GROUND-LEVEL OZONE IN EASTERN CANADA - SEASONAL-VARIATIONS, TRENDS, AND OCCURRENCES OF HIGH-CONCENTRATIONS

Over the past few years, concern has increased in Canada over the heal th and environmental impacts of elevated concentrations of ground-leve l ozone. During the summer the most populated regions of Canada freque ntly record ozone concentrations that exceed the one-hour average maxi mum acceptable air quality objective of 82 parts per billion (ppb). In 1988 the Canadian Council of Ministers of the Environment agreed to d evelop a federal/provincial management plan to control nitrogen oxide and volatile organic compound emissions to reduce ozone concentrations in all affected regions of the country. In addition to the proposed i nterim control measures, the plan recommended that studies be undertak en to acquire the information necessary to develop sound control strat egies. This report represents one of those studies and provides a summ ary of ground-level ozone measurements for eastern Canada for the 1980 to 1991 period with an emphasis on seasonal variations, trends, and o ccurrences of high concentrations.Southwestern Ontario experiences the highest maximum hourly ozone concentrations and the greatest frequenc y of hours greater than the 82 ppb acceptable objective. Urban sites h ave the highest frequencies of ozone concentration measurements in the < 10 ppb range, while rural and remote sites show peaks in frequency distribution in the 20 to 30 ppb range. Trend analysis of summertime ( May to September) average daily maximum ozone concentration showed no consistent pattern for eastern Canadian sites during 1980 to 1991. Sit es in Montreal showed statistically insignificant downward trends whil e sites in Toronto showed small but statistically significant upward t rends. These ozone-increasing trends are associated with reductions in nitric oxide concentrations. At all sites there was large year-to-yea r variability in peak ozone levels and in the frequency of hours with ozone concentrations above the maximum acceptable objective.