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Title: Nitrate chemistry in the northeast US – Part 1: Nitrogen isotope seasonality tracks nitrate formation chemistry
Abstract. Despite significant precursor emission reductions in theUS over recent decades, atmospheric nitrate deposition remains an importantterrestrial stressor. Here, we utilized statistical air mass back trajectoryanalysis and nitrogen stable isotope deltas (δ(15N)) toinvestigate atmospheric nitrate spatiotemporal trends in the northeastern USfrom samples collected at three US EPA Clean Air Status and Trends Network(CASTNET) sites from December 2016–2018. For the considered sites, similarseasonal patterns in nitric acid (HNO3) and particulate nitrate(pNO3) concentrations were observed with spatial differences attributedto nitrogen oxide (NOx) emission densities in source contributingregions that were typically ≤ 1000 km. Significant spatiotemporalδ(15N) variabilities in HNO3 and pNO3 were observedwith higher values during winter relative to summer, like previous reportsfrom CASTNET samples collected in the early 2000s for our study region. Inthe early 2000s, δ(15N) of atmospheric nitrate in the northeastUS had been suggested to be driven by NOx emissions; however, we didnot find significant spatiotemporal changes in the modeled NOxemissions by sector and fuel type or δ(15N, NOx) for thesource regions of the CASTNET sites. Instead, the seasonal and spatialdifferences in the observed δ(15N) of atmospheric nitrate weredriven by nitrate formation pathways (i.e., homogeneous reactions ofNO2 oxidation via hydroxyl radical or heterogeneous reactions ofdinitrogen pentoxide on wetted aerosol surfaces) and their associatedδ(15N) fractionation. Under the field conditions of lowNOx relative to O3 concentrations and when δ(15N,NOx) emission sources do not have significant variability, wedemonstrate that δ(15N) of atmospheric nitrate can be a robusttracer for diagnosing nitrate formation.  more » « less
Award ID(s):
2414561
PAR ID:
10621000
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
Copernicus Publications on behalf of the European Geosciences Union
Date Published:
Journal Name:
Atmospheric Chemistry and Physics
Volume:
23
Issue:
7
ISSN:
1680-7324
Page Range / eLocation ID:
4185 to 4201
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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