Free Amino Acids in Marine Aerosols during Four Cruising Campaigns over the South China Sea: Importance of Biomass Burning and Atmospheric Oxidation

Free amino acids (FAAs) are key organic nitrogen (ON) components in marine aerosols, revealing aerosol sources, transformation processes, and potential impacts over oceanic regions. Total suspended particle samples were collected during four cruise campaigns (2013–2016) over the South China Sea (SCS) to investigate FAAs in marine aerosols. The average concentrations of the total FAAs (TFAAs) ranged from 8.06 to 29.77 ng m–3 in the four campaigns, reflecting the continental influences from the Indo-China Peninsula (ICP) and East Asia (EA). The ICP-influenced samples were predominantly influenced by relatively fresh biomass burning emissions, whereas the EA-influenced samples reflected a mixture of biomass burning and photochemical formation processes. Glycine (Gly) is the major FAA species, accounting for 47.9–64.1% of TFAAs. We estimated the contributions from photochemical degradation and biomass burning to Gly in EA-influenced samples based on Gly/TFAA ratios and correlations between FAAs and continental tracers and found that photochemical degradation significantly enhanced Gly accumulation in continental outflows from EA. This process delivers highly bioavailable ON, with EA-influenced air masses bringing about 10 times higher ON loadings to the SCS atmosphere compared to ICP. This highlights the importance of long-range transport and atmospheric oxidation for SCS nitrogen deposition ecological impact.