Some of these questions have immediate practical importance. For example, Wuebbles worries that many proposed alternatives to CFCs are not so benign as scientists have supposed. Using MOZART-2, he has set his team to examining them more rigorously before an international timetable requires countries to choose which ones to allow. Many of the new compounds that chemists have engineered are like CFCs, yet flimsier, so the molecules degrade before they reach the stratospheric ozone layer almost 100 percent of the time. Nevertheless some kinds of degradation reactions stall, Wuebbles warns, generating not-yet-neutralized compounds that are heartier than their progenitors. Wuebbles says such scenarios need to be considered. To make his case, he points to results his team reported in the July 16 Journal of Geophysical Research.

From manuals and chemistry reports, the researchers culled constants for calculating how fast and in what way n-propyl-bromide (a CFC substitute) and its breakdown products react with different molecules in the atmosphere. Feeding the numbers to MOZART-2, team members then simulated the release of this compound from the equator and points north and south. The high and low latitude launches took n-propyl-bromide on a slow journey to the stratosphere that few molecules survived in any form. But trips from the tropics were fast and delivered many molecules in a merely maimed state—in which they were as dangerous as CFCs.

The results are surprising, says Jeff Cohen, chief of the EPA branch formulating U.S. recommendations on CFC alternatives for the United Nations. He says the inventors of n-propyl-bromide are already manufacturing and selling it in anticipation of a multinational thumbs up. "I think some countries will be inclined not to allow its use," he says.