Aqueous-phase photochemical oxidation of water-soluble brown carbon aerosols arising from solid biomass fuel burning

Abstract

Biomass burning is a major source of short-lived climate forcers, e.g., brown carbon (BrC), that absorb solar radiation and cause warming of the Earth's atmosphere. A large fraction of the population in developing countries is still dependent on wood and animal dung cake burning for household cooking and heating. While wood burning emissions are well studied, a lack of understanding about animal dung burning emissions still persists. This is the first study to explore the aqueous-phase photochemical aging of laboratory-generated water-soluble brown carbon (WS-BrC) aerosols arising from cow dung cake burning, with a focus on cloud-water photochemistry. A comparison with wood burning (pine) was also carried out. We observed that WS-BrC arising from different types of solid biomass fuel burning encountered different evolution pathways, albeit similar UV-light was used during direct photolysis. The photo-enhancement in WS-BrC absorbance was induced more effectively upon exposure to UV-light with shorter wavelengths and coincided with oligomer formation during direct photolysis. During OH oxidation, WS-BrC emitted from burning of all three solid biomass fuels followed similar evolution pathways, with their second-order rate constant values being (25.4 ± 3.9) × 10⁸, (19.2 ± 13.1) × 10⁸ and (23.3 ± 6.5) × 10⁸ M⁻¹ s⁻¹ for WS-BrC arising from burning of Indian cow dung, Canadian cow dung and pine wood, respectively. Combined with a few recent studies, these observations signify that the OH-oxidation lifetime of biomass burning WS-BrC aerosols in cloud-water may not depend on the biomass type.