Pollution Levels Near a Motorway in a Cutting

Abstract

A computer method of estimating traffic generated air pollution near to roads has been developed at TRRL for use in the environmental appraisal of new road schemes. A series of pollution surveys is being carried out to provide data to enable a modification of the method for applications in topographically complex locations. One survey took place near to a heavily trafficked section of the M25 London Orbital motorway, at a site where the road passes through a 7m deep cutting, during Autumn 1986. A range of pollutants was monitored at a mobile laboratory parked close to the hard shoulder and carbon monoxide concentrations were measured at a number of locations inside and outside the cutting using portable analysers. Traffic flows and meteorological conditions were also recorded. The concentrations at the mobile laboratory were typical of a motorway site. Carbon monoxide levels were generally lower and oxides of nitrogen higher than in urban locations, consistent with their relative emission rates from high and low speed traffic. Spatial variations in the carbon monoxide concentrations measured within the cutting, in comparison with those measured outside and with those forecast for a similarly trafficked site in level ground, demonstrated the effect of the cutting in modifying the local dispersion characteristics. At road level the concentration in the cutting was substantially higher than outside, suggesting a restriction to the dispersion. High concentrations were found on the side of the road opposing the prevailing regional wind, probably because of the formation of turbulent eddies in the cutting.