Ozone

Ozone (O₃) is a bluish, relatively dense (1.6 times as heavy as air) gas and strong oxidant. Ozone occurs naturally in relatively large concentrations in the stratosphere, a layer of the upper atmosphere higher than about 3.8-10.7 mi (8-17 km), depending on season and location. Ozone also occurs in the lower atmosphere (or troposphere), where it is by far the most damaging of the photochemical air pollutants. Where these chemicals are abundant, they are known as an oxidizing or photochemical smog. This condition develops in sunny places where there are large emissions of hydrocarbons and oxides of nitrogen from automobiles and industry, especially where atmospheric temperature inversions are common.

Photochemical air pollutants are secondary chemicals, which means they themselves are not emitted, but are synthesized from primary emitted pollutants during complex photochemical reactions occurring in a sunny atmosphere. In addition to ozone, important photochemical air pollutants include peroxy acetyl nitrate (PAN), hydrogen peroxide (H₂O₂), and aldehydes. These secondary gases are the ingredients of oxidizing smogs that are harmful to people and vegetation.

Most countries have set standards for ground-level concentrations of ozone, with a goal of avoiding damage to vegetation and discomfort to humans. Until 1979, the standard for the maximum, one-hour average concentration of O₃ was 80 ppb (parts per billion) in the United States. Thereafter, the standard was raised to 120 ppb because the 80 ppb limit was so frequently exceeded. In a practical sense, the original ozone standard could not be enforced, so it was increased. Large regions of the United States cannot meet the criterion of 120 ppb, especially in the southwestern states. In 1997, the Environmental Protection Agency (EPA) created a new eight-hour standard of 80 ppb to protect against longer exposure periods.

In the vicinity of Los Angeles the maximum one-hour concentration of ozone can exceed 500 ppb, and it is typically greater than 100 ppb for at least 15 days per year. In other cities in North America, the annual maximum one-hour concentration is typically 150-250 ppb, and it is typically 90-180 ppb in London, England.

Humans and other animals are sensitive to ozone. This gas irritates and damages exposed membranes of the respiratory system and eyes. Ozone can also induce asthma. Sensitive people are affected at concentrations that commonly occur during oxidizing smogs. In the United States, ozone air pollution accounts for 10-20% of all summer respiratory related hospital admissions.

Ozone causes substantial damage to both agricultural and wild plants in many places, causing a distinctive, acute injury that reduces the photosynthetic area of foliage. Most plants are acutely injured by a two to four hour exposure to 200-300 ppb ozone, while longer-term exposures to about 100 ppb cause yield decreases, even in the absence of acute injuries. However, some species are relatively sensitive to ozone. In one laboratory experiment, tobacco was acutely injured by exposures to only 50-60 ppb for two to three hours, and spinach by 60-80 ppb for one to two hours. Sensitive species of conifers can be injured by 80 ppb over a 12-hour exposure.

An important field study conducted at various sites throughout the United States involved the exposure of crop plants to either ambient air at each site, or to a typical "background" ozone concentration of 25 ppb. Symptoms of acute ozone injuries were observed at all five of the study sites, although the damages were more frequent and severe in the southwest. On average, it was estimated that exposures to ambient ozone concentrations caused yield decreases of about 53-56% in lettuce, 14-17% in peanut, 10% in soybean, and 7% in turnip. Overall, it has been estimated that ozone causes crop losses equivalent to 2-4% of the potential yield in the United States, resulting in $3 billion in agricultural losses each year.

Trees can also be damaged by ozone, as has been well documented for conifer forests along the western slopes of the Sierra Nevada and San Bernardino Mountains of southern California. In this case, ozone-polluted air is transported eastward from the vicinity of Los Angeles to the mountains, where forests are damaged. The most sensitive species of tree is ponderosa pine (Pinus ponderosa), the naturally dominant species in these forests. Other species of conifers are less sensitive to ozone, and these replace the ponderosa pine when it is killed by the air pollution. The smog damage was first noticed during the 1950s, but the actual cause was not attributed to ozone until 1963. The ozone injuries to pine are diagnostic, characterized initially by a pale-green mottling of foliage, then a tissue death that spreads from the leaf tip, premature loss of foliage, and ultimately death of the tree. Ozone-stressed trees are also vulnerable to secondary damages caused by bark beetles and fungal pathogens, which often kill weakened trees.

The actual mechanism by which plant damage occurs from ozone has recently been discovered. Ozone inhibits the opening of the stoma on leaves, which are the pores that allow carbon dioxide gas into the plant, and through which oxygen gas leaves. The stoma open and close by means of two guard cells, found on either side of the opening. Ozone directly affects the guard cells, inhibiting their ability to open the stoma. Current research is under way to genetically engineer plants with guard cells resistant to the effects of ozone pollution.