A critical review and analysis of the use of exposure- and flux-based ozone indices for predicting vegetation effects

Early studies of plant response to ozone (O 3) utilized concentration-based metrics, primarily by summarizing the commonly monitored hourly average data sets. Research with the O 3 concentration parameter led to the recognition that both peak concentrations and cumulative effects are important when relating plant response to O 3. The US and Canada currently use O 3 concentration-based (exposure-based) parameters for ambient air quality standards for protecting vegetation; the European countries use exposure-based critical levels to relate O 3 to vegetation response. Because plant response is thought to be more closely related to O 3 absorbed into leaf tissue, recent research has been focused on flux-based O 3 parameters. Even though flux-based indices may appear to be more biologically relevant than concentration-based indices, there are limitations associated with their use. The current set of flux-based indices assumes that the plant has no defense mechanism to detoxify O 3. This is a serious limitation. In this paper, we review the literature on exposure- and flux-based indices for predicting plant response. Both exposure- and flux-based metrics may overestimate plant response. At this time, flux-based models that take into consideration detoxification mechanisms (referred to as effective flux) provide the best approach to relate O 3 to plant response. However, because there is considerable uncertainty in quantifying the various defense mechanisms, effective flux at this time is difficult to quantify. Without adequate effective-flux based models, exposure-based O 3 metrics appear to be the only practical measure for use in relating ambient air quality standards to vegetation response.