Charging of mesospheric aerosol particles: the role of photodetachment and photoionization from meteoric smoke and ice particles

摘要

Time constants for photodetachment, photoemission, and electron capture are considered for twoclasses of mesospheric aerosol particles, i.e., meteor smoke particles (MSPs) andpure water ice particles. Assuming that MSPs consist of metal oxides like Fe₂O₃ or SiO,we find that during daytime conditions photodetachment by solar photons is up to 4 ordersof magnitude faster than electron attachment such that MSPs cannot be negatively charged in the presence of sunlight. Rather, even photoemissioncan compete with electron capture unless the electron density becomes very large (1000 cm−3) suchthat MSPs should either be positively charged or neutral in the case of large electron densities. For pure water ice particles,however, both photodetachment and photoemission are negligible due to the wavelength characteristics of itsabsorption cross section and because the flux of solar photons has already dropped significantly at such short wavelengths.This means that water ice particles should normally be negatively charged. Hence, our results can readily explain therepeated observation of the coexistence of positive and negative aerosol particles in the polar summermesopause, i.e., small MSPs should be positively charged and ice particles should be negatively charged.These results have further important implications for our understanding of the nucleation of mesospheric ice particles aswell as for the interpretation of incoherent scatter radar observations of MSPs.