The spatial variability of different fractions of particulate matter (PM) was investigated in the city of Basel, Switzerland, based on measurements performed throughout 1997 with a mobile monitoring station at six sites and permanently recorded measurements from a fixed site. Additionally, PM 10 measurements from the following year, which were concurrently recorded at two urban and two rural sites, were compared.Generally, the spatial variability of PM 4 , PM 10 , and total suspended particulates (TSP) within this Swiss urban IMPLICATIONS It has become popular in recent years to use the concentration of PM as an indicator of air pollution exposure in epidemiologic studies. Since many of these studies assess the exposure of subjects based on one measurement per city, the accuracy of this technique will markedly affect the result of cross-sectional studies. High spatial variability of PM could result in a large non-differential misclassification of exposure that would lead to a smaller recognized health effect of air pollution.The remarkable spatial homogeneity of long-term mean PM levels clearly reduces the error of assigning data from one fixed monitoring site to all study subjects living in Basel, as was done in recent cross-sectional health studies in Switzerland (Swiss Study on Air Pollution and Lung Diseases in Adluts, Swiss Study on Childhood Allergy and Respiratory Systems). In fact, all participants lived in urban Basel, rendering PM 4 , PM 10 , and even TSP useful city-wide surrogates for long-term exposure to outdoor air pollution.
environment (area = 36 km2 ) was rather limited. With the exception of one site in a street canyon next to a traffic light, traffic density had only a weak tendency to increase the levels of PM. Mean PM 10 concentration at six sites with different traffic densities was in the range of less than ±10% of the mean urban PM 10 level. However, comparing the mean PM levels on workdays to that on weekends indicated that the impact of human activities, including traffic, on ambient PM levels may be considerable.Differences in the daily PM 10 concentrations between urban and more elevated rural sites were strongly influenced by the stability of the atmosphere. In summer, when no persistent surface inversions exist, differences between urban and rural sites were rather small. It can therefore be concluded that spatial variability of annual mean PM concentration between urban and rural sites in the Basel area may more likely be caused by varying altitude than by distance to the city center.