The Δ¹⁷O and <i>δ</i>¹⁸O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources – implications for polluted air masses

• Main Authors: M. M. Savard, A. S. Cole, R. Vet, A. Smirnoff
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/10373/2018/acp-18-10373-2018.pdf

<p>There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N) oxides (NO_<i>x</i>) 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 (<i>δ</i>¹⁸O, Δ¹⁷O) 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 (<i>w</i>NO₃⁻) and in particulate (<i>p</i>NO₃⁻) 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 Δ¹⁷O values. Here we have used a wind-sector-based, multi-stage filter sampling system and precipitation collector to simultaneously sample HNO₃ and <i>p</i>NO₃⁻, and co-collect <i>w</i>NO₃⁻. The nitrates are from various distances ( &lt; 1 to  &gt; 125&thinsp;km) downwind of different anthropogenic emitters, and consequently from varying time lapses after emission.</p><p>The separate collection of nitrates shows that the HNO₃ <i>δ</i>¹⁸O ranges are distinct from those of <i>w</i>- and <i>p</i>NO₃⁻. Interestingly, the Δ¹⁷O differences between <i>p</i>NO₃⁻ and HNO₃ shift from positive during cold sampling periods to negative during warm periods. The low <i>p</i>NO₃⁻ Δ¹⁷O 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 NO_<i>x</i> that has not yet reached isotopic equilibrium with O₃. However, these mechanisms, individually or together, cannot explain the observed <i>p</i>NO₃ minus HNO₃ isotopic changes. We propose differences in dry depositional rates, faster for HNO₃, as a mechanism for the observed shifts. Larger proportions of <i>p</i>NO₃⁻ formed via the N₂O₅ pathway would explain the opposite fall–winter patterns. Our results show that the separate HNO₃, <i>w</i>NO₃⁻ and <i>p</i>NO₃⁻ isotopic signals can be used to further our understanding of NO_<i>x</i> oxidation and deposition. Future research should investigate all tropospheric nitrate species as well as NO_<i>x</i> to refine our understanding of nitrate distribution worldwide and to develop effective emission reduction strategies.</p>