A synoptic climatology for surface ozone concentrations in Southern Ontario, 1976-1981

Abstract

The synoptic climatology of ozone (O3) for S Ontario has shown that, over the 1976-1981 period, average summer O3 concentrations follow a relationship similar to that reported for event analysis during periods of high O3 concentration. Highest average concentrations, 36 parts per billion (ppb), occur with 'back of the high' situations while lowest average concentrations (20 ppb) occur with 'front of the high' situations. With similar weather events in the winter, the pattern is reversed with highest average O3 concentrations on the 'front of the high' (19 ppb) and lowest average concentrations on the 'back of the high' (13 ppb). Concentration of O3 in the 'front of the high' sector is due in part to the intrusion of O3 in the vicinity of storms from the stratosphere. The seasonal variation of average concentrations in these situations is low, ranging from 14 to 26 ppb. The very low average concentration during the winter and fall for the 'back of the high' situation may be the result of scavenging by NOx from the urban/industrial areas around the Great Lakes. During the spring and summer, solar energy and warm temperatures cause the photochemical production of O3 from NOx and HCs precursors. In the fall and winter, photochemical production of O3 is either very low or absent, and the NOx consume O3 rather than produce it. Thus, average O3 concentrations for winter 'back of the high' situations are one-third of those in the summer months. The synoptic climatology of events during the months from May to September with maximum O3 concentrations in excess of 80 ppb indicates that 78 % of these events occur under synoptic weather classes generally indicative of back or centre of the high situations. © 1986.