The Δ17O and δ18O values of simultaneously collected atmospheric nitrates from anthropogenic sources – Implications for polluted air masses

摘要

There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N) oxides (NO) emitted from anthropogenic sources into nitrates (NO), two of them being that NO 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 (δO, ΔO) have been applied to infer the chemical pathways leading to the observed distribution of wet (w-NO), particulate (p-NO), and the sum of p-NO and gaseous HNO, while the gaseous form (HNO) has never been separately characterized for O. Previous research studies have investigated w-NO, p-NO or p-NO + HNO from non-polluted or polluted air masses, and inferred seasonal changes in the dominance of oxidation pathways to account for higher δO and ΔO 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 and p-NO, and co-collect w-NO, downwind from five different anthropogenic sources.Overall, the w- and p-NO δO and ΔO values show expected differences between cold and warm seasons, but only the ΔO values of HNO follow this pattern. The HNO δO ranges are distinct from the w- and p-NO patterns. Interestingly, the ΔO differences between p-NO and HNO 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 that has not yet reached isotopic equilibrium with O and subsequent differences in dry deposition rates, while the larger proportion of p-NO formed via the NO pathway can explain the fall-winter pattern. Very low p-NO ΔO values observed during warm months may be due to this non-equilibrated NO, though contribution from RO oxidation remains a possibility. Our results show that the isotopic signals of HNO, w-NO and p-NO are not interchangeable and that their differences can further our understanding of NO oxidation and deposition. Future research should investigate all tropospheric nitrate species as well as NO to refine our understanding of nitrate worldwide and to develop effective emission reduction strategies.