[AAS03-04] The significant role of vehicle ammonia emissions in urban secondary aerosol formation for Chinese megacities
Keywords:Vehicle emissions, Ammonia, Emission inventory, Inorganic secondary aerosol, PM2.5, CMAQ
Tailpipe aftertreatment devices of on-road vehicles are effective to control emissions of regulated pollutants but would result in ammonia (NH3) emissions as unwanted side product. For example, three-way catalyst (TWC) converters used by gasoline vehicles generate NH3 emissions. For diesel vehicles, NH3 emissions are in general related to urea injection for selective catalyst reduction converters (SCR). Based on recent tailpipe emission measurements for gasoline [1] and diesel [2] vehicles in China, we update the vehicle NH3 emission inventory for on-road vehicles in China [3]. The results indicate that, at the national level, on-road vehicles currently are responsible for a minor fraction (~90 kt, 0.8%) of total anthropogenic NH3 emissions in China. The contribution fractions vary from below 0.5% in remote provinces (e.g., Tibet, Xinjiang, Inner Mongolia) to more than 5% in populated municipalities (e.g., Beijing, Shanghai). Of note, supported by detailed street-level traffic data, a high-resolution emission inventory [4] for the urban area of Beijing (note: the urban area of Beijing is about 1000 km2 and denotes the area within the Fifth Ring Road and. By comparison, the total area of the municipality of Beijing is about 16,000 km2) suggest that on-road vehicles are the leading source (approximately 80%) of anthropogenic NH3 emissions, of which gasoline cars are estimated to contribute more than 70%.
We further apply an atmospheric chemical transport model (WRF/CMAQ 5.3; 3 km-and-1 km simulation resolutions) to quantify the role of vehicle NH3 emissions in the formation of secondary inorganic aerosol, in particular focusing on the impact of the spatial heterogeneity of vehicle NH3 emissions. With the Brute-Force method by closing all vehicle NH3 emissions, we find that total PM2.5 concentrations in the urban area of Beijing are lowered by 1 µg/m3 (3%) in the summer. The wintertime reduction of PM2.5 is quite significant, which is up to 10 µg/m3 (9%). The changes in two seasons are both predominantly caused by the reduction in ammonium nitrate. This study, by implication, suggest that vehicle emissions are an important source of NH3 emissions in traffic-dense urban areas of megacities in China, and contribute significantly to urban PM2.5 pollution (particularly in winter). Further source apportionment studies by using atmospheric models should carefully consider the high concentration of vehicle NH3 emissions in urban areas.
References
[1] Huang, C., et al. Ammonia emission measurements for light-duty gasoline vehicles in China and implications for emission modeling. Environmental Science & Technology, 52(19): 11223-11231 (2018)
[2] He, L., et al. On-road measurements of reactive nitrogen compounds from heavy-duty diesel trucks in China, Environmental Pollution (in review)
[3] Wu, Y., et al. On-road vehicle emissions and their control in China: A review and outlook. Science of the Total Environment, 676: 242-251 (2017)
[3] Yang, D. et al. High-resolution mapping of vehicle emissions of atmospheric pollutants based on large-scale, real-world traffic datasets. Atmospheric Chemistry and Physics, 19(13): 8831-8843 (2019)
We further apply an atmospheric chemical transport model (WRF/CMAQ 5.3; 3 km-and-1 km simulation resolutions) to quantify the role of vehicle NH3 emissions in the formation of secondary inorganic aerosol, in particular focusing on the impact of the spatial heterogeneity of vehicle NH3 emissions. With the Brute-Force method by closing all vehicle NH3 emissions, we find that total PM2.5 concentrations in the urban area of Beijing are lowered by 1 µg/m3 (3%) in the summer. The wintertime reduction of PM2.5 is quite significant, which is up to 10 µg/m3 (9%). The changes in two seasons are both predominantly caused by the reduction in ammonium nitrate. This study, by implication, suggest that vehicle emissions are an important source of NH3 emissions in traffic-dense urban areas of megacities in China, and contribute significantly to urban PM2.5 pollution (particularly in winter). Further source apportionment studies by using atmospheric models should carefully consider the high concentration of vehicle NH3 emissions in urban areas.
References
[1] Huang, C., et al. Ammonia emission measurements for light-duty gasoline vehicles in China and implications for emission modeling. Environmental Science & Technology, 52(19): 11223-11231 (2018)
[2] He, L., et al. On-road measurements of reactive nitrogen compounds from heavy-duty diesel trucks in China, Environmental Pollution (in review)
[3] Wu, Y., et al. On-road vehicle emissions and their control in China: A review and outlook. Science of the Total Environment, 676: 242-251 (2017)
[3] Yang, D. et al. High-resolution mapping of vehicle emissions of atmospheric pollutants based on large-scale, real-world traffic datasets. Atmospheric Chemistry and Physics, 19(13): 8831-8843 (2019)