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.

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.

This special issue arises from work conducted by the AMAP Expert Group on short-lived climate forcers for an assessment that will be finalized in 2021. It will include only invited papers.

Atmospheric ozone is one of the most important trace gases in the Earth's atmosphere due to its multiple roles: filtering harmful levels of ultraviolet (UV) solar radiation and shortwave absorption, being a greenhouse gas in the Earth's climate system, and being a powerful oxidant in the troposphere. Also, surface level ozone produces harmful effects on human health, ecosystems, and agricultural production. The latest findings and emerging research on ozone-related topics were presented at the 2021 Quadrennial Ozone Symposium (QOS) held online on October 3–9 2021. This special issue covers studies presented at the QOS and other relevant studies that are broadly related to the following subject areas:

1. stratospheric ozone science;
2. tropospheric ozone science;
3. ozone-depleting substances sources, sinks, and budget;
4. ozone, climate, and meteorology;
5. ozone monitoring and measurement techniques;
6. environmental and human health effects of atmospheric ozone and UV radiation.

This issue is open to all submissions within its scope.

During spring 2020 a German research consortium performed the airborne BLUESKY measurement campaign, using the HALO and DLR Falcon aircraft, to investigate the atmospheric composition changes associated with the COVID-19 lockdown. Comprehensive atmospheric chemistry, aerosol, and cloud measurements were performed with the two aircraft, from the boundary layer to the lower stratosphere during 20 scientific flights over Europe and the North Atlantic. Many groups participated in the campaign (DLR Oberpfaffenhofen, three departments of the MPI for Chemistry, Mainz, University of Frankfurt, KIT Karlsruhe, FZ Jülich). The measurement data are unique and are supplemented by satellite remote sensing data and numerical model simulations. The data analysis also involves comparison with measurement data from many pre-lockdown aircraft campaigns and long-term projects (e.g. CARIBIC). A subject of special interest is the impact of aircraft emissions on atmospheric composition, as the lockdown was associated with a 70–90% reduction of air traffic.

The Clouds, Aerosol Monsoon Processes Philippines Experiment (CAMP2Ex) was a NASA-led study to understand the interdisciplinary relationships between aerosol lifecyle, convection and the radiation budget within the Southeast Asian boreal summer southwest monsoon system. Central to CAMP2Ex was a 25 August–5 October 2019 airborne campaign based at Clark, Philippines, including the NASA P3 and SPEC Inc. Lear 35 research aircraft outfitted with radars, microwave sounders, lidar, radiation, cloud microphysics, composition and state instrumentation, aligned with significant remote sensing data collections and informatics efforts. Operations were the pinnacle of a multi-year monitoring effort that included development of an aerosol and radiation monitoring site at the Manila Observatory, performed in partnership with the ONR-sponsored Propagation of Intraseasonal Tropical Oscillations (PISTON) R/V Sally Ride deployment and contributing to the International Years of the Maritime Continent (YMC) effort. Given its interdisciplinary nature, CAMP2Ex is generating papers on numerous classes of research. We request a Special Issue be granted to the CAMP2Ex team and its partners specifically as a venue for aerosol–cloud–radiation research.

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 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 (http://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 (http://www.iup.uni-bremen.de/emerge/home/home.html), 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.

In April 2017, the Senate of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) established the Priority Programme “Polarimetric Radar Observations meet Atmospheric Modelling (PROM)” (SPP 2115). The programme is designed to run for 6 years and is now in the third funding year. The overall motivation of the SPP PROM is a better understanding and representation of cloud and precipitation processes in numerical weather prediction and climate models. Since 2015 the whole atmosphere over Germany has been monitored by 17 state-of-the-art polarimetric Doppler weather radar observations suitable to challenge the representation of cloud and precipitation processes in atmospheric models. Data assimilation merges observations and models for state estimation as a prerequisite for prediction and can be regarded as a smart interpolation between observations while exploiting the physical consistency of atmospheric models as mathematical constraints. However, considerable knowledge gaps exist both in radar polarimetry and atmospheric models, which impede the full exploitation of the triangle radar-polarimetry–atmospheric-models–data-assimilation. Objectives of SPP PROM and thus the envisioned publications within the special issue are (1) the exploitation of radar polarimetry for quantitative process detection in precipitating clouds and for model evaluation, (2) the improvement of cloud and precipitation schemes in atmospheric models based on process fingerprints detectable in polarimetric observations, (3) monitoring of the energy budget evolution due to phase changes in the cloudy, precipitating atmosphere for a better understanding of its dynamics, (4) the generation of precipitation system analyses by the assimilation of polarimetric radar observations into atmospheric models for weather forecasting, and (5) radar-based detection of the initiation of convection for the improvement of thunderstorm prediction.

