Variability of NO₂/NOx Ratios in Multiple Microenvironments from On-Road and Near-Roadway Measurements

An understanding of the spatial distribution of nitrogen dioxide (NO 2 ) in cities is required to improve health impact assessments for NO 2 exposure. In this study, 32 702 5 s averaged on-road measurements and 125 hourly near-roadway measurements were used to evaluate the variability of NO 2 and nitrogen oxide (NO x ) concentrations in multiple microenvironments during two sampling campaigns (nonholiday and holiday). Microenvironments impacted by heavy-duty diesel vehicles (HDDVs) had large variations in NO 2 /NO x ratios due to the holiday effect. There was a strong linear relationship between NO 2 /NO x ratios and ambient background O 3 in on-road and near-roadway microenvironments. The slopes of the regression lines (Δ(NO 2 /NO x )/ΔO 3 ) provide estimations of the change rate of NO 2 /NO x ratios with respect to ambient background O 3 . Variable slopes occurred in on-road but not for near-roadway microenvironments. This is evidence that the change rate of on-road NO 2 /NO x ratios is strongly affected by vehicle fleet composition (i.e., HDDV%), while the near-roadway NO 2 /NO x ratios are more influenced by the atmospheric turbulence. The findings indicate that HDDVs could be a valid indicator of variability in NO 2 /NO x ratios concerning ambient background O 3 and the effect of HDDVs should be embedded into dispersion models for better prediction of NO 2 concentrations in traffic-related environments.