Nitrogen in marine aerosol and surface seawater over the Northwest Pacific Ocean in spring

Atmospheric deposition of long-range transport of anthropogenic reactive nitrogen (Nr, mainly comprised of NHx , NOy and water-soluble organic nitrogen, WSON) from continents may have profound impact on marine biogeochemistry. However, surface ocean dissolved organic nitrogen (DON) may also contribute to aerosol WSON in the overlying atmosphere. Despite the importance of off-continent dispersion and Nr interactions at the atmosphere–ocean boundary, our knowledge of the sources of various nitrogen species in the atmosphere over the open ocean remains limited due to insufficient observations. We conducted two cruises in the spring of 2014 and 2015 from the coast of China through the East China seas (ECSs, i.e. the Yellow Sea and East China Sea) to the open ocean (i.e. the north-western Pacific Ocean, NWPO). Concentrations of water-soluble total nitrogen (WSTN), NO3- and [NH4]+ , as well as the 15N of WSTN and NO3- in marine aerosol, were measured during both cruises. In the spring of 2015, we also analysed the concentrations and 15N of [NO3]- and the DON of surface seawater (SSW; at a depth of 5m) along the cruise track. Aerosol [NO3]- , [NH4]+ and WSON decreased logarithmically (1–2 orders of magnitude) with distance from the shore, reflecting strong anthropogenic emission sources of [NO3]-, [NH4]+ and WSON in China. Average aerosol [NO3]- and [NH4]+ concentrations were significantly higher in 2014 (even in the remote NWOP) than in 2015 due to the stronger wind field in 2014, underscoring the role of the Asian winter monsoon in the seaward transport of anthropogenic [NO3]- and [NH4]+ . However, the background aerosol WSON over the NWPO in 2015 (13.3±8.5 nmol/m3 was similar to that in 2014 (12.2±6.3 nmol/m3, suggesting an additional non-anthropogenic WSON source in the open ocean. Obviously, marine DON emissions should be considered in model and field assessments of net atmospheric WSON deposition in the open ocean. This study contributes information on parallel isotopic marine DON composition and aerosol Nr datasets, but more research is required to explore complex Nr sources and deposition processes in order to advance our understanding of anthropogenic influences on the marine nitrogen cycle and nitrogen exchange at land–ocean and atmosphere–ocean interfaces.