Yesterday, NASA announced that its scientists have studied the unexpected persistence of an ozone-destroying chemical and have come to the conclusion that there must be some unidentified source of the substance. The item in question, carbon tetrachloride (CCl 4 ), was banned in 1989 as part of the Montreal Protocol, which was intended to reduce the levels of ozone-destroying chemicals in the atmosphere.

In general, the protocol has worked; atmospheric levels of the chemicals covered by the treaty have dropped, and there are indications that Antarctica's annual ozone hole has stabilized. Levels of carbon tetrachloride have also dropped. The hitch is that they're not dropping as fast as we think they should, based on what we know of atmospheric chemistry.

That situation implies that we have one of two things wrong: either there are sources of the chemical that are still leaking it into the atmosphere, or our understanding of what's going on in the atmosphere is wrong. But NASA scientists have now taken data about existing sources and plugged them into a chemistry-climate model and concluded that the data best fits an unknown source. By their own admission, the scientists are mystified about what that source could be. Qing Liang of NASA's Goddard Space Flight Center was quoted as saying, "It is now apparent there are either unidentified industrial leakages, large emissions from contaminated sites, or unknown CCl 4 sources."

Given that carbon tetrachloride is a neurotoxin and causes liver disease, it's generally a highly regulated substance. So the source might be found in locations where regulation is lax, making identifying the source that much more of a challenge.