Sir

The News story “Ozone treaty ‘must tackle CFC smuggling’” is seriously misleading when it states that “Molecule for molecule, methyl bromide is considered at least 50 times more destructive to the ozone layer than chlorine from CFCs”1.

Because of oceanic uptake and removal by oxygen-hydrogen radicals in the lower atmosphere, only about 4 per cent of methyl bromide molecules released at the surface survive the upward journey into the stratospheric ozone layer. Second, the dominant CFCs (chlorofluorocarbons) have two or three chlorine atoms per molecule. Further, only about 35 per cent of atmospheric methyl bromide can be shown to be under human control2. Therefore, to say that atmospheric methyl bromide is 50 times more destructive than CFCs is incorrect and misleading, especially when focusing on anthropogenic effects. Once in the stratosphere, a bromine atom does destroy about 50 times as many ozone molecules as does a chlorine atom.

Atmospheric scientists understand this distinction but it may help many readers and non-scientist representatives to the Montreal Protocol to clarify this point.

Discoveries of the past several years demonstrate that removal of atmospheric methyl bromide is more rapid than had been thought, and imply the existence of unidentified sources. Are other human-controlled sources at play or are the missing sources mostly natural? Research on such questions should continue, partly because of the need to gauge how much ozone protection we purchase by banning the substance, and partly because the questions are becoming more intriguing. The spirit and provisions of the original Montreal Protocol demanded continued research and periodic assessments of the implications of new results. I hope that we can continue to exercise these provisions.