Temporal surface ozone patterns in urban Manitoba, Canada

This study examined temporal surface ozone patterns for two urban centres in Manitoba, Canada by analyzing hourly concentrations at the Winnipeg downtown (1995-1999), Winnipeg residential (1995-1999) and Brandon industrial (1998-1999) monitoring sites. The characteristic annual ozone cycle and diurnal cycles for June and December were attributable to: (1) the annual and diurnal solar radiation cycles, (2) temporal variations in the emissions of precursor chemical compounds, in particular the source strength of nitrogen oxides, (3) temporal variations in the height of the mixed layer, which determine the degree of dilution of these emissions by atmospheric dispersion, and (4) an in situ volatile organic compound sensitive photochemical regime, which resulted in decreased concentrations of ozone in response to increased concentrations of nitrogen oxides. Only one exceedance of the maximum acceptable level of 82 ppb was recorded in the study period; it occurred at the Brandon monitoring site on June 6, 1999. The sequence of weather and the hourly concentrations of ozone and nitrogen oxides indicated that: (1) ozone fumigation, with the transition from the nocturnal boundary layer to the daytime mixed layer, may have supplemented photochemical ozone formation during the morning hours, and (2) during the evening hours, the post cold-frontal downward flux of ozone rich air, which was in the region due to atmospheric transport, stratosphere-troposphere-exchange or, possibly, due to the multiplicity of thunderstorms in the area in late afternoon - early evening, may have been the main cause of this rare exceedance event.