An outdoor adapted quartz crystal microbalance probe for atmospheric corrosivity studies was used to monitor in. situ mass changes during exposure of copper in one rural and two urban sites. Weight gain rates, calculated over consecutive 12 h intervals of the exposures, were compared to 12 h averages in the concentration of aerosol particles, SO2, relative humidity, and temperature. The comparison exhibited coinciding time variations which demonstrated causal relationships. In descending order, the most important factors to influence copper weight gain rates seem to be relative humidity, concentration of aerosol particles, a temperature above -5 degrees C, and possibly the concentration of sulfur dioxide. The weight gain rates show a clear seasonal dependence with the lowest rates (2 ng cm(-2) h(-1)) in June and the highest (10 ng cm(-2) h(-1)) in December. The seasonal dependence found dominates the difference between the investigated rural and urban sites.