Evaluation of a new aerosol model with a Brownian diffusion technique

Abstract

Measurements in Alaska in sub-polar night conditions have indicated that the size distribution of atmospheric aerosols varies significantly and systematically depending upon the type of air mass. Atmospheric aerosol particles are small and numerous in warm Pacific marine air mass systems and large and sparse in cold, Arctic-derived air mass systems. In a previous paper this was hypothesized to be associated with the progressive loss of the smallest particles by attachment to cloud droplets under the driving influence of thermal Brownian motion. A theory involving two parameters, r (mean particle radius), and n 0 (aerosol number concentration) was developed to describe the process. In the previous paper, the relationship r ∞ n 0 ν where ν is the Junge power law exponent (ν ≌ 3) was derived and has recently been confirmed to acceptable accuracy with the use of a simple experiment which employed diffusive separation. The diffusion experiment has also allowed us to estimate that the fraction of time, φ, that the aerosol-laden polar air masses coexist in the presence of cloud is ≌0.01 < φ < 0.1. The submicron aerosol particles in Arctic-derived air masses flowing into central Alaska are deduced to have residence times on the order of ∼10 days.