Chicago's Ground-Level Ozone Has Unsuspected Cause

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Newswise — University of Chicago meteorologists are questioning the widely held explanation for what triggers high concentrations of ground-level ozone, which during the summer can present a serious health threat to asthma sufferers who live in urban areas. Gidon Eshel, Assistant Professor in Geophysical Sciences, and Joe Bernstein, an alumnus of the University's undergraduate College who begins graduate studies at Harvard University next autumn, have submitted a paper on their finding to the Journal of Applied Meteorology.

Scientists have widely accepted the idea that elevated concentrations of ground-level ozone and other pollutants are caused by thermal inversions-the rise, rather than decrease, of atmospheric temperature with height. But the University of Chicago team has found that advective heating-winds that bring in more warmth than they take away-are mostly responsible for ground-level ozone.

"Most of our knowledge about air pollution is mostly based on relatively few locations, such as Los Angeles, which is a closed basin, downwind of the ocean. This configuration lends itself very readily to thermal inversions," Eshel said.

Under normal conditions, warm surface air rises while cool air aloft sinks, thus flushing pollutants that have accumulated near the ground.

"In Illinois, with vast stretches of flat topography, the thermal inversion mechanism doesn't really hold," Eshel said. "We've found that ground-level ozone incidence is very weakly related to the incidence of thermal inversions."

Ground-level ozone differs from naturally occurring stratospheric ozone that forms in the upper atmosphere and protects organisms from the sun's harmful ultraviolet rays. Ground-level ozone, by contrast, forms via a chemical reaction between volatile organic compounds and nitrogen oxides-produced by gasoline-burning engines and other technological processes-in the presence of sunlight.

Once ground-level ozone forms, it can irritate the throat and lungs. Motivating Eshel's research is his desire to develop better forecasting methods for extreme ground-level ozone events.

"By various estimates, up to 15 percent of all Chicago children are affected by asthma. A significant portion of those actually end up in the emergency room under elevated ozone conditions that aggravate their predisposition to asthma," Eshel said.

In Illinois, nearly 730,000 people suffered from asthma in 2001, according to a report issued by the American Lung Association in March 2003. Nationally, more than 20 million Americans suffered from asthma in 2001, including more than 6 million children.

"It's a clear societal problem that disproportionately plagues those who are already plagued by other problems, primarily the lack of money," Eshel said.

The problem has led him to collaborate with his University of Chicago colleagues John Frederick, Professor in Geophysical Sciences, and Edward Naureckas, Assistant Professor in Clinical Medicine, to more establish scientifically the link between ground-level ozone and asthma incidence. Their work is funded by a grant from the Environmental Protection Agency to the University of Chicago's Center for the Integration of Statistical and Environmental Science.

Eshel has devised an elaborate calculation that can predict extreme ground-level ozone events approximately 50 percent of the time. This seems to be an improvement, at least for now, over the method currently used by the EPA. Eshel's approach is statistical. It attempts to make predictions by identifying meteorological patterns that seem to be linked to the formation of ground-level ozone.

The EPA follows what's called a combined dynamical chemical modeling approach. This approach attempts to integrate the characteristics and behavior of the atmosphere at periodic intervals over a given region and project them forward in time. Unfortunately, the dynamical method awaits more powerful computers and better models to improve its accuracy.

"Over the long run there's no question that the dynamical method will carry the day, it's just that we need answers now!" Eshel said.