Atmospheric Reactive Nitrogen Deposition to the Global Ocean During the 2010s: Interannual Variation and Source Attribution

Abstract

This study evaluates atmospheric reactive nitrogen deposition to the global ocean from 2010 to 2019 using the GEOS-Chem model, quantifying the contributions of key anthropogenic and natural sources and examining the impacts of emissions and meteorological changes on interannual variations (IAVs). The average global oceanic nitrogen deposition is 40.7 Tg N yr⁻¹, with 78% deposited in the open ocean. Reduced nitrogen (NH_x) and oxidized nitrogen (NO_y) contribute equally predominantly through wet deposition (62%). Of the total nitrogen deposition to oceans, 39% originates from natural emissions, 33% from fuel combustion including shipping and aviation (11%), and 22% from agricultural activities. Transportation and agriculture are the major contributors in the Northern Hemispheric mid-high latitude and Coastal biomes, whereas natural emissions are dominant in Trades and Southern Hemispheric biomes. Both wet and dry NO_y deposition decreases in the Northern Hemisphere mid-high latitudes but increases in tropical biomes, whereas NH_x shows opposing changes in wet and dry deposition. Anthropogenic emissions drive over 60% of IAVs of oceanic nitrogen deposition except for the NH_x deposition over tropical biomes. Changes in anthropogenic emissions from combustion sources not only influence NO_y deposition through nitrogen oxides (NO_x) emissions from fuel combustions but also lead to opposing trends in wet and dry NH_x deposition due to nonlinear chemistry of secondary inorganic aerosol formations. These findings highlight the need for integrated management strategies targeting multiple pollutants, including NO_x, ammonia (NH₃), and sulfur dioxide (SO₂) along with international regulations on shipping and aviation to effectively control nitrogen deposition over the ocean.