PHOTOCHEMICAL OXIDANT CONCENTRATIONS IN 2 CANADIAN PRAIRIE CITIES - MODEL EVALUATION

It has been previously noted that the cities of Edmonton and Calgary, Alberta (Canada) often act as a sink for ozone, due to removal through reaction with transportation-derived NO, Angle and Sandhu (1989, Atmospheric Environment 23, 215-221). However, an extensive data set of O3 and PAN measurements for these two cities reveals that occasional oxidant episodes occur. To assess the extent to which these oxidant episodes may be the result of local photochemical oxidant production, a box-model was applied to investigation of oxidant formation during the summer months in Edmonton and Calgary. Using model inputs derived from meteorological data, the measured NO(x) concentrations, and NMHC/NO(x) ratios that were consistent with observations, computed concentration-time profiles of ozone and PAN for these cities were generated that were similar to the observed profiles. The model used, containing explicit chemistry for selected representative alkanes, alkenes, and aromatics, indicated that oxidant profiles observed during stagnant conditions were consistent with local photochemical production. The potential for downwind formation of ozone and PAN in an urban plume air parcel was also evaluated with the model. It was found that the ozone concentration downwind from the urban source can be significantly elevated and that the presence of an isoprene flux in rural areas can cause significant elevation of PAN concentrations relative to those produced in the city. It is concluded that the combination of a stagnant air mass with urban NMHC and NO(x) inputs and the solar radiation flux at this latitude (> 51-degrees-N) can lead to substantial oxidant production near the urban source and elevated concentrations in the rural downwind air mass.