Abstract: The invention provides a method for the quantitative and/or qualitative characterization of substances which are contained in a gaseous carrier medium. Modifications of the surface of small solid particles suspended in a carrier gas may be measured with very high sensitivity by the application of the photoelectric effect. Thereby, an aerosol is chemically modified, e.g. by admixing a gaseous agent or a gas mixture, or by subjecting it to an electromagnetic radiation, and subsequently the resulting variation of the photoelectron emission from the suspended particles is recorded. By measuring the variation of the photoelectric effect during the admixing or variation of a gaseous agent to the aerosol to be examined or during a variation of the electromagnetic radiation, to which the aerosol to be examined is subjected, the suspended particles may be classified in one or several steps.