The study, conducted by researchers from Valparaiso University and Rice University, found that ozone pollution levels increased significantly in the air above Houston on July 19-20, 2004. Researchers attributed the increase in part to smoke that was transported to the area over the course of a week from forest fires raging in Alaska and Canada. The study is the first to quantitatively examine the impact of remotely generated pollutants on air quality in the lower atmosphere.
In the summer of 2004, researchers in a NASA-led field research project sampled a variety of trace gases and aerosols ?tiny particles suspended in the air ?across North America. That same summer, physicist Gary Morris, both an associate professor at Valpariaso and an adjunct assistant professor at Rice, launched the Tropospheric Ozone Pollution Project (TOPP) at Rice's Houston campus with funding support from both NASA and Rice's Shell Center for Sustainability. TOPP involved daily launches of weather balloons from Rice's campus south of downtown. The balloons measured ozone levels from the ground to altitudes of more than 100,000 feet, offering the first clear picture of how much ozone was present in each layer of the atmosphere above the city.
While NASA scientists and Morris were collecting their data, forest fires in western Canada and eastern Alaska were consuming more acres than at any time during the past 50 years. Meteorological conditions carried smoke from these intense fires ea stward and southward to the U.S. Gulf Coast.
"The combination of our balloon-borne ozone data and observations by NASA satellites, aircraft, and a network of ground stations provided unprecedented insight into the origins of locally poor air quality in Houston on those two days," Morris said.
Houston frequently exceeds federal standards for ground-level ozone. On 52 days in 2005, Houston's air quality violated the eight-hour Environmental Protection Agency (EPA) standard for ozone, placing Houston among the worst urban areas in the U.S. for ozone pollution. Besides posing a health risk for people with respiratory problems, ground-level ozone has also been linked to increased rates of asthma among children, and it can destroy plants and reduce crop yields. Federal regulators have given the Houston-Galveston region until 2007 to comply with federal air-quality standards for ground-level ozone, under the threat of severe economic sanctions, including loss of federal highway dollars.
In the new study, Morris and colleagues relied on imagery from NASA's Moderate Resolution Imaging Spectroradiometer on the Terra satellite, aerosol data from the Total Ozone Mapping Spectrometer satellite, and carbon monoxide data from the Atmospheric Infrared Sounder on the Aqua spacecraft. They tracked an air mass from the region of forest fires in western Canada and eastern Alaska on July 12-13, 2004, and followed it as it traveled across Canada, through the mid-western United States and into the Houston area on July 19.
"We found that with the arrival of the pollutants associated with these forest fires, ozone levels increased between 50-100 percent in the first five kilometers over Houston," Morris said.
Meteorological conditions, the smoke from the distant forest fires, and the typical urban pollution generated in the Houston area provided a potent mix for increasing local ozone concentrations, he said.
Morris said it's likely that such pollution episodes will continue. Understanding the transport and transformation of gases and aerosols over long distances is needed for improved understanding and air-quality forecasting.
"This event highlights the critical role imported sources can have on local air quality," Morris said.