Studies of the growth and size distribution of aerorosol particles

Abstract

A study is made of the theoretical growth of soluble aerosol particles with increasing humidity. The growth equation is applied to sodium chloride and ammonium sulphate particles, chosen because of their special significance as atmospheric nuclei. The effect of particle growth on visibility is also considered. An experimental investigation of the effect of humidity on particle size is carried out using artificially generated aerosols of sodium chloride and ammonium sulphate. Inaddition a study is made of the variation in the particle size distribution of the atmospheric aerosol measured continuously over a 49 day period at Durham Observatory. A theoretical growth relation, based on Mason's equation (1971) relating the size of a solution droplet to the relative humidity of its environment, is derived. Novel approximations are incorporated in the application of the growth equation to particles of sodium chloride and ammonium sulphate and the results for sodium chloride agree with Mason’s equation to within 0.5% over the dry particle mass range from 3 x l0(^-17) g to 3 x 10(^-11) g. The predicted growth of ammonium sulphate particles with relative humidity indicates agreement with previous work, by Garland (1969). Experimental measurements were made of the growth of sodium chloride ammonium sulphate nuclei with humidity. The particles with initial radius between 2 x 10(^-6) and 3 x 10(^-6) were generated by a collison atomizer and allowed to grow in a storage vessel as the humidity was increased from about 60% to 92% The experimental measurements agree well with the theoretical growth curves, chosen to correspond as closely as possible to the experimental particle size at the lowest humidity. Average percentage increases in the particle radius of 60% and 95% for ammonium sulphate and sodium chloride particles of initial average radii of 3.0 x 10(^-6)cm and 2.6 x 10(^-6)cm respectively were obtained as the humidity was increased from about 60% to 92%. Approximations to Mie's theory of light scattering were used to calculate the extinction coefficient, σ, for monodisperse aerosols of sodium chloride and ammonium sulphate as a function of solute mass per particle in the range between 3 x 10(^-17)g and 3 x 10(^-11)g at humidities of 80%, 95% and 99%. The results indicate that the sodium chloride particles are more efficient in reducing visibility than ammonium sulphate particles by factors varying from about 1.3 to 5.5 over the whole range of humidity and dry particle mass. Calculations show that, for both salts, a Junge particle size distribution is about twice as effective at reducing visibility as a monodisperse aerosol of the same salt with particle radius equal to the mass median of the Junge distribution, A study of the variation in the particle size distribution of the atmospheric aerosol in the radius range 0.25 - 5.0 micrometres, measured at Durham Observatory over a period of 49 days from 21 July to 8 September 1975, indicates that the distribution closely follows the shape of a Junge log-radius distribution with a slope, β, equal to 3.04, The average diurnal variation over the period shows maximum and minimum number concentration occurring between 0200 and 0800 hours B.S.T. and 1400 and 2000 hours B,S.T. respectively. The general increase in the particle number concentration, particularly in the lower size ranges, during the period 0000 to 0600 can be partly attributed to an increase in humidity. This is reflected in the positive correlation coefficient between particle number concentration and relative humidity.