The decrease in size of aerosol particles due to evaporation is calculated for two situations. In the first, the air is saturated with vapour so that evaporation is due to the Kelvin effect and proceeds at the minimal rate; in the second, the air is free from vapour and the rate of evaporation is maximal. The calculation are based upon an interpolation formula covering the transition with decreasing particle size from diffusion control to free molecular flow.