Aqueous Photochemistry of Secondary Organic Aerosol of α-Pinene and α-Humulene Oxidized with Ozone, Hydroxyl Radical, and Nitrate Radical

Formation of secondary organic aerosols (SOA) from biogenic volatile organic compounds (BVOC) occurs via O-3(-) and OH-initiated reactions during the day and reactions with NO3 during the night. We explored the effect of these three oxidation conditions on the molecular composition and aqueous photochemistry of model SOA prepared from two common BVOC. A common monoterpene, beta-pinene, and sesquiterpene, alpha-humulene, were used to form SOA in a smog chamber via BVOC + O-3, BVOC + NO3, and BVOC + OH + NOx oxidation. Samples of SOA were collected on filters, water-soluble compounds from SOA were extracted in water, and the resulting aqueous solutions were photolyzed to simulate the photochemical aqueous processing of SOA. The extent of change in the molecular level composition of SOA over 4 h of photolysis (approximately equivalent to 64 h of photolysis under ambient conditions) was assessed with high-resolution electrospray ionization mass spectrometry. The analysis revealed significant differences in the molecular composition between SOA formed by the different oxidation pathways. The composition further evolved during photolysis with the most notable change corresponding to the nearly complete removal of nitrogen-containing organic compounds. Hydrolysis of SOA compounds also occurred in parallel with photolysis. The preferential loss of larger SOA compounds during photolysis and hydrolysis made the SOA compounds more volatile on average. This study suggests that aqueous processes may under certain conditions lead to a reduction in the SOA loading as opposed to an increase in SOA loading commonly assumed in the literature.