07 Nov 2022
07 Nov 2022
Status: this preprint is currently under review for the journal ACP.

Nitrate chemistry in the northeast US part I: nitrogen isotope seasonality tracks nitrate formation chemistry

Claire Bekker1,a,, Wendell W. Walters2,, Lee T. Murray3, and Meredith G. Hastings1,2 Claire Bekker et al.
  • 1Department of Earth, Environmental, and Planetary Sciences, Brown University; Providence, RI 02912, USA
  • 2Institute at Brown for Environment and Society, Brown University; Providence, RI 02912, USA
  • 3Department of Earth and Environmental Sciences, University of Rochester; Rochester, NY 14627, USA
  • anow at: Department of Environmental Health Sciences, University of California Los Angeles; Los Angeles, CA 90095, USA
  • These authors contributed equally to this work.

Abstract. Despite significant precursor emission reductions in the US over recent decades, atmospheric nitrate deposition remains an important terrestrial stressor. Here we utilized statistical air mass back trajectory analysis and nitrogen stable isotope deltas (δ(15N)) to investigate atmospheric nitrate spatiotemporal trends in the northeastern US from samples collected at three US EPA Clean Air Status and Trends Network (CASTNET) sites from December 2016–2018. For the considered sites, similar seasonal patterns in nitric acid (HNO3) and particulate nitrate (pNO3) concentrations were observed with spatial differences attributed to nitrogen oxide (NOx) emission densities in source contributing regions that were typically ≤1000 km. Significant spatiotemporal δ(15N) variabilities in HNO3 and pNO3 were observed with higher values during winter relative to summer, like previous reports from CASTNET samples collected in the early 2000s for our study region. In the early 2000s, δ(15N) of atmospheric nitrate in the Northeast US had been suggested to be driven by NOx emissions; however, we did not find significant spatiotemporal changes in the modeled NOx emissions by sector and fuel type or δ(15N, NOx) for the source regions of the CASTNET sites. Instead, the spatiotemporal trends were driven by δ(15N) fractionation associated with nitrate formation. Under the field conditions of low NOx relative to O3 concentrations and when δ(15N, NOx) emission sources do not have significant variability, we demonstrate that δ(15N) of atmospheric nitrate can be a robust tracer for diagnosing nitrate formation.

Claire Bekker et al.

Status: open (until 19 Dec 2022)

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  • RC1: 'Comment on acp-2022-621', Eva Stueeken, 15 Nov 2022 reply

Claire Bekker et al.

Claire Bekker et al.


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Short summary
Nitrate is a critical component of the atmosphere that degrades air quality and ecosystem health. We have investigated the nitrogen isotope compositions of nitrate from deposition samples collected across the northeastern United States. Spatiotemporal variability in the nitrogen isotope compositions was found to track with nitrate formation chemistry. Our results highlight that nitrogen isotope compositions may be a robust tool for improving model representation of nitrate chemistry.