Renyi Zhang, associate professor of atmospheric sciences, modeled Houston's air during the summer months. He found that at night, the city's ozone level (rated in PPBs, or parts per billion) was near zero, but during the day it zoomed to more than 200 PPBs, far higher than the U.S. standard of 120.

Zhang's findings are reported in the current issue of the Proceedings of the National Academy of Science.

The reason for the high daytime rating, Zhang says, can be directly traced to the huge refineries and petrochemical complexes in the area, plus the large amount of auto exhaust in Harris County. The highest ozone levels were found around the city's southeastern edge near the huge petrochemical plants.

"These plants emit large amounts of highly reactive volatile organic compounds and nitric oxides," Zhang says. "At midday, the ozone readings are very high because of the industrial emissions, coupled with auto exhaust. It creates very big problems for Houston's air, and ozone levels are far above acceptable federal standards. At night, however, the ozone readings register almost zero, forming an urban scale ozone hole which is caused by nitrogen oxide emissions from the refineries and petrochemical plants and power plants. Nitrogen oxides eat up ozone at night.

"It all means a significant health risk is possible for people in this area unless something is done."

Some studies have previously shown Houston to have some of the worst air pollution in the United States. The city also has one of the worst ozone levels in the country, probably due to it size (No.4 in U.S. population) and the fact that 50 percent of the nation's petroleum refining
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Contact: Keith Randall
kr@univrel.tamu.edu
979-845-4644
Texas A&M University
14-Apr-2004