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Instruments employed for the continuous evaluation of air-borne particulate contaminants in the atmosphere by filtration through paper or other fibrous media are of two main classes. In one class a large measured volume of air is passed through a fibrous filter and the aerosols deposited over a definite time period, usually 24 hr, are collected in sufficient quantity for the determination of mass concentration and for subsequent chemical or spectrographic analysis. In the second class, the air sample is filtered through a paper tape over a time cycle of 1 hr or longer. The aerosols are deposited on the paper in a series of circular spots of constant area, and the degree of darkening or soiling of the surface is evaluated by light transmission or reflectance.
A review is presented of recent developments in this field, including a discussion of various theories which relate the absorbance of the deposit to the mass equivalent per unit volume of air. Attempts have been made to relate the absorbance to mass concentration, carbon content of the deposit, or to number and size distribution of the collected particles.
Consultant, Atmospheric Pollution Services, Ottawa,
Department of National Health and Welfare, Ottawa,