions changes, while for Europe 2009 and 2010 are the designated years to allow comparisons across meteorologically different years. The community presently participating is composed of 15 regional modelling teams and six deposition modules. Subgroups of regional-scale models may be using the same modelling system with a variety of parameterization implementations, or use original models developed in-house. A key feature of the AQMEII multi-model exercise is the submission of all model output on a common grid and at specified observation stations, to a common database at JRC in Ispra, Italy, where a common set of analysis tools which participants can use is available. The variety of the tools used makes possible the comparison of all models with measurements and of specific models with largely used community systems. It also makes it possible to work within the sub-community to exercise different features or modelling options of the community models. Whilst for regional-scale models the evaluation will be based on data on a wide range of pollutants in gas and particle form gathered from standard monitoring networks and soundings in the two continents, in the case of point models, ozone will be the focus, and data have been gathered from sites where high-quality instrumentation has been deployed. Analysis of the ozone dry deposition modules driven by observed site-specific meteorological and biophysical data (i.e., the point models) will focus on process-oriented model evaluation and intercomparison. As mentioned the central topic of the issue is the deposition processes, but differently from the past a deeper level of analyses has been added in AQMEII4. That pertains to the addition of diagnostic analysis to each of the participating models. These diagnostics are allowing participants to compare the parameterizations for deposition pathways across all participating systems in an unprecedented level of detail, with comparisons between individual conductance and resistances being carried out for the first time. The diagnostics have been mapped to a common set of land-use types, in order to allow a common analysis of the different deposition parameterizations for a given land-use type. The AQMEII4 activity goes beyond the comparison of the deposited quantities as has been customary so far and will allow the gathering of insight into the reason for model differences and the efficacy of some schemes compared to others through the comparison with measurements. For the first time the diversity with which the participating models (both point models and the full regional-scale models) describe the underlying surface on which the deposition occurs will be evaluated. This is therefore not a mere model evaluation exercise but one that aims to tackle in a multi-model fashion the most difficult of the four pillars of model evaluation (Dennis et al., 2010), i.e. diagnostic evaluation. The special issue will therefore include papers on process-oriented point model comparisons against deposition supersite data, detailed process-oriented model inter-comparisons of the regional-scale output, and the potential application of model results for ecosystem assessments. An introductory paper (already in preparation) will be the first submission of the special issue, which will describe activity set-up and rationale – an important and crucial preparatory phase which will be referenced by the individual papers that follow.

In recent years, there have been multiple community-level efforts to improve our scientific understanding of the possible effectiveness and effects of solar geoengineering, especially the Geoengineering Model Intercomparison Project (GeoMIP) and the Geoengineering Large Ensemble (GLENS). These projects have wide international participation among climate modeling centers and researchers, including developing country researchers participating in the Developing Country Impacts Modelling Analysis for SRM (DECIMALS) fund. By producing simulations available for any interested researcher, these two projects have been highly effective in advancing the state of knowledge of solar geoengineering.

Several of these projects are reaching critical stages. A new round of GeoMIP simulations has recently been released as part of CMIP6. The DECIMALS teams are maturing and are at the stage where they are ready to produce papers, with the possibility that the project will be extended. GLENS simulations are continuing to be analyzed and supplemented with simulations from additional research efforts. To capture the interrelated research amongst these projects, the special issue is set up jointly between Atmospheric Chemistry and Physics and Earth System Dynamics. In ACP, we invite papers that emphasize process-level understanding and stratospheric dynamics; in ESD, submissions on Earth system effects, feedbacks, and impacts (like agriculture or ecosystem responses) are invited.

The absorption of energy by water vapour keeps the average temperature on Earth at a level that makes life possible. However, the average temperature increased in recent decades due to increased greenhouse gas emissions. This anthropogenic increase is amplified by the water vapour feedback. Water vapour is further an important component of the water cycle and indirectly of the energy cycle. The role of water vapour in these cycles in a changing climate needs to be fully understood and quantified. Thus, high-quality observations of water vapour and the characterisation and validation of associated uncertainties are of high importance for climate applications. Topics covered around atmospheric water vapour include, among others, retrieval development, data records, uncertainty characterisation and validation, inter-comparisons, climate applications, and model validation.

This special issue originates from the GEWEX Water Vapor Assessment (G-VAP, http://gewex-vap.org) and is open for all submissions within scope, not just from G-VAP.

Ice nucleation is a particularly hot topic at the moment in atmospheric sciences, with profound implications for the hydrological cycle and climate. However, the detailed links between the atmospheric aerosol chemical and physical character, ice-nucleating particles (INPs), and the ice nucleation processes in different temperature regimes remain open (e.g. Hoose and Möhler, 2012, Knopf et al., 2018). We performed a comprehensive field study in Hyytiälä, Finland, during 2018 (HyICE campaign) with a clear focus on characterizing the INP concentrations in different temperature ranges with a suite of ice nucleation counters (online and offline). The work was complemented with thorough measurement of physical (size distribution 1 nm–10 um, bioaerosol concentrations) and chemical characteristics (high-resolution AMS) of the atmospheric aerosols from nanometre scale to super-micron sizes done at the Station for Measuring Ecosystem – Atmosphere Relations II (SMEAR II) site. The location provides an excellent facility to study INPs as we can compare our data to the 20-year data set already available at the site. The observation period started in February, with the most intensive INP measurements in March–May 2018. We operated a suite of online ice nucleation counters (SPIN, PINE, PINCII) and performed filter sampling at the ground level, above the canopy, and onboard a research aircraft. We continue the filter sampling providing understanding of the seasonality of INP concentrations in the boreal environment. In SMEAR II we have also the weather radar and lidar capacity to address the cloud microphysics within the clouds. These activities enable us 1) to perform an intercomparison between the ice nucleation chambers, 2) to assess the vertical variability of INPs inside the boundary layer and in the free atmosphere, 3) to connect the observed INP concentrations to physical and chemical parameters and aerosol source apportionment and 4) seasonal variability of the INPs in the boreal context, 5) to connect the ground-based INP measurements to the cloud processes aloft, and 6) to develop parameterizations based on our observations and utilize them in models of various scales.

