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.