Oxidants such as ozone, oxides of nitrogen, and peroxyacetyl nitrate (PAN) cause the most direct damage to plants. These are followed by sulfur dioxide. Fluorides, chlorides, ammonia, and chlorine cause occasional local damage. 

Ozone is the most damaging air pollutant to plants. The action of sunlight (ultraviolet radiation) on molecular oxygen and oxides of nitrogen spontaneously generates ozone. The organic compounds in automobile exhaust enhance ozone accumulation. Ozone can move across great distances to cause damage to plants far from its origin and is therefore classified as a non-point source pollutant. 

Sulfur dioxide is primarily a result of fossil fuel burning for electricity generation. Damage to plants by sulfur dioxide (SO2) has formerly been concentrated around the industrial sources of the pollutant. Where the emitters of this pollutant have tall stacks, intense local damage has ceased, and revegetation has begun. When sulfur dioxide enters the atmosphere from taller stacks, it becomes more widely distributed and greatly diluted before reaching ground level. Therefore, acute injury by sulfur dioxide has become less common. 

Injury from air pollutants can be put into two major categories: (1) chronic injury, which is the cumulative effect of long exposure (often over several seasons) to toxic pollutants at low levels on sensitive species or at moderate to high levels on tolerant species; (2) acute injury occurs after short exposure (a few hours to several days) to pollutants at high levels on tolerant species or at moderate to low levels on sensitive species. Acute injury commonly occurs following accidental leaks or spills of gas or volatile liquids. Acute injury results in moderate to severe foliar injury and is the most frequently diagnosed type of air pollution injury. Chronic injury is difficult to diagnose because the foliar symptoms are mild or nonexistent and the trees decline over several years. 

Most air pollution results from electric power generation, cars and trucks, and a variety of industrial operations. Invisible gasses are the most common form of pollution toxic to plants (phytotoxic) rather than smoke. Air pollution is not restricted to urban and suburban areas but is also found in remote areas. 

Symptoms and Diagnosis

Diagnosis of air pollution damage is difficult. Symptoms can be very similar to those produced by stress from moisture, temperature, or nutrient disorders as well as a variety of biotic factors such as bacteria, fungi, viruses, sucking insects, and mites. Therefore, once these factors are eliminated, one can look to air pollution as the source of the problem. The source, the specific sensitivity as correlated with symptom type, and the nature and the movement of air pollutants must be known before an accurate diagnosis of injury can be achieved. Sensitive chemical analyses are sometimes needed to detect air pollutants. Because of the complexity of symptoms, it is suggested that reference material be used to further investigate symptoms. 

Ozone Damage 

The extent of damage depends on the concentration of ozone, the duration of exposure, and plant sensitivity. Acute damage to deciduous trees causes marginal leaf burn and dot-like, irregular-shaped lesions or spots that may be tan, white, or dark brown. Symptoms may spread over entire leaves. Another common symptom is bleaching of the upper leaf surface. Broad-leaved trees often exhibit these symptoms of stippling or red-purple or necrotic flecks on the upper leaf surface only. Acute damage to conifers causes browning at the same point on all needles in a bundle (needle cluster). Other symptoms include chlorotic (yellow) mottling or necrotic (brown to black) banding, tip burn, dwarfing, and tufted appearance due to early defoliation of older needles.  

Sulfur Dioxide 

Acute foliar injury by sulfur dioxide is indicated by bleached or pigmented (tan to reddish brown or dark brown, depending on species) necrotic interveinal areas on broad-leaved plants and chlorotic spots and bands or brown tips on needles of conifers. Young leaves that have attained full size are generally the most sensitive. 

Life Cycle

Specific effects of air pollutants on plant tissue vary with the pollutant, host, time of year, and numerous meteorological factors such as temperature, relative humidity, wind, and solar radiation. Juvenile leaf tissues are most susceptible to acute injury of fluorides and consequently, most severe injury occurs in the spring. Chronic injury, however, occurs progressively during the entire growing season. 

Ozone Damage 

Ozone builds up to phytotoxic levels in the atmosphere during warm, sunny weather when pollutants accumulate in stagnant air. Accumulation often occurs during atmospheric inversions in valleys and basins bounded by mountains. Ozone may cause damage to plants far from the source of its precursors as masses of polluted air move overland. 

Sulfur Dioxide 

Sulfur dioxide causes injury when it and its sulfite derivative are absorbed faster than they are detoxified. Plants injured by sulfur dioxide have an abnormally high foliar sulfur content for a time, which is diagnostically useful if appropriate standards are available for comparison. Plants vary widely in tolerance to sulfur dioxide because of the differences in the efficiency with which they absorb the gas and the plant’s ability to detoxify the pollutant and dispose of excess sulfur. Much sulfur absorbed by leaves is translocated to other plant parts, thus diluting it. 

Integrated Pest Management Strategies 

1. Maintain plant vigor. Plants in good health resist all types of injury better than weakened plants. Therefore, water in times of drought and fertilize to maintain nutrient balance. Soil should be porous, well-draining, and not compacted. 

2. Select plants. Select plants tolerant of air pollutants. 

3. Reduce emissions. Reduce emissions from the source when possible. 

Organic Strategies 

All of the recommended IPM strategies are strictly organic approaches.