Ammonia emission from real-world in-use vehicle fleets in a megacity in China - based on tunnel measurement.

Ammonia (NH) has been widely recognized as a key precursor of atmospheric secondary aerosol formation. Vehicle emission is a major source of urban atmospheric NH. With the tightening of emission standards and the growing trend of vehicle fleet electrification, it is imperative to update the emission factors for NH from real-world on-road fleets. In this study, a tunnel measurement was conducted in the urban area of Tianjin, China. The fleet-average NH emission factor (EF) was 11.2 mg/(km·veh), significantly lower than those in previous studies, showing the benefit of emission standard updating. Through a multiple linear regression analysis, the EFs of light-duty gasoline vehicles, light-duty diesel vehicles, and heavy-duty diesel vehicles (HDDVs) were estimated to be 5.7 ± 0.6 mg/(km·veh), 40.8 ± 5.1 mg/(km·veh), and 160.2 ± 16.6 mg/(km·veh), respectively. Based on the results from this study, we found that HDDVs, which comprise <3 % of the total vehicles may contribute approximately 22 % of total NH emissions in Tianjin. Our results highlight NH emissions from HDDVs, a previously potentially overlooked source of NH emissions in urban areas. The actual on-road NH emissions from HDDVs may exceed current expectations, posing a growing concern for the future.