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Photodegradation of Secondary Organic Aerosols, the Formation of a Photo-Recalcitrant Fraction and Its Effects on Mass, Composition, and Viscosity
VAHE BABOOMIAN, Giuseppe Crescenzo, Yuanzhou Huang, Fabian Mahrt, Allan Bertram, Sergey Nizkorodov, University of California, Irvine
Abstract Number: 139
Working Group: Aerosol Chemistry
Abstract
The photochemical aging of secondary organic aerosol (SOA) is important for changing their climate and human health-relevant properties. However, these photochemical processes are not well understood to date. One of these properties is viscosity, which is key to predicting lifetimes of particulate air pollutants and understanding their multiphase chemistry. Viscosity is also crucial to predicting the effects of particles on climate through cloud formation. We recently discovered that long-term photodegradation of SOA leads to the formation of a photo-recalcitrant fraction. Due to the unknown viscosity of this non-degrading fraction, considerable uncertainties exist regarding the impact of SOA on climate. To address this, we experimentally measured the humidity dependent viscosity and phase of the photo-recalcitrant fraction of biogenically derived SOA. Our high-resolution mass spectrometry results indicate that the average elemental oxygen-to carbon ratio and molecular weight of the photo-recalcitrant fraction increased by 15% and 10%, respectively, compared to the fresh particles. At the same time, photodegraded particles showed significantly increased viscosity and particle mixing times by as much as 5 orders of magnitude, compared to the fresh particles. The experimentally determined viscosities overlap well with those predicted by parameterizations based on the composition of the recalcitrant fraction, as determined by mass spectrometry. The global distributions of the aerosol mixing time and viscosity were calculated, and the results suggest that glassy aerosols formed at ~ 5 km in altitude for photodegraded particles, compared to ~ 10 km for fresh ones.