Researchers in Finland, have discovered that under some circumstances, needles from trees of Pinus sylvestris can release nitrogen oxides directly into the atmosphere. In theory, these gases could deliver as much air pollution as worldwide emissions from industry and traffic. Nitrogen oxides are a group of trace gases that can induce chemical reactions in the atmosphere, and help to create smog and acid rain. There is confusion about whether plants help or hinder these processes; sometimes plants absorb these gases, but tree canopies can also release nitrogen oxides directly into the atmosphere. The mystery factor that determines what happens is ultraviolet light. Pertti Hari from the University of Helsinki in Finland and colleagues have measured the amount of nitrogen oxides released by Pinus sylvestris shoots. They find that the levels increase when the shoots are exposed to the ultraviolet radiation from the Sun. This may have implications for plant physiology as well as atmospheric chemistry. "Nature" Pertti Hari. E-mail: pertti.hari@helsinki.fi, http://www.AlphaGalileo.org/