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The relation between absorbance values of filtered deposits and mass concentration or carbon content of particulate matter in the atmosphere of Windsor, Ontario, was studied under various meteorological conditions. A method of measuring hourly variations in concentration for small samples that cannot be assessed gravimetrically was developed. The relationship between weight concentration and absorbance was analyzed by regression methods; the data satisfied the equation W = aDb, where W is weight of deposit, D is absorbance of the stain X 100, and a and b are constants. In 24-hour runs, each individual run fitted this equation well, but the values of the constants varied considerably from day to day. Hourly fluctuations in absorbance of stain samples from conventional instruments may be related approximately to the mass of particulates by a power law if meteorological conditions do not vary greatly during a given 24-hour period and a simultaneous determination is made of the mean mass concentration for this period. Alternatively, a plot may be made of the log mass of particulates collected\ area of deposit /

The time constant of a Thomas sulfur dioxide analyzer was determined by measurements at various concentration levels. The arithmetic mean time constant was found to be 46.0 seconds with a standard deviation of 5.5 seconds. A study was made of the peak values observed when known concentrations of sulfur dioxide were injected into the sample air stream for periods of a few seconds to 200 seconds. The area

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.