Atmospheric nitrogen deposition to the northwestern Pacific: seasonal variation and source attribution

Abstract. Rapid Asian industrialization has led to increased downwind atmospheric nitrogen deposition threatening the marine environment. We present an analysis of the sources and processes controlling atmospheric nitrogen deposition to the northwestern Pacific, using the GEOS-Chem global chemistry model and its adjoint model at 1/2° × 2/3° horizontal resolution over East Asia and its adjacent oceans. We focus our analyses on the marginal seas: the Yellow Sea and the South China Sea. Asian nitrogen emissions in the model are 28.6 Tg N a⁻¹ as NH₃ and 15.7 Tg N a⁻¹ as NO_x. China has the largest sources with 12.8 Tg N a⁻¹ as NH₃ and 7.9 Tg N a⁻¹ as NO_x; the high-NH₃ emissions reflect its intensive agricultural activities. We find Asian NH₃ emissions are a factor of 3 higher in summer than winter. The model simulation for 2008–2010 is evaluated with NH₃ and NO₂ column observations from satellite instruments, and wet deposition flux measurements from surface monitoring sites. Simulated atmospheric nitrogen deposition to the northwestern Pacific ranges 0.8–20 kg N ha⁻¹ a⁻¹, decreasing rapidly downwind of the Asian continent. Deposition fluxes average 11.9 kg N ha⁻¹ a⁻¹ (5.0 as reduced nitrogen NH_x and 6.9 as oxidized nitrogen NO_y) to the Yellow Sea, and 5.6 kg N ha⁻¹ a⁻¹ (2.5 as NH_x and 3.1 as NO_y) to the South China Sea. Nitrogen sources over the ocean (ship NO_x and oceanic NH₃) have little contribution to deposition over the Yellow Sea, about 7 % over the South China Sea, and become important (greater than 30 %) further downwind. We find that the seasonality of nitrogen deposition to the northwestern Pacific is determined by variations in meteorology largely controlled by the East Asian monsoon and in nitrogen emissions. The model adjoint further estimates that nitrogen deposition to the Yellow Sea originates from sources over China (92 % contribution) and the Korean peninsula (7 %), and by sectors from fertilizer use (24 %), power plants (22 %), and transportation (18 %). Deposition to the South China Sea shows source contribution from mainland China (66 %), Taiwan (20 %), and the rest (14 %) from the southeast Asian countries and oceanic NH₃ emissions. The adjoint analyses also indicate that reducing Asian NH₃ emissions would increase NO_y dry deposition to the Yellow Sea (28 % offset annually), limiting the effectiveness of NH₃ emission controls on reducing nitrogen deposition to the Yellow Sea.