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 NO efficiency. A radical budget describes the NO volatile - organic compound (VOC) sensitivities of these plumes. Afternoon plumes displayed a rapid transition from VOC-sensitive to NO-sensitive chemistry, driven by HO (=OH + HO₂) production from photolysis of nitrous acid (HONO) (48 ± 20% of primary HO) and formaldehyde (HCHO) (26 ± 9%) emitted directly from the fire. Evening plumes exhibit a slower transition from peak NO efficiency to VOC-sensitive O₃ production caused by a reduction in photolysis rates and fire emissions. HO production in evening plumes is controlled by HONO photolysis (53 ± 7%), HCHO photolysis (18 ± 9%), and alkene ozonolysis (17 ± 9%).