Emission factors of ammonia for on-road vehicles in urban areas from a tunnel study in south China with laser-absorption based measurements.

Vehicle emission is an important source of ammonia (NH) in urban areas. To better address the role of vehicle emission in urban NH sources, the emission factor of NH (NH-EF) from vehicles running on roads under real-world conditions (on-road vehicles) needs to update accordingly with the increasingly tightened vehicle emission standards. In this study, laser-absorption based measurements of NH were conducted during a six-day campaign in 2019 at a busy urban tunnel with a daily traffic flow of nearly 40,000 vehicles in south China's Pearl River Delta (PRD) region. The NH-EF was measured to be 16.6 ± 6.3 mg km for the on-road vehicle fleets and 19.0 ± 7.2 mg km for non-electric vehicles, with an NH to CO ratio of 0.27 ± 0.09 ppbv ppmv. Multiple linear regression revealed that the average NH-EFs for gasoline vehicles (GVs), liquefied petroleum gas vehicles, and heavy-duty diesel vehicles (HDVs) were 18.8, 15.6, and 44.2 mg km, respectively. While NH emissions from GVs were greatly reduced with enhanced performance of engines and catalytic devices to meet stricter emission standards, the application of urea selective catalytic reduction (SCR) in HDVs makes their NH emission an emerging concern. Based on results from this study, HDVs may contribute over 11% of the vehicular NH emissions, although they only share ∼4% by vehicle numbers in China. With the updated NH-EFs, NH emission from on-road vehicles was estimated to be 9 Gg yr in the PRD region in 2019, contributing only 5% of total NH emissions in the region, but still might be a dominant NH source in the urban centers with little agricultural activity.