Aqueous-phase secondary organic aerosol formation on mineral dust

Secondary organic aerosol (SOA) is a significant component of airborne particles that impacts air quality, health, and climate globally. Aqueous-phase reactions contribute substantially to SOA mass. However, this process is primarily treated as occurring in submicron particles that contain water, or within cloud droplets in state-of-the-art models. Here, we challenged this conventional view by showing that >50% of water-soluble organic carbon (WSOC), predominantly SOA, is found in supermicron particles during dust events downwind of Saharan and Asian dust sources. Even on non-dust days, supermicron WSOC contributes 25%-51% of total WSOC. Microscopic analyses revealed that organic matter was only detected on aged dust containing a calcium nitrate coating, which contains water at typical ambient relative humidity conditions. This suggests that it is the water-containing nitrate coating that facilitates aqueous-phase SOA formation. By incorporating the reactive uptake of glyoxal, a key precursor of SOA, into a global model, we significantly improved the model's performance in reproducing supermicron particle contributions to total WSOC observed in the field. Using this improved model, aqueous-phase SOA formed on dust particles over the land contributes to 16% of total SOA and 28% of total aqueous-phase SOA, with levels reaching up to 67% and 74% across the 'dust belt', respectively. These results underscore the important role of aqueous-phase reactions in aged nitrate-containing dust in SOA formation, which should be incorporated into global models to quantify their potential implications for air quality, health, and climate.