Geostationary Environment Monitoring Spectrometer (GEMS) is the first instrument to observe air quality from a geostationary Earth orbit (GEO) successfully launched on 19 February 2020 and in initial operation after in-orbit tests (IOTs). GEMS provides hourly air quality information on both aerosols and gases at unprecedented spatial resolution of 7 km × 8 km. GEMS is a scanning UV–visible spectrometer measuring the hyperspectral spectrum in the ultraviolet and visible, which allows for the observation of key atmospheric constituents including O₃, NO₂, CO, SO₂, CH₂O, CHOCHO, aerosols, clouds, and UV indices. The mission opened a new era of air quality monitoring from space and will be joined by NASA’s TEMPO and ESA’s Sentinel-4 to form the GEO Air Quality Constellation in ~3 years to cover the most polluted region in the Northern Hemisphere. In this ACP–AMT special issue, the first results will be presented including instruments, IOT results, initial products, algorithm, calibration, validation from the recent GMAP field campaign, applications in modelling, etc.

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 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.

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.

Unlike in traditional bulk and bin microphysical approaches, atmospheric particles like hydrometeors and aerosol particles are not described by field variables but by computational particles. Over the last years, such particle-based methods have become more and more popular in aerosol and cloud physics. The special issue is open to all submissions with a clear focus on model description or evaluation or on studies specifically exploiting the advantages of particle-based methods. Contributions of any particle-based model related to aerosol and cloud modelling are welcome. This inter-journal special issue of GMD and ACP serves as a thematic collection and intends to increase visibility and foster collaboration among researchers.

ACTRIS statements of the purpose of the special issue on stay-at-home policies in response to the COVID-19 pandemic have resulted in an unprecedented decrease in pollutant emissions and in a well-publicized improvement of air quality in many cities in Asia, Europe and America. While the impact of lockdown on air quality was unambiguously detected in the urban areas through both in situ and space remote-sensing observations and its spatial and temporal extents, the specific role of meteorology and the cascade responses from indirect and non-linear effects are far from being fully evaluated. Throughout the very specific year 2020, ACTRIS and its partner institutions engaged in a pan-European effort to document the impact of governmental policies on atmospheric composition. ACTRIS maintained its operations at fully nominal standards, even increasing its sampling capacity in some cases. The ACTRIS data collection containing measurements of aerosol, cloud and trace gas properties across Europe measured during the year 2020 has been compiled and made available by the ACTRIS Data Centre units. Data from the year 2021 will soon follow. A community of scientists started evaluating the impact of the repeated lockdowns over the European regions. Because concentration and properties of short-lived atmospheric species are highly variable in space and time, evaluating the impact of reduced emissions is not straightforward. The special issue “Quantifying the impacts of stay-at-home policies on atmospheric composition and properties of aerosol and clouds over the European regions using ACTRIS related observations” will gather a series of scientific papers dealing with the measurable effects of lockdown measures over Europe. The special issue will particularly be dealing with • quantifying the spatial and temporal extent of stay-at-home policies on the European atmosphere, at both local and regional scales, • evaluating the impact of lockdown measures on the formation of secondary pollutants, • documenting the impact of reduced emissions (including air-traffic emissions) on cloud properties and occurrence, and • estimating the “missing” emissions using observation–model approaches. The outcome from the special issue aims to provide an in-depth analysis of the perturbation induced by the repeated lockdowns on the complex atmospheric system. The special issue is open for all submissions within its scope.

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 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.

Clouds over the ocean and their radiative properties are partially influenced by the biology, biogeochemistry and physics of the surface ocean. For example, the production of sea spray via bubble bursting associated with breaking waves can be influenced by biogenic material present in surface seawater, and biogenic material carried in sea spray aerosol can also influence ice-nucleating properties in air overlying the ocean. Furthermore, phytoplankton produce a range of compounds, including gases and particles, which when emitted to the atmosphere participate in the atmospheric chemistry. Our recent ship campaign Sea2Cloud had a primary focus of investigating the link between marine particle emissions and the biogeochemical properties of the seawater in subantarctic and subtropical waters, examining new particle formation processes and the relationship between surface ocean biogeochemistry and sea spray aerosol in dedicated ship-borne experiments. In parallel, ambient air aerosol, gas and cloud properties were measured along the ship track, which will allow for testing of these relationships in modelling exercises. Along with the ship campaign, longer-term measurements of aerosol properties were monitored at the Baring Head GAW research station (NZ) as well as continuously ongoing onboard the R/V Marion Dufresne over the years 2021–2022. These measurements will be used for the inspection of the representativeness of the ship campaign's process-orientated results. The Sea2Cloud project is running from July 2018 to July 2023, with potential additional laboratory and ship-borne measurements in the Southern Ocean by January 2023.

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.

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 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.

