Appendix 4: Formation of ozone

Ozone formation in the lower atmosphere is a highly complex interaction between VOCs and NOx in the presence of sunlight. The ratio of the carbon content of VOCs to NOx of about 8:1 is optimal for ozone production. Whenever the concentration of VOCs is substantially more than 8 times that of NOx, the reaction that creates ozone is limited by the amount of NOx in the air (NOx-limited); similarly, when the concentration of VOCs is substantially less than 8 times that of NOx, the reaction is said to be VOC-limited.

If ozone production is limited by NOx, reducing emissions of VOCs may have little or no effect in reducing the level of ozone. Naturally occurring VOCs from trees and other vegetation causes the formation of ozone to be NOx-limited in many regions. The United States National Research Council suggested that the failure to consider the effect of naturally occurring VOCs was a significant factor in the failure of the US EPA's ozone-reduction strategy (US NRC 1991: 4, 6). This strategy includes I/M programs, which have never been effective in reducing NOx and may actually have increased it in some areas (Coninx 1996a: sec. 2). In Canada, it is not always clear whether the formation of ozone is limited by NOx or VOCs: the formation of ozone tends to be VOC-limited in urban areas but as urban air moves over rural areas, it typically becomes more NOx-limited (CCME 1997: 285-86).

In many ways, limitation on the formation of ozone by the relative concentrations of VOCs and NOx is a moot point in Canada. Ozone levels are already very low all over Canada except in southern Ontario where 50 to 60 percent of the ozone or its precursors (VOCs and NOx) is transported in the air coming from the United States (CCME 1997: 258). I/M programs have been shown to be largely ineffective in reducing emissions of NOx by vehicles so I/M programs are not likely to have any significant effect in reducing NOx-limited ozone. There are alternatives that are clearly more cost-effective than I/M programs in reducing VOCs and these alternatives should be in place in regions where the formation of ozone is VOC-limited before an I/M program is developed.

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