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Getting the Big Picture on Houston's Air Pollution

Houston on a smoggy day (left) and a clear day (right). Image courtesy: The Batelle Institute
Houston on a smoggy day (left) and a clear day (right). Image courtesy: The Batelle Institute

Houston has a serious air quality problem. Since 1999, the Texas city has exchanged titles with Los Angeles as having the most polluted air in the United States defined by the number of days each city violates federal smog standards.

Now the Multi-angle Imaging SpectroRadiometer, an instrument that flies on NASA's Terra satellite and is managed by the Jet Propulsion Laboratory, is stepping in to help researchers find out exactly where, how much and what type of pollution is in Houston. The instrument acquires images of Earth at nine angles simultaneously, using nine separate cameras pointed forward, downward and backward along its flight path. By incorporating the instrument's data on airborne particles with local air quality information, researchers hope to increase their understanding of air pollution models. Such models could then be used to map other urban areas.

The instrument is monitoring Houston "as a validation test, a challenge, to see if the instrument can provide a regional picture of air pollution," said Dr. Ralph Kahn, aerosol scientist for the instrument and JPL's lead scientist for Earth and planetary atmospheres. "If it is successful, we may be able to use the instrument to conduct regional pollution studies of any city in the world."

Typically, air pollution studies are conducted through combined measurements taken at ground sites and by aircraft that sample at fixed points along flight paths. By including satellite data in this campaign, researchers can view an entire region in detail at once. In addition, they are using state-of-the-art measurement methods for an enormous array of gases and particles.

In the Houston area, several air-monitoring stations on the ground measure basic weather, gases and particulate matter - pollution particles that arise from various sources, such as the burning of coal for electricity or fuel for transportation. These particles, also called aerosols, are often composed of carbon or sulfate compounds and can reach tiny, concentrated states that can harm the human respiratory system. Data from the JPL instrument help identify the size, shape and radiative properties of aerosols. The instrument also takes images at multiple angles, giving it the unique ability to measure aerosols over land and obtain detailed particle properties over dark water.

"The Multi-angle Imaging SpectroRadiometer measurements provide an incomparable regional view of the air quality," said Dr. Martin Buhr, a scientist working with Baylor University to perform airborne measurements in support of both the Texas air quality studies and validation of the satellite measurements. "One direct benefit of using this type of observation is the insight it provides to the regional extent of the problem, which is important both from the perspective of what is coming in to the region as well as what goes out."

Houston is an ideal prototype in that it possesses a combination of the many potential sources that contribute to increased pollution: a growing population already in the millions, an enormous amount of automobile use and an abundance of chemical industry and power plants. Periodic impact by storms such as the recent tropical storm Allison, upstream sources like other metropolitan areas and additional factors may also contribute to Houston's pollution.

"A challenge for the instrument, which looks at the whole atmosphere from top to bottom, is that the air quality managers are only interested in what is happening near the ground," Kahn said. "That's where air pollution begins and generally stays."

Because the state of Texas is responsible for the health of its residents, any time the measured levels of air pollutants exceed the Federal health-based standards, the state (acting through the Texas Commission on Environmental Quality) must develop a plan to reduce emissions to meet the standards. (This is accomplished via a document called a state implementation plan on a pollutant-by-pollutant basis.) Before the NASA instrument became involved, the University of Houston and the Battelle Memorial Institute began assessing the potential use of satellite sensors to monitor urban air quality, and the Texas Commission on Environmental Quality was planning aircraft flights to collect air quality information in Houston and other locations.

"The experiment with the Multi-angle Imaging SpectroRadiometer came about because people were interested in a new technique and were willing and able to adjust existing projects to contribute to this experiment," said Dr. Jill Engel-Cox, a principal research scientist at Battelle Memorial Institute, Columbus, Ohio.

Now, the Texas Commission on Environmental Quality flies a special flight pattern when the satellite instrument is scheduled to pass over Houston on cloud-free days. Simultaneously, the team managing the instrument collects local area data over Houston.

"We are thrilled to have a chance to work directly with the JPL science team," said Dr. Anthony Haymet, a scientist and Chemistry professor at the University of Houston. "The fantastic thing about this collaboration is that they have been able to work on these very tough problems in the context of a genuine and urgent need for understanding one of the worst polluted urban areas in the United States of America."

The experiment is a collaborative effort of several existing projects involving Houston's air pollution and part of an ongoing series of experiments conducted by the state of Texas. The first of these efforts was the Texas 2000 Air Quality Study involving more than 150 scientists and engineers from 40 public, private, and academic institutions across the United States.

The Multi-angle Imaging SpectroRadiometer is one of several Earth-observing experiments aboard Terra, launched in December 1999. More information is available online at http://www-misr.jpl.nasa.gov. The Terra mission is part of NASA's Earth Science Enterprise, a long-term research effort dedicated to understanding and protecting our home planet. JPL is a division of the California Institute of Technology in Pasadena.