Impact of anthropogenic atmospheric nitrogen and sulfur deposition on ocean acidification and the inorganic carbon system

Fossil fuel combustion and agriculture result in atmospheric deposition of 0.8 Tmol/yr reactive sulfur and 2.7 Tmol/yr nitrogen to the coastal and open ocean near major source regions in North America, Europe, and South and East Asia. Atmospheric inputs of dissociation products of strong acids (HNO3 and H2SO4) and bases (NH3) alter surface seawater alkalinity, pH, and inorganic carbon storage. We quantify the biogeochemical impacts by using atmosphere and ocean models. The direct acid/base flux to the ocean is predominately acidic (reducing total alkalinity) in the temperate Northern Hemisphere and alkaline in the tropics because of ammonia inputs. However, because most of the excess ammonia is nitrified to nitrate (NO3−) in the upper ocean, the effective net atmospheric input is acidic almost everywhere. The decrease in surface alkalinity drives a net air–sea efflux of CO2, reducing surface dissolved inorganic carbon (DIC); the alkalinity and DIC changes mostly offset each other, and the decline in surface pH is small. Additional impacts arise from nitrogen fertilization, leading to elevated primary production and biological DIC drawdown that reverses in some places the sign of the surface pH and air–sea CO2 flux perturbations. On a global scale, the alterations in surface water chemistry from anthropogenic nitrogen and sulfur deposition are a few percent of the acidification and DIC increases due to the oceanic uptake of anthropogenic CO2. However, the impacts are more substantial in coastal waters, where the ecosystem responses to ocean acidification could have the most severe implications for mankind.