Variability and Time of Day Dependence of Ozone Photochemistry in Western Wildfire Plumes

Understanding the efficiency and variability of photochemical ozone (O3) production from western wildfire plumes is important to accurately estimate their influence on North American air quality. A set of photochemical measurements were made from the NOAA Twin Otter research aircraft as a part of the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) experiment. We use a zero-dimensional (0-D) box model to investigate the chemistry driving O3 production in modeled plumes. Modeled afternoon plumes reached a maximum O3 mixing ratio of 140 ± 50 ppbv (average ± standard deviation) within 20 ± 10 min of emission compared to 76 ± 12 ppbv in 60 ± 30 min in evening plumes. Afternoon and evening maximum O3 isopleths indicate that plumes were near their peak in NOx efficiency. A radical budget describes the NOx volatile - organic compound (VOC) sensitivities of these plumes. Afternoon plumes displayed a rapid transition from VOC-sensitive to NOx-sensitive chemistry, driven by HOx (=OH + HO2) production from photolysis of nitrous acid (HONO) (48 ± 20% of primary HOx) and formaldehyde (HCHO) (26 ± 9%) emitted directly from the fire. Evening plumes exhibit a slower transition from peak NOx efficiency to VOC-sensitive O3 production caused by a reduction in photolysis rates and fire emissions. HOx production in evening plumes is controlled by HONO photolysis (53 ± 7%), HCHO photolysis (18 ± 9%), and alkene ozonolysis (17 ± 9%).