Known emitters of chloromethane do not produce enough of it to account for its atmospheric abundance.

Tropical forests emit much lower amounts of an ozone-depleting compound than scientists previously realized, suggesting the presence of another, as-yet unidentified source producing the destructive chemical.

The most prevalent natural compound that attacks Earth’s ozone layer is chloromethane, emitted by the Amazon rainforest and other ecosystems. Compared with chloromethane molecules that waft from forests, those that originate from fires, soils and oceans tend to include more carbon atoms of a heavier variety, or isotope. This distinction enables researchers to trace the origins of the gas in the atmosphere.

Enno Bahlmann at the University of Hamburg, Germany, and his colleagues ran experiments in a smog chamber to determine how chloromethane molecules of varying masses break down in the atmosphere. The team then used a computer model to analyse the composition of atmospheric chloromethane and to calculate its emissions from an array of sources.

The results suggest that the chloromethane emitted by tropical vegetation amounts to only around 670,000 tonnes each year — three times less than previous estimates. The revised figure implies that roughly 1.5 million tonnes of the chemical come from unknown sources.