Geostationary Environment Monitoring Spectrometer (GEMS) is the first instrument to observe air quality from a geostationary Earth orbit (GEO) successfully launched on 19 February 2020 and in initial operation after in-orbit tests (IOTs). GEMS provides hourly air quality information on both aerosols and gases at unprecedented spatial resolution of 7 km × 8 km. GEMS is a scanning UV–visible spectrometer measuring the hyperspectral spectrum in the ultraviolet and visible, which allows for the observation of key atmospheric constituents including O₃, NO₂, CO, SO₂, CH₂O, CHOCHO, aerosols, clouds, and UV indices. The mission opened a new era of air quality monitoring from space and will be joined by NASA’s TEMPO and ESA’s Sentinel-4 to form the GEO Air Quality Constellation in ~3 years to cover the most polluted region in the Northern Hemisphere. In this ACP–AMT special issue, the first results will be presented including instruments, IOT results, initial products, algorithm, calibration, validation from the recent GMAP field campaign, applications in modelling, etc.

Atmospheric ozone is one of the most important trace gases in the Earth's atmosphere due to its multiple roles: filtering harmful levels of ultraviolet (UV) solar radiation and shortwave absorption, being a greenhouse gas in the Earth's climate system, and being a powerful oxidant in the troposphere. Also, surface level ozone produces harmful effects on human health, ecosystems, and agricultural production. The latest findings and emerging research on ozone-related topics were presented at the 2021 Quadrennial Ozone Symposium (QOS) held online on October 3–9 2021. This special issue covers studies presented at the QOS and other relevant studies that are broadly related to the following subject areas:

1. stratospheric ozone science;
2. tropospheric ozone science;
3. ozone-depleting substances sources, sinks, and budget;
4. ozone, climate, and meteorology;
5. ozone monitoring and measurement techniques;
6. environmental and human health effects of atmospheric ozone and UV radiation.

This issue is open to all submissions within its scope.

Clouds over the ocean and their radiative properties are partially influenced by the biology, biogeochemistry and physics of the surface ocean. For example, the production of sea spray via bubble bursting associated with breaking waves can be influenced by biogenic material present in surface seawater, and biogenic material carried in sea spray aerosol can also influence ice-nucleating properties in air overlying the ocean. Furthermore, phytoplankton produce a range of compounds, including gases and particles, which when emitted to the atmosphere participate in the atmospheric chemistry. Our recent ship campaign Sea2Cloud had a primary focus of investigating the link between marine particle emissions and the biogeochemical properties of the seawater in subantarctic and subtropical waters, examining new particle formation processes and the relationship between surface ocean biogeochemistry and sea spray aerosol in dedicated ship-borne experiments. In parallel, ambient air aerosol, gas and cloud properties were measured along the ship track, which will allow for testing of these relationships in modelling exercises. Along with the ship campaign, longer-term measurements of aerosol properties were monitored at the Baring Head GAW research station (NZ) as well as continuously ongoing onboard the R/V Marion Dufresne over the years 2021–2022. These measurements will be used for the inspection of the representativeness of the ship campaign's process-orientated results. The Sea2Cloud project is running from July 2018 to July 2023, with potential additional laboratory and ship-borne measurements in the Southern Ocean by January 2023.

ACTRIS statements of the purpose of the special issue on stay-at-home policies in response to the COVID-19 pandemic have resulted in an unprecedented decrease in pollutant emissions and in a well-publicized improvement of air quality in many cities in Asia, Europe and America. While the impact of lockdown on air quality was unambiguously detected in the urban areas through both in situ and space remote-sensing observations and its spatial and temporal extents, the specific role of meteorology and the cascade responses from indirect and non-linear effects are far from being fully evaluated. Throughout the very specific year 2020, ACTRIS and its partner institutions engaged in a pan-European effort to document the impact of governmental policies on atmospheric composition. ACTRIS maintained its operations at fully nominal standards, even increasing its sampling capacity in some cases. The ACTRIS data collection containing measurements of aerosol, cloud and trace gas properties across Europe measured during the year 2020 has been compiled and made available by the ACTRIS Data Centre units. Data from the year 2021 will soon follow. A community of scientists started evaluating the impact of the repeated lockdowns over the European regions. Because concentration and properties of short-lived atmospheric species are highly variable in space and time, evaluating the impact of reduced emissions is not straightforward. The special issue “Quantifying the impacts of stay-at-home policies on atmospheric composition and properties of aerosol and clouds over the European regions using ACTRIS related observations” will gather a series of scientific papers dealing with the measurable effects of lockdown measures over Europe. The special issue will particularly be dealing with • quantifying the spatial and temporal extent of stay-at-home policies on the European atmosphere, at both local and regional scales, • evaluating the impact of lockdown measures on the formation of secondary pollutants, • documenting the impact of reduced emissions (including air-traffic emissions) on cloud properties and occurrence, and • estimating the “missing” emissions using observation–model approaches. The outcome from the special issue aims to provide an in-depth analysis of the perturbation induced by the repeated lockdowns on the complex atmospheric system. The special issue is open for all submissions within its scope.

