LAUSR

Abstract. There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N) oxides (NO x ) emitted from anthropogenic sources into nitrates (NO 3 − ), two of them being that NO 3 − contributes to acidification and eutrophication of terrestrial and aquatic ecosystems, and particulate nitrate may play a role in climate dynamics. For these reasons, oxygen isotope ratios (δ 18 O, Δ 17 O) have been applied to infer the chemical pathways leading to the observed distribution of wet (w-NO 3 − ), particulate (p-NO 3 − ), and the sum of p-NO 3 − and gaseous HNO 3 , while the gaseous form (HNO 3 ) has never been separately characterized for 17 O. Previous research studies have investigated w-NO 3 − , p-NO 3 − or p-NO 3 −  + HNO 3 from non-polluted or polluted air masses, and inferred seasonal changes in the dominance of oxidation pathways to account for higher δ 18 O and Δ 17 O values in winter relative to summer. However, none of the polluted air studies collected samples specific to targeted emission sources. Here we have used a wind-sector-based, multi-stage filter sampling system and precipitation collector to simultaneously sample HNO 3 and p-NO 3 − , and co-collect w-NO 3 − , downwind from five different anthropogenic sources. Overall, the w- and p-NO 3 δ 18 O and Δ 17 O values show expected differences between cold and warm seasons, but only the Δ 17 O values of HNO 3 follow this pattern. The HNO 3 δ 18 O ranges are distinct from the w- and p-NO 3 − patterns. Interestingly, the Δ 17 O differences between p-NO 3 − and HNO 3 shifts from positive during cold sampling periods to negative during warm periods. The summer pattern may be due to the presence of nitrates derived from NO x that has not yet reached isotopic equilibrium with O 3 and subsequent differences in dry deposition rates, while the larger proportion of p-NO 3 − formed via the N 2 O 5 pathway can explain the fall-winter pattern. Very low p-NO 3 − Δ 17 O values observed during warm months may be due to this non-equilibrated NO x , though contribution from RO 2 oxidation remains a possibility. Our results show that the isotopic signals of HNO 3 , w-NO 3 − and p-NO 3 − are not interchangeable and that their differences can further our understanding of NO x oxidation and deposition. Future research should investigate all tropospheric nitrate species as well as NO x to refine our understanding of nitrate worldwide and to develop effective emission reduction strategies.