Significant NO2 Formation in Truck Exhaust Plumes and Its Association with Ambient O3: Evidence from Extensive Plume-Chasing Measurements

Abstract

Vehicle nitrogen oxides (NO_x) significantly increase nitrogen dioxide (NO₂) exposure in traffic-related environments. The NO₂/NO_x ratios are crucial for accurate NO₂ 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 NO₂/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 NO₂/NO_x ratios were also found, attributed to the after-treatment technologies. The NO₂/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 NO₂ formation in exhaust plumes. We developed a quantitative relationship between NO₂/NO_x ratios from tailpipe and plume-chasing measurements and demonstrated a robust correlation (R² > 0.90). Since the NO₂ formation in the exhaust plume is not explicitly accounted for in NO₂ modeling, the quantitative relationship (O₃–NO₂/NO_x) could improve the estimation of NO₂ exposure when local emission inventory (tailpipe emissions) is available.