During spring 2020 a German research consortium performed the airborne BLUESKY measurement campaign, using the HALO and DLR Falcon aircraft, to investigate the atmospheric composition changes associated with the COVID-19 lockdown. Comprehensive atmospheric chemistry, aerosol, and cloud measurements were performed with the two aircraft, from the boundary layer to the lower stratosphere during 20 scientific flights over Europe and the North Atlantic. Many groups participated in the campaign (DLR Oberpfaffenhofen, three departments of the MPI for Chemistry, Mainz, University of Frankfurt, KIT Karlsruhe, FZ Jülich). The measurement data are unique and are supplemented by satellite remote sensing data and numerical model simulations. The data analysis also involves comparison with measurement data from many pre-lockdown aircraft campaigns and long-term projects (e.g. CARIBIC). A subject of special interest is the impact of aircraft emissions on atmospheric composition, as the lockdown was associated with a 70–90% reduction of air traffic.

Unlike in traditional bulk and bin microphysical approaches, atmospheric particles like hydrometeors and aerosol particles are not described by field variables but by computational particles. Over the last years, such particle-based methods have become more and more popular in aerosol and cloud physics. The special issue is open to all submissions with a clear focus on model description or evaluation or on studies specifically exploiting the advantages of particle-based methods. Contributions of any particle-based model related to aerosol and cloud modelling are welcome. This inter-journal special issue of GMD and ACP serves as a thematic collection and intends to increase visibility and foster collaboration among researchers.

In April 2017, the Senate of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) established the Priority Programme “Polarimetric Radar Observations meet Atmospheric Modelling (PROM)” (SPP 2115). The programme is designed to run for 6 years and is now in the third funding year. The overall motivation of the SPP PROM is a better understanding and representation of cloud and precipitation processes in numerical weather prediction and climate models. Since 2015 the whole atmosphere over Germany has been monitored by 17 state-of-the-art polarimetric Doppler weather radar observations suitable to challenge the representation of cloud and precipitation processes in atmospheric models. Data assimilation merges observations and models for state estimation as a prerequisite for prediction and can be regarded as a smart interpolation between observations while exploiting the physical consistency of atmospheric models as mathematical constraints. However, considerable knowledge gaps exist both in radar polarimetry and atmospheric models, which impede the full exploitation of the triangle radar-polarimetry–atmospheric-models–data-assimilation. Objectives of SPP PROM and thus the envisioned publications within the special issue are (1) the exploitation of radar polarimetry for quantitative process detection in precipitating clouds and for model evaluation, (2) the improvement of cloud and precipitation schemes in atmospheric models based on process fingerprints detectable in polarimetric observations, (3) monitoring of the energy budget evolution due to phase changes in the cloudy, precipitating atmosphere for a better understanding of its dynamics, (4) the generation of precipitation system analyses by the assimilation of polarimetric radar observations into atmospheric models for weather forecasting, and (5) radar-based detection of the initiation of convection for the improvement of thunderstorm prediction.

The Clouds, Aerosol Monsoon Processes Philippines Experiment (CAMP2Ex) was a NASA-led study to understand the interdisciplinary relationships between aerosol lifecyle, convection and the radiation budget within the Southeast Asian boreal summer southwest monsoon system. Central to CAMP2Ex was a 25 August–5 October 2019 airborne campaign based at Clark, Philippines, including the NASA P3 and SPEC Inc. Lear 35 research aircraft outfitted with radars, microwave sounders, lidar, radiation, cloud microphysics, composition and state instrumentation, aligned with significant remote sensing data collections and informatics efforts. Operations were the pinnacle of a multi-year monitoring effort that included development of an aerosol and radiation monitoring site at the Manila Observatory, performed in partnership with the ONR-sponsored Propagation of Intraseasonal Tropical Oscillations (PISTON) R/V Sally Ride deployment and contributing to the International Years of the Maritime Continent (YMC) effort. Given its interdisciplinary nature, CAMP2Ex is generating papers on numerous classes of research. We request a Special Issue be granted to the CAMP2Ex team and its partners specifically as a venue for aerosol–cloud–radiation research.

This special issue arises from work conducted by the AMAP Expert Group on short-lived climate forcers for an assessment that will be finalized in 2021. It will include only invited papers.

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).

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 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.

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 (http://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 (http://www.iup.uni-bremen.de/emerge/home/home.html), 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.

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.

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.

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.

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.

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.

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 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.