Abstract
Critical levels to determine plant response to ozone (O3) have been used in Europe since the 1980s, utilizing the concentration-based AOT40 to relate plant response to ambient O3 exposure. More recently, there has been progress in Europe toward utilizing flux-based critical levels, because plant response is more closely related to O3 uptake than to the amount of O3 in ambient air. Flux-based critical levels are plant species specific; data for parameterization of flux-based critical levels models are lacking for most plant species. Although flux-based critical levels are now being used for a limited number of agricultural crops and tree species where data are available, the use of flux-based critical levels is limited by the lack of adequate consideration and incorporation of plant internal detoxification mechanisms in flux modeling. Critical levels have not been used in North America; however, recent interest in the U.S. and Canada for using critical loads for nitrogen and sulfur has generated interest in using critical levels for O3. A major obstacle for utilization of critical levels in North America is that ambient air quality standards for O3 in the U.S. and Canada are concentration based. It appears that cumulative exposure-based metrics, particularly when implemented with a quantification of peak concentrations and environmental variables, such as a drought index, are currently the most useful to relate O3 to vegetation response. Because data are unavailable to quantify detoxification potential of vegetation, effective flux models are not available to determine plant response to O3.
Highlights
Critical levels to determine plant response to ozone (O3) have been used in Europe since the 1980s, utilizing the concentration-based AOT40 to relate plant response to ambient O3 exposure
The EPA understands the usefulness of flux-based metrics for relating ambient O3 to plant response, it believes that there is insufficient scientific information available at this time to apply a fluxbased metric in the U.S Data are lacking for plant species in North America for parameterization of flux models
The current U.S National Ambient Air Quality Standards (NAAQS) concentration metrics are not biologically based in terms of parameters related to vegetation effects, it has been reported that the 8-h average concentration-based standards do perform well compared to the exposure-based metrics SUM00 and AOT40 for yield of aspen and birch[24]
Summary
Critical levels to determine plant response to ozone (O3) have been used in Europe since the 1980s, utilizing the concentration-based AOT40 to relate plant response to ambient O3 exposure. There has been progress in Europe toward utilizing flux-based critical levels, because plant response is more closely related to O3 uptake than to the amount of O3 in ambient air.
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