ACP was launched in September 2001 as the first open-access journal with open peer review in its field. To celebrate its success and immense growth since then, we compile in this special issue a collection of opinion articles to reflect on the developments of the field of atmospheric chemistry and physics in the last 20 years and point to exciting directions for the future, which will stimulate the scientific debate in our field for the next 20 years. Only invited contributions to the special issue will be considered, and the review process will be overseen by the ACP executive and senior editors.

The fourth phase of the Air Quality Model Evaluation International Initiative (AQMEII) focuses on deposition processes. Since 2008, AQMEII has analysed a number of critical aspects of regional-scale air quality models that require international collaboration and evaluation. Each of the previous three phases have resulted in multi-paper “special issues” in the peer-reviewed literature (two in Atmospheric Environment and one in ACP). AQMEII leverages the long-term experience in model development and application of the two largest communities of such models, namely the North American community and the European one. These two communities have been engaged in AQMEII model evaluation activities that tackled issues such as operational and probabilistic evaluations (phase 1), online model coupling between air quality and meteorology (phase 2), intercontinental transport of air pollutants (phase 3, ACP SI “Global and regional assessment of intercontinental transport of air pollution: results from HTAP, AQMEII and MICS”, 41 papers), and now dry and wet deposition processes (phase 4).

The subject tackled in phase 4 is central to air quality modelling at any scale, and it has never been examined in the systematic and the detailed manner currently underway in AQMEII4. Given the complexity of deposition processes and the variety of approaches adopted by different groups, we opted for an analysis that comprises the individual deposition modules and the full regional-scale models that used those modules as part of a more complex computational system. As customary, for AQMEII two regional modelling domains are considered, a European and a North American one, and simulations are underway for 2 representative years for the processes of interest in the two continents. The process-oriented character of the activity is defined already in the selection of the simulation year. For the North American domain, the air quality in 2010 and 2016 will be simulated to allow comparisons across emissions changes, while for Europe 2009 and 2010 are the designated years to allow comparisons across meteorologically different years. The community presently participating is composed of 15 regional modelling teams and six deposition modules. Subgroups of regional-scale models may be using the same modelling system with a variety of parameterization implementations, or use original models developed in-house. A key feature of the AQMEII multi-model exercise is the submission of all model output on a common grid and at specified observation stations, to a common database at JRC in Ispra, Italy, where a common set of analysis tools which participants can use is available. The variety of the tools used makes possible the comparison of all models with measurements and of specific models with largely used community systems. It also makes it possible to work within the sub-community to exercise different features or modelling options of the community models. Whilst for regional-scale models the evaluation will be based on data on a wide range of pollutants in gas and particle form gathered from standard monitoring networks and soundings in the two continents, in the case of point models, ozone will be the focus, and data have been gathered from sites where high-quality instrumentation has been deployed. Analysis of the ozone dry deposition modules driven by observed site-specific meteorological and biophysical data (i.e., the point models) will focus on process-oriented model evaluation and intercomparison.

As mentioned the central topic of the issue is the deposition processes, but differently from the past a deeper level of analyses has been added in AQMEII4. That pertains to the addition of diagnostic analysis to each of the participating models. These diagnostics are allowing participants to compare the parameterizations for deposition pathways across all participating systems in an unprecedented level of detail, with comparisons between individual conductance and resistances being carried out for the first time. The diagnostics have been mapped to a common set of land-use types, in order to allow a common analysis of the different deposition parameterizations for a given land-use type.

The AQMEII4 activity goes beyond the comparison of the deposited quantities as has been customary so far and will allow the gathering of insight into the reason for model differences and the efficacy of some schemes compared to others through the comparison with measurements. For the first time the diversity with which the participating models (both point models and the full regional-scale models) describe the underlying surface on which the deposition occurs will be evaluated. This is therefore not a mere model evaluation exercise but one that aims to tackle in a multi-model fashion the most difficult of the four pillars of model evaluation (Dennis et al., 2010), i.e. diagnostic evaluation. The special issue will therefore include papers on process-oriented point model comparisons against deposition supersite data, detailed process-oriented model inter-comparisons of the regional-scale output, and the potential application of model results for ecosystem assessments. An introductory paper (already in preparation) will be the first submission of the special issue, which will describe activity set-up and rationale – an important and crucial preparatory phase which will be referenced by the individual papers that follow.

