Characterization and Source Attribution of Free Amino Acids in Marine Aerosols across the South China Sea to the Eastern Indian Ocean

Free amino acids (FAAs) are crucial components of organic nitrogen (ON) in aerosols. We investigated the characteristics of FAAs in PM_2.5 in a cruise campaign from the South China Sea (SCS) to the Eastern Indian Ocean (EIO). The contribution of marine and continental sources to FAAs was assessed in both the continent-surrounded sea area and the open-sea area. Higher total FAA concentrations were observed in the continental-influenced samples than in the oceanic-influenced samples. Among the FAAs, the abundance of Glycine (Gly) was significantly enhanced (71.5%) due to the transport of continental air masses. Moreover, the d/l-aspartate acid ratio exhibited an elevation under the impact of biomass burning. Under marine influence, the abundance of l-tryptophan (l-Trp) increased by 94.8% in the EIO. We propose that Gly and l-Trp can serve as the indicators for continental and marine sources, respectively, and we applied the l-Trp/Gly ratio to quantify their relative contributions to FAAs in marine aerosols. The results reveal the impact of continental transport and marine emissions on atmospheric ON. This study deepens the understanding of features of the nitrogen cycle and its implications for biogeochemical cycling over the ocean.