Use of isotopic compositions of nitrate in TSP to identify sources and chemistry in South China Sea

NO3− concentration, nitrogen and oxygen isotopic compositions (δ15N and δ18O) of NO3− were measured in total suspended particulates (TSP) at Yongxing Island in the South China Sea (SCS) between Feb. 2013 and Jan. 2014, as well as on two cruises in the northern South China Sea (NSCS). Measurements aimed to identify NO3− sources, and possible chemical formation processes of NO3−. The δ15N and δ18O of NO3− in TSP at Yongxing Island ranged from −2.5 to +4.9‰, and +48.1 to +99.0‰, with annual weighted averages of +1.5‰ and +83.2‰, respectively. Both δ15N and δ18O had higher values in cool months, indicating that NOx sources and oxidants were different between seasons. In cool months, NOx was mainly from anthropogenic sources, particularly from coal combustion in South China, resulting in high nitrogen deposition that was oxidized by O3 to NO3−. In warm months, natural emissions were an important source of NOx. TSP samples in the NSCS had higher NO3− concentrations, higher δ15N and lower δ18O values than samples from Yongxing Island over the same period. This suggests that atmospheric processes caused a decrease in NO3− concentrations and δ15N but increase in δ18O from coast to remote marine. Assuming to oxygen atoms were derived from O3 during transport in cool months, the mean ratio of NO3− formed by NOx to total NO3− was calculated to be 47.9%. This suggests the mean loss ratio of NOx was 89% while the loss ratio of NO3− was 87% during transport between Chinese coastal areas and Yongxing Island in Nov., 2013.