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

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 and acid rain that has been a focus of regulatory-driven emissions reductions. Detailing the chemistry of atmospheric nitrate formation is critical for improving the model representation of atmospheric chemistry and air quality. The oxygen isotopic compositions of atmospheric nitrate are useful indicators in tracking nitrate formation pathways. Here, we measured oxygen isotope deltas (Delta(O-17) and delta(O-18)) for nitric acid (HNO3) and particulate nitrate (pNO(3)) from three US EPA Clean Air Status and Trends Network (CASTNET) sites in the northeastern US from December 2016 to 2018. The Delta(O-17, HNO3) and delta(O-18, HNO3) values ranged from 12.9% to 30.9% and from 46.9% to 82.1 %, and the Delta(O-17, pNO3) and delta(O-18, pNO(3)) ranged from 16.6% to 33.7% and from 43.6% to 85.3 %, respectively. There was distinct seasonality of delta(O-18) and Delta(O-17), with higher values observed during winter compared to during summer, suggesting a shift in O-3 to HO x radical chemistry, as expected. Unexpectedly, there was a statistical difference in Delta(O-17) between HNO3 and pNO(3), with higher values observed for pNO(3) (27.1 +/- 3.8)% relative to HNO3 (22.7 +/- 3.6) %, and significant differences in the relationship between delta(O-18) and Delta(O-17). This difference suggests atmospheric nitrate phase-dependent oxidation chemistry that is not predicted in models. Based on the output from GEOS-Chem and both the delta(O-18) and Delta(O-17) 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 delta(O-18) and Delta(O-17) of HNO3 and pNO(3), though large uncertainties remain in the quantitative transfer of delta(O-18) from major atmospheric oxidants. This comparison provides important insight into the role of oxidation chemistry in reconciling a commonly observed positive bias for modeled atmospheric nitrate concentrations in the northeastern US.