The prediction of the winter weather over complex terrain is quite challenging due to the highly variable nature of winds, visibility, and snowfall. As a World Meteorological Organization (WMO) World Weather Research Program (WWRP) Research Demonstration Project (RDP) and Forecast Demonstration Project (FDP), ICE-POP 2018 (International Collaborative Experiments for PyeongChang 2018 Olympic and Paralympic winter games) was held in the PyeongChang region from November 2017 to April 2018 with contributions from 29 agencies from 12 countries. The region was quite unique for observing winter weathers that are influenced by cold air and warm ocean interaction, sudden uplifting by steep terrains near the coast, and modulation by complex terrains. The main scientific goal was to understand the precipitation processes in this unique region during the cold season and to evaluate/improve forecasting from numerical models based on intensive observations. Dense observational networks of upper air observation (eight soundings, two wind profilers, shipborne sounding, and dropsonde), remote sensing (three X-Pol radars, one Ku/Ka-Pol radar and three S-Pol, one S-band, two C-band, and three Doppler lidars), microphysical observation (2DVD, MASC, PIP, Parsivel, MRR, POSS, Pluvio), and surface stations (64 stations) were implemented, in particular, to observe the evolution of precipitation along and across atmospheric flows. The field experiment and real-time forecast demonstration ended and the second phase of the experiment has started for better understanding of the microphysical processes, their better representation in the numerical modeling, and further improvement of winter weather prediction through various international collaborations.

The main purposes of the special issue are

1) to document the scientific findings on the winter weather during the forecast demonstration project

2) to share scientific knowledge on processes of winter weathers that have been investigated with unprecedented dense observational networks,

3) to share current status and improved knowledge of forecasting of winter weathers, and

4) to document new retrieval and quality control methods of the operational and advanced instruments.

The special issue will include all manuscripts related to observational data, products, NWP modeling, researches on observational instrumentation, process/mechanism study, reanalysis, integration of observation and numerical modeling, and prediction of the winter weathers.

Fire is an essential feature of terrestrial ecosystems and plays an important role in the Earth system. Fires are regulated by climate, vegetation characteristics, and human activity and also feedback to them in multiple ways. For example, fires can shape vegetation composition and structure; adjust land carbon, nutrient, water, and energy cycles; change atmospheric composition, chemistry, and physics; and affect air quality and human health. Elevated fire activity in 2019 across the arctic, Amazon, and other regions underscores the urgency of a quantitative understanding of fire as an Earth system process that interacts with vegetation, climate, and humans. The EGU2020 B3.17 session on the role of fire in the Earth system received the most abstracts in the Biogeosciences division this year and was highly attended during the live chat. The abstracts cover several aspects of interactions between fire and the biosphere, atmosphere, and humans across various temporal and spatial scales using modelling, field and laboratory observations, and remote sensing. Given that the topic is highly interdisciplinary and overarching, we propose an inter-journal (ESD/BG/ACP/GMD/NHESS) special issue for this session with ESD as the lead journal, and we also encourage the submission of topic-related studies outside the EGU fire session. The special issue will bring together new advances in understanding the feedbacks and interactions between fire and other components of the Earth system at all temporal and spatial scales using various methods, including (1) impacts of fire on weather, climate, and atmospheric chemistry; (2) interactions between fire, the biogeochemical cycle, vegetation composition and structure, and land water and energy budgets; (3) influence of humans on fire and vice versa; (4) fire characteristics (e.g., fire duration, emission factor, emission height, smoke transport); (5) spatial and temporal changes of fires in the past, present, and future; (6) fire products and models, their validation, and error/bias assessment; and (7) analytical tools designed to enhance situational awareness among fire practitioners and early warning systems, addressing specific needs of operational fire behaviour modelling.

The 2019 Raikoke eruption emitted an estimated 1.5±0.2 Tg of volcanic sulfur dioxide (SO₂) into the atmosphere, which makes it the largest volcanic SO₂ perturbation in the upper troposphere and lower stratosphere since the 2011 eruption of Nabro. The 2019 Raikoke eruption triggered numerous discussions among scientists involved in the SPARC Volcano Response (VolRes) initiative and provides an excellent opportunity to test our understanding of processes involved in the formation, growth and removal of volcanic aerosols in the atmosphere. Remote sensing measurements from geostationary and low-orbiting satellites with ultraviolet, visible, and infrared sensors (HIMAWARI-8, OMPS/NPP, OMI/Aura, TROPOMI/SP5, IASI/MeteopA-B-C, AIRS/Aqua, CALIOP/CALIPSO, SAGE III/ISS) provided a wealth of information on the SO₂ concentration, ash and sulfate aerosol particle optical properties from the initial injection through to the growth and removal of aerosols. This eruption therefore provides an excellent opportunity to inter-compare satellite observations and validate the different retrievals for SO₂ and volcanic aerosol properties from new satellite sensors such as TROPOMI. After the initial injection into altitudes up to 15–16 km, the plume reached 25–26 km altitude in late 2019, which corresponds to a fast vertical ascent that appears unusual compared to previous eruptions. The fast vertical ascent of the 2019 Raikoke plume may suggest an influence of smoke aerosol from large wildfires that took place in Canada and Siberia around the time of the eruption. The veil of aerosol particles from Raikoke 2019 has noticeably reduced the transparency of the stratosphere and detailed information and measurements on the aerosol properties will be useful to constrain atmospheric and climate model simulations.

