The role of Toronto urban emissions in regional ozone episodes

To study the impact of the Greater Toronto urban emissions on O3 levels in southern Ontario, the ambient ozone measurements made in Ontario during the time period of 1979–1988 were analysed. Statistics indicate an O3 depression associated with the Greater Toronto urban plume under the conditions of regional O3 episodes. An analysis of the 03 data at Dorset and Stouffville, two rural monitoring sites on the NE to NNE side of Toronto, with screening based on wind measurements, shows a possible negative impact of the Greater Toronto urban plume on the O3 levels at 40 km downwind under regional episodic conditions. On average, the impact led to an O3 depression of 22–27 ppbv within the Greater Toronto urban plume in comparison with the background air. A photochemical transport model was used further to investigate the impact of the Greater Torontoʹs anthropogenic emissions on O3 levels downwind. The model includes a photochemical module, a vertical transport module and a horizontal mixing algorithm. Two sets of initial conditions were derived by running the model in the Eulerian mode, and by adjusting emissions to fit the ambient measurements of O3, NOx and NMHCs under regional episodic conditions. The adjusted anthropogenic emission rates for the Greater Toronto urban area were 72.4 and 83.3 % of their original 1985 inventory values for NOx and NMHCs, respectively. The adjustment may reflect the uncertainties in the emissions inventory. Diurnal variations of the species at virtual receptors located at different downwind distances from Toronto were calculated by running the model following 25 plume puffs consecutively released at 60-minute intervals. The calculated O3 depression at 40 km downwind is in good agreement with the historical ambient data. Calculated spatial distributions of the daily maximum O3 levels indicate that, under the regional episodic conditions, there is an 03 depression of about 20 ppbv extending from the Greater Toronto urban core to about 100 km downwind. Beyond 170 km, the daily maximum O3 levels in the urban plume exceed their counterparts in the background air by only a few ppbv. The small O3 enhancement at a medium range downwind of Toronto under the regional episode conditions is due to the lower NMHCs/NOx emission ratio in the Greater Toronto area.