The Δ<sup>17</sup>O and <i>δ</i><sup>18</sup>O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources – implications for polluted air masses
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Published:2018-07-20
Issue:14
Volume:18
Page:10373-10389
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Savard Martine M., Cole Amanda S.ORCID, Vet Robert, Smirnoff Anna
Abstract
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 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 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 Δ17O 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 δ18O ranges are distinct from those of w- and
pNO3-. Interestingly, the Δ17O differences
between pNO3- and HNO3 shift from positive during
cold sampling periods to negative during warm periods. The low
pNO3-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 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.
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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