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.

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

This special issue consists of papers that describe the modelling, observations, and analysis of data related to the ACROSS (Atmospheric Chemistry of the Suburban Forest) field measurement campaign that took place in summer 2022 in the Paris region. It could also include papers describing new instrumentation or new instrumental configurations used during the campaign. Any papers directly related to the ACROSS project are welcome for submittal to this special issue.

The climate crisis is one of the grand challenges of the 21st century. The increase in Earth's surface temperature, commonly known as global warming or anthropogenically induced climate change, is primarily driven by the rise in greenhouse gases (GHGs) in the atmosphere. The two most significant greenhouse gases affected by human activity are carbon dioxide (CO₂) and methane (CH₄).

While human activities such as fossil fuel combustion, oil and gas exploration, waste management, and agriculture are major sources of GHGs, natural sources also play a significant role. Extensive wetlands, for example, are the largest natural source of CH₄ globally. In wetlands, methane is produced by soil microbes and plants that metabolize under anaerobic conditions and is then released into the atmosphere through diffusion, transport via plant tissues, and gas bubble emissions. These processes make global wetlands among the most important yet least understood sources and sinks in the global methane and CO₂ budget.

A major scientific challenge in this context is distinguishing between methane emissions from natural sources and those resulting from human activities. Our understanding of these processes, their relative magnitudes, and the associated feedback mechanisms – such as increased wildfire activity, permafrost thaw, or changes in inundation patterns – is still insufficient to fully meet the needs of scientists and policymakers in predicting and mitigating climate warming.

To enhance our understanding of greenhouse gas budgets, a series of airborne measurement campaigns, known as CoMet (Carbon Dioxide and Methane Mission), have been conducted using the unique capabilities of the German research aircraft HALO. The CoMet campaigns integrate active airborne remote sensing measurements with lasers, passive remote sensing with spectrometers and solar radiation, and advanced in situ greenhouse gas concentration measurements, alongside an extensive suite of meteorological parameters. These observations are further supported by extensive modelling activities that also contribute to validating existing GHG satellite data and preparing for the next generation of such missions.

The first CoMet campaign took place in 2018, and its findings were published in a special inter-journal issue of AMT/ACP/GMD. The follow-up campaign, CoMet 2.0 Arctic (https://comet2arctic.de), was successfully conducted during a 6-week intensive operation period in August and September 2022 in Canada. The research flights focused on greenhouse gas emissions from boreal wetlands, permafrost areas in the Canadian Arctic, and wildfires, as well as anthropogenic sources like oil, gas, and coal extraction sites and landfills (in Canada and, during a test flight, in Spain). This campaign provided a valuable dataset for understanding methane and carbon dioxide cycles, particularly at high northern latitudes.

CoMet 2.0 Arctic is also part of the transatlantic AMPAC (Arctic Methane and Permafrost Challenge) initiative, a collaborative effort between NASA and ESA that fosters cooperation among US, Canadian, and European research institutes in this crucial area of research.

The special issue is open to all contributions that fit the topic from participants of the CoMet 2.0 Arctic field mission, the AMPAC community, and associated research partners.

The Asian summer monsoon (ASM) plays a key role in the vertical transport of material (including anthropogenic pollutants, aerosols, aerosol precursors, and other important trace gases) across the tropopause, with significant impacts on stratospheric chemistry and dynamics regionally and globally. Additionally, the Asian region is experiencing increasingly frequent and severe weather extremes connected to the ASM that are causing unprecedented damage to public property and loss of life. Predicting localized extreme events with sufficient lead times using numerical weather prediction (NWP) models remains challenging, and recent research suggests that better representation of stratospheric processes in NWP models can help to improve the prediction of monsoon extremes. On the other hand, it is well recognized that the complexities of the underlying mechanisms of stratosphere–troposphere coupling processes are difficult to incorporate in NWP models.

Observational and modelling aspects of the stratosphere–troposphere coupling processes and extreme weather events associated with the Asian summer monsoon were addressed at an international workshop, Stratosphere-Troposphere Interactions and Prediction of Monsoon weather EXtremes (STIPMEX), in Pune, India, from 2 to 7 June 2024. The STIPMEX workshop provided a platform for discussions on dynamical, chemical, radiative, and convective processes of the atmosphere during the ASM and fostered knowledge exchange and collaboration between experts on stratosphere–troposphere interactions and extreme weather prediction. The workshop aimed to promote and improve the inclusion of stratospheric and tropospheric processes in NWP models for better predictability of monsoon extremes. Full information on the STIPMEX workshop including a detailed list of themes and topics as well as an overview of the delivered presentations can be found at https://sparc-extreme.tropmet.res.in/.

