An international team of scientists conducted the study, which was part of the Department of Energy's Atmospheric Sciences in Complex Terrain program -- a long-term effort to study the effects that geographical features such as mountains and valleys have on air movement and pollution. The study was funded by DOE and the Mexican government.
Pacific Northwest National Laboratory organized the meteorological portion of the project and staffed one of six data-collection sites. "The complex movement of air in the high basin makes it difficult to predict where the pollution will concentrate at any given time," says Pacific Northwest?s Chris Doran, science director of the project. "The close coupling of meteorological and chemistry measurements and analysis is necessary to understand how the surrounding terrain affects the transport and diffusion of pollutants."
The study may help explain how fine particles form in the atmosphere. These particles are potentially the most hazardous because they can lodge in the lungs, and the U.S. Environmental Protection Agency has proposed new standards for fine particles. "This study will be relevant to our understanding of the nature and causes of fine particle air pollution throughout the world, including that found in U.S. cities," said Pacific Northwest's Sylvia Edgerton, project director.
The data will be used to improve computer models designed to predict atmospheric conditions and potentially guide responses to pollution -- such as changing the location or timing of pollution releases.
Mexico City, with 20 million residents, sits at 2,255 meters (7,400 feet) and is largely surrounded by mountains, creating a basin where inversions of colder air can be trapped by warmer air. Pollution from human activities collects in the air over four- to six-day cycles, similar to Western U.S. cities.
In addition to Pacific Northwest, collaborators included Los Alamos and Argonne National Laboratory, the National Oceanic and Atmospheric Administration's Environmental Technology Laboratory, Desert Research Institute and several Mexican institutions including the Mexican Petroleum Institute and the University of Mexico.
Scientists at four sites around the basin released radiosondes -- balloons with sensors to measure temperature, pressure and humidity -- into the atmosphere five times a day beginning at 8 a.m. Each site also used radar wind profilers, which can measure wind up to four kilometers above the ground. Another tool, called SODAR, provided high resolution measurements of wind flow up to 500 meters above the city. Ground stations at each site measured wind, temperature and humidity. A number of other sites collected concentration data. Other study findings include:
- Particle concentrations varied widely from site to site.
- Concentrations peaked around 9 a.m. and 7 p.m.
- Average concentrations of particles exceeded the EPA standard seven times, once by more than three times the standard.
- Between 40 and 60 percent of the particles were small enough to lodge in the lungs.
- Reduced visibility can be tied to high concentrations of the small particles. Historically, low visibility has correlated with sulfur content of fuels.
- Wind directions varied significantly with elevation and location in the valley, producing pollutant transport patterns that are difficult to model accurately.