Impact of Toronto urban emissions on ozone levels downwind

During the 1992 Southern Ontario Oxidants Study (SONTOS 92), an impingement of the Greater Toronto urban plume was observed in the late afternoon of 6 August at the Hastings monitoring site, 140 km to the northeast of the Toronto urban core. Associated with the start of the impingement, sharp increases in the concentration of O₃, and other species were observed. A 1D photochemical transport model was used to investigate this event further. The 1D model was first exercised in an Eulerian mode to generate two initial chemical systems, one for the air parcel associated with the Greater Toronto urban core and the other characterizing the background. Lagrangian calculations with the 1D model were then conducted following both the urban plume and the background air parcels. When consecutive plume puffs passed over preset virtual receptors, the time of passage and species concentrations, both within the plume and in the background air, were registered. The calculated and measured changes in the concentrations of O₃, PAN, NO(x) and hydrocarbon compounds due to the urban plume impingement are in reasonable agreement. Further sensitivity studies were conducted and the factors affecting the impact of the Greater Toronto urban plume are discussed.