Organic matter in dust-dominated aerosols over Namibia influenced by biomass burning and marine emissions

The southern African region, particularly the Atlantic coast of Namibia, is an ideal natural laboratory influenced by contrasting aerosol sources that represent endmembers of aerosol formation processes, including aged marine aerosols, mineral dust resuspension, and long-range transported biomass-burning aerosols (BBA), with only moderate local anthropogenic influence. This setting enables direct linkage between aerosol chemical composition and optical properties under near-pristine conditions, thereby improving the representation and projection of aerosol–radiation and aerosol–cloud interactions and associated climate feedbacks in close proximity to the highly sensitive southeastern Atlantic stratocumulus deck, one of the major regulators of planetary albedo.

Although organic matter (OM) typically accounts for only a minor fraction of aerosol mass, it demonstrates a disproportional contribution to optical properties, hygroscopicity and cloud-formation, especially in a dust-dominated environment. Here, we present a comprehensive characterization of OM with respect to its sources, molecular composition, aging processes, and relationships with aerosol optical properties. Organic aerosol (OA) was extracted from daily particulate matter (PM₁₀) samples collected during a month-long field campaign at Gobabeb, Namibia, and analyzed using high-resolution mass spectrometry (HRMS), complemented by inorganic ion analysis, organic and elemental carbon quantification, and multivariate statistical methods. These approaches were used to distinguish three dominant aerosol regimes: dust-dominated, BBA-influenced, and marine-dominated periods.

Organic molecules associated with BBA events exhibited elevated O/C ratios and double-bond equivalent (DBE) values, consistent with enhanced light absorption in the UV–visible range. These molecular features show strong correlations with bulk aerosol extinction and scattering coefficients, highlighting the optical relevance of OM despite its limited mass contribution. To better constrain the contribution of dust-derived OM, laboratory resuspension experiments were conducted using local soils. Comparison of OM extracted from parent soils and resuspended aerosols revealed substantial compositional differences, indicating selective transfer of specific organic components into the aerosol phase. This selectivity allowed identification of soil-derived OM fractions that systematically contribute to atmospheric aerosols and their optical properties.

Finally, we applied a novel formula-difference approach to the HRMS data to resolve aerosol aging processes across the different aerosol regimes. By comparing molecular transformation patterns, we explored period-specific aging pathways reflected in characteristic gains and losses of functional groups and molecular connectivity. These chemical fingerprints indicate periods with higher influence of oxidation, condensation and aromatisation of OM, which provides additional insights into the fate of organic matter in mixed aerosol systems and its role in modifying aerosol optical properties during atmospheric aging.