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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 08 May 2019

Submitted as: research article | 08 May 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Global inorganic nitrate production mechanisms: Comparison of a global model with nitrate isotope observations

Becky Alexander1, Tomás Sherwen2,3, Christopher D. Holmes4, Jenny A. Fisher5, Qianjie Chen1,a, Mat J. Evans2,3, and Prasad Kasibhatla6 Becky Alexander et al.
  • 1Departmentof Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
  • 2Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York YO10 5DD, UK
  • 3National Center for Atmospheric Science, University of York, York YO10 5DD, UK
  • 4Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA
  • 5Centre for Atmospheric Chemistry, University of Wollongong, Wollongong, New South Wales 2522, Australia
  • 6Nicholas School of the Environment, Duke University, Durham, NC 27708, USA
  • anow at: Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, USA

Abstract. The formation of inorganic nitrate is the main sink for nitrogen oxides (NOx = NO + NO2). Due to the importance of NOx for the formation of tropospheric oxidants such as the hydroxyl radical (OH) and ozone, understanding the mechanisms and rates of nitrate formation is paramount for our ability to predict the atmospheric lifetimes of most reduced trace gases in the atmosphere. The oxygen isotopic composition of nitrate (Δ17O(nitrate)) is determined by the relative importance of NOx sinks, and thus can provide an observational constraint for NOx chemistry. Until recently, the ability to utilize Δ17O(nitrate) observations for this purpose was hindered by our lack of knowledge about the oxygen isotopic composition of ozone (Δ17O(O3)). Recent and spatially widespread observations of Δ17O(O3) have greatly reduced this uncertainty, and allow for an updated comparison of modeled and observed Δ17O(nitrate) and a reassessment of modeled nitrate formation pathways. Model updates based on recent laboratory studies of heterogeneous reactions renders dinitrogen pentoxide (N2O5) hydrolysis as important as NO2 + OH (both 41 %) for global inorganic nitrate production near the surface. All other nitrate production mechanisms represent less than 6 % of global nitrate production near the surface, but can be dominant locally. Updated reaction rates for aerosol uptake of NO2 result in significant reduction of nitrate and nitrous acid (HONO) formed through this pathway in the model, and render NO2 hydrolysis a negligible pathway for nitrate formation globally. Although photolysis of aerosol nitrate may have implications for NOx, HONO and oxidant abundances, it does not significantly impact the relative importance of nitrate formation pathways. Modeled Δ17O(nitrate) (28.6 ± 4.5 ‰) compares well with the average of a global compilation of observations (27.6 ± 5.0 ‰), giving confidence in the model's representation of the relative importance of ozone versus HOx (= OH + HO2 + RO2) in NOx cycling and nitrate formation.

Becky Alexander et al.
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Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Becky Alexander et al.
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Becky Alexander et al.
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