Daytime PAN and PPN Production Mechanisms in Western U.S. Wildfire Smoke Plumes

Abstract:

Acyl peroxy nitrates (APNs) are formed rapidly in fire plumes through the oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxide radicals (NO_x= NO + NO₂). The formation and decomposition of APNs are important pathways by which NO_x emissions from pyrogenic sources are sequestered and then subsequently released to contribute to downwind ozone (O₃) formation. Predicting the formation rate of APNs in smoke has been a challenge to models, indicating potential gaps in our understanding of this chemistry and insufficient constraints on key input parameters. Here we use observations of PAN (peroxyacetic nitric anhydride; CH₃C(O)O₂NO₂), PPN (peroxypropionic nitric anhydride; CH₃CH₂C(O)O₂NO₂) and their precursors from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) campaign/study to test our ability to predict PAN and PPN formation in smoke using a simple empirical production model. We show that acetaldehyde and biacetyl are important immediate PAN precursors in large western wildfire smoke plumes, and conclude that models should include the emissions, formation, and loss of these compounds in order to accurately predict PAN formation and the sequestration of NO_x. We show that propanal is unlikely to be the only significant PPN precursor in smoke plumes; we hypothesize that ethylglyoxal could contribute to the PPN formation observed during WE-CAN that cannot be attributed to propanal, but very few measurements of ethylglyoxal exist, suggesting an area of future research.