Air Pollution as a Climate Forcing: A Workshop
Day 4 Presentations
The Adverse Impact of Surface Ozone on Agricultural Crops
Denise L. Mauzerall
Princeton University, Princeton, NJ, U.S.A.
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Ozone (O3) is well documented as an air pollutant that is extremely damaging to agricultural crops and other plants. Most crops are grown in summer when O3 concentrations are elevated and frequently high enough to reduce yields. Uptake of O3 by plants is a complex process involving transport of O3 into the plant canopy where 3 can then be absorbed into the leaves via the stomata (openings on the underside of the leaf whose aperture is controlled by the plant). Stomata open in response to light and increasing temperature and close in response to decreasing humidity, water stress and increased CO2 or air pollution. Physiological effects of O3 uptake are manifest by a) reduced net photosynthesis, b) increased senescence and c) damage to reproductive processes. Thus O3 uptake has an impact on both plant growth and crop yields. Dose-response relationships vary by plant species, crop cultivar, developmental stage, and external environmental factors, such as water availability and temperature, which influence the opening and closing of stomata.
Extensive field studies conducted in the United States during the 1980s as part of the National Crop Loss Assessment Network (NCLAN) found reductions in crop yields due to O3 exposure to total approximately 2%-4% of US crop production with an estimated value of $2 billion (1980 US$). Current secondary standards (to protect public welfare which includes agriculture and natural ecosystems) in the United States have been set equal to the primary standards (to protect human health). The new primary standards set the frequency at which a peak O3 concentration is permitted to occur (0.08 ppm averaged over 8 hours not to be exceeded more than three times in three years). However, in the NCLAN study, when peak and seasonal mean statistics were compared, only seasonal mean statistics were found useful for estimating yield reductions.
A comparison of different exposure-response indices found that indices that weight peak concentrations using a sigmoid (or discrete 0-1) weighting scheme and accumulate exceedances over a threshold concentration give a better fit to yield data than do indices that use mean concentrations over a growing season or peak values alone. Examples of three cumulative statistics determined by an empirical fit of agricultural yield data and O3 concentration are shown in Figure 1. SUM06 has been used to estimate yield reductions in the United States while AOT40 has been used in Europe. W126 is believed to provide a better fit of the dose-response data, but is viewed as too complex for use in regulatory standards. Figure 2 shows relative yield losses due to O3 exposure for several crops.
In Europe, research in the 1990s has found that because exposure to high O3 levels is correlated with high temperatures and humidity, during hot, dry conditions, plants usually close their stomata, hence protecting themselves from O3 exposure. Thus, consideration of parameters that influence the flux of O3 into the plant such as soil moisture conditions, vapor pressure deficit and temperature are critical in determining O3 dose, and hence harm, to the plant. For example, in northern Europe where O3 concentrations are lower than in southern and central Europe, the potential O3 uptake at a given O3 concentration is higher because of higher levels of humidity. Thus net O3 uptake may vary according to a different geographical pattern than indicated by the AOT40 or SUM06 measures. As a result, the European Community is considering the implementation of a modified AOT40 critical level (i.e., a cumulative exposure above which an unacceptable level of harm is incurred) which includes vapor pressure deficit criteria as a first step toward a feasible standard that takes O3 flux and hence dose to the plant into account.
Substantially less research on the effect of O3 on plants has been conducted in developing countries. Increasing impacts of O3 on crops are likely in developing countries as they continue to industrialize and their emissions of air pollutants increase. More research is needed on surface O3 concentrations in developing countries, on their projected increase, and on the sensitivity that crop cultivars used in developing countries have to O3. The threat of reduced agricultural yields due to increasing O3 concentrations may encourage developing countries to increase their energy efficiency and to use different energy sources. This could simultaneously achieve a local benefit through improved regional air quality and a global benefit through a reduction in the emission of greenhouse gases.