资源环境科技发展态势分析平台

There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N)ox-ides (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 O-18, Delta O-17) are frequently applied to infer the chemical pathways leading to the observed mass-independent isotopic anomalies from interaction with O-17-rich ozone (O-3). Recent laboratory experiments suggest that the isotopic equilibrium between NO2 (the main precursor of NO3-) and O-3 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 (wNO(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 Delta O-17 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.

The separate collection of nitrates shows that the HNO3 delta O-18 ranges are distinct from those of w- and pNO(3)(-). Interestingly, the Delta O-17 differences between pNO(3)(-) and HNO3 shift from positive during cold sampling periods to negative during warm periods. The low pNO(3)(-) Delta O-17 values observed during warm periods may partly derive from the involvement of O-17-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 O-3. However, these mechanisms, individually or together, cannot explain the observed pNO(3) minus HNO3 isotopic changes. We propose differences in dry depositional rates, faster for HNO3, as a mechanism for the observed shifts. Larger proportions of pNO(3)(-) formed via the N2O5 pathway would explain the opposite fall-winter patterns. Our results show that the separate HNO3, wNO(3)(-) and pNO(3)(-) 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.