Nitrate Chemistry in the Northeast US Part II: Oxygen Isotopes Reveal Differences in Particulate and Gas Phase Formation.

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(¹⁸O) and Δ(¹⁷O)) of atmospheric nitrate are useful indicators in tracking nitrate formation pathways. Here, we measured Δ(¹⁷O) and δ(¹⁸O) for nitric acid (HNO₃) and particulate nitrate (pNO₃) from three US EPA Clean Air Status and Trends Network (CASTNET) sites in the northeastern US from December 2016 to 2018. The Δ(¹⁷O, HNO₃) and δ(¹⁸O, HNO₃) values ranged from 12.9 ‰ to 30.9 ‰ and from 46.9 ‰ to 82.1 ‰, and the Δ(¹⁷O, pNO₃) and δ(¹⁸O, pNO) ranged from 16.6 ‰ to 33.7 ‰ and from 43.6 ‰ to 85.3 ‰, respectively. There was distinct seasonality of δ(¹⁸O) and Δ(¹⁷O) with higher values observed during winter compared to summer, suggesting a shift in O₃ to HO_x radical chemistry, as expected. Unexpectedly, there was a statistical difference in Δ(¹⁷O) between HNO₃ and pNO₃, with higher values observed for pNO₃ (27.1±3.8) ‰ relative to HNO₃ (22.7±3.6) ‰, and significant differences in the relationship between δ(¹⁸O) and Δ(¹⁷O). 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(¹⁸O) and Δ(¹⁷O) observations, we quantify the production pathways of atmospheric nitrate. The model significantly overestimated the heterogeneous N₂O₅ hydrolysis production for both HNO₃ and pNO₃, a finding consistent with observed seasonal changes in δ(¹⁸O), Δ(¹⁷O) and δ(¹⁵N) of HNO₃ and pNO₃, though large uncertainties remain in the quantitative transfer of δ(¹⁸O) 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]