The international Network for the Detection of Atmospheric Composition Change (NDACC) began official network operations in January 1991. Today it is composed of more than 110 globally distributed, ground-based, remote-sensing stations with more than 160 currently active instruments. The overriding goal of NDACC is to collect and maintain high-quality data for achieving the following scientific objectives:

• establishing long-term databases for the characterization of the state and behaviour of the atmosphere
• establishing links and feedbacks between changes in atmospheric composition, climate, and air quality
• validating atmospheric measurements from other platforms
• providing critical datasets to fill gaps in satellite observations
• collaboratively supporting field campaigns and other observing networks
• providing validation and development support for atmospheric models
• contributing to assessments of the state of the atmosphere (WMO, IPCC, etc.)

To achieve its objectives, NDACC collaborates with 10 cooperating networks, satellite teams, and model developers.

This special Issue marks the 35th anniversary of NDACC (https://ndacc.org). Its purpose is to present past scientific achievements and to inform readers about the updated strategy of the network in light of the evolution of the scientific questions regarding the state and behaviour of the atmosphere by taking advantage of the current landscape of observations. We also welcome papers that focus on related studies with important implications for our understanding of the state and behaviour of the atmosphere.

The introductory paper to the special issue will provide an overview of NDACC's revised strategy for the next decade.

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.

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.