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Photodegradation of Secondary Organic Aerosols by Long Term Exposure to Solar Actinic Radiation
VAHE BABOOMIAN, Yiran Gu, Sergey Nizkorodov, University of California, Irvine
Abstract Number: 68
Working Group: Aerosol Chemistry
Abstract
The photochemical aging of secondary organic aerosols (SOA) are crucial to understand the climate and health-related properties of particulate matter, but these photochemical processes are not well understood. We measured the photodegradation rates of SOA by condensed phase photochemical reactions over long time periods of multiple days of ultraviolet exposure. The photodegradation rates of various SOA types were quantified using a quartz crystal microbalance. We observed that 254 nm irradiation degraded SOA almost entirely after 24 h. In contrast, 305 nm irradiation led to initially high mass loss rates (corresponding to 1-5 % fractional mass loss per hour) but slowed down after 24 h of irradiation with a photorecalcitrant fraction of SOA degrading much slower (< 1% fractional mass loss per hour). The mass loss rates were observed to increase at higher relative humidity due to increased diffusivity. Long term changes in chemical composition were observed using a high-resolution electrospray ionization mass spectrometer and suggests a more complex mixture of species remain after degradation. The compounds in the degraded samples had on average lower H:C and higher O:C compared to the compounds in fresh SOA. These experiments confirm that condensed phase photochemical processes are an important SOA aging mechanism during long range transport.