Ion—Aerosol Attachment Coefficients and the Steady-State Charge Distribution on Aerosols in a Bipolar Ion Environment

Abstract

Calculations of the ion—aerosol attachment coefficients are carried out for Fuchs' theory (as corrected in this paper) and for a theory which includes three body trapping. The resulting charge distributions agree quite well for particles with radii greater than about 0.007 μm. For smaller particles three-body trapping becomes increasingly important. Comparison of theoretically predicted charge distributions with recently measured charge distributions at radii smaller than 0.02 μm show good agreement. Asymmetric charging due to differences in the physical properties of positive and negative ions can result in large differences in the number of positively and negatively charged particles, particularly at larger radii. The asymmetric charge distribution is also shown to depend on the ionization rate. For the case when aerosol concentrations are comparable to the ion concentrations the effect of polydispersity on the charge distribution is difficult to predict. It is shown that a dominant size particle can establish a positive to negative ion ratio which, in turn, will determine a charge distribution at other sizes, different from that which would exist in the absence of the dominant species.