Reevaluating Isoprene Oxidation Pathways and Their Influence on Secondary Organic Aerosol Formation

CHUANYANG SHEN, Haofei Zhang, University of California, Riverside

     Abstract Number: 125
     Working Group: Aerosol Chemistry

Abstract
Isoprene is the largest global non-methane hydrocarbon emission, and the chemical reactions of isoprene with atmospheric oxidants play a crucial role in the formation of secondary organic aerosols (SOA), particularly under low-NOx conditions. Two primary pathways contribute to SOA formation from isoprene low-NOx OH oxidation: (1) the production of low-volatility compounds through OH oxidation of intermediates such as ISOPOOH (LV pathway) and (2) the reactive uptake of isoprene epoxidiol (IEPOX) on acidic or aqueous particle surfaces, resulting in the formation of 2-methyltetrols and organosulfates (IEPOX pathway). In this study, we develop a condensed gas-phase chemical mechanism for isoprene oxidation based on the most recent observation-constrained chemistry, and update the zero-dimensional F0AM-WAM model to predict SOA formation from both pathways under chamber and field conditions. We applied our revised isoprene mechanism in simulating chamber studies to evaluate its performance against existing mechanisms by comparing simulations to the observed SOA data. Modeling of the Southern Oxidant and Aerosol Studies (SOAS) field measurements using F0AM-WAM indicate that low-volatility compounds contribute to isoprene SOA more significantly than previously thought, with the modeled SOA mass ratio from the LV pathway of 10-15%, with the rest from the IEPOX pathway. Our findings emphasize that the mass yield from the LV pathway should be considered to enhance the accuracy of model estimations for isoprene SOA formation. The new condensed isoprene chemical mechanism will be further incorporated into regional-scale air quality models, such as the Community Multiscale Air Quality Modelling System (CMAQ), to assess the influence of the LV pathway on a larger scale.