Atmospheric ammonia in China: Long-term spatiotemporal variation, urban-rural gradient, and influencing factors.

In recent years, atmospheric ammonia (NH) concentrations have increased in China. Ammonia control has become one of the next hot topics in air pollution mitigation with the increasing cost of acid gas emission reduction. In this study, using Infrared Atmospheric Sounding Interferometer (IASI) satellite observations, we analyzed the spatiotemporal distribution, the urban-rural gradient of the vertical column densities (VCDs) of NH and the contribution of influencing factors (meteorology, social, atmospheric acid gases, and NH emissions) in China from 2008 to 2019 using hotspot analysis, circular gradient analysis, geographical and temporal weighted regression, and some other methods. Our results showed that NH VCDs in China have significantly increased (31.88 %) from 2008 to 2019, with the highest occurring in North China Plain. The average NH VCDs in urban areas were significantly higher than those in rural areas, and the urban-rural gap in NH VCDs was widening. The results of circular gradient analysis showed an overall decreasing trend in NH VCDs along the urban-rural gradient. We used a geographically and temporally weighted regression model to analyze the contribution of various influencing factors to NH VCDs: meteorology (30.13 %), social (27.40 %), atmospheric acid gases (23.20 %), and NH emissions (19.28 %) factors. The results showed substantial spatiotemporal differences in the influencing factors. Atmospheric acid gas was the main reason for the increase in NH VCDs from 2008 to 2019. A more thorough understanding of the spatiotemporal distribution, urban-rural variations, and factors influencing NH in China will aid in developing control strategies to reduce PM.