The δ17O and δ18O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources - implications for polluted air masses

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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 NO-3 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 (δ18O, 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 NO-3 ) 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 NO-3 in precipitation (wN-3 ) and in particulate (pNO-3 ) 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 117O values. Here we have used a wind-sector-based, multi-stage filter sampling system and precipitation collector to simultaneously sample HNO3 and pNO-3 , and co-collect wNO-3 . The nitrates are from various distances (< 1 to > 125 km) downwind of different anthropogenic emitters, and consequently from varying time lapses after emission.

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Savard, M. M., Cole, A. S., Vet, R., & Smirnoff, A. (2018). The δ17O and δ18O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources - implications for polluted air masses. Atmospheric Chemistry and Physics, 18(14), 10373–10389. https://doi.org/10.5194/acp-18-10373-2018

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