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We present a novel method, the Gaussian observational model for edge to center heterogeneity (GOMECH), to quantify the horizontal chemical structure of plumes. GOMECH fits observations of short-lived emissions or products against a long-lived tracer (e.g., CO) to provide relative metrics for the plume width ( w i / w CO ) and center ( b i / w CO ). To validate GOMECH, we investigate OH and NO 3 oxidation processes in smoke plumes sampled during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality, a 2019 wildfire smoke study). An analysis of 430 crosswind transects demonstrates that nitrous acid (HONO), a primary source of OH, is narrower than CO ( w HONO / w CO = 0.73-0.84 ± 0.01) and maleic anhydride (an OH oxidation product) is enhanced on plume edges ( w maleicanhydride / w CO = 1.06-1.12 ± 0.01). By contrast, NO 3 production [P(NO 3 )] occurs mainly at the plume center ( w P(NO 3 ) / w CO = 0.91-1.00 ± 0.01). Phenolic emissions, highly reactive to OH and NO 3 , are narrower than CO ( w phenol / w CO = 0.96 ± 0.03, w catechol / w CO = 0.91 ± 0.01, and w methylcatechol / w CO = 0.84 ± 0.01), suggesting that plume edge phenolic losses are the greatest. Yet, nitrophenolic aerosol, their oxidation product, is the greatest at the plume center ( w nitrophenolicaerosol / w CO = 0.95 ± 0.02). In a large plume case study, GOMECH suggests that nitrocatechol aerosol is most associated with P(NO 3 ). Last, we corroborate GOMECH with a large eddy simulation model which suggests most (55%) of nitrocatechol is produced through NO 3 in our case study.

Understanding the efficiency and variability of photochemical ozone (O 3 ) 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 O 3 production in modeled plumes. Modeled afternoon plumes reached a maximum O 3 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 O 3 isopleths indicate that plumes were near their peak in NO x efficiency. A radical budget describes the NO x volatile - organic compound (VOC) sensitivities of these plumes. Afternoon plumes displayed a rapid transition from VOC-sensitive to NO x -sensitive chemistry, driven by HO x (=OH + HO 2 ) production from photolysis of nitrous acid (HONO) (48 ± 20% of primary HO x ) and formaldehyde (HCHO) (26 ± 9%) emitted directly from the fire. Evening plumes exhibit a slower transition from peak NO x efficiency to VOC-sensitive O 3 production caused by a reduction in photolysis rates and fire emissions. HO x production in evening plumes is controlled by HONO photolysis (53 ± 7%), HCHO photolysis (18 ± 9%), and alkene ozonolysis (17 ± 9%).

Wildfires are an important source of nitrous acid (HONO), a photolabile radical precursor, yet in situ measurements and quantification of primary HONO emissions from open wildfires have been scarce. We present airborne observations of HONO within wildfire plumes sampled during the Western Wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) campaign. ΔHONO/ΔCO close to the fire locations ranged from 0.7 to 17 pptv ppbv -1 using a maximum enhancement method, with the median similar to previous observations of temperate forest fire plumes. Measured HONO to NO x enhancement ratios were generally factors of 2, or higher, at early plume ages than previous studies. Enhancement ratios scale with modified combustion efficiency and certain nitrogenous trace gases, which may be useful to estimate HONO release when HONO observations are lacking or plumes have photochemical exposures exceeding an hour as emitted HONO is rapidly photolyzed. We find that HONO photolysis is the dominant contributor to hydrogen oxide radicals (HO x = OH + HO 2 ) in early stage (<3 h) wildfire plume evolution. These results highlight the role of HONO as a major component of reactive nitrogen emissions from wildfires and the main driver of initial photochemical oxidation.