Comparative nitrogen speciation in marine and coastal urban aerosols of the Greater Bay Area and implications for PM _2.5 -bound nitrogen deposition

We present a summer 2024 comparative study of aerosol nitrogen speciation in the Guangdong–Hong Kong–Macau Greater Bay Area (GBA), examining how nitrogen abundance and chemical composition vary between coastal urban air and adjacent marine air, and their implications for nitrogen deposition. Concurrent shipboard and urban observations from 12 July to 3 August 2024 included bulk inorganic nitrogen (IN) and organic nitrogen (ON) measurements in PM _2.5 and offline aerosol mass spectrometry (AMS) of water extracts. Regional mean PM _2.5 -bound total nitrogen and IN concentrations increased from western to eastern coastal waters, reflecting both spatial and temporal variability. Bulk PM _2.5 -bound ON concentrations decreased offshore by 12.2 % per 10 km, with estimates ranging from 10.1 % to 14.2 % across sensitivity analyses. Beyond these concentration patterns, AMS mass spectra revealed contrasting nitrogen chemistry. Water extracts of urban PM _2.5 exhibited higher NO+/NO2+ ratios than marine PM _2.5 extracts (7.9 ±  2.6 versus 5.4 ± 1.2) and more pronounced organic nitrate-related fragments. These complementary signatures indicate more prominent organic-nitrate-related oxidized-N functionality in urban PM _2.5 , consistent with NO _x –VOC photochemistry at the urban site. Four ship-influenced samples showed enhanced imidazole-like fragment signatures, identifying an additional compositional feature warranting targeted investigation. Estimated PM _2.5 -bound nitrogen dry-deposition fluxes were 0.036 and 0.040 mg N m ⁻²  d ⁻¹ for marine and coastal urban environments, respectively, with overlapping model-sensitivity ranges. These results demonstrate a notable coastal transition in nitrogen chemical form and suggest that ON speciation should be considered when modelling nitrogen deposition to coastal waters and assessing potential ecosystem responses.