Non-agricultural sources dominate the atmospheric NH₃ in Xi'an, a megacity in the semi-arid region of China

Ammonia (NH₃), 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 NH₃ in the semi-arid region of East Asia, real-time measurements of NH₃ and NH₄ ⁺ 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. NH₃ was 38.0 ± 9.4 μg/m³ in the summer, which is 1.5 times higher than that in the winter. Concentration of NH₃ 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 NH₃ in Xi'an. Moreover, diurnal variation pattern of NH₃ 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 NH₃ in the city. Based on the source apportionment by using isotope mixing model, we found that 66.4% and 62.5% of NH₃ 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 NH₃ of Xi'an, where haze pollution is still severe, and suggested that emission controls of non-agricultural NH₃ could be an effective way to mitigate the air pollution problem in the semi-arid region of East Asia.