Forest fires that ravaged parts of eastern Alaska and western Canada in 2004 exacerbated the already-high levels of ozone pollution in Houston, Texas, some 5,000 kilometers [3,000 miles] away. The Houston area average eight-hour ozone concentrations on 19 and 20 July 2004 were the highest of any July day in the last five years. Scientists have now tracked their source to smoke from major fires along the Alaska-Yukon border that reached their greatest intensity a week earlier, 12 to 14 July.

A team of 15 scientists headed by Gary Morris of Valparaiso University in Indiana analyzed data from a variety of sources, including weather balloons launched from the campus of Rice University in Houston, NASA satellite instruments, aircraft, and computer models. Their findings will be published on 26 September in the Journal of Geophysical Research (Atmospheres).

Houston frequently fails to meet federal standards for ground-level ozone. 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.

In the new study, Morris and colleagues tracked an air mass from the region of Alaskan-Canadian forest fires as it traveled across Canada, through the mid-western United States and down the Mississippi valley into the Houston area. "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 [three miles] over Houston," Morris said.

Morris, Scott Hersey from Rice, Anne Thompson from Pennsylvania State University, and the other researchers employed imagery from NASA's Moderate Resolution Imaging Spectroradiometer on the Terra satellite, aerosol (fine particle) data from the Total Ozone Mapping Spectrometer satellite, and carbon monoxide data from the Atmospheric Infrared Sounder on the Aqua spacecraft. The weather balloons in the study were launched as part of the Tropospheric Ozone Pollution Project, to measure ozone levels from the ground up to more than 30,000 meters [100,000 feet]. Fortuitously, the project coincided with the arrival of pollution from the northern fires.

Normal summer meteorological conditions, smoke from the distant forest fires, and the typical urban pollution generated in the Houston area combined to greatly increase local ozone concentrations during this two-day event in July 2004. The scientists believe that such pollution episodes will continue. Understanding the transport and transformation of gases and aerosols over long distances is needed for improved air quality forecasting, they say.

"This event highlights the critical role imported sources can have on local air quality," said Morris. "Environmental managers at the state level need to take into consideration the impact of such sources as they develop their comprehensive plans to come into compliance with federal regulations. Continuous operation of a network that provides data on ozone concentrations above the surface would be helpful in accurately identifying the sources of pollutants and improving air quality forecasts."

The study was funded by the Shell Center for Sustainability at Rice University, NASA, and the Texas Commission on Environmental Quality.

SOURCE: American Geophysical Union