Quantifying present and future impacts of reactive soil nitrogen emissions on global air quality

Abstract:

Soil is a major global source of both oxidized and reduced reactive nitrogen to the atmosphere, mostly in the form of nitric oxide (NO) and ammonia (NH₃), with important implications for air quality. In addition to acting as precursors of secondary pollution, the emitted reactive nitrogen will eventually return to Earth’s surface through deposition, where excessive nitrogen deposition to ecosystems can lead to problems like eutrophication and soil acidification. Soil reactive nitrogen emissions can be both altered by anthropogenic (e.g. fertilizer application, land management) and natural (e.g. climate change, land cover change) processes. Our research explores both the anthropogenic and natural dimensions of soil reactive nitrogen emission changes and their impact on air quality. We simulate the impact of increasing agricultural NO_x and NH₃ emissions on PM_2.5 air quality and nitrogen deposition over 1992 – 2014 using the GEOS-Chem model and the CEDS anthropogenic emission inventory. We find that the large increase in agricultural NH₃ emission have enhanced annual mean population-weighted surface PM_2.5 concentration over China (+1.77 μg m^-3) and India (+1.56 μg m^-3), while the opposite happens over Former Soviet Union (-1.02 μg m^-3) and central Europe (-0.99 μg m^-3). Globally, we simulate +7.2 TgN yr^-1 of increase in nitrogen deposition, and a net increase of 3.67 x 10⁶ km² of land area that potentially faces excessive nitrogen deposition ( > 5 kgN ha^-1 yr^-1). We also use an Earth system model (CESM) with newly implemented soil nitrogen biogeochemistry to explore the response of soil NO and NH₃ to climate change, and the subsequent effects on air quality.