The growth of atmospheric aerosol particles as a function of the relative humidity—II. An improved concept of mixed nuclei

Abstract

This paper deals with processes influencing the growth of continental atmospheric aerosol particles ( r > 0·1 μm) with relative humidity. The results presented are valid for average conditions, individual cases may of course yield deviations. The main results are: 1. (1) Atmospheric aerosol particles each consist of a mixture of different substances. 2. (2) Atmospheric aerosol particles show continuous smooth growth, without suddenly increased or reduced absorption of water, unlike pure salt particles whose growth is characterized by a sharp increase at the critical humidity. This feature is controlled by three properties of the atmospheric aerosol particles: The steady and smooth growth character is caused by 2.1. (a) gradual deliquescence of soluble matter with increasing humidity, the reverse being observed for decreasing humidity. Reduced absorption of water is caused by 2.2. (b) insoluble matter which amounts to about 40 per cent of the aerosol mass and 2.3. (c) increase of the solubility of the various ions due to their mutual interaction in the special salt mixture forming the water soluble fraction of the aerosol particles. 2.4. (3) Variations in content of insoluble matter and in chemical composition contribute almost equally to the effect on the growth curve. 2.5. (4) Growth curves can be described by the following equation r/r o=(1+ C ∗F(ƒ)/(1−ƒ)) 1 3 r being the radius at the relative vapour pressure ƒ, r o the particle radius at ƒ = 0, F the dissolving characteristic describing the gradual deliquescence of the water soluble matter with relative humidity, and C = sav, a constant where s is the fraction of water soluble matter within the sample, v the ratio of the density of the aerosol particles to the density of water, and a a dimensionless constant ( a ≈ 0·14 on the average) which is a measure of the solubility of the soluble matter.