Routine air monitoring data taken by the Los Angeles Air Pollution Control District are related to visibility at downtown Los Angeles over the decade 1965 through 1974. The relationship between light extinction and total suspended particulate mass implied by the historical data base is shown to be consistent with the findings of previous short-term studies.
A non-linear regression model for light extinction at Los Angeles is constructed which combines available information on aerosol chemical composition with relative humidity and NO_2 data. It is shown that there is a pronounced increase in light scattering per unit sulfate solute mass on days of high relative humidity, as would be expected for a hygroscopic or deliquescent substance.
Using the chemically resolved regression model, estimates are made of the long-run visibility impact of reducing sulfates to one half and to one quarter of their measured historic values on each past day of record. It is found that the effect of such a sulfate concentration reduction would have been manifested most clearly in a decline in the number of days per year with average visibility less than three miles. The number of days per year with average visibility less than ten miles would be little affected. One reason for the disproportionate impact of sulfates on the days of the worst visibility is found in the high positive correlation between sulfate mass concentration and relative humidity. High values of light scattering per unit sulfate mass thus occur on days of high sulfate mass concentration
