While trees need carbon dioxide to live, as they absorb it and release oxygen, they do not need air pollutants such as sulfur dioxide and oxidants, since such compounds are harmful to trees, according to the U.S. Department of Agriculture. Air pollution affect trees in many negative ways, including browning, damage and eventual death.
Smog in urban areas causes chlorosis in trees. Smog blows into pine forests, such as those in the San Bernardino Mountains in California or the Blue Ridge Mountains of Virginia, and causes browning of the leaves and premature needle drop. It also leaves trees more susceptible to pests and shortens their life spans.
Ozone is an oxidant that occurs when car pollution and sunlight mix together and form a gas. Ozone damages trees, both deciduous and coniferous, by causing slow growth, browning leaves or molted needles. According to Colorado State University, "white ash, aspen, Ohio buckeye, Kentucky coffee, Crimean linden, imperial honey locust, weeping willow and hybrid poplar trees" are all affected by ozone damage.
Sulfur dioxide comes from burning coal and smelting ores. Besides being the main cause of acid rain, sulfur dioxide also affects both deciduous and coniferous trees by causing leaf or needle damage, such as orange tips on pine needles. Sometimes sulfur dioxide can damage trees in as little as three to four days of exposure.