This special issue has been initiated to publish the new and original research presented during STIPMEX and make it available to the wider community, and all STIPMEX presenters are encouraged to submit full write-ups of their novel and so far unpublished scientific studies. Submissions of follow-on studies or material representing a product of conference discussions and knowledge exchange are also welcome, as are any other submissions or related and relevant work that fits the scope of the workshop and special issue.

To best accommodate the two overarching STIPMEX themes, (i) dynamical, chemical, radiative, and convective processes in general, with a particular emphasis on recent changes and trends in stratosphere–troposphere coupling and linkages between stratospheric aerosol variability (e.g. due to volcanic eruptions) and the Asian summer monsoon, and (ii) the challenges of forecasting extreme weather events during the Asian summer monsoon, the special issue is organized as an inter-journal special issue of Atmospheric Chemistry and Physics (ACP) and Weather and Climate Dynamics (WCD).

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 purpose of the Special Issue is to publish main results from the airborne field campaign HALO-(AC)³ studying warm-air intrusions and cold-air outbreaks and corresponding air mass transformations. HALO-(AC)³ took place in the area around Svalbard in March and April 2022. The campaign is embedded into a Transregional Collaborative Research Centre ( http://www.ac3-tr.de/) that investigates causes and consequences of the currently ongoing dramatic climate changes in the Arctic.

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.

Mercury (Hg) is a chemical pollutant of human health concern worldwide; a consequence of anthropogenic activities; and the focus of the Minamata Convention on Mercury (MC; https://minamataconvention.org/en), an international treaty to protect human health and the environment from the adverse effects of mercury. The MC entered into force on 16 August 2017 and committed to limiting the use and environmental release of mercury. Also, the 1998 Protocol on Heavy Metals of the 1979 Convention on Long-Range Transboundary Air Pollution (LRTAP) commits parties to mitigating emissions of mercury (as well as cadmium and lead) from a variety of point sources and provides guidance on mitigating emissions associated with heavy metal use in manufactured products. The MC framework requires an evaluation of the effectiveness of its measures in meeting the objectives beginning no later than 6 years after the convention’s entry into force and periodically thereafter. The Protocol on Heavy Metals requires a periodic review of the progress towards meeting the obligations in the protocol and the sufficiency and effectiveness of those obligations and an evaluation of whether additional emission reductions are warranted.

This multi-journal special issue (SI) is intended to develop the required information that can be scientifically exploited to address key policy questions of the conventions: (1) what are the contributions of anthropogenic emissions and releases and other Hg sources to current Hg levels observed in air, biota, humans, and other media? (2) How have these contribution levels changed over time and over the timeline of the convention? (3) How do the contribution levels and their trends vary geographically at the global scale? (4) What are the contributions of anthropogenic emissions and releases and other drivers to the temporal trends in observed Hg levels across global regions? (5) How are observed Hg levels expected to change in the future?

The special issue aims at collecting relevant research advances arising from the design, implementation, and results of the Multi-Compartment Hg Modeling and Analysis Project (MCHgMAP) and from the scientific community on all aspects of biogeochemical mercury cycling, including primary and secondary emissions, observations, process studies, and single to multi-compartmental and statistical model development and application. A challenge of analysing the fate of emitted mercury is that it can recycle between the atmosphere, land, and ocean, and as a result, past and present emissions can continue to affect the environment on timescales of decades to centuries. MCHgMAP is an ensemble modelling initiative developed to inform the effectiveness of evaluations of the MC and LRTAP, utilizing a coordinated modelling approach between single-medium (atmosphere, land, and ocean) and multi-media mercury models to consistently simulate the changing global and regional environmental Hg cycling and analyse its drivers. The SI includes an overview paper on MCHgMAP, describing its scientific background and design (an important and crucial preparatory stage), which will be referenced by the individual papers on this project that follow.

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.

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.

