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

Vehicle nitrogen oxides (NOx) significantly increase nitrogen dioxide (NO2) exposure in traffic-related environments. The NO2/NOx 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 NOx emission factors (EFs), and the NO2/NOx ratios, were calculated at distinct stages (i.e., tailpipe and on-road). A significant reduction in NOx EFs (>64%) was observed with normal operating after-treatment devices, except for trucks equipped with diesel particulate filter (DPF). Disparities in tailpipe NO2/NOx ratios were also found, attributed to the after-treatment technologies. The NO2/NOx 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/NOx 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/NOx) could improve the estimation of NO2 exposure when local emission inventory (tailpipe emissions) is available.