On the relationship between sulfate air quality and visibility with examples in Los Angeles

Abstract

Routine air monitoring data are related to visibility in downtown Los Angeles over the decade 1965–1974. A non-linear regression model for light extinction is employed which uses particle chemical composition as a key to particle size and solubility. Sulfates and oxides of nitrogen (NO_2 and nitrates) plus particulates correlated with NO_2 are implicated as the major contributors to visibility reduction in downtown Los Angeles. 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 by a substantial reduction in the number of days per year with average visibility less than 3 miles (4.8 km).

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