OZONE

• Source of ground level ozone
• Effect of ozone on plants
• Ozone exposure and absorption by crops
• Measuring plant response to ozone
• Examples of crop response to ozone
• Environmental interactions

OZONE

Ozone is a bluish gas, with a sharp, irritating odor. It occurs naturally in the upper atmosphere ("stratosphere"), where it forms continually from reactions promoted by the sun's radiation. Unlike the more common gas oxygen (O₂), O₃ is highly unstable, reacting with other molecules in the atmosphere, so that its lifetime is only hours or days. The O₃ in the upper atmosphere serves a useful function by filtering out harmful UV radiation. However, pollutants entering the upper atmosphere deplete the O₃, thereby increasing the intensity of UV radiation at the earth's surface.

Ozone also occurs naturally near ground level, where it occurs at concentrations of 25­40 parts per billion by volume (ppbv). Along with other pollutants (e.g., nitrogen oxides, peroxides, peroxyacetyl nitrate, and particulate matter), ground-level O₃ forms smog. The ill effects of smog on human health are reasonably well known, but its effect on plants has received little publicity. Yet, according to some estimates, O₃ causes tens of millions of dollars worth of damage to crops in Canada annually, mainly in the Fraser Valley of British Columbia, the Quebec­Windsor corridor, and the southern Atlantic region.

Thus O₃ is unique among atmospheric gases: in the upper layer, it is highly beneficial; near ground level, it is a serious pollutant. Ironically, human activity has depleted O₃ in the upper atmosphere but increased its concentration at ground level. In this section, we describe the problem of ground-level O₃; the problems arising from depleting O₃ in the upper atmosphere we discuss later.