Jan. 16, 2001 Albuquerque, N.M. -- The chemical cycles in the troposphere along with pollutants of human and natural origin can alter the composition of the air and effect local, regional and global environmental quality, according to a Penn State researcher.
The troposphere -- the area of the Earth's atmosphere from the surface to ten miles above the surface where weather exists -- is also where pollution becomes a problem. In the atmosphere, a complex series of chemical reactions can alter some pollutants so that they rain out as aerosol particles or acid rain and clear the air. Other compounds remain in the air, changing, and changing again as other chemicals cause reactions.
"Pollution from megacities and biomass burning, including precursor gases to hydrogen oxides such as acetone and formaldehyde, lofted into the upper troposphere, can become the dominant hydrogen oxide source and result in efficient ozone production," says Dr. William Brune, professor of meteorology and head of Penn State's meteorology department. "These compounds can also be transported great distances before descent, possibly influencing the chemistry of remote regions."
Ozone is complicated. In the stratosphere it serves to protect life from the detrimental effects of the sun's ultraviolet radiation. At ground level, it is a pollutant implicated in respiratory problems and eye irritation. Sunlight breaks ozone apart resulting in the creation of the very reactive hydroxyl radical which begins the process that removes some pollutants from the air. However, when hydroxyl radicals break down some compounds, they produce other hydrogen oxides, which react with other pollutants and form ozone.
"The hydroxyl radical drives atmospheric oxidation by reacting with chemicals emitted from Earth‚s surface, thus creating new chemicals that are more easily scavenged and removed by aerosols, clouds and rain," Brune told attendees today (Jan. 15) at the annual meeting of the American Meteorological Society in Albuquerque, N.M.
"But in the oxidation process, hydroperoxyl radicals form and combined with the industrial pollutant nitric oxide produces ozone. The sun then breaks down this ozone, creating hydroxl radicals and starting the cycle all over again."
Brune is part of ongoing studies to measure the amounts of hydroxyl radical and hydroperoxyl radical in the atmosphere over various areas of the globe during different times of the day. So far, using airplane-mounted equipment, they have tested air over the south Pacific near Hawaii, Fiji, Tahiti and Easter Island, over the North Atlantic flight corridor, and are preparing for flights over the western pacific from Hong Kong and Tokyo. Measures of hydroxyl and hydroperoxyl radicals reflect the outflow of air carrying pollutants off China and other industrialized nations. The study over the North Atlantic flight corridor assessed the contribution of air travel to this type of pollution. Brune has also tested air from ground towers in lower Michigan and Houston, Texas.
To measure these radicals, special equipment samples the air and uses a laser to excite the hydroxyl radicals so that they fluoresce. The hydroxyl count is proportional to this fluorescence. The researchers count hydroperoxyl radicals by releasing nitrogen oxide that rapidly reacts with hydroperoxyl to form the hydroxyl radical. They then count the hydroxyl and subtract the hydroxyl that was there before the nitric oxide.
"The Michigan environment was dominated by trees that produce an organic compound called isoprene," says Brune. The hydroxyl radical reacts with isoprene to form a hydrocarbon oxide radical. Whether the next reaction produces or reduces hydrogen oxides depends on the amount of nitrogen oxide in the air.
"Isoprene, a naturally occurring compound, in the presence of nitrogen oxide produced by power plants is very reactive and can create a lot of ozone," says Brune. "We want to test the understanding of the basic chemical reactions for isoprene." The complex nature of the chemistries of ozone, the hydroxyl radical and the hydroperoxyl radical is not always as simple as sunlight and human-produced pollution. Even naturally occurring organic compounds, like isoprene, under the right circumstances can produce unexpected results. Observations of hydrogen oxide levels, nitrogen oxide levels and other meteorological factors over areas as different as an urban city, a rural forest and the Pacific ocean shed light on the fundamental relationships among atmospheric oxidation, ozone production, nitrogen oxide and hydrogen oxides.
"The major thrust of atmospheric research is tied into the Earth science," says Brune. "We look at things locally, then regionally and globally. It all comes back to affecting us locally."
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