The sources of atmospheric NH₃ in marine environments were investigated through year-round observations of NH₄⁺ in PM_2.5 and δ¹⁵N-NH₄⁺ on Yongxing Island in the South China Sea from 2023 to 2024. Concentrations exhibited pronounced seasonal variability,  and a marked increases in spring 2024. δ¹⁵N-NH₄⁺ values displayed seasoanl difference. A significant negative correlation between δ¹⁵N and NH₄⁺ concentrations (P<0.01) suggests higher spring concentrations correspond to isotopically light sources, whereas lower concentrations in other seasons are linked to heavier signatures. Back-trajectory analysis with a Bayesian isotope mixing model confirmed these sources shifts. However, marine release contributed only ~5% through the year.