Ozone exposure and absorption by crops

Air pollution monitoring sites across Canada routinely measure ground-level O₃ concentrations. But concentrations alone are insufficient to evaluate potential damage to plants. Plants are less sensitive at night and during periods of slower growth. Temperature and moisture conditions also affect sensitivity. Consequently, we must measure actual O₃ absorption to assess effects on plants.

One way of estimating O₃ absorption is to measure the instantaneous O₃ concentration in downward- and upward-moving air, using sensors mounted on towers. If the concentration is greater in air moving down than in air moving up, that indicates O₃ absorption: the greater the difference, the higher is the absorption rate. This approach allows almost continuous measurement of O₃ flux and provides daily and seasonal patterns of absorption. In one study, for example, the O₃ flux above a soybean field increased during the day but then dropped sharply when the stomata began closing (Fig. 26). Because the opening and closing of stomata is controlled by water stress, there is a strong relationship between O₃ absorption and transpiration (the amount of water lost from the plants).

For larger-scale measurements, instruments can be mounted on aircraft, as described for CO₂, N₂O, and CH₄ measurements. Aerial O₃ surveys have already been made for many crops, weather conditions, and O₃ concentrations. One observation from this approach is the strong relationship between O₃ absorption and the amount of green vegetation.

The scale can be increased still further by using satellites. Scientists can calculate transpiration from environmental conditions and can obtain a "greenness" index from satellite images. Because of its close relationship to transpiration, O₃ absorption can then be estimated for the entire growing season on large areas, using O₃ concentrations from measurement networks (e.g., Fig. 27).

Volatile organic compounds and agriculture

Volatile organic compounds (VOCs) include natural and artificial chemical compounds that contain carbon as a main constituent. Volatile organic compounds and nitrogen oxides combine in the presence of sunlight to form ozone at ground level. In rural areas, the VOCs are largely contributed by vegetation. Crops that emit VOCs include tomatoes, potatoes, soybeans, wheat, lettuce, and rice. Even if artificial VOCs were eliminated completely, ozone would still form from VOCs released from vegetation.

Air-quality objectives

Air-quality objectives are national goals for outdoor air quality that protect human health and the environment. These objectives are developed by a working group for various atmospheric pollutants under the Canadian Environmental Protection Act. The working group reviews the most recent scientific studies.

The current "maximum acceptable" air-quality objective for ozone is 82 ppbv averaged over a 1-hour period. The current ground-level ozone objective was established in 1976, based on the best scientific information. It was reaffirmed in 1989, but a new assessment of the science of ground-level ozone is now nearing completion.

One aspect of the Harmonization Accord, recently signed by the Canadian Council of Ministers of the Environment, identified ground-level ozone as a priority. Work currently under way will develop a Canada-wide standard for ambient ozone levels.

(M. Shepard, Atmospheric Environment Service, Environment Canada)

This approach, however, only estimates average absorption over the long term and cannot describe the short-term fluctuations associated with daily changes in moisture stress or plant development. Furthermore, it tends to "dilute" relatively brief exposures to high concentrations that are likely to be most harmful to plants. Nevertheless, these estimates provide a useful indicator of potential plant damage.