"The concentration of bromine is much smaller in the tropics, so it does not work," he says. "We want to understand why it's so low. What controls ozone on a global scale?"

But Jacob is not discouraged. "We look for anomalies like this and try to explain them," Jacob says. "When you do, it is often indicative of a phenomenon on a larger scale."

Jacob has some other ideas in the works to explain the situation in the tropics, including one which involves sea salt.

"Sodium chloride from sea spray releases chlorine which may go through a series of steps, similar to the steps of bromine, to deplete ozone," Jacob says. "It's all theoretical work here [at Harvard], but experimentalists at [the University of Virginia] and MIT have observed ozone-eating chlorine gases over the ocean."

Other atmospheric chemists are skeptical. California State University at Fullerton Professor of Chemistry Barbara Finlayson-Pitts, who is currently researching the chlorine hypothesis, says chlorine is more likely to create ozone than destroy it.

According to Finlayson-Pitts, organic molecules such as hydrocarbons participate in a string of reactions involving hydroxide (OH) to eventually form nitrogen dioxide and ozone.

The only ozone-creating molecule generated by humans is nitric oxide, but the molecule must be first converted to nitrogen dioxide before it can form ozone.

Finlayson-Pitts and her colleagues are studying the possibility that chlorine can serve as a substitute for hydroxide, reacting with organics to hasten the formation of ozone. And, she says, the chlorine reaction might take place instantaneously, while the hydroxide reaction can take up to two weeks.

But Finlayson-Pitts concedes that chlorine can react to destroy ozone as well as to create it.

"It depends on the conditions," says Finlayson-Pitts. "If there are enough organics, the chlorine will react with them quickly to produce ozone rather than destroying it."

The Role of Tropospheric Ozone

Despite its beneficial effects, tropospheric ozone has gained far more publicity for its role as a major contributor to air pollution and smog in Los Angeles and other large cities.

Ozone in the troposphere and the stratosphere play very different roles, which initially seem unconnected. Jacob says this is not the case. "Ozone is a very complicated molecule, just in terms of its environmental effects," he says.

One of ozone's chief tasks is to clean up tons of junk thrown up into the air by human consumption.

"We release on the order of a billion tons of material into the atmosphere," Jacob says. "The atmosphere can't hold that much, and what goes up must come down."