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One proxy is methyl chloroform, banned years ago for contributing to the ozone hole.

But a close look at the methyl chloroform trend shows that the decline isn’t as steady or certain as many have assumed.

However, the concentration of atmospheric hydroxyl radicals is extremely difficult to measure, so it must be inferred from measurements of the abundance of other species, like methyl chloroform.

Past studies of atmospheric methyl chloroform have indicated that OH can undergo large annual shifts in concentration, although atmospheric models predict a less variable history.

In 1990 the Montreal Protocol was broadened to include two potent industrial solvents not covered in the original agreement: methyl chloroform and carbon tetrachloride.