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

Rapid population growth and urbanization worldwide accelerate eco-environmental and socio-economic stress as well as adverse climatic and health impacts on urban dwellers. Atmospheric modelling research has largely been performed on a horizontal grid spacing of kilometres or larger due to a lack of understanding of the local-scale phenomena, appropriate parameterizations, and adequate modelling tools and computer resources. Urban- to hyperlocal-scale (at street or city block level) air pollution, climate change, and their impacts on population exposure and human health have increasingly received attention from both researchers and policy makers around the world. Several state-of-the-science models have recently been developed for urban- to hyperlocal-scale air pollution modelling, including the street-network model, the Model of Urban Network of Intersecting Canyons and Highways (MUNICH) that incorporates detailed representations of gas-phase chemistry and secondary aerosol formation pathways, and the Street-in-Grid (SinG) model that dynamically combines a 3-D Eulerian chemical–transport model (CTM), Polair3D, with MUNICH. There have been increasing numbers of developers and users for MUNICH, SinG, and other similar coupled 3-D CTMs and urban canyon models for street-level air pollution modelling worldwide, such as CALIOPE-Urban, the Operational Street Pollution Model (OSPM) coupled with the Danish Eulerian Hemispheric Model (DEHM), and the Parallelized Large-Eddy Simulation Model (PALM). Meanwhile, air quality measurement data at hyperlocal scales have become increasingly available for model validation and improvement. Recognizing the urgent need for scientific advancement, pollution and exposure assessment, policy making, and public health protection at urban to hyperlocal scales, we launched a special issue on air quality research at street level in 2018, in which we have published 20 journal papers: https://acp.copernicus.org/articles/special_issue994.html. This special issue (Part II) will continue to advance scientific understanding of local-scale atmospheric phenomena, promote discussion on state-of-the-science urban as well as hyperlocal street- and city-block-level air quality research including measurements, emissions, and model development, and encourage application for complex interactions among urban air pollution, climate, and health. The special issue (Part II) is open for all submissions which address the following themes.

• 3-D Street-in-Grid (SinG) model development and application
• Urban canyon and network model development and its incorporation into 3-D CTMs
• Urban and street-level air quality modelling in support of human exposure assessment
• Impact of urban traffic emissions on air quality and human health at a street level
• Hyperlocal (street and city block scales) air quality measurement and modelling
• Urban infrastructure-induced circulation and its impact on city planning

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.

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

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

In 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 Tibetan Plateau is referred to as the third pole of the world and is representative of background region of the Earth’s atmosphere. Atmospheric photochemistry, in conjunction with the unique circulation pattern of Asian Monsoon outflow and prevailing westerly wind, leads to, as of yet, unidentified implications for climate change, the oxidative capacity of the atmosphere, local air quality, and health effects on local residents and visitors.

For a better scientific understanding of the background atmospheric chemistry and its impacts on climate change, air quality, and human health, multiple international campaigns, referred to as the @Tibet field campaigns, have been conducted since 2019 under the umbrella of the second Tibetan Plateau Scientific Expedition and Research Program (STEP). As supported by the major research plan of the National Natural Science Foundation of China (NSFC) entitled fundamental researches on the formation and response mechanism of air pollution complex, the research conducted in the Tibetan Plateau and eastern China was underpinned by a number of instrument developments required to meet the specific demands of exploring air pollution and fundamental atmospheric photochemistry. This AMT–ACP joint special issue is therefore designed to intensively discuss data from @Tibet field campaigns and instrument development progress.

The purpose of the special issue (SI) is the collection of relevant contributions on the detection, analysis, and prediction of long-term changes and trends of trace gases and physical parameters of the atmosphere from the upper troposphere up into the ionosphere as presented at the TRENDS2022 workshop (11th International Workshop on Long-Term Changes and Trends in the Atmosphere). The SI will be open for contributions to the TRENDS2022 conference and co-located SPARC LOTUS and SPARC ATC working group meetings, as well as for other publications directly related to the scope of the SI.

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.

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.

