Non-agricultural sources dominate the atmospheric NH_3 in Xi'an, a megacity in the semi-arid region of China

摘要

Ammonia (NH_3). as a dominant alkaline gas in the atmosphere, plays a vital role in Chinese urban haze formation process, but its source in urban areas of China is controversial. To identify the sources of urban NH_3 in the semi-arid region of East Asia, real-time measurements of NH_3 and NH_4~+ of PM_(2.5) in the urban atmosphere of Xi'an, inland China during the winter and summer of 2017 were performed and their stable nitrogen isotope composition were analyzed. NH_3 was 38.0 ± 9.4 ug/m~3 in the summer, which is 1.5 times higher than that in the winter. Concentration of NH_3 in both seasons well correlated with that of PAHs in PM_(2.5) and the mass ratio of (BbF + BeP + IP + BghiP) to the total PAHs, suggesting that fossil fuel combustion is an important source of NH_3 in Xi'an. Moreover, diurnal variation pattern of NH_3 was consistent with that of CO in the summer, peaking in the morning and evening rush hours, respectively, further indicating an importance of the contribution of traffic emissions to NH_3 in the city. Based on the source apportionment by using isotope mixing model, we found that 66.4% and 62.5% of NH_3 in the urban atmosphere were contributed by non-agricultural sources in the summer and winter, respectively. Our work revealed that non-agricultural sources dominate the atmospheric NH_3 of Xi'an, where haze pollution is still severe, and suggested that emission controls of non-agricultural NH_3 could be an effective way to mitigate the air pollution problem in the semi-arid region of East Asia.