The purpose of this special issue is the compilation of modelling and observational studies on the role of atmospheric aerosols in the life cycle and composition of fog and low clouds in southern Africa as well as their mutual links with climate and biogeochemistry.

The special issue is motivated by new results from the AeroFog project, focusing on the hyperarid coastal deserts of Namibia, on the western coast of southern Africa. AeroFog is based on two intensive, ground-based field deployments, which took place in May and September 2024 along the coast and in the desert, and the long-term monitoring and analysis of remote-sensing products. Observations, satellite measurements, and modelling address the aerosol radiative, chemical, and microphysical interactions relevant to fog as well as the meteorological fields and dynamical processes that influence aerosol emission, transport, and deposition. Building on the diverse expertise of scientists in both the Northern and the Southern hemispheres, AeroFog aims to develop a holistic understanding of the mechanisms by which nutrients and pollutants are found in fog and the effect of fog on the biogeochemistry of regional ecosystems. The special issue comprises papers with complementary goals so as to encourage the exchange of ideas among previous, current, and planned large-scale projects and activities in the region.

The purpose of this special issue is to bring together papers summarizing the results of the Radio Occultation Modeling Experiment (ROMEX) in a special issue of Atmospheric Measurement Techniques (AMT). ROMEX is an international collaboration the purpose of which is to test the impact of varying numbers of vertical profiles of radio occultation (RO) observations in numerical weather prediction (NWP) models. An average of 35 000 RO profiles per day for September–November 2022 from 13 different missions are being used in experiments at major international weather prediction centres along with other atmospheric case studies demonstrating their effectiveness in observing atmospheric phenomena.

The primary focus of the special issue is the evaluation and intercomparison of the quality, characteristics, and impact on NWP models of an unprecedented number of RO observations from multiple missions across the public and private sectors in the US, Europe, and China. ROMEX represents a coordinated measurement campaign with contributions from data providers, processing centres, space agencies, and operational forecasting centres.

Central to this special issue are studies involving ROMEX-related datasets, with topics including but not limited to observation data quality and characteristics, advances in data processing methodologies, data evaluation and validation efforts, and impact studies on numerical weather prediction and other applications. Collectively, these topics – particularly the impact assessments – offer critical insights into the strengths and limitations of radio occultation as an atmospheric measurement technique.

The special issue is open to all studies evaluating or using ROMEX observations in NWP and other areas of atmospheric research. Papers for the special issue may be submitted between 1 May 2025 and 31 December 2026.

Through their interaction with solar and thermal infrared radiation, clouds and aerosols have been implicated as two of the greatest sources of uncertainty in climate projections. The EarthCARE (Earth Cloud, Aerosol and Radiation Explorer) satellite is a joint ESA–JAXA mission designed to provide key new information to tackle this crucial challenge. Launched on 28 May 2024, it carries a Doppler cloud profiling radar (CPR), a high-spectral-resolution atmospheric lidar (ATLID), a multi-spectral imager (MSI), and a three-view broadband radiometer (BBR). CPR is the first radar in space that can measure how fast cloud and precipitation particles are falling in the atmosphere or rising in the cores of thunderstorms, while the ability of ATLID to separately measure the backscatter of particles and air molecules enables it to retrieve the all-important profile of the cloud and aerosol extinction coefficient without having to make any assumption regarding the scattering properties of the particles. A large number of cloud, aerosol, precipitation, and radiation data products are being produced, some derived from individual EarthCARE instruments and others from the synergy of multiple instruments.

This inter-journal special issue (SI) is dedicated to presenting early scientific results using data from the satellite. It is open to the submission of papers that use EarthCARE data and fall into the remit of one of the three journals participating in the SI: Atmospheric Measurement Techniques, Atmospheric Chemistry and Physics, and Geoscientific Model Development. Topics can include, but are not limited to, the following: calibration and validation of EarthCARE data, development of new retrieval algorithms and improvement of existing algorithms, generation of new open-access datasets, improvement of the understanding of atmospheric properties and processes, evaluation of atmospheric models, assimilation of EarthCARE data into weather forecast and air quality models, and combination of EarthCARE with other datasets to estimate climate trends.

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.