SCOR (Scientific Committee on Oceanic Research) Working Group 167 (Reducing Uncertainty in Soluble aerosol Trace Element Deposition, RUSTED), appointed in October 2022, brings together experts from the atmospheric chemistry, ocean biogeochemistry, and modelling communities. Aiming to reduce uncertainties in soluble aerosol trace element deposition, RUSTED will quantitatively assess different aerosol leaching schemes; formulate standard operating procedures (SOPs) for frequently used aerosol leaching schemes; and develop a user-friendly, open-access database of aerosol trace element data which includes advice on the use of the data in Earth system models.

In this special issue, we propose to curate cutting-edge studies which advance our knowledge of the deposition of soluble aerosol trace elements and their impacts on marine ecosystems. We also encourage the submission of manuscripts which address challenges and/or report recent advances in the field of aerosol trace element deposition from researchers outside the working group.

The Joint Aeolus Tropical Atlantic Campaign (JATAC) used ground-based, aircraft, and balloon measurements to validate data provided by ESA's Aeolus satellite and support related science activities on the interaction of wind, dust, and clouds. ESA’s Aeolus satellite observations are expected to have the biggest impact on the improvement of numerical weather prediction in the tropics. An important case relating to the predictability of tropical weather systems is the outflow of Saharan dust, its interaction with cloud microphysics, and its impact on the development of tropical storms over the Atlantic Ocean. JATAC, deployed over Cabo Verde (2021–2022) and the US Virgin Islands (2021), supported the validation and preparation of the ESA Aeolus, EarthCARE, and WIVERN missions. It also addressed science objectives regarding the Saharan aerosol layer, the African easterly waves and jet, the tropical easterly jet, and the Intertropical Convergence Zone (including their relation to the formation of convective systems) as well as the long-range transport of dust and its impact on air quality.

This special issue (SI) collects the studies that utilized the synergy of remote sensing, surface-based, and airborne observations to address the satellite validation objectives and spatio-temporal representativeness of the different atmospheric measurement techniques. The SI studies bring together different observations from the individual ground-based and airborne campaign activities that have taken place in the frame of JATAC, to demonstrate the added value of the synergistic use of different measurements and platforms to address open science questions related to dynamics and the interactions of aerosols with clouds and radiation.

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 climate crisis is one of the grand challenges of the 21st century. The increase in Earth's surface temperature, commonly known as global warming or anthropogenically induced climate change, is primarily driven by the rise in greenhouse gases (GHGs) in the atmosphere. The two most significant greenhouse gases affected by human activity are carbon dioxide (CO₂) and methane (CH₄).

While human activities such as fossil fuel combustion, oil and gas exploration, waste management, and agriculture are major sources of GHGs, natural sources also play a significant role. Extensive wetlands, for example, are the largest natural source of CH₄ globally. In wetlands, methane is produced by soil microbes and plants that metabolize under anaerobic conditions and is then released into the atmosphere through diffusion, transport via plant tissues, and gas bubble emissions. These processes make global wetlands among the most important yet least understood sources and sinks in the global methane and CO₂ budget.

A major scientific challenge in this context is distinguishing between methane emissions from natural sources and those resulting from human activities. Our understanding of these processes, their relative magnitudes, and the associated feedback mechanisms – such as increased wildfire activity, permafrost thaw, or changes in inundation patterns – is still insufficient to fully meet the needs of scientists and policymakers in predicting and mitigating climate warming.

To enhance our understanding of greenhouse gas budgets, a series of airborne measurement campaigns, known as CoMet (Carbon Dioxide and Methane Mission), have been conducted using the unique capabilities of the German research aircraft HALO. The CoMet campaigns integrate active airborne remote sensing measurements with lasers, passive remote sensing with spectrometers and solar radiation, and advanced in situ greenhouse gas concentration measurements, alongside an extensive suite of meteorological parameters. These observations are further supported by extensive modelling activities that also contribute to validating existing GHG satellite data and preparing for the next generation of such missions.

The first CoMet campaign took place in 2018, and its findings were published in a special inter-journal issue of AMT/ACP/GMD. The follow-up campaign, CoMet 2.0 Arctic (https://comet2arctic.de), was successfully conducted during a 6-week intensive operation period in August and September 2022 in Canada. The research flights focused on greenhouse gas emissions from boreal wetlands, permafrost areas in the Canadian Arctic, and wildfires, as well as anthropogenic sources like oil, gas, and coal extraction sites and landfills (in Canada and, during a test flight, in Spain). This campaign provided a valuable dataset for understanding methane and carbon dioxide cycles, particularly at high northern latitudes.

