Seasonal variations of nitrate dual isotopes in wet deposition in a tropical city in China

Abstract

Monthly analytical results for more than two years long of δ15N and δ18O of nitrate in wet deposition are reported for a tropical city of Zhanjiang in the southernmost mainland China, in an effort to elucidate NOx sources and its oxidation pathways to nitrate. The results showed that monthly variations of δ18ONO3- responded well to changes in sunshine hours, with lower δ18ONO3- values corresponding to longer sunshine hours. This pattern suggests that NOx oxidation via the OH radical was the predominant pathway, which, by estimate, accounted for 87% in winter and 94% in summer, for nitrate formation. Remarkably, we found that the δ18ONO3- here are prominently low relative to previous studies, likely due to that annual sunshine hours in this tropical city is relatively long. Moreover, available data of reported δ18ONO3- in wet deposition showed an increasing trend with latitude, reflecting a shift of the predominant NOx oxidation pathways from those via the OH radical in low latitudes to those via O3 in high latitudes, corresponding to the decrease of sunshine hours. A temporarily co-variation of δ15NNO3- and δ18ONO3- was observed, exhibiting higher values in dry winter and lower values in wet summer, which was attributed to the washout effect on the dual isotopes. During wet seasons, complete nitrate washout suggests that δ15NNO3- in precipitation should be similar to the nitrate formed in the atmosphere, and thereby can be used for source apportionment. A Bayesian model showed that the source of atmospheric nitrate deposition is mainly natural (61%) in Zhanjiang, with less anthropogenic contribution.