Nitrate chemistry in the northeast US – Part 1: Nitrogen isotope seasonality tracks nitrate formation chemistry
<p>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 (<span class="inline-formula&...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/23/4185/2023/acp-23-4185-2023.pdf |
| Summary: | <p>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 (<span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><sup>15</sup></span>N)) 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 (HNO<span class="inline-formula"><sub>3</sub></span>) and particulate nitrate
(pNO<span class="inline-formula"><sub>3</sub></span>) concentrations were observed with spatial differences attributed
to nitrogen oxide (NO<span class="inline-formula"><sub><i>x</i></sub></span>) emission densities in source contributing
regions that were typically <span class="inline-formula">≤</span> 1000 km. Significant spatiotemporal
<span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><sup>15</sup></span>N) variabilities in HNO<span class="inline-formula"><sub>3</sub></span> and pNO<span class="inline-formula"><sub>3</sub></span> 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, <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><sup>15</sup></span>N) of atmospheric nitrate in the northeast
US had been suggested to be driven by NO<span class="inline-formula"><sub><i>x</i></sub></span> emissions; however, we did
not find significant spatiotemporal changes in the modeled NO<span class="inline-formula"><sub><i>x</i></sub></span>
emissions by sector and fuel type or <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><sup>15</sup></span>N, NO<span class="inline-formula"><sub><i>x</i></sub></span>) for the
source regions of the CASTNET sites. Instead, the seasonal and spatial
differences in the observed <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><sup>15</sup></span>N) of atmospheric nitrate were
driven by nitrate formation pathways (i.e., homogeneous reactions of
NO<span class="inline-formula"><sub>2</sub></span> oxidation via hydroxyl radical or heterogeneous reactions of
dinitrogen pentoxide on wetted aerosol surfaces) and their associated
<span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><sup>15</sup></span>N) fractionation. Under the field conditions of low
NO<span class="inline-formula"><sub><i>x</i></sub></span> relative to O<span class="inline-formula"><sub>3</sub></span> concentrations and when <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><sup>15</sup></span>N,
NO<span class="inline-formula"><sub><i>x</i></sub></span>) emission sources do not have significant variability, we
demonstrate that <span class="inline-formula"><i>δ</i></span>(<span class="inline-formula"><sup>15</sup></span>N) of atmospheric nitrate can be a robust
tracer for diagnosing nitrate formation.</p> |
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| ISSN: | 1680-7316 1680-7324 |