ABSTRACT
Both natural sources and human activities put pollutants into the air. Not only do these contribute to an unhealthy atmosphere, but they can interact in the presence of ultraviolet radiation from the sun, forming ozone, oxidized organics such as formaldehyde and peroxyacetylnitrate (PAN), and acidic gases. The brownish photochemical smog which hangs over so many of our larger cities is one of the most obvious effects of air pollution. Dying evergreens in the Black Forest of Germany and on the hillsides in the eastern United States attest to the effect of acid rain, caused by the acidic components of air pollution washing down in precipitation.
