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

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 (T g) 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 T g of dry SOA, and the dominant species that regulates the dry T g variation is dependent on location and season. At the surface, the T g 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 T g 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.

Particle phase-state variability in the North Atlantic free troposphere during summertime is determined by atmospheric transport patterns and sources

“…S11), implying that organic particles would likely be solid in the FT. These results are consistent with a previous study at the site (Schum et al, 2018) and other modeling studies (Li et al, 2021;Schmedding et al, 2020;Shiraiwa et al, 2017;Shrivastava et al, 2017) indicating that organic particles in the FT are likely in solid and semisolid states. We also calculated the T g,org of organic particles at the OMP using meteorological data collected at the OMP (Table S2), and this also showed that the organics in our samples should have been in a solid state at the time they were sampled at the OMP, since the predicted T g,org is higher than the measured temperature.…”

Global Distribution of the Phase State and Mixing Times within Secondary Organic Aerosol Particles in the Troposphere Based on Room-Temperature Viscosity Measurements

“…131,132 In the PBL, RH and temperature will vary significantly with the time of day and season, and hence the phase state and mixing times within SOA are also expected to vary with the time of day and season. 54…”

“…Additional studies are needed to determine how the global distributions of the phase state and mixing times within SOA change with the time of day and season. The RH and temperature fields in the FT will depend less on the time of the day than at the surface. , In the PBL, RH and temperature will vary significantly with the time of day and season, and hence the phase state and mixing times within SOA are also expected to vary with the time of day and season. ,− …”

“…They found that SOA is mostly in a glassy state, and the mixing times of water most often exceed 1 h in the middle and upper FT, while mixing of organic molecules often exceed 1 h throughout most of the FT. Regional air quality models were also applied to simulate SOA viscosity and T g over the U.S., finding that a glassy state is ubiquitous in the FT. 53,54 In the following, we investigate the global distribution of the phase state and mixing times of water and organic molecules within 200 nm SOA for both the PBL and FT. We focused on SOA with diameters of 200 nm since this is a common size of SOA particles in the troposphere. 55−57 Only one study, Shiraiwa et al, 52 previously investigated global distributions in the FT. Our approach for calculating the phase state and mixing times is different from the approach taken by Shiraiwa et al 52 In our case, we developed a parameterization for the viscosity of simulated pine tree SOA (proxy for biogenic SOA over the boreal forest) and toluene SOA (proxy for anthropogenic SOA) as a function of RH and temperature based on room-temperature viscosity measurements.…”

“…They found that SOA is mostly in a glassy state, and the mixing times of water most often exceed 1 h in the middle and upper FT, while mixing of organic molecules often exceed 1 h throughout most of the FT. Regional air quality models were also applied to simulate SOA viscosity and T g over the U.S., finding that a glassy state is ubiquitous in the FT. , …”