Particulate air pollution, with emphasis on traffic generated aerosols

Experimental methods for identifying particles generated from the wear of automobile tires and roadway asphalts have been developed. The methods have been employed on aerosols, collected with Berner low pressure cascade impactors, in Copenhagen and Risø and aerosols collected with medium volume samplers on two locations in Copenhagen. Furthermore the deposited particulate matter has been measured in soil near highways and at remote sites, and finally the ad- and absorbed particulate matter has been measured on plant leaves sampled in Copenhagen and at remote sites. Tire and bitumen particles constitute each about 5 wt-% of the collected suspended particulate matter in inner city air. The particle size distribution shows that 92 % of the mass of airborne particulate tire debris have aerodynamic diameters smaller than 1 µm. The mean aerodynamic diameter is about 1 µm for the bitumen particles. Soil concentrations in the vicinity of a highway indicate an approximate exponential decrease with increasing distance from the road. Constant values are reached after about 5 m for the tire particles and 10 m for the bitumen particles. This implies a presence of larger particles, typically larger than 20 µm, that deposit immediately and that are not collected by the aerosol samplers. Concentrations in soil that has not been touched for at least 30 years show a decrease in tire concentration by a factor of 30 when moving from the top soil to a depth of 3 cm. The bitumen concentration is approximately constant to a depth of 10 cm. The leaf samples indicate a slightly higher tire particle concentration on the adaxial side compared to the abaxial side and an increased surface retention for pubescent leaves. About 0.5 m g tire pr. cm2 leaf is found near a highway, and about 65 % of this concentration derives from adsorbed particles on both leaf sides. The remainder is either respired through stomata or incorporated in the epicuticular wax layer. The fact that a substantial amount of the airborne tire and bitumen particles occur in the submicron range permits long range transportation and uptake and assimilation in the human respiratory system as well as absorption in plant tissue.