Non-agricultural sources dominate the atmospheric NH 3 in Xi'an, a megacity in the semi-arid region of China.

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 μg/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.
Competing Interests: Declaration of competing interest We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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