Havala Olson Taylor Pye, United States Environmental Protection Agency (US EPA)
Organic aerosol is a significant component of PM2.5 and is an increasing fraction of the total in many locations. Organic aerosol is largely secondary in nature and forms from the gas to particle conversion of low volatility or semi-soluble species. In the southeast US, monoterpene oxidation is responsible for half of the total organic particulate matter in summer. In this work, we examine multiple ways in which nitrogen oxides (NOx) affect the efficiency with which monoterpenes are converted to PM2.5. First, we explore prompt, first-generation unimolecular autoxidation reactions. Using ambient observations downwind of Atlanta, an explicit mechanism, and CMAQ model calculations, we show that reductions in oxidants that come with reducing NOx allow for reductions in PM2.5 from first generation autoxidation. Second, we expand the detailed explicit mechanism to understand how first vs second generation monoterpene chemistry leading to PM2.5 changes as a function of NOx. Third, we estimate the degree to which semi-volatile and semi-soluble organic nitrates contribute to PM2.5 via heterogeneous reaction. Overall, we predict a positive coupling between anthropogenic NOx and regional biogenic SOA from monoterpene oxidation pathways implying that NOx emission controls should reduce organic PM2.5 from monoterpenes.
Kelvin Bates and Viral Shah