Significant NO<sub>2</sub> Formation in Truck Exhaust Plumes and Its Association with Ambient O<sub>3</sub>: Evidence from Extensive Plume-Chasing Measurements
Sheng Xiang, Shaojun Zhang, Yu Ting Yu, Hui Wang, Ke Hao, Ye Wu
Abstract
Vehicle nitrogen oxides (NO x ) significantly increase nitrogen dioxide (NO 2 ) exposure in traffic-related environments. The NO 2 /NO x ratios are crucial for accurate NO 2 modeling and are closely linked to public health concerns. In 2020, we used a mobile platform to follow test trucks (plume-chasing) that were installed with a portable emission measuring system (PEMS) on two restricted driving tracts. Six hundred eighteen exhaust plumes were collected through the PEMS-chasing measurements from seven trucks. The NO x emission factors (EFs), and the NO 2 /NO x ratios, were calculated at distinct stages (i.e., tailpipe and on-road). A significant reduction in NO x EFs (>64%) was observed with normal operating after-treatment devices, except for trucks equipped with diesel particulate filter (DPF). Disparities in tailpipe NO 2 /NO x ratios were also found, attributed to the after-treatment technologies. The NO 2 /NO x ratios measured from plume-chasing were significantly higher (3–4 times, p < 0.001) than the tailpipe measurements, providing field evidence of substantial NO 2 formation in exhaust plumes. We developed a quantitative relationship between NO 2 /NO x ratios from tailpipe and plume-chasing measurements and demonstrated a robust correlation ( R 2 > 0.90). Since the NO 2 formation in the exhaust plume is not explicitly accounted for in NO 2 modeling, the quantitative relationship (O 3 –NO 2 /NO x ) could improve the estimation of NO 2 exposure when local emission inventory (tailpipe emissions) is available.