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.
Forests and industrial pollutants sometimes interact unpredictably, however. For example, researchers found that a hy