Aerosol Wall Losses in Electrically Charged Chambers

Abstract

Wall deposition of aerosol particles in spherical, electrically charged chambers is investigated theoretically and experimentally. The theory accounts for transport by convection, Brownian diffusion, gravitational sedimentation, and electrostatic drift. Experiments involved measurements of loss rates for particles of known size and charge in 250-liter FEP Teflon film bags. Agreement between theory and experiment is satisfactory provided that charging and neutralization of aerosol particles by ions is taken into account. It is shown that under these experimental conditions, charge can have a dominant effect on deposition rates, depending on particle size and on electric field strength. For example, wall loss rates of singly charged 0.1 μm diameter particles were a factor of 100 greater than wall loss rates of neutral particles of the same size in these experiments. The theory is valid for any distribution of electric fields near the interior surface of the chamber. A general result of the theory is that positively and negatively charged particles are lost at equal rates; this was also confirmed experimentally.

The theory was also used to calculate size-dependent wall deposition rates in a large (60 m³) Teflon film smog chamber. The aerosol was assumed to be initially in Boltzmann charge equilibrium, as might be the case if the chamber were filled with ambient aerosol at time zero. In this case, particles of a given size can be either neutral or positively or negatively charged, and the overall wall deposition rate for particles includes contributions from each charge state. Furthermore, because charged particles are lost faster than neutrals, the charge distribution (and therefore the overall size-dependent wall deposition rate) is time dependent. Theory predicts that Brownian and turbulent diffusion are the dominant removal mechanisms for particles smaller than 0.05 μm and that gravitational sedimentation dominates for particles larger than 1.0 μm. Electrostatic effects dominate for intermediate-sized particles. Theory is in good agreement with data that have been reported for overall loss rates in large Teflon film smog chambers.