Back to Search
Start Over
Nitrate Chemistry in the Northeast US Part II: Oxygen Isotopes Reveal Differences in Particulate and Gas Phase Formation.
- Source :
- Atmospheric Chemistry & Physics Discussions; 11/8/2022, p1-33, 33p
- Publication Year :
- 2022
-
Abstract
- The northeastern US represents a mostly urban corridor impacted by high population density, high emissions density and degraded air quality and acid rain that has been a focus of regulatory-driven emissions reductions. Detailing the chemistry of atmospheric nitrate formation is critical for improving model representation of atmospheric chemistry and air quality. The oxygen isotope deltas (δ(<superscript>18</superscript>O) and Δ(<superscript>17</superscript>O)) of atmospheric nitrate are useful indicators in tracking nitrate formation pathways. Here, we measured Δ(<superscript>17</superscript>O) and δ(<superscript>18</superscript>O) for nitric acid (HNO<subscript>3</subscript>) and particulate nitrate (pNO<subscript>3</subscript>) from three US EPA Clean Air Status and Trends Network (CASTNET) sites in the northeastern US from December 2016 to 2018. The Δ(<superscript>17</superscript>O, HNO<subscript>3</subscript>) and δ(<superscript>18</superscript>O, HNO<subscript>3</subscript>) values ranged from 12.9 ‰ to 30.9 ‰ and from 46.9 ‰ to 82.1 ‰, and the Δ(<superscript>17</superscript>O, pNO<subscript>3</subscript>) and δ(<superscript>18</superscript>O, pNO) ranged from 16.6 ‰ to 33.7 ‰ and from 43.6 ‰ to 85.3 ‰, respectively. There was distinct seasonality of δ(<superscript>18</superscript>O) and Δ(<superscript>17</superscript>O) with higher values observed during winter compared to summer, suggesting a shift in O<subscript>3</subscript> to HO<subscript>x</subscript> radical chemistry, as expected. Unexpectedly, there was a statistical difference in Δ(<superscript>17</superscript>O) between HNO<subscript>3</subscript> and pNO<subscript>3</subscript>, with higher values observed for pNO<subscript>3</subscript> (27.1±3.8) ‰ relative to HNO<subscript>3</subscript> (22.7±3.6) ‰, and significant differences in the relationship between δ(<superscript>18</superscript>O) and Δ(<superscript>17</superscript>O). This difference suggests atmospheric nitrate phase-dependent 29 oxidation chemistry that is not predicted in models. Based on output from GEOS-Chem, and both the δ(<superscript>18</superscript>O) and Δ(<superscript>17</superscript>O) observations, we quantify the production pathways of atmospheric nitrate. The model significantly overestimated the heterogeneous N<subscript>2</subscript>O<subscript>5</subscript> hydrolysis production for both HNO<subscript>3</subscript> and pNO<subscript>3</subscript>, a finding consistent with observed seasonal changes in δ(<superscript>18</superscript>O), Δ(<superscript>17</superscript>O) and δ(<superscript>15</superscript>N) of HNO<subscript>3</subscript> and pNO<subscript>3</subscript>, though large uncertainties remain in the quantitative transfer of δ(<superscript>18</superscript>O) from major atmospheric oxidants. This comparison provides important insight into the role of oxidation chemistry in reconciling a commonly observed positive bias for model atmospheric nitrate concentrations in the northeastern US. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 16807367
- Database :
- Complementary Index
- Journal :
- Atmospheric Chemistry & Physics Discussions
- Publication Type :
- Academic Journal
- Accession number :
- 160118876
- Full Text :
- https://doi.org/10.5194/acp-2022-622