Controlling ozone levels within cities.

• Position: Air Pollution - Brief Article

The most critical factor affecting ozone concentrations in U.S. cities is beyond the control of local regulators. It is the amount of ozone that drifts into a city from outside its boundaries, according to an Ohio State University, Columbus, nationwide study. Moreover, the number and placement of road intersections within a city is the largest determinant of carbon monoxide levels.

The researchers used a comprehensive statistical analysis to determine what most influenced levels of ozone and carbon monoxide--both major pollutants--in more than 100 U.S. cities, ranging from car emissions to weather to population. Their findings also indicated the positive impact that public transportation, forests, and farmland in metropolitan areas can have in lowering levels of ozone and carbon monoxide.

Jean-Michel Guldmann, coauthor of the study and professor of city and regional planning, pointed out that researchers knew ozone was a national problem because it decays slowly and can travel for long distances. However, the study confirmed that so-called background ozone--that which could not be attributed to local sources--was the most-important factor in determining total ozone levels in cities. "Our results confirm that ozone is transported over long distances with minimal decay. Local efforts are not going to control ozone in a city. There has to be a...