Elucidating sources of atmospheric NO pollution in a heavily urbanized environment using multiple stable isotopes.

Atmospheric deposition of nitrogen (N) from rain and aerosols can be a significant non-point source - particularly in urbanized coastal areas and contribute to coastal eutrophication and hypoxia. Here, we present geochemical and isotopic data from surface waters coupled with an 18-month time series of geochemical and isotopic data measured on wet and dry deposition over Hong Kong from June 2018. Dual stable isotopes of nitrate (δN-NO and δO-NO) of rain and total suspended particulates (TSP) were analyzed to trace the sources and understand seasonal pattern of atmospheric nitrate. The δN of TSP, δN-NO in rain and TSP ranged from +0.94 to +17.6‰, -4.1 to +3.0‰ and -1.3 to +9.0‰ respectively. δN varied seasonally with higher values in winter and lower values in summer. This variation can be explained by a change in the sources of atmospheric NO driven by the East Asian Monsoon. It was found that most NO comes from coal burning in winter and a mix of vehicle emissions, fossil fuel combustion and lightning in summer. Moreover, the estimated dry and wet deposition of nitrate and ammonium in Hong Kong is around 18 kg N ha annually, which is of the same order of magnitude as N released by sewage effluents and groundwater. This implies that atmospheric N deposition over the N-limited waters of the eastern side of Hong Kong could contribute significantly to the N budget. Therefore, atmospheric N deposition may alter the local N marine cycling, thus monitoring its impact is crucial for water quality in Southern China.