29 Mar 2021

29 Mar 2021

Review status: this preprint is currently under review for the journal ACP.

Isotopic constraints on wildfire derived HONO

Jiajue Chai1,2, Jack E. Dibb3, Bruce E. Anderson4, Claire Bekker1,2, Danielle E. Blum1,5, Eric Heim3, Carolyn E. Jordan4,6, Emily E. Joyce1,2, Jackson H. Kaspari7, Hannah Munro3, Wendell W. Walters1,2, and Meredith G. Hastings1,2 Jiajue Chai et al.
  • 1Institute at Brown for Environment and Society, Brown University, Providence, RI
  • 2Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI
  • 3Institute for the Study of Earth, Ocean and Space, University of New Hampshire, Durham, NH
  • 4NASA Langley Research Center, Hampton, VA
  • 5Department of Chemistry, Brown University, Providence, RI
  • 6National Institute of Aerospace, Hampton, VA
  • 7Department of Chemistry, University of New Hampshire, Durham, NH

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, but it also results in highly polluted conditions that favour heterogeneous formation of HONO from nitrogen oxides (NOx = NO + NO2) and nitrate on both ground and particle surfaces. However, these processes remain poorly constrained. To quantitatively constrain the HONO budget under various fire/smoke conditions, we combine a unique dataset of field concentrations and isotopic ratios (15N/14N and 18O/16O) of NOx and HONO, with an isotopic box model. Here we report the first isotopic evidence of secondary HONO production in near-ground wildfire plumes, and the subsequent quantification of 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 photo-enhanced NO2-to-HONO heterogeneous conversion contributes 85–95 % to total HONO production, followed by OH+NO (5–15 %). In nighttime, heterogeneous reduction of NO2 catalysed by redox active species (e.g., iron oxide and/or quinone) is essential (≥ 75 %) for HONO production in addition to surface NO2 hydrolysis. Additionally, the 18O/16O of HONO is used for the first time to constrain the NO-to-NO2 oxidation branching ratio between ozone and peroxy radicals. Our approach provides a new and critical way to mechanistically constrain atmospheric chemistry/air quality models.

Jiajue Chai et al.

Status: open (until 24 May 2021)

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Jiajue Chai et al.

Jiajue Chai et al.


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Short summary
Nitrous acid (HONO) derived from wildfire emissions plays a key role in controlling the atmospheric oxidation chemistry. However, HONO budget remains poorly constrained. By combining the field-observed concentrations and novel isotopic composition (N and O) of HONO and nitrogen oxides (NOx), we quantitatively constrained the relative contribution of each pathway to secondary HONO production, and the relative importance of major atmospheric oxidants (ozone versus peroxy) in aged wildfire smokes.