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 NO_x sources and oxidants were different between seasons. In cool months, NO_x 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 NO_x. 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 NO_x to total NO₃⁻ was calculated to be 47.9%. This suggests the mean loss ratio of NO_x was 89% while the loss ratio of NO₃⁻ was 87% during transport between Chinese coastal areas and Yongxing Island in Nov., 2013.