PNNL

Abstract

Accurate predictions of chemical lifetime and mass of organic aerosols (OA) in atmospheric models are critical to quantify impacts of OA on air quality, public health, and climate more precisely. Heterogeneous oxidation of OA by OH radicals is a key process governing degradation and lifetime of OA. Combined impacts of OH concentrations and phase state/viscosity (i.e., liquid, semi-solid, solid) of OA on the heterogeneous chemical degradation rate of OA and its lifetime in the atmosphere are investigated in this study. In this study, most recent parametrization to predict OA decay rates and lifetimes due to aging by heterogeneous OH oxidation kinetics is implemented in the Weather Research and Forecasting Model coupled to chemistry (WRF-Chem) model, with focus on the GoAmazon 2014/15 dry-to-wet transition season influenced by significant biogenic and biomass burning emissions. OA lifetime from OH aging is characterized through OH uptake coefficient (?_OH) and resultant heterogeneous reaction rate (k_(het,OH)) as a function of OA viscosity (?_(org )). Updated volatility-viscosity prediction framework recently implemented in WRF-Chem has been leveraged for this study. This parametrization is robustly validated with and based on observations of ?_OH and k_(het,OH) for varied OA systems, and is relevant for all ambient tropospheric conditions. OH uptake coefficients and reaction rates with OA (?_OH and k_(het,OH), respectively) are much higher near surface compared to the upper troposphere (factor of ~7 and ~2, respectively), due to liquid OA dominant near surface but solid in the upper troposphere. Our box model suggests that away from source regions and in polluted regions compared to Amazon, heterogeneous OH oxidation will strongly affect OA concentrations (full decay in 7-10 days). However, WRF-Chem results for OA mass change with this OH uptake parametrization also highlights that in vicinity of multiple sources generating fresh OA (as is the case for dry-to-wet transition GoAmazon 2014/15 season) OA decay from heterogeneous OH oxidation is impeded. Our work has large scope to further the understanding on how urbanization and deforestation in the Amazon might impact the chemical lifetime of OA in the atmosphere, and thereby have implications on its ability to serve as cloud condensation nuclei (CCN) and feedback on climate.