Ground-Level Ozone and Regional Transport of Air Pollutants

Abstract

Photochemical air pollution was studied in the lower Great Lakes region of southern Ontario, Canada, using O3 data for 1973–76 inclusive. High O3 concentrations in the boundary layer are correlated primarily with the several meteorological variables in warm seasons: pressure, temperature, radiation, wind speed and direction. Fluctuations of O3 are highly dependent on the synoptic-, regional- and small-scale air flows at low levels. In general, O3 concentrations were observed to increase when the ridge of a surface anticyclone was passing over the region. Relatively high values (>80 ppb) were observed on the rear sides of high pressure centers and in the warm sectors of cyclones (well ahead of cold fronts). In turn, high concentrations of surface O3 usually occurred during periods of high temperature with southerly and/or southwesterly airflows. After the passage of a cold front, when strong north-northwest flows developed in the low levels (e.g., at the forward side of a “new” anticyclone with general subsidence), O3 concentrations decreased considerably. These observations were also based on results obtained from using a regional-scale trajectory model. Also, temperature changes due to cold air advection, local rainshowers and thunderstorm activity, as well as the direct import of NOx, were associated with decreases in O3. This study also suggests that lake effects influence concentrations of photochemical air pollution. Frequently, high O3 values were associated with lake breezes on the rear sides of anticyclones. In addition, it was found that nocturnal O3 maxima were present in the diurnal cycles for urban stations in Toronto and Montreal only. For high O3 situations the daily maxima occurred about 30 km downwind of a large urban area, Toronto (population: 2.5 millions). On the average, this city appeared to produce ground-level O3 downstream in the range 40–60 ppb. Air pollutants crossing the Canada-United States international boundary from the northeastern Midwest were also found to be important factors for high O3 situations. In these cases, O3 in the order of ∼100 ppb was readily contained in air parcels arriving in southern Ontario. The results of this study do not agree with earlier theories which assumed that the peaks in surface O3 are due to transport downward from the stratosphere. In many cases, high surface O3 concentrations were mainly the result of photochemically active precursors (of anthropogenic origin) which were transported over medium (10–100 km) and long distances (∼1000 km). The time for the photochemical production of surface O3 required a few hours under favorable meteorological conditions.s