In Los Angeles, the city with the worst air pollution problem in the country, even the brown smog clouds may have silver linings.
Not only does the city's smog cover appear to reduce dramatically the intensity of ultraviolet radiation that reaches the palm trees and pavement in the City of Angels, but the UV attenuation also appears to cut down the concentration of ozone below what would otherwise exist. These results are beneficial because ultraviolet light is known to cause skin cancer, and ozone exposure contributes to respiratory and cardiovascular illnesses.
This picture comes from the first detailed study of the causes and effects of major UV reductions that were originally measured in two studies performed in the area in 1973 and 1987. Both found reductions of UV radiation ranging from 22 to 46 percent.
Now Mark Z. Jacobson, an assistant professor of civil and environmental engineering at Stanford who specializes in creating computer models of the atmosphere, has identified the key chemical compounds in L.A. smog that appear to act as specialized UV filters. He has determined that the resulting reduction in UV levels has a significant side effect, that of lowering ozone levels on the ground by 5 to 8 percent. He is presenting these findings in a poster paper on Dec. 11 at the annual meeting of the American Geophysical Union in San Francisco.
The adverse health effects of smog are well known. A 1995 study in the Los Angeles basin found that increases in the concentration of particulates -- airborne, microscopic particles and droplets that are a major constituent of smog -- are associated with increases in the number of patients checking into local hospitals with chronic respiratory problems, cardiovascular complaints and acute respiratory ailments.
Similarly, the health effects of exposure to UV light are well established. The general rule of thumb is that a 1 percent increase in UV radiation is accompanied by a 2 percent increase in the incidence of skin cancer. For that reason, Jacobson found the two studies that reported such significant reductions in UV light levels of particular interest.
"Although the UV data were collected a long time ago, no study to date has examined the causes or effects of the observed reductions. So I decided to look into it," he said.
As Jacobson analyzed the information from the earlier studies, he determined that the scattering and absorption of UV light by the various gases present in the atmosphere was too small to explain the effect. The researcher also realized that the reductions of UV light were considerably higher than those for sunlight as a whole, which was attenuated by only 6 to 7 percent. This disparity allowed him to eliminate light scattering by aerosols because the concentrations required to reduce UV light by these amounts by scattering would also have reduced the total sunlight much more than was observed.
That left aerosol absorption as the most plausible cause of the UV reduction. To date, only elemental carbon and large soil compounds, such as iron and aluminum oxides, are considered important absorbers of visible and ultraviolet radiation. Both these candidates had problems, however. There wasn't enough elemental carbon in the L.A. atmosphere to account for such large UV reductions. Nor did the characteristics of the soil compounds fit the prevailing conditions. Because of their large size, these compounds do not stay suspended in the air for a long time and their concentrations were not high enough to reduce UV radiation significantly.
That led Jacobson to so-called secondary organics and nitrated inorganic particulates. Primary organics are carbon particulates emitted from factory smokestacks, vehicle tailpipes, backyard barbecues and other similar sources. Secondary organics are created when organic gases, which are also emitted from these sources, undergo chemical reactions in the atmosphere and condense onto or dissolve within existing particles.
Going to the scientific literature, he determined that large numbers of secondary organics, most notably those that have nitrate groups attached, have the desired characteristic of absorbing weakly in visible light and strongly in the ultraviolet. Studies have found that these nitrated aromatic compounds make up from 12 to 21 percent of the organic material found in particulates in the area. When he assumed that this material was evenly mixed within particles, the computer simulations did not show the magnitude of UV reductions that had been observed. When he assumed that secondary organic material coated the particles, however, the amount of UV reduction predicted was very close to what was observed.
Ozone is another urban air pollutant that can be harmful. The 1995 Los Angeles study also found that increased concentrations of ozone were accompanied with increased hospital check-ins by people with chronic and acute respiratory problems and cardiovascular complaints.
The reduction in ozone caused by reduced levels of ultraviolet light is relatively straightforward to explain. Ultraviolet radiation strongly affects the rates of chemical reactions that produce and destroy ozone. When UV radiation is reduced, the rate of production of ozone decreases more than the rate at which ozone is destroyed. This causes a net decrease in ozone levels.
Similar reductions in UV radiation should also be taking place in other polluted urban areas located in sunny, dry climates where there are high emissions of nitric oxides and reactive organic gases, Jacobson said. But in more humid climates, like that of the eastern United States, water tends to coat airborne particles, which keeps them from absorbing UV radiation and so weakens the filtering effect.
Jacobson's research is supported by the U.S. Environmental Protection Agency and the National Science Foundation.
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