Observation of O3 and Precursors Levels at Two Sites Around Toronto-Ontario, During SONTOS 92

Characteristic air quality features observed at two ground level air quality monitoring stations operated during the Southern Ontario Oxidant Study field campaign (SONTOS 92) are described. The two sites, Binbrook and Hastings, are located 60 km SSW and 140 km NE of Metropolitan Toronto, respectively. The analysis of 5-min average concentrations in relation with local wind directions-suggests that a strong contrast exists between conditions of northerly clean air and polluted southwesterly flows at both locations. This contrast is shown to deeply influence the shape and amplitude of the diurnal cycles of O3, NO and NOz. The analysis indicates that the downward transport of either clean or polluted background air during the first part of the day, as the boundary layer develops, is responsible for the alteration of the diurnal patterns with the prevailing wind direction between 08:00 and 10:00 (EDT). At Hastings, NMHC/NOx ratios are found to be relatively elevated ( > 8 ppbC ppb−1) and highly variable for both the clean northerly and the southwesterly polluted regimes. At this site and for the polluted southwesterly flow, an average of 11.2 molecules of O3 appear to be formed for each molecule of (NOx) oxidized. Estimated values for “backgrounde” ozone levels associated with the different sectors, as well as for the contribution of major urban centres in the region are discussed.