Peak Levels of Nitrogen Dioxide

Map of monitoring stations

Maximum levels of a contaminant are important because of potential health or environmental effects of high concentrations. Scientists often find the "99th percentile" concentration more useful in tracking trends than the maximum reading. The annual 99th percentile concentration is the concentration that is higher than 99% of the concentrations recorded throughout the year. In other words, only 1% of the ambient concentrations observed during the year fall above the 99th percentile concentration.

The 99th percentile nitrogen dioxide concentrations have decreased (by 2 to 48%) or remained relatively consistent since 1990 at most Alberta stations. Significant decreasing trends were found at Edmonton Central (22% decrease), Calgary Central (24%), Calgary Northwest (16%), Tomahawk (15%), Carrot Creek (16%), Caroline (2%) and Fort Saskatchewan (12%). Increasing trends in 99th percentile nitrogen dioxide concentrations were found in the oilsands area north of Fort McMurray (22 to 37%), as well as at the Beaverlodge (10%) and Range Road 220 (54%) stations. In 2007, the highest 99th percentile nitrogen dioxide concentrations were found in Calgary and Edmonton.

Calgary Central exceeded the one-hour nitrogen dioxide objective once in 1991, and the 24-hour objective in the same year (data not shown). The one-hour average objective is 212 parts per billion, while the 24-hour objective is 106 parts per billion. There have been no exceedances of the one-hour or 24-hour objectives in Alberta since 1991.