Comparative nitrogen speciation in marine and coastal urban aerosols of the Greater Bay Area and implications for PM 2.5 -bound nitrogen deposition

Abstract. We present a summer 2024 comparative study of aerosol nitrogen speciation across the Guangdong–Hong Kong–Macau Greater Bay Area (GBA), contrasting marine air over the coastal ocean with a coastal urban site in Hong Kong. Inorganic nitrogen (IN) and organic nitrogen (ON) were quantified, and a high-resolution time-of-flight aerosol mass spectrometer was operated offline to characterize water-soluble nitrogen-containing organics. Total nitrogen and IN showed a west-to-east increase along the coastal ocean, indicating stronger anthropogenic influence in the more populated eastern GBA. ON showed a contrasting pattern: while its concentration decreased offshore, its fraction in total nitrogen peaked in the western marine region (34.6 ± 12.4 %), highlighting the relative importance of ON under lower PM2.5 loadings. Urban aerosols were enriched in ammonium and exhibited more oxidized ON signatures, including higher NO+/NO2+ ratios (7.9 ± 2.6), consistent with NOx–VOC photochemistry. Marine aerosols showed lower NO+/NO2+ ratios (5.3 ± 1.3) and molecular signatures consistent with reduced, amine-related ON, reflecting marine biogenic inputs in the marine boundary layer. Using an inferential approach with deposition velocity (Vd) assumptions, PM2.5-bound nitrogen deposition over the ocean was estimated to be comparable to that at the urban site (0.14 vs. 0.15 kg N ha−1 yr−1), indicating non-negligible fine-particle nitrogen input to adjacent coastal waters. These results demonstrate a notable coastal transition in nitrogen chemical form and suggest that ON speciation should be considered when assessing nitrogen deposition to coastal waters and potential ecosystem responses in the South China Sea.