Aerosols are an important atmospheric component. They affect the Earth’s energy balance and the exposure of living organisms to incoming light from the Sun by scattering and absorbing radiation [IPCC 2023, Stamatis et al. 2025]. They also participate in cloud formation and alter their properties [Maloney et al. 2022]. They are also dangerous when inhaled in large quantities and remain one of the most important air pollutants, with 99% of the human population being exposed to levels higher than the safety guidelines of the World Health Organisation [https://www.who.int/data/gho/data/themes/air-pollution].
Aerosols are a highly heterogeneous mixture of non-gaseous particles with different shapes, sizes and chemical composition. There are various ways to study aerosols and different methods to observationally quantify their properties. To study the total aerosol column (over the entire atmosphere instead of a specific altitude) we use aerosol remote sensing. Remote sensing is based on radiation measurements that provide information about the state and composition of the atmosphere. It is separated into passive and active. Passive uses measurements of natural radiation sources such as the Sun and active remote sensing uses anthropogenic light sources such as a laser beam. Solar passive remote sensing is the main technique to retrieve aerosol optical and microphysical properties corresponding to the overall aerosol column, while active remote sensing is the main method to retrieve their vertical profile. This study is focused on solar passive remote sensing to retrieve information about the origin of aerosols.
There are different columnar properties, which are typically retrieved by measurements of direct solar irradiance (radiation coming from the solar disk only) and sky radiance at different angles (radiation coming from different parts of the sky). The information on how radiation changes in different parts of the sky combined with inversion modelling results in the retrieval of aerosol properties related to their size, scattering efficiency per direction and absorptivity. The Aerosol Robotic NETwork (AERONET) deploys the CIMEL sun and sky photometers to provide such aerosol properties in hundreds of locations worldwide [Sinyuk et al. 2020].
Aerosols from different sources differ also in certain properties. For example, dust aerosols tend to be larger than smoke aerosols [Mona et al., 2014; Masoom et al., 2023]. In this work, we use AERONET observations and an updated analysis based on the methodology of Hamill et al. [2016] to study the contribution of aerosol sources to their effect on solar radiation and how this changed over the years. This method uses 5 AERONET parameters:
1. Extinction Ångström Exponent. Describes the spectral dependence of radiation extinction due to aerosols and is related to aerosol size.
2. Absorption Ångström Exponent. It describes the spectral dependence of radiation absorption.
3. Single scattering albedo. Describes the balance of radiation scattering and absorption by aerosols.
4. Real part of refractive index: Describes the tendency of aerosols to scatter radiation.
5. Imaginary part of refractive index: It describes the tendency of aerosols to absorb radiation.
Through careful selection of locations, we form clusters of values of these parameters corresponding to each aerosol source under study (urban/industrial, biomass burning, desert dust, maritime and mixed). Then, for any measurement of those properties, we can identify the predominant source according to its nearest reference cluster.
Aerosols remain one of the main sources of uncertainties in the attribution of the causes of Earth energy imbalance (IPCC, 2023) and in weather forecasting [Glotfelty et al. 2019]. They are also one of the primary causes of premature deaths [WHO, 2021]. Studying the effects of different sources from an observational point of view provides valuable information to better understand their emissions, transport and effects of related policies.
References
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