Abstract View
Diurnal and Seasonal Variations of Secondary Organic Aerosol Phase State over the Continent US Simulated in CMAQ
YING LI, Annmarie Carlton, Kirk Baker, Manabu Shiraiwa, University of California, Irvine
Abstract Number: 328
Working Group: Aerosol Physics
Abstract
Secondary organic aerosol (SOA) accounts for a substantial portion of atmospheric particulate matter. Accurate descriptions of formation and evolution of SOA remain a grand challenge in air quality models in which SOA particles have often been assumed to be homogeneous and well-mixed liquids and the semi-volatile oxidation products rapidly establish a gas-particle equilibrium partitioning. Recent laboratory experiments and atmospheric measurements, however, have demonstrated that SOA can occur in amorphous solid or semi-solid phase states, which may result in kinetic limitations in gas-particle partitioning. A phase transition between amorphous solid and semi-solid state occurs at glass transition temperature (Tg), which can be predicted based on organic molecular composition (Shiraiwa et al., 2017; Li et al., 2020). In this study we simulate Tg and SOA phase state over the contiguous US in 2016 using the Community Multiscale Air Quality (CMAQ) model to investigate potential locations where thermodynamic gas-particle partitioning should be more thoroughly evaluated. Simulations show that Tg and SOA viscosity are higher in the west than the eastern US which is due to higher mass fractions of accretion products and lower relative humidity in the western US. SOA viscosity is predicted to be higher in the daytime than the nighttime, and higher in Spring than other seasons. The diurnal and seasonal variations in SOA viscosity are substantially smaller than the difference between the viscosity in the western and eastern US. Vertical profiles show that SOA occur mainly as semi-solid and solid in the middle and upper troposphere, consistent with our previous global simulations (Shiraiwa et al., 2017).
References:
[1] Shiraiwa, et al. (2017), Global distribution of particle phase state in atmospheric secondary organic aerosols, Nat. Commun., 8:15002.
[2] Li, et al. (2020), Predictions of the glass transition temperature and viscosity of organic aerosols by volatility distributions, Atmos. Chem. Phys., https://doi.org/10.5194/acp-2019-1132, accepted.