On-Road Chemical Transformation as an Important Mechanism of NO Formation.

Nitrogen dioxide (NO) not only is linked to adverse effects on the respiratory system but also contributes to the formation of ground-level ozone (O) and fine particulate matter (PM). Our curbside monitoring data analysis in Detroit, MI, and Atlanta, GA, strongly suggests that a large fraction of NO is produced during the "tailpipe-to-road" stage. To substantiate this finding, we designed and carried out a field campaign to measure the same exhaust plumes at the tailpipe-level by a portable emissions measurement system (PEMS) and at the on-road level by an electric vehicle-based mobile platform. Furthermore, we employed a turbulent reacting flow model, CTAG, to simulate the on-road chemistry behind a single vehicle. We found that a three-reaction (NO-NO-O) system can largely capture the rapid NO to NO conversion (with time scale ≈ seconds) observed in the field studies. To distinguish the contributions from different mechanisms to near-road NO, we clearly defined a set of NO/NO ratios at different plume evolution stages, namely tailpipe, on-road, curbside, near-road, and ambient background. Our findings from curbside monitoring, on-road experiments, and simulations imply the on-road oxidation of NO by ambient O is a significant, but so far ignored, contributor to curbside and near-road NO.