The Pan-Eurasian Experiment (PEEX) (www.atm.helsinki.fi/peex) is a multi-disciplinary, multi-scale and multi-component research infrastructure and capacity building programme. The PEEX originated from a bottom–up approach by the science communities in 2012. The programme aims to resolve the major uncertainties in Earth system science and global sustainability issues concerning the Arctic and boreal Pan-Eurasian regions as well as China. The PEEX solves interlinked global grand challenges influencing human well-being and societies in northern Eurasia and China by establishing and maintaining long-term, coherent and coordinated research activities as well as continuous, comprehensive research and educational infrastructures. The scientific issues covered by PEEX include climate change, air quality, biodiversity loss, chemicalization, food supply, freshwater and use of natural resources by mining, industry, energy production and transport. Our approach is integrative and interdisciplinary, recognizing the important role of the Arctic and boreal ecosystems in the Earth system.

The programme has had an open special issue in Atmospheric Chemistry and Physics in 2015–2019/2020 and has published 55 papers in total. A major part of the published papers originated from the PEEX collaboration, including organization of the PEEX science conferences, special sessions in the international conferences and on INAR Univ. Helsinki (PEEX-HQ) international collaboration, especially with the air quality research groups from China. The papers contributed to the large-scale research questions addressed by the PEEX Science Plan (www.atm.helsinki.fi/peex/images/PEEX_Science_Plan.pdf). PEEX is currently creating an overview of the research carried out during the last 5 years and is re-visiting its research agenda and large-scale research questions. PEEX continues organizing scientific conferences and workshops on a regular basis and is connected to the Arctic research community by U-Arctic thematic network "Arctic - boreal hub" (www.uarctic.org/organization/thematic-networks/arctic-boreal-hub/) coordinated by INAR Univ.Helsinki. The applied new PEEX- Part II special issue in ACP will be open for all submissions within these frameworks.

Sand and dust storms (SDSs) are extreme meteorological phenomena that generate significant amounts of airborne mineral dust particles. SDSs play a significant role in different aspects of weather, climate and atmospheric chemistry. In addition, SDSs represent a severe hazard for life, health, property, the environment and the economy, which is aligned with several Sustainable Developed Goal (SDG) targets established by the United Nations (UN). Understanding, managing and mitigating SDS risks and effects requires fundamental and cross-disciplinary knowledge and filling current knowledge gaps on the dust cycle. Recent developments in Earth observation have led to big advances in the observational capabilities of desert dust particles. Research products show the possibility of exploring the wide dimensional range of desert dust particles by integrating different observational techniques on the ground, while satellite observations are currently providing aerosol description with finer and finer spatiotemporal resolution. Experts on dust observations and modelling participating in the EU-funded COST Action InDust (www.cost-indust.eu, CA16202) have identified critical scientific gaps in our current knowledge of dust aerosol measurements, modelling, processes and effects and have also identified a critical mass of scientists working towards addressing them. Such gaps include at least issues such as - the need for improvement of surface-based dust aerosol measurements (in situ, sun photometric, lasers), including the use of new technologies, improved algorithms and combined approaches; - the improvement of the global coverage on aerosol dust observation through improvement of satellite-based retrievals; - advancements in dust aerosol modelling (sources, mechanisms, deposition); - the presentation of intensive campaign results with a focus on dust aerosols; - dust mineralogy; - the assessment of the dust particle size distribution with an emphasis on giant particles; - enhancements regarding dust aerosol effects on different communities (e.g. solar energy, aviation, health, climate, air quality, agriculture, ocean life). This special issue focuses on addressing the above-mentioned gaps through individual and combined efforts from at least the COST InDust community (0000108:0000100 scientists from 52 countries). A list of papers to be submitted has been created including results related to SDS-WAS, ACTRIS/EARLINET activities, long-term measurement analysis of measured and modelled data, dust retrieval algorithm and techniques, dust effects, and foreseen results of the ESA Aeolus Cal/Val activity “ASKOS” (Cabo Verde, June - July, 2020).

Satellite limb measurements have become an essential tool to remotely sense the chemical composition of the Earth's atmosphere with a particular focus on the stratosphere. They enable combining near global sampling on a daily basis with vertical profiling capability and thus overcome some of the limitations of nadir or occultation measurements. Although the number of satellite limb instruments is currently declining, several missions are still operational (e.g., OSIRIS, SMR, ACE-FTS, MAESTRO, OMPS-LP and SAGE III) and several new missions are planned or will be launched in the near future (e.g., MATS, ICON, ALTIUS and several mini satellites). In addition, the data sets of several past limb sounders, particularly the Envisat instruments SCIAMACHY, MIPAS and GOMOS are still used for many scientific studies and new data products are developed based on measurements with these instruments.

This combined ACP/AMT special issue is open for submissions dealing with all aspects related to limb measurements (including limb scatter, limb emission and occultation) in the optical, IR and microwave spectral regions. This includes instrument development and characterization, retrieval algorithm development, validation studies, as well as scientific applications based on the retrieved data sets. We note that radio occultation measurements are outside the scope of the special issue.

The version 8 spectral data of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), released in January 2019, are supposed to be the final set. Besides the ORM reprocessed version 8 level-2 MIPAS data distributed by ESA, a research processor has been run at IMK and IAA to generate temperature and trace gas concentrations. The IMK–IAA data product includes a larger list of atmospheric constituents and covers a wider altitude range in the case of the special observation modes than the operational data. The processing is based on independent algorithms. This special issue collects articles which 1) document the retrieval strategies used to derive these distributions along with uncertainty estimates, 2) report validation studies, and 3) use the data to answer current questions in atmospheric research.

