Spatial variability of different fractions of particulate matter within anurban environment and between urban and rural sites

The spatial variability of different fractions of particulate matter (PM) w as investigated in the city of Basel, Switzerland, based on measurements pe rformed throughout 1997 with a mobile monitoring station at six sites and p ermanently recorded measurements from a fixed site. Additionally, PM10 meas urements from the following year, which were concurrently recorded at two u rban and two rural sites, were compared. Generally, the spatial variability of PM4, PM10, and total suspended partic ulates (TSP) within this Swiss urban environment (area = 36 km(2)) was rath er limited. With the exception of one site in a street canyon next to a tra ffic light, traffic density had only a weak tendency to increase the levels of PM. Mean PM10 concentration at six sites with different traffic densiti es was in the range of less than +/-10% of the mean urban PM10 level. Howev er, comparing the mean PM levels on workdays to that on weekends indicated that the impact of human activities, including traffic, on ambient PM level s may be considerable. Differences in the daily PM10 concentrations between urban and more elevate d 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 tha t spatial variability of annual mean PM concentration between urban and rur al sites in the Basel area may more likely be caused by varying altitude th an by distance to the city center.