CoMet 2.0 Arctic is also part of the transatlantic AMPAC (Arctic Methane and Permafrost Challenge) initiative, a collaborative effort between NASA and ESA that fosters cooperation among US, Canadian, and European research institutes in this crucial area of research.

The special issue is open to all contributions that fit the topic from participants of the CoMet 2.0 Arctic field mission, the AMPAC community, and associated research partners.

The Asian summer monsoon (ASM) plays a key role in the vertical transport of material (including anthropogenic pollutants, aerosols, aerosol precursors, and other important trace gases) across the tropopause, with significant impacts on stratospheric chemistry and dynamics regionally and globally. Additionally, the Asian region is experiencing increasingly frequent and severe weather extremes connected to the ASM that are causing unprecedented damage to public property and loss of life. Predicting localized extreme events with sufficient lead times using numerical weather prediction (NWP) models remains challenging, and recent research suggests that better representation of stratospheric processes in NWP models can help to improve the prediction of monsoon extremes. On the other hand, it is well recognized that the complexities of the underlying mechanisms of stratosphere–troposphere coupling processes are difficult to incorporate in NWP models.

Observational and modelling aspects of the stratosphere–troposphere coupling processes and extreme weather events associated with the Asian summer monsoon were addressed at an international workshop, Stratosphere-Troposphere Interactions and Prediction of Monsoon weather EXtremes (STIPMEX), in Pune, India, from 2 to 7 June 2024. The STIPMEX workshop provided a platform for discussions on dynamical, chemical, radiative, and convective processes of the atmosphere during the ASM and fostered knowledge exchange and collaboration between experts on stratosphere–troposphere interactions and extreme weather prediction. The workshop aimed to promote and improve the inclusion of stratospheric and tropospheric processes in NWP models for better predictability of monsoon extremes. Full information on the STIPMEX workshop including a detailed list of themes and topics as well as an overview of the delivered presentations can be found at https://sparc-extreme.tropmet.res.in/.

This special issue has been initiated to publish the new and original research presented during STIPMEX and make it available to the wider community, and all STIPMEX presenters are encouraged to submit full write-ups of their novel and so far unpublished scientific studies. Submissions of follow-on studies or material representing a product of conference discussions and knowledge exchange are also welcome, as are any other submissions or related and relevant work that fits the scope of the workshop and special issue.

To best accommodate the two overarching STIPMEX themes, (i) dynamical, chemical, radiative, and convective processes in general, with a particular emphasis on recent changes and trends in stratosphere–troposphere coupling and linkages between stratospheric aerosol variability (e.g. due to volcanic eruptions) and the Asian summer monsoon, and (ii) the challenges of forecasting extreme weather events during the Asian summer monsoon, the special issue is organized as an inter-journal special issue of Atmospheric Chemistry and Physics (ACP) and Weather and Climate Dynamics (WCD).

The Joint Aeolus Tropical Atlantic Campaign (JATAC) used ground-based, aircraft, and balloon measurements to validate data provided by ESA's Aeolus satellite and support related science activities on the interaction of wind, dust, and clouds. ESA’s Aeolus satellite observations are expected to have the biggest impact on the improvement of numerical weather prediction in the tropics. An important case relating to the predictability of tropical weather systems is the outflow of Saharan dust, its interaction with cloud microphysics, and its impact on the development of tropical storms over the Atlantic Ocean. JATAC, deployed over Cabo Verde (2021–2022) and the US Virgin Islands (2021), supported the validation and preparation of the ESA Aeolus, EarthCARE, and WIVERN missions. It also addressed science objectives regarding the Saharan aerosol layer, the African easterly waves and jet, the tropical easterly jet, and the Intertropical Convergence Zone (including their relation to the formation of convective systems) as well as the long-range transport of dust and its impact on air quality.

This special issue (SI) collects the studies that utilized the synergy of remote sensing, surface-based, and airborne observations to address the satellite validation objectives and spatio-temporal representativeness of the different atmospheric measurement techniques. The SI studies bring together different observations from the individual ground-based and airborne campaign activities that have taken place in the frame of JATAC, to demonstrate the added value of the synergistic use of different measurements and platforms to address open science questions related to dynamics and the interactions of aerosols with clouds and radiation.

