Sizes or size distribution of aerosol particles of an aerosol gas stream is detected. A particle-free carrier flow is saturated in a saturator by a first condensing medium, after which, the carrier flow is mixed in a mixing section turbulently with aerosol particle flow. Advantageously, a plurality of separate mixtures with different saturation ratios are provided and introduced to the first condensing environment in order to condensate the condensing medium onto the aerosol particles. Depending on the saturation ratio, the particles with different lowest original size are activated and grown in a growing section and/or in the condensing environment. After activation and growing, the particles are detected by a detecting means.