With the launch of the novel Aeolus wind profiling mission, a wealth of direct wind and atmospheric backscatter profile observations has become available across the globe, revealing new information on atmospheric dynamics and optical properties to the benefit of a number of applications. These applications include, for example, improvements in numerical weather prediction (NWP), general circulation models (GCMs) and their included parameterized processes, such as land and ocean drag, convection, stratosphere–troposphere interaction and atmospheric waves, air quality and air pollution transport, etc. In this special issue, new data products and their validation as well as studies using the Aeolus data for application in meteorology, air quality, and climate studies are discussed. Topics vary from observation interpretation to GCM or NWP model diagnostics, parameterization, and data assimilation experiments. This is the first special issue on scientific analysis of Aeolus mission data, including inter-comparisons with different collocated wind profiling, aerosol and cloud in situ and remote-sensing measurements, and atmospheric models. Contributions concerning Aeolus data calibration, processing, and tools for data access and exploitation are also provided.

This ACP Special Issue seeks to report research advancements in the understanding of mercury sources, distribution pathways, levels, physicochemical characterization, measurement traceability, impacts on global ecosystems, and policy-relevant information presented at the ICMGP 2019 and being developed under the MercOx (Metrology of oxidized mercury) project and two ERA-PLANET initiatives – Integrative and Comprehensive Understanding on Polar Environments (iCURE) and Integrated Global Observing Systems for Persistent Pollutants (iGOSP). The Special Issue is open to all publications related to the scope of the Special Issue.

This special issue includes papers based on recent field campaigns, including US Department of Energy Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA), NASA North Atlantic Aerosols and Marine Ecosystems Study (NAAMES), and the North Atlantic Climate System Integrated Study (ACSIS), and long-term measurements from observatories (e.g., the Atmospheric Radiation Measurement Eastern North Atlantic site on Graciosa Island in the Azores). Presently the responses of marine low-cloud systems to changes in atmospheric greenhouse gases and aerosols are major sources of uncertainty that limit our ability to predict future climate. Major contributions to this uncertainty derive from poor understanding of the cloud responses to aerosol changes and the natural aerosol state that is being perturbed by anthropogenic emissions. One overarching scientific objective of these recent field campaigns is to understand key processes that drive the properties and interactions of aerosol and clouds under representative meteorological and cloud conditions. Another important focus is to validate and improve ground-based retrieval algorithms using comprehensive in situ aircraft measurements. This will lead to high-quality, long-term datasets from the routine ground-based remote sensing, which allow greater statistical reliability in the observed properties and relationships among aerosols, clouds, and precipitation than is possible with the aircraft measurements alone.

With a wide variety of aerosol optical, physical, and chemical properties, China offers a test bed for the study of aerosol processes and techniques leading to a better understanding of aerosol properties. Aerosol concentrations in the southeast of China are periodically among the highest in the world, while at other times they are relatively low. The prevailing aerosol chemical composition (e.g., sulfate, nitrate, black carbon, brown carbon, particulate organic matter, sea salt) also changes significantly with the quick evolution of both the economy and society, along with the government’s strong environmental actions. The complex mechanisms of the secondary formation of aerosols and haze-fog interaction are still not well understood in this important region. In areas other than the southeast of China, topography determines the aerosol concentrations, which are consistently high. Examples are Sichuan Province or the Guanzhong Basin, i.e. large basins surrounded by high mountains. Furthermore, the large deserts in the west and north of China are strong sources of dust particles influencing the aerosol properties over a large part of the country. There are however also large areas with relatively low aerosol loading, in particular in mountainous areas and over the high plateaus such as Tibet. Aerosol chemical properties in the center and east of China still need to be clarified both geographically and temporally, e.g. distinguish spectrally absorbing components like mineral dust and brown carbon from remote sensing observations. Anthropogenic emissions of aerosols and precursor gases have a large influence on aerosol, in regards to both chemical and physical properties, and consequently optical properties, and their effects can be augmented by the transport of aerosol from elsewhere, such as carbonaceous particles from biomass burning aerosol and dust particles emitted from the deserts. The complex mechanisms and intrinsic relations of aerosol chemistry and physics, and their impacts on optical properties, need better interpretation based on comprehensive observation and analysis. Weather conditions have a large influence on the formation of haze, and haze episodes occur more and more frequently. The influence of large-scale weather systems such as the monsoon or the Siberian high may result in inter-annual variations in different regions of China. In particular the progress of the monsoon from south to north during spring and summer, and back in the autumn, results in strong variations in aerosol content.

In this special issue we focus on the application of remote sensing techniques using both ground-based and satellite measurements to study the temporal and spatial variations in aerosol properties over China on different scales. Results from campaigns in the Taklamakan Desert (Kashi), on the Tibetan Plateau, and other remote areas such as the NE of China are contrasted with those from densely populated and industrialized urban areas in the east of China. The special issue aims to bring together results from different projects such as Kashi, MOST, and regional air quality studies. Contributions are solicited on aerosol optical, physical, and chemical properties inferred from remote sensing observations and related optical techniques, and the spatial and temporal variations in these properties across China, with emphasis on processes leading to the observed differences, including the influence of meteorological conditions and large-scale weather systems.

The project Effect of Megacities on the Transport and Transformation of Pollutants on the Regional and Global Scales, EMeRGe, was created to experimentally investigate the transport and transformation of air plumes coming from major population centres, MPCs. It comprises a set of unique airborne measurement campaigns carried out in two parts of the world having MPCs of very different characteristics and emissions. The two measurement campaigns using the HALO research aircraft (www.halo-spp.de) were conducted in Europe in summer 2017 and in East Asia during the intermonsoon period in March-April 2018. The initial scientific team of EMeRGe, comprising scientists from four German universities and four research centres in 2016 (www.iup.uni-bremen.de/emerge/), was extended within the EMeRGe international membership, incorporating more than 50 research partners from 16 different countries. The combined and synergetic set of information obtained from airborne and ground-based data, satellite products, and their comparison with models enables the patterns of atmospheric composition and meteorological parameters observed during the campaigns to be constrained and thereby our understanding of the emissions, their transport, and physico-chemical transformation of the outflow from MPC emissions to be tested and assessed. The EMeRGe special issue aims to publish the scientific results achieved by the project EMeRGe. This comprises contributions of the partners of the national and international EMeRGe scientific community. It solicits all relevant submissions addressing the EMeRGe science objectives.

