Rapid SO 2 emission reductions significantly increase tropospheric ammonia concentrations over the North China Plain

Liu, Mingxu; Huang, Xin; Song, Yu; Xu, Tingting; Wang, Shuxiao; Wu, Zhijun; Hu, Min; Zhang, Lin; Zhang, Qiang; Pan, Yuepeng; Liu, Xuejun; Zhu, Tong

The North China Plain has been identified as a significant hotspot of ammonia (inline-formulaNH3) due to extensive agricultural activities. Satellite observations suggest a significant increase of about 30 % in tropospheric gas-phase inline-formulaNH3 concentrations in this area during 2008–2016. However, the estimated inline-formulaNH3 emissions decreased slightly by 7 % because of changes in Chinese agricultural practices, i.e., the transition in fertilizer types from ammonium carbonate fertilizer to urea, and in the livestock rearing system from free-range to intensive farming. We note that the emissions of sulfur dioxide (inline-formulaSO2) have rapidly declined by about 60 % over the recent few years. By integrating measurements from ground and satellite, a long-term anthropogenic inline-formulaNH3 emission inventory, and chemical transport model simulations, we find that this large inline-formulaSO2 emission reduction is responsible for the inline-formulaNH3 increase over the North China Plain. The simulations for the period 2008–2016 demonstrate that the annual average sulfate concentrations decreased by about 50 %, which significantly weakens the formation of ammonium sulfate and increases the average proportions of gas-phase inline-formulaNH3 within the total inline-formulaNH3 column concentrations from 26 % (2008) to 37 % (2016). By fixing inline-formulaSO2 emissions of 2008 in those multi-year simulations, the increasing trend of the tropospheric inline-formulaNH3 concentrations is not observed. Both the decreases in sulfate and increases in inline-formulaNH3 concentrations show highest values in summer, possibly because the formation of sulfate aerosols is more sensitive to inline-formulaSO2 emission reductions in summer than in other seasons. Besides, the changes in inline-formulaNOx emissions and meteorological conditions both decreased the inline-formulaNH3 column concentrations by about 3 % in the study period. Our simulations suggest that the moderate reduction in inline-formulaNOx emissions (16 %) favors the formation of particulate nitrate by elevating ozone concentrations in the lower troposphere.

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Liu, Mingxu / Huang, Xin / Song, Yu / et al: Rapid SO2 emission reductions significantly increase tropospheric ammonia concentrations over the North China Plain. 2018. Copernicus Publications.

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