Mercury (Hg) is a chemical pollutant of human health concern worldwide; a consequence of anthropogenic activities; and the focus of the Minamata Convention on Mercury (MC; https://minamataconvention.org/en), an international treaty to protect human health and the environment from the adverse effects of mercury. The MC entered into force on 16 August 2017 and committed to limiting the use and environmental release of mercury. Also, the 1998 Protocol on Heavy Metals of the 1979 Convention on Long-Range Transboundary Air Pollution (LRTAP) commits parties to mitigating emissions of mercury (as well as cadmium and lead) from a variety of point sources and provides guidance on mitigating emissions associated with heavy metal use in manufactured products. The MC framework requires an evaluation of the effectiveness of its measures in meeting the objectives beginning no later than 6 years after the convention’s entry into force and periodically thereafter. The Protocol on Heavy Metals requires a periodic review of the progress towards meeting the obligations in the protocol and the sufficiency and effectiveness of those obligations and an evaluation of whether additional emission reductions are warranted.

This multi-journal special issue (SI) is intended to develop the required information that can be scientifically exploited to address key policy questions of the conventions: (1) what are the contributions of anthropogenic emissions and releases and other Hg sources to current Hg levels observed in air, biota, humans, and other media? (2) How have these contribution levels changed over time and over the timeline of the convention? (3) How do the contribution levels and their trends vary geographically at the global scale? (4) What are the contributions of anthropogenic emissions and releases and other drivers to the temporal trends in observed Hg levels across global regions? (5) How are observed Hg levels expected to change in the future?

The special issue aims at collecting relevant research advances arising from the design, implementation, and results of the Multi-Compartment Hg Modeling and Analysis Project (MCHgMAP) and from the scientific community on all aspects of biogeochemical mercury cycling, including primary and secondary emissions, observations, process studies, and single to multi-compartmental and statistical model development and application. A challenge of analysing the fate of emitted mercury is that it can recycle between the atmosphere, land, and ocean, and as a result, past and present emissions can continue to affect the environment on timescales of decades to centuries. MCHgMAP is an ensemble modelling initiative developed to inform the effectiveness of evaluations of the MC and LRTAP, utilizing a coordinated modelling approach between single-medium (atmosphere, land, and ocean) and multi-media mercury models to consistently simulate the changing global and regional environmental Hg cycling and analyse its drivers. The SI includes an overview paper on MCHgMAP, describing its scientific background and design (an important and crucial preparatory stage), which will be referenced by the individual papers on this project that follow.

SCOR (Scientific Committee on Oceanic Research) Working Group 167 (Reducing Uncertainty in Soluble aerosol Trace Element Deposition, RUSTED), appointed in October 2022, brings together experts from the atmospheric chemistry, ocean biogeochemistry, and modelling communities. Aiming to reduce uncertainties in soluble aerosol trace element deposition, RUSTED will quantitatively assess different aerosol leaching schemes; formulate standard operating procedures (SOPs) for frequently used aerosol leaching schemes; and develop a user-friendly, open-access database of aerosol trace element data which includes advice on the use of the data in Earth system models.

In this special issue, we propose to curate cutting-edge studies which advance our knowledge of the deposition of soluble aerosol trace elements and their impacts on marine ecosystems. We also encourage the submission of manuscripts which address challenges and/or report recent advances in the field of aerosol trace element deposition from researchers outside the working group.

This special issue consists of papers that describe the modelling, observations, and analysis of data related to the ACROSS (Atmospheric Chemistry of the Suburban Forest) field measurement campaign that took place in summer 2022 in the Paris region. It could also include papers describing new instrumentation or new instrumental configurations used during the campaign. Any papers directly related to the ACROSS project are welcome for submittal to this special issue.

The purpose of the Special Issue is to publish main results from the airborne field campaign HALO-(AC)³ studying warm-air intrusions and cold-air outbreaks and corresponding air mass transformations. HALO-(AC)³ took place in the area around Svalbard in March and April 2022. The campaign is embedded into a Transregional Collaborative Research Centre ( http://www.ac3-tr.de/) that investigates causes and consequences of the currently ongoing dramatic climate changes in the Arctic.

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

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 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.

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.

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 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 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.