Nitrate sources and formation in aerosol and precipitation in a tropical city in South China: Insight from nitrate dual isotopes

Nitrate dual isotopes (δ15N–NO3- and δ18O–NO3-) and NO3- concentration were measured in aerosol and precipitation in Zhanjiang, a tropical city in the southmost tip of mainland China, between November 2018 and October 2019. Measurements aimed to identify possible formation processes of NO3-, sources and the difference of nitrate dual isotopes between aerosol and precipitation in such tropical area. The values of δ15N–NO3- and δ18O–NO3- varied from −5.4‰ to 3.8‰ and from 50.2‰ to 84.3‰ in aerosol, respectively, and varied from −5.3‰ to 5.3‰ and from 53.7‰ to 84.4‰ in precipitation, respectively. The values of nitrate dual isotopes had a similar seasonal change in both aerosol and precipitation, showing that higher values in the dry season and lower values in the rainy season. This indicated that nitrate in the city had different sources and chemical formation pathway for different seasons. Similar to the precipitation, the OH radical pathway is the dominant pathway to produce atmospheric NO3- in such tropical city (accounting for 75% in the rainy season and 50% in the dry season). However, the differences of δ15N–NO3- and δ18O–NO3- between aerosol and precipitation were also found in Zhanjiang, suggesting that precipitation frequency is the dominant factor for the difference in δ15N–NO3- whereas peroxy radicals are the dominant factor for the difference in δ18O–NO3-. The complete nitrate washout that occurred from May to July suggested that δ15N–NO3- in the aerosol can be applied to identify the nitrate sources during these periods. The atmospheric nitrate sources calculated by a Bayesian model showed that vehicle emission (35%), lightning (27%) and biomass burning (26%) were the main atmospheric nitrate sources in Zhanjiang.