Variability of NO<sub>2</sub>/NO<i><sub>x</sub></i> Ratios in Multiple Microenvironments from On-Road and Near-Roadway Measurements
Sheng Xiang, Shaojun Zhang, Hui Wang, Yu Ting Yu, Timothy J. Wallington, Wei Shen, U. Kirchner, Ye Deng, Qinwen Tan, Zihang Zhou, Ye Wu
Abstract
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.