Even pristine forests can contribute to air pollution. In fact, researchers now say that northern pine forests exude a family of nitrogen oxides and do so in quantities that may rival those produced worldwide by industry and traffic.
Nitrogen oxides can react with hydrocarbons to yield nitric acid, a primary ingredient in acid rain. They can also help produce smog-causing ozone. Scientists generally peg automobiles as the prime source of nitrogen oxides. Trees, on the other hand, are usually credited with sopping up air pollutants.