Spatial variation of aerosol number concentration in Helsinki city

Aerosol number concentration was measured continuously in Helsinki from 1 of November, 1996 till 1 May, 1997. In addition to that number concentrations were measured simultaneously for 14 days in several places in a downtown area and in a remote site close to the city. The measured data allows us to investigate spatial variation of urban aerosol number concentration. In general, the number concentration time series measured in different places show high correlation. In areas, where traffic follows similar pattern and provides dominant local isotropic aerosol source, correlation in our case is high (about 0.8). Correlation mainly depends on the traffic intensity. During the working days concentration averages of 10 min - 1 h are good representatives of concentration variation in relatively large area of the city. The place for the sampling point must be chosen carefully. (C) 1999 Elsevier Science Ltd. All rights reserved.