Isotopic evidence for dominant secondary production of HONO in near-ground wildfire plumes.

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From: Atmospheric Chemistry and Physics(Vol. 21, Issue 17)
Publisher: Copernicus GmbH
Document Type: Brief article
Length: 335 words

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Abstract :

Nitrous acid (HONO) is an important precursor to hydroxyl radical (OH) that determines atmospheric oxidative capacity and thus impacts climate and air quality. Wildfire is not only a major direct source of HONO, it also results in highly polluted conditions that favor the heterogeneous formation of HONO from nitrogen oxides (NO.sub.x = NO + NO.sub.2) and nitrate on both ground and particle surfaces. However, these processes remain poorly constrained. To quantitatively constrain the HONO budget under various fire and/or smoke conditions, we combine a unique dataset of field concentrations and isotopic ratios (.sup.15 N / .sup.14 N and .sup.18 O / .sup.16 O) of NO.sub.x and HONO with an isotopic box model. Here we report the first isotopic evidence of secondary HONO production in near-ground wildfire plumes (over a sample integration time of hours) and the subsequent quantification of the relative importance of each pathway to total HONO production. Most importantly, our results reveal that nitrate photolysis plays a minor role (5 %) in HONO formation in daytime aged smoke, while NO.sub.2 -to-HONO heterogeneous conversion contributes 85 %-95 % to total HONO production, followed by OH + NO (5 %-15 %). At nighttime, heterogeneous reduction of NO.sub.2 catalyzed by redox active species (e.g., iron oxide and/or quinone) is essential (⥠75 %) for HONO production in addition to surface NO.sub.2 hydrolysis. Additionally, the .sup.18 O / .sup.16 O of HONO is used for the first time to constrain the NO-to-NO.sub.2 oxidation branching ratio between ozone and peroxy radicals. Our approach provides a new and critical way to mechanistically constrain atmospheric chemistry and/or air quality models on a diurnal timescale.

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Gale Document Number: GALE|A674201786