This paper deals with methods for determining the distribution of sizes and mass fractions of airborne particulate matter (aerosols) in the diameter range 3 μ ≥ d ≥ 0.03 μ, which prevails in the lower atmosphere as hazes and smogs. The analytical methods refer to size-classified deposits produced by the exposure of a continuous, laminar air flow to a high centrifugal field in the aerosol spectrometer. The particle size-frequency function is determined by progressive-integral counting along the deposit with the use of special instruments described in some detail. The distribution function of particle mass in the size range results from dividing the foil bearing the deposits into sections according to a defined geometrical pattern. The analytical data (for example, of photometric extinction) obtained from a sequence of such deposit sections are then correlated and result in the distribution of the mass for specific components that the analytical method indicates. Comparison with the size-frequency distribution, obtained independently by the counting method, yields information about the composition of the individual particle—for example, the relationship between the nucleating mass and the accumulated condensate thereon, such as radioactive or photoactivated components.
At the same time, the stability of such deposits—particularly with regard to particle size—is obtained by microphotometric determination of the light-scattering capacity and particle counts before and after exposing the deposit to increased temperature or intense irradiation. This method has proven useful for determining the limited life of various atmospheric aerosol components, particularly of those resulting from photochemica interaction between traces of oxides of nitrogen dioxide, and certain hydrocarbons.