Vehicular Ammonia Emissions Significantly Contribute to Urban PM 2.5 Pollution in Two Chinese Megacities.

Ammonia (NH 3 ) plays a vital role in the formation of fine particulate matter (PM 2.5 ). Prior studies have primarily focused on the control of agricultural NH 3 emissions, the dominant source of anthropogenic NH 3 emissions. The air quality impact from vehicular NH 3 emissions, which could be particularly important in urban areas, has not been adequately evaluated. We developed high-resolution vehicular NH 3 emission inventories for Beijing and Shanghai based on detailed link-level traffic profiles and conducted atmospheric simulations of ambient PM 2.5 concentrations contributed by vehicular NH 3 emissions. We found that vehicular NH 3 emissions shared high proportions among total anthropogenic NH 3 emissions in the urban areas of Beijing (86%) and Shanghai (45%), where vehicular NH 3 was primarily emitted by gasoline vehicles. Local vehicular NH 3 emissions could be responsible for approximately 3% of urban PM 2.5 concentrations during wintertime, and the contributions could be much higher during polluted periods (∼3 μg m -3 ). We also showed that controlling vehicular NH 3 emissions will be effective and feasible to alleviate urban PM 2.5 pollution for megacities in the near future.