A comparative study of traffic related air pollution next to a motorway and a motorway flyover

Abstract

The aim of this study was to investigate the influence of the vertical position of a motorway on the pollutant concentrations in the vicinity of the motorway. Therefore, a near-road monitoring campaign was performed along two adjoining motorway stretches: one motorway at ground level and a motorway flyover. PM2.5 and nitrogen oxides (NO, NO2 and NOx) were measured at both sides of the motorway on a 30-min resolution. In addition, EC (elemental carbon) was measured on both sides of the road on a daily basis using low volume samplers and thermal-optical transmission (TOT) analysis of filters. PNC (particle number concentration) was measured at one side. The measurement locations on each side of the road were located at 29 m and at 102 m from the motorway at ground level and at 60 m and 120 m from the motorway flyover. Pollution roses,showed increased average concentrations in the direction of the road for nitrogen oxides and PM2.5. The impact of road traffic appeared to be higher at groundlevel compared to the flyover. The difference in concentration between both sides of the road for PM2.5, NO, NO2 and NOx was analysed using linear mixed models. The concentration difference, which can be seen as the contribution of road traffic to the pollutant concentration, was calculated taking into account prevailing wind directions. The results indicate that taking into account confounding parameters(time of day, day of the week, distance to the road and windspeed), the contribution of the motorway traffic to pollutant concentrations is significantly higher when the motorway is at ground level. Furthermore, time of day and day of the week also have a significant effect on the concentration difference. The linear mixed model estimated a reduced road increment at the motorway flyover compared to the ground level motorway of 119 ppb for NO, 29 ppb for NO2 and 3.5 mg m 3for PM2.5, taking into account confounding parameters. The highest EC and PNC concentrations were measured downwind of the motorway at ground level.