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TitleThe delta-17O and delta-18O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources - implications for polluted air masses
AuthorSavard, M M; Cole, A S; Vet, R; Smirnoff, A
SourceAtmospheric Chemistry and Physics vol. 18, issue 14, 2018 p. 10373-10389, (Open Access)
Alt SeriesNatural Resources Canada, Contribution Series 20170310
PublisherCopernicus GmbH
Mediaon-line; digital
File formatpdf
Subjectsenvironmental geology; geochemistry; pollutants; isotopic studies; oxygen isotopes; isotope ratios; ozone; atmospheric geochemistry; ecosystems; geochemical anomalies; meteorology; oxidation; methodology; air pollution; anthropogenic sources; nitrogen oxides; nitrates; nitric acid; peroxy radicals; Air quality; Air pollution
Illustrationslocation maps; tables; plots; diagrams
ProgramEnvironmental Geoscience, Sources
Released2018 07 20
AbstractThere 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 delta values (delta-18O, delta-17O) are frequently applied to infer the chemical pathways leading to the observed mass-independent isotopic anomalies from interaction with 17O-rich ozone (O3). Recent laboratory experiments suggest that the isotopic equilibrium between NO2 (the main precursor of NO3-) and O3 may take long enough under certain field conditions that nitrates may be formed near emission sources with lower isotopic values than those formed further downwind. Indeed, previously published field measurements of oxygen isotopes in NO3- in precipitation (wNO3-) and in particulate (pNO3-) samples suggest that abnormally low isotopic values might characterize polluted air masses. However, none of the air studies have deployed systems allowing collection of samples specific to anthropogenic sources in order to avoid shifts in isotopic signature due to changing wind directions, or separately characterized gaseous HNO3 with delta-17O values. Here we have used a wind-sector-based, multi-stage filter sampling system and precipitation collector to simultaneously sample HNO3 and pNO3-, and co-collect wNO3-. The nitrates are from various distances (< 1 to > 125 km) downwind of different anthropogenic emitters, and consequently from varying time lapses after emission.
The separate collection of nitrates shows that the HNO3 delta-18O ranges are distinct from those of w- and pNO3-. Interestingly, the delta-17O differences between pNO3- and HNO3 shift from positive during cold sampling periods to negative during warm periods. The low pNO3- delta-17O values observed during warm periods may partly derive from the involvement of 17O-depleted peroxy radicals (RO2) oxidizing NO during that season. Another possibility is that nitrates derive from NOx that has not yet reached isotopic equilibrium with O3. However, these mechanisms, individually or together, cannot explain the observed pNO3 minus HNO3 isotopic changes. We propose differences in dry depositional rates, faster for HNO3, as a mechanism for the observed shifts. Larger proportions of pNO3- formed via the N2O5 pathway would explain the opposite fall-winter patterns. Our results show that the separate HNO3, wNO3- and pNO3- isotopic signals can be used to 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 distribution worldwide and to develop effective emission reduction strategies.
Summary(Plain Language Summary, not published)
We need to understand better the atmospheric processes transforming nitrous oxides (NOx) emitted from human activities into nitrates (NO3-), because NO3- may degrade terrestrial and aquatic ecosystems, and particulate nitrate may influence the Earth climate. While yearly NOx emissions in Canada declined steadily from 2000 to 2015, emissions in Alberta have remained relatively constant, forming about 35% of Canadian emissions. We have then studied isotopic indicators (sigma-18O, delta-17O) in wet (w-), particulate (p-) and gaseous (HNO3) NO3-forms, which have never been separately characterized with delta-17O values, downwind from five contamination plumes in Alberta, without the influence from other sources. An important finding is that the HNO3 sigma-18O and delta-17O ranges are not interchangeable with those of w- and p-NO3. Our results also show that the plume dynamics is crucial in interpreting the results, which do not characterize the different sources, but the processes responsible for the distribution of nitrates.