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

M Rapp

doi:10.5194/angeo-27-2417-2009

Abstract

Abstract. Time constants for photodetachment, photoemission, and electron capture are considered for two classes of mesospheric aerosol particles, i.e., meteor smoke particles (MSPs) and pure 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 orders of magnitude faster than electron attachment such that MSPs cannot be negatively charged in the presence of sunlight. Rather, even photoemission can compete with electron capture unless the electron density becomes very large (>>1000 cm−3) such that 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 its absorption 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 the repeated observation of the coexistence of positive and negative aerosol particles in the polar summer mesopause, 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 as well as for the interpretation of incoherent scatter radar observations of MSPs.

Highlights

In recent years it has been realized that charged aerosol particles play an important role in the physics of the mesosphere
We have shown above that photodetachment of electrons from charged meteor smoke particles (MSPs) is a dominant process which must be taken into account for calculations of the charge state of MSPs
If MSPs do consist of Fe2O3, silicon oxide (SiO) or a material with comparable absorption properties, MSPs cannot be charged negatively under conditions of sunlight such as in the polar summer mesopause region

Summary

Introduction

In recent years it has been realized that charged aerosol particles play an important role in the physics of the mesosphere. Photoemission as an additional charging mechanism was suggested for mesospheric ice particles (Havnes et al, 1990) but quantitative calculations on the basis of laboratory measurements showed that the work function of pure ice is too large to allow photoemission to compete with electron capture and result in a positive particle charge This applies unless the particles become contaminated for example by atomic sodium as the consequence of a fresh input of metals by a rather large meteoroid (e.g., Vondrak et al, 2006).

Time constant analysis

Findings

Conclusions