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 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) began in 2013 as a coordinated activity to compare numerous key diagnostics in reanalysis data sets. The objectives of this project were

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

Phase 1 of the S-RIP project culminated with the publication of the S-RIP report (https://www.sparc-climate.org/sparc-report-no-10/) in January 2022 and a very successful special issue (https://acp.copernicus.org/articles/special_issue829.html) in ACP and ESSD with over 50 papers. Phase 1 was very successful in achieving the above objectives, and in doing so it taught us the value and importance of continuing reanalysis intercomparisons and communications between the reanalysis centres and the SPARC community. The above objectives thus remain the primary aims of S-RIP as the project moves into Phase 2.

This special issue is being initiated in the early stages of S-RIP Phase 2. The community is continuing to produce valuable papers including both updates using new reanalyses of diagnostics studied in Phase 1 and evaluation of diagnostics for processes and atmospheric regions that were not emphasized in Phase 1. This special issue welcomes papers both during this transitional period and in the following years of Phase 2 and both updates of work on processes studied in Phase 1 and new studies focused on additional processes and/or atmospheric regions.

The S-RIP project focuses primarily on differences among reanalyses, but studies that include operational analyses and studies comparing reanalyses with observations or model outputs are encouraged. Phase 1 of S-RIP emphasized diagnostics in the upper troposphere, stratosphere, and mesosphere. This special issue will collect research relevant to S-RIP, including broadening of the scope to, for example, evaluation of new reanalyses and of chemical reanalyses; more comprehensive evaluation of processes in the upper stratosphere and mesosphere; evaluation of tropospheric processes such as blocking, jet stream variations, and temperature anomalies; and more comprehensive evaluation of links between the stratospheric, upper tropospheric, and near-surface circulation and implications for extreme weather events.

All researchers are encouraged to submit to this issue regardless of past participation in S-RIP; we further encourage researchers to participate in and help guide S-RIP Phase 2.

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.

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

The main purposes of the special issue are

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

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

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

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

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

The climate research community uses reanalyses widely to understand atmospheric processes and variability in the middle atmosphere, yet different reanalyses 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) began in 2013 as a coordinated activity to compare numerous key diagnostics in reanalysis data sets. The objectives of this project were

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

Phase 1 of the S-RIP project culminated with the publication of the S-RIP report (https://www.sparc-climate.org/sparc-report-no-10/) in January 2022 and a very successful special issue (https://acp.copernicus.org/articles/special_issue829.html) in ACP and ESSD with over 50 papers. Phase 1 was very successful in achieving the above objectives, and in doing so it taught us the value and importance of continuing reanalysis intercomparisons and communications between the reanalysis centres and the SPARC community. The above objectives thus remain the primary aims of S-RIP as the project moves into Phase 2.

This special issue is being initiated in the early stages of S-RIP Phase 2. The community is continuing to produce valuable papers including both updates using new reanalyses of diagnostics studied in Phase 1 and evaluation of diagnostics for processes and atmospheric regions that were not emphasized in Phase 1. This special issue welcomes papers both during this transitional period and in the following years of Phase 2 and both updates of work on processes studied in Phase 1 and new studies focused on additional processes and/or atmospheric regions.

The S-RIP project focuses primarily on differences among reanalyses, but studies that include operational analyses and studies comparing reanalyses with observations or model outputs are encouraged. Phase 1 of S-RIP emphasized diagnostics in the upper troposphere, stratosphere, and mesosphere. This special issue will collect research relevant to S-RIP, including broadening of the scope to, for example, evaluation of new reanalyses and of chemical reanalyses; more comprehensive evaluation of processes in the upper stratosphere and mesosphere; evaluation of tropospheric processes such as blocking, jet stream variations, and temperature anomalies; and more comprehensive evaluation of links between the stratospheric, upper tropospheric, and near-surface circulation and implications for extreme weather events.

All researchers are encouraged to submit to this issue regardless of past participation in S-RIP; we further encourage researchers to participate in and help guide S-RIP Phase 2.

