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

An understanding of the spatial distribution of nitrogen dioxide (NO2) in cities is required to improve health impact assessments for NO2 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 NO2 and nitrogen oxide (NOx) concentrations in multiple microenvironments during two sampling campaigns (nonholiday and holiday). Microenvironments impacted by heavy-duty diesel vehicles (HDDVs) had large variations in NO2/NOx ratios due to the holiday effect. There was a strong linear relationship between NO2/NOx ratios and ambient background O3 in on-road and near-roadway microenvironments. The slopes of the regression lines (Δ(NO2/NOx)/ΔO3) provide estimations of the change rate of NO2/NOx ratios with respect to ambient background O3. Variable slopes occurred in on-road but not for near-roadway microenvironments. This is evidence that the change rate of on-road NO2/NOx ratios is strongly affected by vehicle fleet composition (i.e., HDDV%), while the near-roadway NO2/NOx ratios are more influenced by the atmospheric turbulence. The findings indicate that HDDVs could be a valid indicator of variability in NO2/NOx ratios concerning ambient background O3 and the effect of HDDVs should be embedded into dispersion models for better prediction of NO2 concentrations in traffic-related environments.