Nonanthropogenic Ozone in California Urban Areas

Abstract

Ozone concentrations of a few parts per hundred million (pphm) sometimes occur in urban areas, unaccompanied by significant concentrations of other pollutants. The Santa Ana winds provide an important example of such ozone episodes, for they invariably contain 3 to 6 pphm of ozone with no diurnal variation. The origin of these winds in a large-scale weather system, along with the absence of other pollutants and the lack of a diurnal cycle, indicates that this ozone is of natural origin. Occasionally the sea breeze develops a sharp front as it moves inland from the Pacific Ocean. In such episodes, the air which the marine sea breeze replaces is free of pollution except for several parts per hundred million of ozone. This is also regarded as not man-made because of the absence of other pollutants. Elevated ground-level concentrations of ozone have also been occasionally reported from other areas in association with storms. Downward transport from the ozonosphere seems the most probable source for these ozone episodes.