The Δ 17O and δ18O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources – implications for polluted air masses

Savard, Martine M.; Cole, Amanda S.; Vet, Robert; Smirnoff, Anna

There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N) oxides (inline-formulaNOx) emitted from anthropogenic sources into nitrates (inline-formula M6inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimga33a7d42b70ca1fe513ac92c5832eec2 acp-18-10373-2018-ie00001.svg25pt16ptacp-18-10373-2018-ie00001.png ), two of them being that inline-formula M7inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimga02883d0956e7dc256b9fe9fffa70b09 acp-18-10373-2018-ie00002.svg25pt16ptacp-18-10373-2018-ie00002.png 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 (inline-formulaδ18O, inline-formulaΔ17O) are frequently applied to infer the chemical pathways leading to the observed mass-independent isotopic anomalies from interaction with inline-formula17O-rich ozone (inline-formulaO3). Recent laboratory experiments suggest that the isotopic equilibrium between inline-formulaNO2 (the main precursor of inline-formula M13inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg9712381780fcc4de6c4d72f703a8771c acp-18-10373-2018-ie00003.svg25pt16ptacp-18-10373-2018-ie00003.png ) and inline-formulaO3 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 inline-formula M15inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimgecc3e6dd5af0ffb1da8bfbfcb16b8e8b acp-18-10373-2018-ie00004.svg25pt16ptacp-18-10373-2018-ie00004.png in precipitation (inline-formula M16inlinescrollmathml chem w normal NO normal 3 - 34pt16ptsvg-formulamathimg0f0f7dc62ac3673cbbb22d5d5ba00435 acp-18-10373-2018-ie00005.svg34pt16ptacp-18-10373-2018-ie00005.png ) and in particulate (inline-formula M17inlinescrollmathml chem p normal NO normal 3 - 32pt16ptsvg-formulamathimg14d1970394e6dbadf091d7564dfafd47 acp-18-10373-2018-ie00006.svg32pt16ptacp-18-10373-2018-ie00006.png ) 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 inline-formulaHNO3 with inline-formulaΔ17O values. Here we have used a wind-sector-based, multi-stage filter sampling system and precipitation collector to simultaneously sample inline-formulaHNO3 and inline-formula M21inlinescrollmathml chem p normal NO normal 3 - 32pt16ptsvg-formulamathimg78ae31051e5270db81572442a6c8a3b8 acp-18-10373-2018-ie00007.svg32pt16ptacp-18-10373-2018-ie00007.png , and co-collect inline-formula M22inlinescrollmathml chem w normal NO normal 3 - 34pt16ptsvg-formulamathimg67de8c81f54e611630f885555caced60 acp-18-10373-2018-ie00008.svg34pt16ptacp-18-10373-2018-ie00008.png . The nitrates are from various distances (inline-formula<1 to inline-formula>125 km) downwind of different anthropogenic emitters, and consequently from varying time lapses after emission.

The separate collection of nitrates shows that the inline-formulaHNO3δ18O ranges are distinct from those of inline-formulaw- and inline-formula M27inlinescrollmathml chem p normal NO normal 3 - 32pt16ptsvg-formulamathimge6c5460001240179641eacc5af2081f3 acp-18-10373-2018-ie00009.svg32pt16ptacp-18-10373-2018-ie00009.png . Interestingly, the inline-formulaΔ17O differences between inline-formula M29inlinescrollmathml chem p normal NO normal 3 - 32pt16ptsvg-formulamathimg48fff2cf597a625b655ed000be709fa9 acp-18-10373-2018-ie00010.svg32pt16ptacp-18-10373-2018-ie00010.png and inline-formulaHNO3 shift from positive during cold sampling periods to negative during warm periods. The low inline-formula M31inlinescrollmathml chem p normal NO normal 3 - nobreak0.25em normal Δ chem normal 17 normal O 60pt17ptsvg-formulamathimgb27f11457d3a36d4e4df259191c2a158 acp-18-10373-2018-ie00011.svg60pt17ptacp-18-10373-2018-ie00011.png values observed during warm periods may partly derive from the involvement of inline-formula17O-depleted peroxy radicals (inline-formulaRO2) oxidizing NO during that season. Another possibility is that nitrates derive from inline-formulaNOx that has not yet reached isotopic equilibrium with inline-formulaO3. However, these mechanisms, individually or together, cannot explain the observed inline-formulapNO3 minus inline-formulaHNO3 isotopic changes. We propose differences in dry depositional rates, faster for inline-formulaHNO3, as a mechanism for the observed shifts. Larger proportions of inline-formula M39inlinescrollmathml chem p normal NO normal 3 - 32pt16ptsvg-formulamathimgb6eabb62847bcef14c9d6cb26004a8dc acp-18-10373-2018-ie00012.svg32pt16ptacp-18-10373-2018-ie00012.png formed via the inline-formulaN2O5 pathway would explain the opposite fall–winter patterns. Our results show that the separate inline-formulaHNO3, inline-formula M42inlinescrollmathml chem w normal NO normal 3 - 34pt16ptsvg-formulamathimg29837830551a099200d512efef6a8619 acp-18-10373-2018-ie00013.svg34pt16ptacp-18-10373-2018-ie00013.png and inline-formula M43inlinescrollmathml chem p normal NO normal 3 - 32pt16ptsvg-formulamathimge1bfbcafbb2f4b5fc292f0c6cb065234 acp-18-10373-2018-ie00014.svg32pt16ptacp-18-10373-2018-ie00014.png isotopic signals can be used to further our understanding of inline-formulaNOx oxidation and deposition. Future research should investigate all tropospheric nitrate species as well as inline-formulaNOx to refine our understanding of nitrate distribution worldwide and to develop effective emission reduction strategies.

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Savard, Martine M. / Cole, Amanda S. / Vet, Robert / et al: The Δ17O and δ18O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources – implications for polluted air masses. 2018. Copernicus Publications.

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