Rapid population growth and urbanization worldwide accelerate eco-environmental and socio-economic stress as well as adverse climatic and health impacts on urban dwellers. Atmospheric modelling research has largely been performed on a horizontal grid spacing of kilometres or larger due to a lack of understanding of the local-scale phenomena, appropriate parameterizations, and adequate modelling tools and computer resources. Urban- to hyperlocal-scale (at street or city block level) air pollution, climate change, and their impacts on population exposure and human health have increasingly received attention from both researchers and policy makers around the world. Several state-of-the-science models have recently been developed for urban- to hyperlocal-scale air pollution modelling, including the street-network model, the Model of Urban Network of Intersecting Canyons and Highways (MUNICH) that incorporates detailed representations of gas-phase chemistry and secondary aerosol formation pathways, and the Street-in-Grid (SinG) model that dynamically combines a 3-D Eulerian chemical–transport model (CTM), Polair3D, with MUNICH. There have been increasing numbers of developers and users for MUNICH, SinG, and other similar coupled 3-D CTMs and urban canyon models for street-level air pollution modelling worldwide, such as CALIOPE-Urban, the Operational Street Pollution Model (OSPM) coupled with the Danish Eulerian Hemispheric Model (DEHM), and the Parallelized Large-Eddy Simulation Model (PALM). Meanwhile, air quality measurement data at hyperlocal scales have become increasingly available for model validation and improvement. Recognizing the urgent need for scientific advancement, pollution and exposure assessment, policy making, and public health protection at urban to hyperlocal scales, we launched a special issue on air quality research at street level in 2018, in which we have published 20 journal papers: https://acp.copernicus.org/articles/special_issue994.html. This special issue (Part II) will continue to advance scientific understanding of local-scale atmospheric phenomena, promote discussion on state-of-the-science urban as well as hyperlocal street- and city-block-level air quality research including measurements, emissions, and model development, and encourage application for complex interactions among urban air pollution, climate, and health. The special issue (Part II) is open for all submissions which address the following themes.

• 3-D Street-in-Grid (SinG) model development and application
• Urban canyon and network model development and its incorporation into 3-D CTMs
• Urban and street-level air quality modelling in support of human exposure assessment
• Impact of urban traffic emissions on air quality and human health at a street level
• Hyperlocal (street and city block scales) air quality measurement and modelling
• Urban infrastructure-induced circulation and its impact on city planning

ACP was launched in September 2001 as the first open-access journal with open peer review in its field. To celebrate its success and immense growth since then, we compile in this special issue a collection of opinion articles to reflect on the developments of the field of atmospheric chemistry and physics in the last 20 years and point to exciting directions for the future, which will stimulate the scientific debate in our field for the next 20 years. Only invited contributions to the special issue will be considered, and the review process will be overseen by the ACP executive and senior editors.

The purpose of the special issue (SI) is the collection of relevant contributions on the detection, analysis, and prediction of long-term changes and trends of trace gases and physical parameters of the atmosphere from the upper troposphere up into the ionosphere as presented at the TRENDS2022 workshop (11th International Workshop on Long-Term Changes and Trends in the Atmosphere). The SI will be open for contributions to the TRENDS2022 conference and co-located SPARC LOTUS and SPARC ATC working group meetings, as well as for other publications directly related to the scope of the SI.

The Tibetan Plateau is referred to as the third pole of the world and is representative of background region of the Earth’s atmosphere. Atmospheric photochemistry, in conjunction with the unique circulation pattern of Asian Monsoon outflow and prevailing westerly wind, leads to, as of yet, unidentified implications for climate change, the oxidative capacity of the atmosphere, local air quality, and health effects on local residents and visitors.

For a better scientific understanding of the background atmospheric chemistry and its impacts on climate change, air quality, and human health, multiple international campaigns, referred to as the @Tibet field campaigns, have been conducted since 2019 under the umbrella of the second Tibetan Plateau Scientific Expedition and Research Program (STEP). As supported by the major research plan of the National Natural Science Foundation of China (NSFC) entitled fundamental researches on the formation and response mechanism of air pollution complex, the research conducted in the Tibetan Plateau and eastern China was underpinned by a number of instrument developments required to meet the specific demands of exploring air pollution and fundamental atmospheric photochemistry. This AMT–ACP joint special issue is therefore designed to intensively discuss data from @Tibet field campaigns and instrument development progress.

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 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 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.

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.

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.

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.