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 /