Atmospheric ammonia and its effect on PM2.5 pollution in urban Chengdu, Sichuan Basin, China

Controlling ammonia (NH3) emissions has been proposed as a strategy to mitigate haze pollution. To explore the role of NH3 in haze pollution in Sichuan Basin, where agricultural activities are intense, hourly in situ data of NH3, as well as nitric acid and secondary inorganic aerosols (SIAs) were gathered in Chengdu from April 2017 to March 2018. We found that NH3 had an annual mean concentration of 9.7 +/- 3.5 (mean +/- standard deviation) mu g m-3, and exhibited seasonal variations (spring > summer > autumn and winter) due to changes in emission sources and meteorological conditions (particularly temperature). Chengdu's atmosphere is generally NH3-suf-ficient, especially in the warm seasons, implying that the formation of SIAs is more sensitive to the availability of nitric acid. However, an NH3 "sufficient-to-deficient" transition was found to occur during winter pollution periods, and the frequency of NH3 deficiency increased with the aggravation of pollution. Under NH3-deficient conditions, the nitrogen oxidation ratio increased linearly with the increase in free NH3, implying that NH3 contributes appreciably to the formation of nitrate and thus to high PM2.5 loadings. No relationships of NH3 with fossil fuel combustion-related pollutants were found. The NH3 emissions from farmland and livestock waste in the suburbs of Chengdu and regional transport from west of Chengdu probably contribute to the occurrence of high PM2.5loading in winter and spring, respectively. These results suggest that to achieve effective mitigation of PM2.5 in Chengdu, local and regional emission control of NH3 and NOx synergistically would be effective.