Volatile suspended particles are eliminated from an aerosol by heating the aerosol to a temperature at which the volatile suspended particles evaporate and diluting the aerosol with a dilution gas. The sequence heating and diluting may be performed in any order. According to another aspect of the invention, dilution of the raw gas takes place such that raw gas is led to a measurement gas channel through which measurement gas flows, wherein the quantity of raw gas transferred per unit of time is dependent on the volume flow in the raw gas channel. According to a further aspect of the invention, in a condensation nucleus counter the number and intensity of scattered light pulses evaluated in order to be able to draw conclusions on the reliability of the measurement.