Aerosols are particles floating in the Earth's atmosphere and are linked with the largest uncertainty in estimates and interpretations of the Earth's changing energy budget. Measurement principles differ depending on the desired derived aerosol optical parameter and on the measurement platform (surface or space). Common aerosol columnar properties' retrieval techniques consist of direct measurement of a bright source of radiation (sun, star, moon, sky) with multi-wavelength photometers. Several global photometric aerosol networks exist. However, there are several instrumental, algorithm-based, and hardware-based differences in their related aerosol products, and global standardization is needed. In addition, in order to improve and optimize sun- and moon-photometric aerosol measurements, a network of aerosol scientists and operators, aerosol measurement users and software, and hardware developers is needed.

In the frame of the EU COST Action Harmonia, International network for harmonisation of atmospheric aerosol retrievals from ground based photometers (https://harmonia-cost.eu/), a network of 150 people has been established who are working towards improving sun-photometric aerosol measurements and also linking measurements with various effects and applications dealing with aerosol columnar properties. During the first 2 years of the project, various aspects were identified as scientific gaps in our current knowledge on aerosol measurements, modelling, processes, and effects, with a critical mass of scientists working towards addressing them. Such gaps include issues such as

• a need for improvement of the aerosol measurements from surface-based instruments, including use of new technologies, improved algorithms, and combined approaches;
• the harmonization of aerosol optical properties from different aerosol networks (filter radiometers, spectroradiometers);
• aerosol trend analysis and satellite validation;
• sun-photometric synergies with in situ and lidar aerosol measurements and also modelling assimilation;
• the presentation of intensive campaign results with a focus on aerosols and aerosol–cloud–radiation interactions;
• enhancements regarding aerosol effects on different communities (e.g. solar energy, aviation, health, climate, air quality, agriculture).

This special issue focuses on addressing the above-mentioned gaps through individual and combined efforts of the Harmonia community and others. A list of papers to be submitted has been created, including results related to Harmonia and also ACTRIS research infrastructure (SK2) activities, long-term measurement analysis of measured data, aerosol retrieval algorithms and techniques, and aerosol effects.

We invite original research, reviews, and case studies that focus on atmospheric greenhouse gas (GHG) monitoring, emission estimates, and mitigation strategies, particularly in the Asia–Pacific region.

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 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.

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.