Novel Analysis to Quantify Plume Crosswind Heterogeneity Applied to Biomass Burning Smoke

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 NO3 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, NO3 production [P­(NO3)] occurs mainly at the plume center (w P(NO3)/w CO = 0.91–1.00 ± 0.01). Phenolic emissions, highly reactive to OH and NO3, 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(NO3). Last, we corroborate GOMECH with a large eddy simulation model which suggests most (55%) of nitrocatechol is produced through NO3 in our case study.