As ozone restricts carbohydrate movement in leaves, the resulting higher sugar concentration makes the leaves more susceptible to insect attacks. High levels of PAN and ozone are toxic to trees, but ozone is more abundant and injurious. Container-grown eastern white pine with ozone damaged needles. Ozone-caused brown bands at the same point on all needles of an eastern white pine. Sulfur dioxide Sulfur dioxide is primarily a result of fossil fuel burning for electricity generation, and to a lesser degree, the processing of steel and other ores.
Sulfur dioxide is classified as a point-source pollutant.
Acute sulfur dioxide damage causes severe leaf scorch, usually on upper interveinal leaf surfaces. Younger leaves are generally more sensitive. Moisture in the air or on leaf surfaces may combine with sulfur dioxide to form sulfuric acid. Sulfuric acid causes leaf scorch, spotting defoliation, and can also cause tree death over a large geographic area with many affected species. Particulates are not extremely damaging, but can inhibit or reduce photosynthesis by plugging stomates.