Trees and air pollution
Ronald Reagan did not say, "Trees cause smog" but rather, "Trees cause more pollution than automobiles do." In a discussion of smoggy city skies, it could be easy to overlook what Reagan was actually referring to:
- According to a study by the University of Helsinki, coniferous forests—that is to say, those composed of trees such as pines—release nitrogen oxides into the atmosphere that combine with other pollutants to form smog.
- Nitrogen oxides (NOx), along with hydrocarbon gases and sunlight, are the three ingredients needed to make photochemical smog that clogs the skies of many urban centers, as well as acid rain. The researchers were surprised to find NOx produced by the pine needles, and that levels rose when shoots were exposed to ultraviolet radiation from the Sun. While the amount produced was insignificant on a local scale, "our findings suggest that global NOx from boreal coniferous forests may be comparable to those produced by worldwide industrial and traffic sources," said the authors.