Fundamentals of particle charging by free electrons in electrical coronas

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The electrical charging of aerosols is an important process in many technical applications and in aerosol measurements. The kinetics of aerosol charging by positive and negative gas ions based on the mechanisms of diffusion charging and field charging is well known. But in some situations aerosols are being charged much faster and to a much higher level than expected. As a rule, this effect can be attributed to aerosol charging by free electrons. This so called “Electronic Charging” of aerosols is to be found especially with low concentrations of electronegative gases or with elevated temperatures. In these cases the formation of negative gas ions is delayed or does not take place. Hence, the current transport in a corona discharge will occur in part or almost completely by free electrons. In electrostatic fields, the free electrons will reach very high (non-equilibrium) electron temperatures, and aerosol particles can reach very high charging levels by electronic diffusion charging. So far, there is a lack of systematic investigations on electronic charging of aerosols. In addition, there is a lack of basic data which would be needed for a theoretical prediction of electronic charging in technically relevant gas mixtures. Within the project, the fundamentals for modelling quantitatively the electronic charging of aerosols in corona discharges shall be provided and validated. This will be done first with the system N2-O2-H2O as a technically relevant gas mixture model and with submicron aerosols. For measurements of the charge carrier kinetics, classical drift tube experiments will be adapted to the conditions. As well, measurements of aerosol charging kinetics will be done first in a drift tube within a homogeneous electrostatic field. Further validations will be done with corona discharges in a wire-tube-geometry.