THE SIZE DISTRIBUTION AND AGING OF NATURAL AEROSOLS AS DETERMINED FROM ELECTRICAL AND OPTICAL DATA ON THE ATMOSPHERE

Abstract

A first attempt at surveying the complete size distribution of natural aerosol particles is made. The size range covers more than three orders of magnitude in radius, with approximate limits of 2 ? 10?3 and 5 ? 10?7 cm. Previously only parts of this spectrum had been carefully investigated, due to the size limits of the various experimental methods used. Particle spectra down to radii below 10&-4 cm are well known, having been obtained by direct count under the microscope. Particles of radius less than 10?5 cm are counted in the Aitken counter, but their size distribution can only be determined from mobility measurements on those particles which are charged. However, to deduce nuclei spectra from ion spectra, the fraction of charged particles must be known. Previous determinations of this fraction are examined and are shown to contain inaccuracies or errors. The expression derived here theoretically, is used to convert some of the ion-spectra data into nuclei spectra. The particle-size range between radii of 10?5 and 10?4 cm has hardly been investigated by direct measurements, because of the experimental difficulties encountered. Valuable information can be gained here from the dependence of haze scattering on wavelength. The available data seem to indicate that the maximum number concentration is located in the size region between 10?5 and 10?6 cm radius, and that the number concentration drops to zero between 10?6cm and 10?7 cm radius. The size range greater than 10&-5 cm can be well represented by a power law which seems to hold for large parts of the world. It can be shown further, where ion and nuclei counts or other electrical data are available from a large number of places all over the world, that the average radius of all particles increases when the total number decreases. This can be explained by coagulation processes. It should be emphasized that the results obtained in this article are to be considered as a rough first approach, valid only for average conditions in time and space, and that more detailed information is needed to improve our knowledge of this field.