The Wave-driven ISentropic Exchange (WISE) mission is a collaborative research project to investigate transport and mixing processes in the upper troposphere and the lower stratosphere (UTLS) over the Atlantic. Headed by Forschungszentrum Jülich GmbH and the Johannes Gutenberg University of Mainz, 14 measurement flights were conducted with the high-altitude research aircraft HALO in September and October 2017 from Shannon, Ireland. Further partners providing instrumentation were the Karlsruhe Institute for Technology (KIT), the German Aerospace Center (DLR), the universities of Heidelberg, Frankfurt am Main, and Wuppertal, and the German National Metrology Institute. The scientific flights were supported by a team of about 90 persons.

The main objective of this project is to study the relation between chemical composition and the dynamical structure of the UTLS with a focus on the following topics: (i) interrelation of the tropopause inversion layer (TIL) and trace gas distribution, (ii) role of planetary wave breaking for water vapor transport into the extratropical lower stratosphere, (iii) role of halogenated substances for ozone and radiative forcing in the UTLS region, and (iv) occurrence and effects of sub-visual cirrus (SVC) in the lowermost stratosphere.

The special issue mainly includes papers from the airborne measurement campaign itself but is also open to other studies close to the topic. This includes both advanced scientific analyses of observational and measured data during the campaign as well as related climatological studies, but also manuscripts based on instrumental developments.

The Aerosol Chemistry Model Intercomparison Project (AerChemMIP) is endorsed by the Coupled-Model Intercomparison Project 6 (CMIP6) and is designed to quantify the climate and air-quality impacts of aerosols and chemically reactive gases. Specifically, the aim of AerChemMIP is to answer four scientific questions. 1. How have anthropogenic emissions contributed to global radiative forcing and affected regional climate over the historical period? 2. How might future policies (on climate, air quality, and land use) affect the abundances of NTCFs and their climate impacts? 3. How do uncertainties in historical NTCF emissions affect radiative forcing estimates? 4. How important are climate feedbacks to natural NTCF emissions, atmospheric composition, and radiative effects? This special issue is to be devoted to papers that analyse targeted simulations with CMIP6 climate models to address these questions as well as papers that evaluate these models against observations. It also includes papers from other CMIP6-related activities (e.g. RFMIP, DAMIP), containing contributions to the understanding of climate forcing and feedbacks involving aerosols, reactive gases, and associated biogeochemical cycles.

The ISTP11 AMT special issue aims at collecting original papers from the 11th Edition of the International Symposium on Tropospheric Profiling (ISTP11) as well as additional contributions fitting well within the scope of tropospheric profiling. The main topics will cover new algorithms, instrumental techniques and synergy to retrieve atmospheric profiling of state parameters and consituents (clouds, aerosols and trace gazes), improved understanding of tropospheric processes thanks to innovative dataset of tropospheric profiles and the use of experimental data to improve numerical weather prediction (model evaluation, advances in modelling and data assimilation).

The last 50 years have enabled significant progress in our ability to describe and model both the oceanic and atmospheric environments of the Indian Ocean and their interactions with the sea floor and coastal regimes. However, our understanding of the major processes of the Indian Ocean is still far from complete and is rudimentary in many respects. This SI compiles various synthesis articles about the recent results of the 2nd International Indian Ocean Expedition Program (IIOE-2: https://iioe-2.incois.gov.in/). The synthesis articles will review and highlight results of physical, biogeochemical, ecological, geological, coastal and atmospheric studies from all parts of the Indian Ocean. Moreover, the SI will serve to illustrate some of the key questions that are relevant for future research on the grand challenges in the Indian Ocean system.

In order to improve our current knowledge on the budgets of the two most important anthropogenic greenhouse gases, CO₂ and CH₄, a co-ordinated measurement campaign in the central European region was successfully carried out in May-June 2018 with the German research aircraft HALO and two smaller Cessna aircraft as its main experimental platforms. The goal of CoMet is to combine a suite of state-of-the-art airborne active (lidar) and passive remote sensors (spectrometer) with in situ instruments to provide regional-scale data about greenhouse gases (GHGs) which are urgently required for their accurate modelling.

During the intensive observation period, HALO research flights were performed along extended latitudinal transects to capture GHG gradients, over known regions of strong emissions, and over the ground-based remote-sensing sites of the Total Carbon Column Observing Network (TCCON). While HALO provides a larger-scale picture, the two Cessna aircraft concentrated on a region of prime interest: the Upper Silesian Coal Basin in Poland, which, due to hard coal mining activities, is one of the hot spots of anthropogenic methane emissions in Europe. Here, a variety of ground-based instruments additionally supported the CoMet mission: FTIR spectrometers, wind lidars, mobile vans, and small drones provided valuable information to quantify CH₄ emissions from coal mining activities. A model infrastructure (regional inverse modelling, chemistry-climate modelling with regional refinement) has been employed in forecast mode to optimize flight strategies and is now used to hindcast the campaign period to evaluate GHG fluxes and transport. The CoMet mission is also part of validation activities for existing passive remote sounding GHG satellites (GOSAT, Sentinel-5P, and OCO-2) and preparation of the first active CH₄ satellite mission MERLIN. For that reason, the CHARM-F lidar developed at DLR as an airborne MERLIN demonstrator was one of the key instruments. In addition, CoMet is intended to investigate methodologies for the synergistic combination for GHG measurements using lidar and passive remote sensing.

