Modeling the Formation of Secondary Organic Aerosols from Semi-volatile Organic Vapors in the Mexico City Region

Manishkumar Shrivastava Pacific Northwest National Laboratory
Rahul Zaveri Pacific Northwest National Laboratory
Jerome Fast Pacific Northwest National Laboratory
Dick Easter Pacific Northwest National Laboratory
Alma Hodzic NCAR

Category: Aerosol Properties

Working Group: Aerosol Life Cycle

Organic aerosols are an important component of ambient particulate matter affecting both climate and human health. Secondary organic aerosols (SOA) are formed in the atmosphere through oxidation of semi-volatile organic vapors emitted from a variety of combustion and biogenic sources. Many studies have shown that formation of anthropogenic and biomass burning SOA is highly under-predicted in current air quality models. Recent work has shown that evolution of organics in the atmosphere is highly dynamic, resulting from photochemical oxidation and gas-particle partitioning downwind of emission sources. The objective of this study is to improve the process representation of anthropogenic and biomass burning organic aerosols in models. The volatility basis-set framework has been coupled to the MOSAIC aerosol model and used to model the dynamic evolution of organics in the atmosphere. A box-model framework is used to do various sensitivity tests on the prediction of organic aerosols and relevant parameters, such as elemental oxygen-to-carbon ratios O:C of organics. In addition to the widely used equilibrium gas-particle partitioning scheme, a kinetics limited gas-particle mass transfer scheme is evaluated for the first time using the volatility basis-set approach. Sensitivity to parameters such as mass accommodation coefficients, volatility distribution, and reaction scheme on the kinetically limited gas-particle mass transfer rates are evaluated first in the box model. Results from these tests are used to infer the most relevant parameters for implementation of SOA treatment in the three-dimensional WRF-Chem model. WRF-Chem simulation results of SOA are evaluated against various measurements, such as those from Aerosol Mass Spectrometers (AMS), taken during the MILAGRO campaign in the vicinity of Mexico City during March 2006, to guide future research in predicting SOA in models. The improved process representation of organics in WRF-Chem will affect simulated light absorption and cloud condensation nuclei (CCN) formation and consequently shed light on the role of organics on direct and indirect forcing of climate. For example, the CCN formation has been recently shown to strongly correlate with O:C ratios that could be explicitly tracked as function of photochemical aging, and kinetically limited gas-particle partitioning in WRF-Chem.

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