Another Pollution Culprit

Abstract

Pitts: Singlet oxygen may be crucial in specific cases. General recipe for a 20th century urban atmosphere: Begin with pure air. From millions of autos, take several thousand tons of nitrogen oxides yearly. Sift in sulfur oxides from burned fuel oil. Sprinkle liberally with hydrocarbons, carbon monoxide and ozone. Season with aldehydes, ketones and peroxides. Garnish with soot particles from incinerators. Let simmer in direct sunlight. After a few hours, a choking eyesearing and poisonous haze called photochemical smog will be ready for consumption. Chemists know many of the ingredients of the toxic brew and even some of the major reactions, but there is still much to learn about what happens chemically in that turbulent soup. Before 1967, only a handful of scientistseven air pollution experts-considered as significant the role of something called singlet oxygen. Even today, it is a relative stranger, but it is becoming more prominent. In October, 135 U.S. and foreign scientists gathered in New York for the first international conference on the role of singlet oxygen in the environment. A few years earlier, the meeting's sponsor, the New York Academy of Sciences, would have been lucky to have attracted a tenth of that number. Singlet oxygen is molecular oxygen in an excited, or higher-energy state. A source of energy is sunlight, and energy is the reason for the emerging concern. In terms of electronic structure, singlet oxygen is characterized by a pair of electrons spinning in opposite directions. By contrast, in the normal or ground-state condition of molecular oxygen the electrons spin in the same direction. The result of these spins, when acted upon by a magnetic field and observed on a spectrometer, is three distinct spin states for the ground state (also called a triplet state) and one spin state for the singlet. The presence of that single state opens an atmospheric Pandora's box. Its higher energy makes singlet oxygen a potential reactive species, since to get rid of its excess energy and return to the ground state it could enter readily into reactions with other substances. Since it almost certainly exists in the urban atmosphere, scientists now reason that it must react with pollutants to play a role in smog formation, as well as produce its own harmful biological effects. Researchers emphasize, however, that the elimination of singlet oxygen will not end air pollution. Its importance cannot be measured in terms of tons of pollutants converted, says Dr. James N. Pitts Jr., professor of chemistry at the University of California at Riverside. I'm not saying it's the major source of air pollution, he explains. I'm not saying it's the reason we have air pollution. But it could be among the most critical contaminants in terms of specific reactions. Its health effects could be serious and they warrant detailed examination. Dr. Pitts ranks singlet oxygen as a secondary pollutant, one not directly emitted in the air but formed by chemical and photochemical processes. One obvious reason for its not being a major source of air pollution is its quantity in the air. Dr. R.H. Kummler of General Electric's space sciences laboratory in Philadelphia estimates that the singlet oxygen concentration in the air is at most two hundredths of a part per billion, and a thousandth of that at a minimum. Though it may exist in the lower atmosphere, its lifetime is estimated at only 0.1 second before its extra energy is released by quenching in collisions with atmospheric substances. This may seem like a short life, but in relation to other particles whose lifetimes are measured in thousandths and millionths of a second, it is long enough to enter into reactions that are biologically and environmentally significant. Because of its relatively brief life, singlet oxygen wasn't discovered in the atmosphere until 1958, when electronic telemetry equipment in rockets detected its presence in the upper atmosphere, where it exists for a longer time because there are fewer molecules to quench it. It still has not been detected in the lower atmosphere, because of interference from other air pollutants and scattered sunlight. Its presence in the lower atmosphere was postulated as early as 1960 but not much attention was paid to the idea since the only way scientists could envision singlet oxygen being produced was by the direct irradiation of ordinary oxygen by sunlight, a process that could not produce significant amounts. So there the idea sat until 1967, when Dr. Pitts and his co-workers at Riverside showed that singlet oxygen could be produced in a much more efficient way. They suggested that a