Analysis of traffic-related NOx and EC concentrations at various distances from major roads in Japan

Traffic-related air pollutants were monitored near major roads at 10 sites in Japan. Nitrogen oxides (NOx), suspended particulate matter (100% cut-off aerodynamic diameter at 10 μm), PM2.5 (50% cut-off aerodynamic diameter at 2.5 μm), and black carbon, from which elemental carbon (EC) content was calculated, were instantaneously and continuously monitored at four stations at various distances (about 5, 35, 70, and 150 m) from each of the target roads. We analyzed concentration data from a 1-year monitoring period (Jan. 1–Dec. 31, 2006) at nine sites where credible estimation of emission rates was possible. For conditions of wind directions nearly perpendicular to the target roads, neutral atmospheric stability, and sufficiently high wind speed (>1 m s−1), we compared the observed concentrations with concentrations calculated by means of the conventional Gaussian plume model. Except for a site with densely packed high-rise buildings and another site with suspected additional emission sources that were not included in the model, the NOx and EC concentrations normalized by the values at the stations closest to the road agreed well between the Gaussian plume model and the observation. By assuming that the emission factor of EC was proportional to that of PM (total particulate matter at emission) and by using the emission factor of NOx  , we estimated the emission factor of EC by evaluating the ratio (CNOx−CNOx,background)/(CEC−CEC,background)(CNOx−CNOx,background)/(CEC−CEC,background). Good agreement between the observed and estimated ratios was obtained with a proportionality constant (EC/PM) of 0.4, indicating that the emission factor of EC was 0.4 times that of PM.