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

Ammonia (NH₃) has been widely recognized as a key precursor of atmospheric secondary aerosol formation. Vehicle emission is a major source of urban atmospheric NH₃. With the tightening of emission standards and the growing trend of vehicle fleet electrification, it is imperative to update the emission factors for NH₃ from real-world on-road fleets. In this study, a tunnel measurement was conducted in the urban area of Tianjin, China. The fleet-average NH₃ 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 NH₃ emissions in Tianjin. Our results highlight NH₃ emissions from HDDVs, a previously potentially overlooked source of NH₃ emissions in urban areas. The actual on-road NH₃ emissions from HDDVs may exceed current expectations, posing a growing concern for the future.