30 May 1999

Ozone Linked To Warmer Weekends In Toronto

Better weekend weather is regularly being experienced in Toronto, and it may not be just good fortune. Rather, a new University of Toronto study suggests higher amounts of ground-level ozone on weekends compared to weekdays are causing the warmer weather.

Professor William Gough and graduate student Gary Beaney of environmental science at the University of Toronto have found that, contrary to expectation, the lack of a Saturday morning rush hour in the Greater Toronto Area increases the amount of harmful ozone in the atmosphere. Ground-level ozone forms when air pollutants such as car exhaust mix with sunlight, but on weekday mornings when there is little sun the rush hour pollutants actually destroy much of the previous day's ozone.

While this phenomenon has been documented in other major cities, Gough and Beaney also found that when ozone levels are very high on weekends the temperatures are one degree Celsius higher than during the week. "This strong correlation between ozone levels and temperature challenges the assumption that ozone is a minor contributor to greenhouse warming compared to carbon dioxide," says Gough. "It may have implications for climate change assessment and strategies to reduce urban smog - for example reducing emissions in the morning rush hour may not be as important as reducing emissions later in the day."

Beaney analysed data over approximately 30 years from Environment Canada and the Ontario Ministry of the Environment.