Abstract
Twenty-four hour averaged concentrations of PM
2.5 and PM
10 have been measured at UK urban sites using dichotomous samplers. Four site pairs have been selected, each of which comprises a busy roadside site and a nearby urban background location. Differences between roadside and urban background concentrations are interpreted in terms of street geometry and atmospheric circulations. Markedly different patterns of behaviour are observed between the more enclosed and more open sites. At the former, the traffic increment in particle mass (difference between roadside and background sites) tends to be independent of the magnitude of concentrations. At the more open sites, there is a tendency for the traffic increment in particle mass to increase with increasing concentration. At one site, with an asymmetric building configuration across the street, the behaviour is wind-direction dependent with higher traffic increments observed when the air above the urban canopy passes first over the taller buildings before apparently creating a recirculating vortex over the roadway. Incremental coarse to incremental fine particle ratios vary greatly between sites (0.21–0.61 on average), the values increasing with the total traffic volume at the site.