There are many measuring techniques for combustion aerosol particles. In-situ techniques analyze the particles in their natural environment. They are fast, do not require work-up procedures and allow a continuous monitoring of the aerosol. It is important to determine and monitor a toxicological relevant property of the particles. We will therefore present photoelectric charging as fast, reliable and highly relevant method to observe particles from combustion engines. It delivers a realistic picture of the heavier Polycyclic Aromatic Hydrocarbons (PAH) adsorbed on the particle surface and correlates well with the usual black carbon measurements. Photoelectric charging meets all requirements for exhaust particle measurement, such as simplicity, speed and toxicological relevance.Most of the nanoparticle properties are governed by the chemical and physical nature of the surface. However, the relevant surface area is not the geometrical surface but only a fraction of it called the Active Surface. Experimentally, one obtains this Active Surface either from a measurement of the mobility b or equivalently from the mass transfer coefficient K. This coefficient has been determined either by attachment of radioactive Pb-atoms or, as in this studies, by diffusion charging of neutralized particles. We show that K · b = const independent of particle material or shape. This empirical rule is useful for intercomparison and calibration of various instruments based on different physical principles. Dynamical changes of the Active Surface can be observed by measuring the photoelectric yield of the particles while suspended in their proper carrier gas.