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.