In the vast low-nutrient—low-chlorophyll (LNLC) ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be 1 order of magnitude greater over short timescales. Recently, significant experimental, field and modeling work allowed us to better link atmospheric deposition with nutrient availability, ocean productivity, carbon cycling and marine emissions. By improving our understanding on how atmospheric inputs are impacting biological activity and the carbon balance in oligotrophic environments, their representation in biogeochemical models at different timescales and space scales will be better assessed. In particular, processes may be affected by ongoing environmental changes in the ocean such as a decrease in pH and an increase in temperature.

This special issue aims at gathering contributions from both experimental and modeling approaches showing how natural or anthropogenic inputs from the atmosphere can impact biogeochemical and ecological processes in the present and future oligotrophic ocean. It will present the results obtained during the PEACETIME (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea) cruise conducted in 2017 in the Mediterranean Sea, but the special issue is open to any submissions provided that the subject is consistent with the objectives defined above.

The atmospheric chemistry box modeling system CAABA/MECCA is open-source code published under the GNU General Public License (GPL). The system is based on the box model CAABA (Chemistry As A Boxmodel Application). The chemistry code is provided by the sub-model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere). Additional sub-models provide the code to calculate photolysis, trajectories, isotope tagging and other features.

StratoClim is a collaborative research project funded by the European Commission 7th Framework programme. The main objective of the project is to improve the understanding of key processes in the upper troposphere and stratosphere (UTS) for more reliable projections of climate change and stratospheric ozone. StratoClim focuses on the understanding of the microphysical, chemical, and dynamical processes that determine the composition of the UTS, such as the formation, loss, and redistribution of aerosol, ozone, and water vapour; how these processes are affected by climate change; and how they feed back on global climate.

The issue will include papers coming from all sectors included in the project: in situ and sonde measurement deployment, satellite data products and analysis, mesoscale and transport modelling, and CCM experiments and analysis. We expect a special emphasis on the results from the high-altitude research aircraft campaign in the Asian monsoon area carried out in 2017 deploying an innovative and comprehensive payload. Due to the expected contribution of instrumental manuscripts, the issue will also include AMT submissions.

In this special issue papers resulting from the project “Marine biological production, organic aerosol particles and marine clouds: a Process Chain – MarParCloud” and closely related projects shall be aggregated. MarParCloud is a highly interdisciplinary project funded by the German Leibniz Society (https://www.leibniz-gemeinschaft.de/en/about-us/leibniz-competition/projekte-2016/funding-line-2/), which aims to improve our knowledge on the nature, formation, transfer and effects of organic matter (OM) in the marine environment. To this end the biological formation of OM in the ocean, its accumulation in the sea surface microlayer (SML), the export to marine aerosol particles and, finally, the importance of OM for the formation of cloud droplets and ice crystals are investigated. Within MarParCloud several field and tank studies were performed to investigate ambient seawater, the SML, aerosol particles and cloud water as well as water and air samples obtained during artificial bubble experiments. Closely related campaigns comprise the EU project MARSU (“Advanced analytical and mass spectrometric techniques to study ocean-atmosphere interactions”) and the MILAN study (“sea-surface MIcroLAyer at Night”), focusing on diel dynamics of the uppermost layer of the ocean, as well as the cruise EMB184 “Aerosol” performed with the R/V Elisabeth Mann Borgese. In addition to results from these projects and campaigns, this special issue is open for all submissions concerning the cycling of OM in the marine environment with potential atmospheric implications.

This inter-journal special issue of AMT, ACP, and GMD serves as a thematic collection for manuscripts arising from the work of Horizon 2020 integrated infrastructure project Eurochamp-2020; cf. https://www.eurochamp.org/ . As Eurochamp-2020 has project parts dealing with new instrumentation, with atmospheric chemistry and physics studies as well as with modelling, contributions are expected for all three journals as part of the inter-journal special issue.

Rapid population growth and urbanisation worldwide accelerate eco-environmental/socio-economic stress as well as adverse climatic and health impacts on urban dwellers. Atmospheric modelling research has largely been performed on a horizontal grid spacing of kilometres or larger due to a lack of understanding of the local-scale phenomena, appropriate parameterisations, and adequate modelling tools, and computer resources. Urban/local street-level air pollution, climate change, and their impacts on population exposure and human health have increasingly received attentions by both researchers and policymakers around the world. Several state-of-the-science models have been recently developed for urban/local street-level air pollution modelling, including the street-network model, the Model of Urban Network of Intersecting Canyons and Highways (MUNICH) that incorporates detailed representations of gas-phase chemistry and secondary aerosol formation pathways, and the Street-in-Grid model (SinG) that dynamically combines a 3-D Eulerian chemical-transport model (CTM), Polair3D, with MUNICH. There have been increasing numbers of developers and users for MUNICH, SinG, and other similar coupled 3-D CTMs and urban canyon models for street-level air pollution modelling worldwide. Recognising the urgent need for scientific advancement, pollution/exposure assessment, policymaking, and public health protection at urban/local-street scales, we launch this special issue to advance scientific understanding of local-scale atmospheric phenomena and promote research and discussion on state-of-the-science urban/local-street-level air quality model development and application for complex interactions among urban air pollution, climate, and health. The special issue is open for all submissions which address the following themes:

• 3-D Street-in-Grid (SinG) model development and application;
• urban canyon/network model development and its incorporation into 3-D CTMs;
• urban/street-level air quality modelling in support of human exposure assessment;
• impact of urban traffic emissions on air quality and human health at a street level.

