Spatial and temporal variation in nitrogen dioxide pollution adjacent to rural roads

Concentrations of nitrogen dioxide were measured near to a major, minor and unclassified (serving only a few farmhouses) road in Snowdonia National Park, North Wales. Diffusion tubes were place along transects, at right angles to the roads, and changed weekly. Background concentrations of NO2, recorded for comparison at a remote site, 2 km from the nearest road, were up to 40% higher in winter than in summer. Close to the major road this pattern was reversed, with concentrations 50–60% higher in summer than winter. Near the major road, monthly mean concentrations were consistently higher than those found 250 m away, by up to 20 nl L−1 NO2. Near the minor road, concentrations of NO2 were 6–10 nl L−1 higher than at 250 m away but only during summer months (tourist season). There were no differences between mean NO2 concentrations at 1 and 250 m from the unclassified road. The concentrations of NO2 near to the major and minor roads exceeded UN-ECE critical levels. It is concluded that many roadside plant communities are threatened by NO2 pollution toxicity.