Abstract
Secondary organic aerosol (SOA) accounts for a substantial portion of atmospheric particulate matter. Phase state plays an important role in the formation and evolution of SOA, while current air quality models usually assume that SOA particles are homogeneous and well-mixed liquids. In this study, we simulate glass transition temperature (Tg) and particle viscosity of SOA based on organic molecular composition over the contiguous US in 2016 using the Community Multiscale Air Quality (CMAQ) model. Simulations show that oligomers from anthropogenic and biogenic SOA and acid-catalyzed isoprene SOA are large contributors to Tg of dry SOA, and the dominant species that regulates the dry Tg variation is dependent on location and season. At the surface, the Tg of dry SOA is higher in the western than in the eastern US, which is due to higher mass fractions of accretion products in the western US. Taking into account the water uptake by SOA, the estimated SOA viscosity shows a prominent geospatial gradient, which is nearly a mirror image of relative humidity. SOA viscosity exhibits a strong diel cycle, and the phase state tends to be more viscous in daytime. The seasonal variations in SOA viscosity are substantially smaller than the diurnal variations. Simulations for four diverse field sites show that Tg and SOA viscosity exhibit significant vertical variations that increase with the altitude. SOA in winter undergoes glass transition at lower altitude (∼3 km) than the other three seasons, and SOA occurs as a non-liquid phase at lower altitude at night than during the daytime. This suggests that chemical transport models may need to consider the bulk diffusion limitations in partitioning into viscous SOA in dry western areas of US and aloft in the humid eastern US as well as dry periods during the day to accurately predict SOA formation, size distribution dynamics, and subsequent impacts.
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