Nitrate chemistry in the northeast US – Part 2: Oxygen isotopes reveal differences in particulate and gas-phase formation

Abstract. The northeastern US represents a mostly urban corridor impacted by high population and fossil fuel combustion emission density. This has led to historically degraded air quality acid rain that been focus of regulatory-driven emissions reductions. Detailing the chemistry atmospheric nitrate formation is critical for improving model representation quality. oxygen isotopic compositions are useful indicators in tracking pathways. Here, we measured isotope deltas (Δ(17O) δ(18O)) nitric (HNO3) particulate (pNO3) from three EPA Clean Air Status Trends Network (CASTNET) sites December 2016 2018. Δ(17O, HNO3) δ(18O, values ranged 12.9 ‰ 30.9 46.9 82.1 ‰, pNO3) 16.6 33.7 43.6 85.3 respectively. There was distinct seasonality δ(18O) Δ(17O), with higher observed during winter compared summer, suggesting shift O3 HOx radical chemistry, as expected. Unexpectedly, there statistical difference Δ(17O) between HNO3 pNO3, pNO3 (27.1 ± 3.8) relative (22.7 3.6) significant differences relationship Δ(17O). suggests phase-dependent oxidation not predicted models. Based on output GEOS-Chem both observations, quantify production pathways nitrate. significantly overestimated heterogeneous N2O5 hydrolysis finding consistent seasonal changes though large uncertainties remain quantitative transfer major oxidants. comparison provides important insight into role reconciling commonly positive bias modeled concentrations US.