A 3 week measurement campaign was undertaken to study the effect of local weather parameters, transportation from an urban area, structure of boundary layer, and precipitation on submicron (8-450 nm in mobility diameter) aerosol particles in urban background area in Finland. Also, the concentrations of NOx, O3, and SO2 were monitored. The most important meteorological factor affecting aerosol particles was shown to be local wind direction. It was also seen that the diurnal behavior of boundary layer plays an important role for aerosol particle concentration and size distribution and gas phase chemistry at the ground level. Even the few occurrences of new particle formation that were observed seem to be connected with changes in the boundary layer. Clear indications of the possible effect of precipitation (rain or snow) on aerosol size distributions could not be detected in this study. The effect is obviously small compared to the influences of other meteorological processes. ¿ 2000 American Geophysical Union