Tokyo, like other major cities, has too many people, too many cars, and smog. But Koji Takeuchi thinks he can alleviate the smog problem. Takeuchi, a chemist at the National Institute for Resources and Environment in Tsukuba, wants to line Japanese highways with smog-collecting panels he and his colleagues have designed. Initial field tests have been promising, and Takeuchi says the panels should be inexpensive to install and maintain.
A bit over a foot long, the panels are made of Teflon fibers that have been mixed with carbon powder and titanium dioxide (a chemical commonly found in paint) and rolled into sheets. Sunlight striking a panel boosts electrons in the titanium dioxide to higher energy levels, making the molecules more reactive. The molecules grab oxygen and water vapor from the air and hold them on the panel. The water and oxygen then react with nitrogen oxides and sulfur oxides--smog components released in car exhaust- -that drift over the panels. The carbon, Takeuchi believes, also attracts pollutants, holding on to them temporarily until they eventually react with the water vapor and oxygen.
The reaction converts the pollutants into nitric acid and sulfuric acid, which collect on the surface of the panel. About 60 percent of the acids run off the panels when it rains and drain into a ditch filled with lime, where they are neutralized. The remainder of the acids stick to the panels, which need to be cleaned periodically.
Of course, Takeuchi says, the panels are not the ultimate answer to smog control. And they may not work everywhere. For example, some parts of the American Southwest may be too dry. Without rain, the panels will become clogged with acidic gunk. The sheets require sunlight and rainwater to work, says Takeuchi. Fortunately, we have a lot of rain in Japan.
In a field test in Tokyo, the filters lowered nitrogen oxide and sulfur oxide levels along a 30-foot stretch of highway by at least 20 percent, says Takeuchi. He says he and his colleagues have recently made their sheets three times more efficient--at least in the lab. Even a 20 percent reduction of nitrogen oxide will greatly increase the areas where the environmental standards are satisfied in Japan, says Takeuchi. In the future, he hopes to use the filters in tunnels with artificial sunlight and to coat parts of buildings and highway dividers so they can act like filters.