Interannual variation of reactive nitrogen emissions and their impacts on PM2.5 air pollution in China during 2005-2015

Emissions of reactive nitrogen as ammonia (NH3) and nitrogen oxides (NOx), together with sulfur dioxide (SO2), contribute to formation of secondary PM2.5 in the atmosphere. Satellite observations of atmospheric NH3, NO2, and SO2 levels since the 2000s provide valuable information to constrain the spatial and temporal variability of their emissions. Here we present a bottom-up Chinese NH3 emission inventory combined with top-down estimates of Chinese NOx and SO2 emissions using OMI satellite observations, aiming to quantify the interannual variations of reactive nitrogen emissions in China and their contributions to PM2.5 air pollution over 2005–2015. We find small interannual changes in the total Chinese anthropogenic NH3 emissions during 2005–2016 (12.0–13.3 Tg with over 85% from agricultural sources), but large interannual change in top-down Chinese NOx and SO2 emissions. Chinese NOx emissions peaked around 2011 and declined by 22% during 2011–2015, and Chinese SO2 emissions declined by 55% in 2015 relative to that in 2007. Using the GEOS-Chem chemical transport model simulations, we find that rising atmospheric NH3 levels in eastern China since 2011 as observed by IASI and AIRS satellites are mainly driven by rapid reductions in SO2 emissions. The 2011–2015 Chinese NOx emission reductions have decreased regional annual mean PM2.5 by 2.3–3.8 μg m-3. Interannual PM2.5 changes due to NH3 emission changes are relatively small, but further control of agricultural NH3 emissions can be effective for PM2.5 pollution mitigation in eastern China.