Ground level ozone monitored by NAPS should not be confused with stratospheric ozone. Ground level ozone is the major component of photochemical smog, and is not emitted into the atmosphere. High ozone concentrations tend to occur under conditions of bright sunlight, high temperature and a stationary air mass. The highest smog concentrations are normally found in the Windsor-Quebec City corridor, the Saint John area of the Southern Atlantic region and the Lower Fraser Valley in British Columbia. Ground level ozone concentrations are strongly influenced by meteorology and thus are highly variable from year to year.
Sulphur dioxide is a colourless gas with a strong odour.
Oil and gas processing, ore smelting and the burning of coal and heavy oil are the major generators of sulphur dioxide. From 1974 to 1992, the annual mean sulphur dioxide concentration decreased 61 per cent. As a result, sulphur dioxide levels rarely exceed maximum acceptable levels. Carbon monoxide is a toxic, colourless and odourless gas generated from burning material containing carbon. Most carbon monoxide is created by motor vehicles, heating of dwellings and industrial pollution.