CHARACTERISTICS OF SUMMER MIDDAY LOW-VISIBILITY EVENTS IN THE LOS ANGELES AREA.

The reduction of visibility is one of the most easily perceived features of a polluted atmosphere. Suspended particles and gaseous pollutants can scatter and absorb light, causing a decrease in visual range, lower contrast between objects and their backgrounds, and apparent discoloration of objects in the field of view. The summer 1984 experiments are used to describe the distribution and the nature of summer low-visibility events in the Los Angeles area. From the size distribution and the measured chemical composition of the aerosol, a volume averaged value for the index of refraction for the fine and coarse aerosol is calculated. Then the aerosol size distribution and refractive index are used to compute the extinction coefficient during each sampling event. Theoretically predicted extinction coefficient values are compared to measured values in order to validate the calculation and to account for the distribution of visibilities during the summer. Then the chemical nature of the aerosol is examined to deduce the relative importance of various pollutant emission sources that contribute to the Los Angeles visibility problem.