The sensitivity of computed ozone concentrations to U.V. radiation in the Los Angeles area

Abstract

A three-dimensional photochemical diffusion model was used to investigate the dependence of calculated ozone concentrations throughout the Los Angeles Basin on incident u.v. radiation. The u.v. radiation was varied to simulate a seasonal change from July to November and a reduction due to stratus and cirrus cloudiness. The model results suggested that variations of u.v. radiation as small as 10% can have a noticeable effect on ambient ozone concentrations. The seasonal effect of decreasing solar elevations caused significantly lower ozone values; the greatest changes occurred in the western part of the Basin. The presence of cirrus clouds, in comparison to cloud free conditions, generally resulted in about 10% lower ozone levels. Overcast stratus clouds persisting all day yielded substantially reduced ozone concentrations. However, when the stratus dissipated by mid-morning their effect was minimal over the eastern sections of the model grid. Finally, the model simulations showed that calculated ozone levels are sensitive to the manner in which u.v. radiation is coupled to the NO 2 photodissociation rate constant.