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Urban air quality — The difference between night and day

A collaboration of researchers measured the differences in air quality at sunrise in Phoenix, Arizona, in one of the most comprehensive studies ever done on the vertical structure of air pollution over a major urban area



Air quality over Phoenix, Arizona.

March 25, 2002—Air pollution is no laughing matter. For large urban areas, it can be a coughing, sneezing, wheezing matter. When air pollution exceeds federal air quality standards, it can trigger a host of human health problems, not to mention serious regulatory issues for a municipality. Now, armed with data from an air quality study in Phoenix, Arizona, conducted by the Pacific Northwest National Laboratory (PNNL) and collaborators in the summer of 2001, researchers are studying air pollution at sunrise in an effort that could help urban areas better manage their air quality problems.

During a two-week field campaign, researchers measured pollution buildup close to the ground at night to understand its effect on air quality the following day. The full extent of this effect still remains to be analyzed, but data reviewed to date adds an important piece to the puzzle as scientists seek to better understand how ozone and other air pollutants mix and form near the ground.

Data were collected from scores of ground-based and elevated sensors in and around Phoenix, making it one of the most comprehensive studies ever done on the vertical structure of air pollution over a major urban area. Air chemistry and meteorological measurements were taken with instruments installed on the mountaintops to the north and south of Phoenix, but they were also installed at various elevations using high-rise buildings in downtown areas of Phoenix. In addition tethered and free-flying balloons, remote sensing wind radars, and other research devices were used to gather chemical and meteorological information.

These measurements were augmented with data collected by the Department of Energy's (DOE's) Gulfstream-1 aircraft, operated by PNNL, which made repeated early morning, low-level flights at several altitudes. Collectively these instruments monitored the changes in air quality as they occurred, from ground level to more than 10,000 feet above the ground.

"We were able to see changes in air quality taking place near the surface as well as aloft during the morning hours and learned that these changes are determined by a complex combination of processes. We saw pollutants trapped near the ground mixing upward, while pollutants from above mixed downward at the same time. These two actions were combined with a large number of chemical changes triggered by sunlight," said PNNL atmospheric scientist Carl Berkowitz.

The data show that cooler nighttime temperatures trap auto emissions from early morning commuters and other pollutants in a shallow layer close to the ground in the Phoenix basin. When the sun comes up and warms the ground, vertical mixing becomes vigorous in a growing layer near the surface and the pollutants are quickly mixed upward throughout this layer. Concentrations of ozone at approximately 6,000 feet above ground were as much as (or more than) 50% higher than those found at lower elevations during the morning hours. On a number of days, the observed rate of increase in ozone appeared to be related to the period of time that vertical mixing became well developed, providing evidence of a close link between chemical processes and the onset of mixing.

The DOE is concerned about air pollution derived from energy consumption as it impacts National energy policy. Most of the air pollution in the Phoenix area is the result of the use of automobiles. The dry, Southwest climate keeps night time pollutants trapped near the ground until warmed by the sun the following day, making it an excellent place to do such a study. The scientific knowledge to come from this study is expected to be applicable to other cities in the arid Southwest as well.

The data collected from the Phoenix study are still being analyzed to determine how these various mechanisms interact under a wide range of meteorological conditions. "Sorting out those interactions will provide a better scientific basis for the design of strategies to help improve air quality over large urban areas," added PNNL scientist Chris Doran.

Collaborators with PNNL in this study include Argonne National Laboratory, the Arizona Department of Environmental Quality (ADEQ), Arizona State University, Battelle Columbus, Brookhaven National Laboratory, Lawrence Livermore National Laboratory, Loyola University at Chicago, the University of Alaska at Fairbanks, and the University of California at Los Angeles.

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