The research foci in the MICS-Asia III study include the following aspects: development of reliable emission inventories in Asia; ability and uncertainty of current air quality models in Asia in predicting ozone, nitrogen oxides, VOC, sulfur oxides, PM_2.5, etc.; ability of online coupled models to simulate aerosol radiative forcing and ozone and aerosol–weather/climate interaction through aerosol–radiation–cloud feedback; and prediction of future air pollution and climate. The special issue is open to all publications that inform about these aspects and provide answers to the MICS-Asia III scientific questions.

• What is the ability of models to reproduce pollutant concentrations of photochemical smog under highly polluted conditions (regional haze) and to model a sharp PM_2.5 increase such as the episodes between January 2010 and March 2018?
• What are the uncertainties of process (physical and chemical processes) and model resolutions (horizontal and vertical) in air quality modelling? In particular, uncertainties in key boundary layer parameters, which have both direct and indirect impacts on modelling results by transporting meteorological fields and providing parameters in physical/chemical modules, respectively, need to be addressed.
• What are the air quality responses to specific emission perturbations in a common case?
• What are the impacts of anthropogenic emissions on radiation and climate change over East and Southeast Asia?
We especially welcome the publications related to MICS-Asia Phase III studies to inform the research community about the recent findings and progress of air quality and regional climate change studies over East and Southeast Asia. The findings from these publications will also help to better understand capability, uncertainty, and future direction within the current CTMs.

This special issue will include papers that guide the path forward in making reliable predictions of aerosol effects on the heat budget of the South East Asian subcontinent by improving our knowledge of aerosol processes and their influence on the Indian monsoon. Several recent major studies have aimed to provide a detailed determination of aerosol physical and chemical properties across India prior to and during the Indian monsoon, and results from these studies will be welcomed. Most recently the South West Asian Monsoon Interactions project used a combination of UK and Indian research aircraft and a network of ground-based measurement sites. The analysis of data will enable assessment of aerosol composition and mixing state, provide source characterization, and help quantify aerosol optical properties such as extinction, absorption, and single scattering albedo that drive regional climate. Results that use this detailed characterization to test representations of aerosol properties in regional and global climate models will be included. The special issue will report on evaluations of and improvements in model representations of aerosol properties with data over India.

The purpose of this special issue is the compilation of modelling and observational studies in connection with five international field deployments (AEROCLO-sA, CLARIFY, LASIC, ORACLES, and NaFoLiCA) that focus on the interactions of natural and anthropogenic aerosols with radiation, clouds, and regional climate in the South Atlantic Ocean and the southern African region. These deployments, based in Namibia, Ascension Island, and São Tomé, took place between 2016 and 2018 and support a significant number of investigations extending beyond just the individual science teams. The airborne and ground-based observations, as well as the related satellite measurements and climate modelling studies, address all aspects of aerosol–cloud–climate interactions, including the link of aerosol properties to meteorological fields and dynamical processes that influence aerosol emission and transport. The projects also target the advancement of remote sensing of aerosols for complex scenes over land, sea, and clouds. The special issue will be open to all submissions, with complementary goals to the five mentioned deployments, so as to encourage the exchange of ideas from inside and outside the science teams of all projects.

In this special issue papers resulting from two major combined field campaigns shall be aggregated: (i) the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD), and (ii) the Physical feedbacks of Arctic boundary layer, Sea ice, Cloud and AerosoL (PASCAL). These two concurrent campaigns took place in the vicinity of Svalbard in May and June 2017. They were designed to study processes important for explaining Arctic amplification, and, in particular, for investigating the role of microphysical and dynamical properties of Arctic low- and mid-level, mixed-phase clouds, and their interactions with atmospheric radiation and aerosol particles. Ground-based, ship-borne, tethered balloon, aircraft, and satellite observations have been combined. The research vessel (RV) Polarstern, an ice floe camp (erected close to the icebreaker) including an instrumented tethered balloon, and the two research aircraft, Polar 5 and Polar 6, were jointly operated. Polar 5 served as a mobile remote sensing observatory looking at the clouds from above, whereas Polar 6 operated as a flying in situ measurement laboratory mostly sampling inside the clouds. The permanent ground station of Ny-Ålesund observed the clouds from below, applying similar but upward-looking remote sensing equipment as Polar 5. Some of the flights were performed underneath respective satellite tracks. In this special issue we compile a number of papers reporting about the results of the observations conducted during ACLOUD/PASCAL within the framework of the (AC)³ project (http://www.ac3-tr.de/).

The Hydrological cycle in the Mediterranean Experiment (HyMeX, https://www.hymex.org/) programme is a 10-year concerted effort at the international level started in 2010 with aims to advance the understanding of the water cycle, and with emphases on the predictability and evolution of high-impact weather events, as well as on evaluating social vulnerability to these extreme events. The special issue is jointly organized between the Atmospheric Chemistry and Physics, Hydrology and Earth System Sciences, Ocean Science, Natural Hazards and Earth System Sciences, Atmospheric Measurement Techniques, and Geoscientific Model Development journals. It aims at gathering contributions to the areas of understanding, modelling, and predicting at various timescales and spatial scales of the Mediterranean water cycle and its related extreme events, including cyclones, heavy precipitation, flash floods and impacts, drought and water resources, strong winds, and dense water formation. The special issue is not limited to studies conducted within HyMeX: any multiscale or multidisciplinary approaches related to the Mediterranean water cycle are encouraged.

