An example of the influence of meteorological conditions on ground-level ozone concentrations in southern Ontario

An example of the influence of meteorological conditions on ground-level ozone concentrations is illustrated with a plot of hourly measured ground-level ozone and nitrogen dioxide concentrations for several days in June, 2002 in five southern Ontario locations. In this example of a summer day followed by a cloudy, rainy day, the diurnal variation and inter-dependence of ozone and nitrogen dioxide are clearly illustrated for the urban locations, and similar ozone variations are seen for all five locations used in this case study.

This case is typical for hot summer days in southern Ontario, with daytime mixing of the atmospheric boundary layer increasing during and following the breakup of a low-level nocturnal inversion several hours after sunrise, and increasing ozone concentrations due to generation from photolytic reactions as well as from ozone-rich air above the boundary layer mixing down to ground level as the inversion breaks up and mixing increases. Ozone concentrations are seen to diminish during the night due to loss of generation and due to titration by nitric oxide within the stagnant layer as the nocturnal inversion strengthens and lowers, accompanied by a corresponding increase in nitrogen dioxide concentrations during the night.