|Air Quality Standards for Ozone
Ozone is an air pollutant which can lead to plant tissue injury
and reductions in growth and productivity. Plants respond to ozone
by compensatory or defensive reactions such as avoidance by stomatal
closure, detoxification of ozone by chemical reaction, adjustment
by alteration of metabolic pathways or repair of injured tissue.
The response is dependent on intensity of the ozone exposure, environmental
factors influencing both plant photosynthesis and ozone deposition
to plant surfaces, and defensive mechanisms inherent within plants.
Hence the exposure of the plant to ozone is critical in the plant
response to and damage from ozone.
The current air quality standards for ozone are based on the measured
eight hour average ambient concentration. However, some biologists
have recommended a cumulative ozone exposure parameter where the
ozone concentrations over a set period of time are used to protect
vegetation. Musselman and Massman (1999) describe a third alternative
which uses quantifiable flux-based numerical parameters as a replacement
for measured ambient ozone concentration.
This alternative contains parameters that are much more closely
related to plant response than are the current ambient ozone concentrations.
Massman et al. (2000) develop the dose-based standard further by
using physical reasoning based on plant defenses and general resistance
concepts of dry deposition. The model is developed in terms of plant
injury, but could be extended to include damage, which historically
has been the focus of air quality standards. With this dose-based
approach, Massman et al. (2000) clarify some issues concerning the
plant response to ozone and the use of fluxes to determine an ozone
standard. They demonstrate that using a dose-based standard is likely
to prove more useful in protecting vegetation from ozone, to be
more precise and more discriminating than a cumulative exposure-based
standard. In addition, they explored the interactions of the daily
cycles of ozone concentration and plant stomatal conductance. In
a related paper, Musselman and Minnick (2000) focused on the importance
of night time conductance to ozone air quality standards. These
research findings should help improve the standards associated with
ozone for vegetation.
Massman, William J.; Musselman, Robert C.; Lefohn, Allen S. 2000.
A conceptual ozone dose-response model to develop a standard to
protect vegetation. Atmospheric Environment. 34: 745-759.View
Musselman, Robert C.; Massman, William J. 1999. Ozone flux to vegetation
and its relationship to plant response and ambient air quality standards.
Atmospheric Environment. 33: 65-73.View
Musselman, Robert C.; Minnick, Tamera J. 2000. Nocturnal stomatal
conductance and ambient air quality standards for ozone. Atmospheric
Environment. 34: 719-733.View