Diurnal and Seasonal Variations in the Phase State of Secondary Organic Aerosol Material over the Contiguous US Simulated in CMAQ

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.