Volcanic eruptions are one of the major natural factors influencing climate variability at interannual to multidecadal timescales. However, simulating volcanically forced climate variability is a challenging task for climate models and one of the major uncertainties in near-term climate predictions. The Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP) is an endorsed contribution to the sixth phase of the Coupled Model Intercomparison Project. This multi-journal special issue on VolMIP aims at collecting relevant research results obtained within the VolMIP framework, and specifically concerning different aspects of the radiative and dynamical climatic response to volcanic forcing, detailed description of effects of different implementation of volcanic forcing in current climate models, aspects concerning the dynamical and chemical atmospheric response to volcanic aerosols simulated by global aerosol models, and comparison between reconstructed and simulated climate evolution after major eruptions. Articles in the special issue should contain the following statement: “This article is part of the multi-journal special issue “The Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP)”. It does not belong to a conference.”

The EARLINET ACP Special Issue aims to collect high-quality aerosol studies employing advanced lidar remote sensing techniques as developed within EARLINET, the European Aerosol Research Lidar Network. EARLINET was established in 2000 with the main goal of providing a comprehensive, quantitative, and statistically significant database for aerosol distribution on a continental scale. The 5-year EARLINET-ASOS (Advanced Sustainable Observation System) EC Project (2006–2011) has strongly contributed to optimizing the operation of the network. EARLINET is now a key component of the ACTRIS (Aerosols, Clouds and Trace gases Research InfraStructure Network) research infrastructure project that aims to integrate European ground-based stations equipped with advanced atmospheric probing instrumentation for aerosols, clouds and short-lived gas-phase species. The 15-year EARLINET database of aerosol profiles over Europe will be utilized in this Special Issue to reveal the network’s contribution to atmospheric and climate research.

The Water Vapour Phase II (WAVAS II), a SPARC activity, started in 2008 (SPARC Newsletter No. 30 (2008) p. 16: SPARC Water Vapour Initiative, by C. Schiller et al.). The activity includes satellite assessment and in situ comparison components. This international activity encompasses:

1) Providing a quality assessment of upper tropospheric to lower mesospheric satellite records since the early 1990s
2) Providing, as far as possible, absolute validation against ground-truth instruments
3) Assessing inter-instrument biases, depending on altitude, location, and season
4) Assessing the representation of temporal variations on various scales
5) Including data records on isotopologues
6) Providing recommendations for usage of available data records and for future observation systems


The main objective of WAVAS II is to assess and extend our knowledge and understanding of measurements of the vertical distribution of water vapor in the upper troposphere and middle atmosphere (UT/MA), where water has small concentrations, but significant radiative impact. This is a follow-up of the SPARC WAVAS activity, whose report was published in 2000 (SPARC Report No. 2 (2000) Upper Tropospheric and Stratospheric Water Vapour. D. Kley, J.M. Russell III, and C. Philips (eds.). WCRP-113, WMO/TD – No. 1043). Information gained from this activity will improve our ability to estimate long-term changes with associated uncertainties in UT/MA water as well as make recommendations as to what data would be most valuable for model validation and how such data should be used.

Papers will be accepted for this special issue according to the following guidelines, independent if they originate from the WAVAS II activity or other activities.

Guidelines for submissions:

• Papers covering existing UT/MA satellite water vapour measurements;
• Papers discussing comparisons of UT/MA satellite measurements, including discussion of quantities derived from these measurements, such as seasonal cycles, estimates of transport, or estimates of drifts, trends and variability.
• Papers discussing merging of water vapour measurements will be considered, although this topic is not specifically part of the WAVAS-II activity.
• Model papers that incorporate the datasets discussed and the uncertainty estimates resulting from the WAVAS-II activity will also be considered for inclusion.

The climate research community uses reanalyses widely to understand atmospheric processes and variability in the middle atmosphere, yet different reanalyses may give very different results for the same diagnostics. For example, the global energy budget and hydrological cycle, the Brewer–Dobson circulation, stratospheric vortex weakening and intensification events, and large-scale wave activity at the tropical tropopause are known to differ among reanalyses.

The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) is a coordinated activity to compare reanalysis data sets with respect to a variety of key diagnostics. The objectives of this project are

1. to understand the causes of differences among reanalyses;
2. to provide guidance on the appropriate usage of various reanalysis products in scientific studies;
3. to contribute to future improvements in the reanalysis products by establishing collaborative links between the reanalysis centres and the SPARC community.

The project focuses predominantly on differences among reanalyses (although studies that include operational analyses are welcome and studies comparing reanalyses with observations are encouraged), with an emphasis on diagnostics in the upper troposphere, stratosphere and mesosphere. This special issue serves to collect research with relevance to S-RIP in preparation for the publication of the S-RIP report in 2018. Although participation in S-RIP is not a prerequisite for submission to this special issue, authors contributing to this collection are encouraged to consider contributing to the preparation of the S-RIP report.

The Chemistry and Aerosol Mediterranean Experiment (ChArMEx) special issue will be simultaneously presented in the ACP and AMT journals. It aims at gathering experimental and modelling contributions to the field of atmospheric chemistry in the Mediterranean region and its impacts on regional air quality and climate, both in the recent past, present and future decades. It addresses natural and anthropogenic emissions of tropospheric reactive species, source apportionment, chemical transformations, transport processes, atmospheric deposition, aerosol optical properties and interactions with radiation, water vapour and clouds, variability and trends, and future conditions following climate change and increasing anthropogenic pressure.

The Modular Earth Submodel System (MESSy) is a multi-institutional project providing a strategy and the software for developing Earth System Models (ESMs) with highly flexible complexity.

The strategy follows a bottom-up approach, meaning that the various processes and diagnostic tools are implemented as so-called submodels, which are technically independent of each other and strictly separated from the underlying technical model infrastructure, such as memory management, input/output, flow-control, etc.

The MESSy software provides generalized interfaces for the standardized control and interconnection (coupling) of these submodels.

The present time-unlimited Special Issue hosts scientific and technical documentation and evaluation manuscripts concerned with the Modular Earth Submodel System and the models build upon it. Moreover, it comprises manuscripts about scientific applications involving these models.