Aerosol Photoemission (APE) is used to monitor chemical processes on gas-borne particles and to show the influences of process parameters. For this purpose, a system is presented which is able to detect changes in the photoemission behavior by means of APE, which can be brought directly into a relationship with surface modifications of the particles using further measurement techniques. This is shown by the example of the conversion of metal nitrates to oxides and the coating of particles in the gaseous phase.