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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-1103
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
09 Jan 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
The Δ17O and δ18O values of simultaneously collected atmospheric nitrates from anthropogenic sources – Implications for polluted air masses
Martine M. Savard1, Amanda Cole2, Robert Vet2, and Anna Smirnoff1 1Geological Survey of Canada (Natural Resources Canada), 490 de la Couronne, Québec (QC), G1K 9A9, Canada
2Air Quality Research Division, Environment and Climate Change Canada, 4905 Dufferin St., Toronto (ON), M3H 5T4, Canada
Abstract. There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N) oxides (NOx) emitted from anthropogenic sources into nitrates (NO3), two of them being that NO3 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 (δ18O, Δ17O) have been applied to infer the chemical pathways leading to the observed distribution of wet (w-NO3), particulate (p-NO3), and the sum of p-NO3 and gaseous HNO3, while the gaseous form (HNO3) has never been separately characterized for 17O. Previous research studies have investigated w-NO3, p-NO3 or p-NO3 + HNO3 from non-polluted or polluted air masses, and inferred seasonal changes in the dominance of oxidation pathways to account for higher δ18O and Δ17O 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 HNO3 and p-NO3, and co-collect w-NO3, downwind from five different anthropogenic sources.

Overall, the w- and p-NO3 δ18O and Δ17O values show expected differences between cold and warm seasons, but only the Δ17O values of HNO3 follow this pattern. The HNO3 δ18O ranges are distinct from the w- and p-NO3 patterns. Interestingly, the Δ17O differences between p-NO3 and HNO3 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 NOx that has not yet reached isotopic equilibrium with O3 and subsequent differences in dry deposition rates, while the larger proportion of p-NO3 formed via the N2O5 pathway can explain the fall-winter pattern. Very low p-NO3 Δ17O values observed during warm months may be due to this non-equilibrated NOx, though contribution from RO2 oxidation remains a possibility. Our results show that the isotopic signals of HNO3, w-NO3 and p-NO3 are not interchangeable and that their differences can further our understanding of NOx oxidation and deposition. Future research should investigate all tropospheric nitrate species as well as NOx to refine our understanding of nitrate worldwide and to develop effective emission reduction strategies.


Citation: Savard, M. M., Cole, A., Vet, R., and Smirnoff, A.: The Δ17O and δ18O values of simultaneously collected atmospheric nitrates from anthropogenic sources – Implications for polluted air masses, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-1103, in review, 2018.
Martine M. Savard et al.
Martine M. Savard et al.
Martine M. Savard et al.

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Short summary
Improving air quality requires understanding the atmospheric processes transforming nitrous oxides emitted by human activities into nitrates, an N-form that may degrade natural ecosystems. Isotopes (∆17O, δ18O) are characterized in separate wet, particulate and gaseous nitrates for the first time. The gas ranges are distinct from those of the other nitrates, the plume dynamics emerge as crucial in interpreting the results, which unravel key processes behind the distribution of nitrates.
Improving air quality requires understanding the atmospheric processes transforming nitrous...
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