Determining organic nitrogen emission sources and secondary formations in an urban coastal airshed via stable isotope...

Organic nitrogen (ON) has been excluded in the majority of atmospheric N studies. However, dissolved organic nitrogen (DON) deposition influences coastal water quality and primary production creating an urgent need for comprehensive atmospheric ON characterization, especially in coastal airsheds. This study measured the concentration and isotopic composition of rainwater DON (δ¹⁵N-DON) and applied stable isotope mixing models to determine the ON emission source apportionments in a small-sized coastal city. The DON concentration averaged 10.6 ± 7.6 μM (n = 42), which was 29% of the total dissolved nitrogen in rainwater and produced a deposition flux of 1.5 kg N·ha^-1·yr^-1. The average rainwater δ¹⁵N-DON value was 8.3 ± 5.3‰ and isotope mixing model results suggested vehicles as a dominant source, overall contributing 35 ± 15% of ON emissions, followed by marine emissions (24 ± 16%), organic amines (18 ± 11%), organic nitrates (17 ± 11%), and biomass burning (8 ± 3%). Although secondary ON formations (i.e., organic amines and nitrates) had less contributions than primary emission sources (i.e., vehicles, marine, and biomass burning), it can be significant and rival primary emissions when the fertilizer application started. Our results fill knowledge gaps of ON wet deposition and emission sources in small-sized coastal cities and inform future atmospheric N mitigation strategies and coastal watershed restoration plans in similar regions. We call for further research determining the isotopic composition of ON emission sources and fractionation associated with primary emission and secondary formation in anticipation of creating a similar isotope-based foundation that has been used for decades to investigate inorganic nitrogen emissions.