A method and an apparatus for determining an aerosol particle size distribution that includes the steps of modulating an aerosol particle beam with an aerosol particle gate which is controlled by a modulation function for generating a modulated aerosol particle beam, guiding the modulated aerosol particle beam through a drifting region, measuring a signal of the modulated aerosol particle beam after the modulated aerosol particle beam has passed the drifting region and calculating a correlation of the modulation function and the signal in order to determine the size distribution of the aerosol particles.