Vehicular ammonia emissions: an underappreciated emission source indensely populated areas

On-road ammonia (NH3) emissions play a significant role in fine particulate matter (PM2.5) formation in urban areas, posing severe risks for human health. Limited studies have depicted the spatial and temporal variations of on-road NH3 emissions, in particular lacking detailed quantification of their contributions within densely populated areas. In this study, we established a comprehensive vehicular NH3 emission model and compiled a gridded on-road NH3 emission inventory with high spatial (3 km x 3 km) and temporal (monthly) resolution for mainland China. China's annual vehicular NH3 emissions are estimated to increase from 32.8 +/- 1.7 to 87.1 +/- 37.5 kt during the period of 2000-2019. Vehicular NH3 emissions are significantly concentrated in densely populated areas, where agricultural emissions have relatively lower intensity. It is found that vehicular NH3 emissions could exceed agricultural emissions in the grids containing 23.0 +/- 2.1% of the Chinese population in 2019 (approximately 326.6 +/- 25.4 million people), and this ratio is up to 29.4 +/- 3.0% in winter. For extremely populous megacities such as Beijing and Shanghai, vehicular NH3 emissions exceed agricultural emissions, where 69.2 +/- 1.2% and 72.0 +/- 1.3% of the population resides, respectively. Thus, the significant role of on-road NH3 emissions in populated areas may have been underappreciated. This study gave a better insight into the absolute value and relative importance of on-road NH3 emissions for different regions, seasons and population densities in China; this is important in terms of the implications for air quality.