Multifilter technique for examination of the size distribution of the natural aerosol in the submicrometer size range.
In recent years, greater attention has been given to the question of “typical” size distributions of natural aerosols, and demand has arisen for practical methods for obtaining size distributions in natural and artificial aerosols. Direct sampling and observation of particles individually, whether by optical or electron microscopy, is a time-consuming process, and, for submicrometer particles observable only by electron microscopy, possible evaporation of collected particles before and during observations poses a question which as yet is not fully resolved. Uncertainty about size distribution exists particularly in the submicrometer region-an inspection of the rather sparse literature suggests that even the position of the maximum of the size distribution of number concentration (i.e., the modal radius) is uncertain. It has been estimated to be as high as 0.1 pm cm) by some workers ( I ) , while others (2) have published data which indicate a modal radius closer to 0.001 pm cm), results differing in radius by 2 orders of magnitude (6 orders in mass). Even though the contrasting results just cited were obtained in different geographic regions, one can hardly conclude that there is general agreement concerning even the gross features and general shape of the particle size distribution in natural atmospheric aerosols, especially in the size range below 0.1 pm. The concept of employing multiple filters to obtain optical spectra is a very old one, and naturally analogous concepts have been introduced for obtaining size distributions in aerosols. A heterogeneous aerosol passing through any tube, hole, or array of such elements loses particles by diffusive and inertial effects, and, since the fraction lost depends on particle size, the process is analogous to optical filtering. The emerging particle concentration is given by