June 3, 2002
Air-Sampling Study IDs Source of Excessive Ozone Pollution
Findings may lead to more effective regulations for protecting public health
UPTON, NY — Using data from one of the most comprehensive U.S. air pollution studies ever conducted, scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have identified specific volatile organic compounds (VOCs) as key sources of excess ozone smog in industrial areas of Houston, Texas — which appear to be different from traditional sources of ozone pollution in typical urban areas around the country. Specific efforts to control these industrial emissions of VOCs might be necessary to control Houston’s ozone problem, say the authors, whose findings will appear in an upcoming issue of Geophysical Research Letters (published on-line May 28, 2002).
“A clear understanding of the complex causes of ozone pollution will help to identify cost-effective ways to control smog and protect public health,” said atmospheric chemist Larry Kleinman, one of the lead Brookhaven researchers on the study.
Traditional efforts to control ozone have focused on limiting emissions of precursor chemicals such nitrogen oxides (NOx) and/or volatile organic compounds (VOCs), which are emitted from automobiles, power plants, and other industrial sources and form ozone when they react with sunlight in Earth’s atmosphere. But despite improvements in air quality due to more stringent emission standards, many areas still exceed ozone standards.
To get a better understanding of the ozone problem, the Brookhaven team participated in the Texas 2000 Air Quality Study, a collaborative air pollution study involving hundreds of researchers from more than 40 public, private, and academic institutions, which was led by Peter Daum, another atmospheric chemist at Brookhaven.
During August and September of 2000, the scientists conducted air-sampling flights over the Houston-Galveston area — which experiences the country’s highest ozone levels — in a specially equipped aircraft operated by the Department of Energy (DOE). The scientists flew over “clean” background areas and over urban and industrial areas with high emission rates of nitrogen oxides and volatile organic compounds, as well as downwind from these sources in regions where ozone is expected to form.
On each flight, the scientists measured levels of ozone, ozone precursors, and photochemical oxidation products. They were then able to calculate the ozone production rate for each of the flight areas. For the present paper, they compared the Houston findings with data collected during several previous DOE-sponsored air quality studies over Nashville, Tennessee; New York, New York; Phoenix, Arizona; and Philadelphia, Pennsylvania.
Ninety two air-sampling flights were conducted in the five-city study. On 13 flights, ozone concentration exceeded the 120 parts per billion (ppb) federal standard set by the Environmental Protection Agency (EPA) to protect human health. Nine of those flights were in Houston. These results agree with data collected by the EPA at ground level. Over the past five years, 15 of the highest 25 ozone concentrations recorded in all of the U.S. were in the Houston-Galveston area.
“We found that most of Houston resembles other urban areas in its concentration of ozone precursors and ozone production rates,” said Daum. “The industrial Houston Ship Channel region, however, the location of one of the largest petrochemical complexes in the world, has a distinctive chemistry,” he said. There, very high concentrations of VOCs not seen in the other cities, nor in the other parts of Houston — specifically ethene, propene, and butenes — lead to excessive production of ozone.
“Calculations based on the aircraft measurements show that the ozone production rate in the Houston Ship Channel region can be as much as five times higher than occurs in the other four cities or in nonindustrial parts of Houston,” said Kleinman. “This extra kick in the photochemistry is a direct result of the high concentrations of VOCs emitted by industrial facilities.”
This work was funded by the U.S. Department of Energy, which supports basic research in a variety of scientific fields; the U.S. Environmental Protection Agency; and the Texas Natural Resource Conservation Commission.