Trees do bear some blame for acid rain

Ronald Reagan was right. Well, nearly. One of the former US president's most ridiculed statements was that acid rain came from trees. Up to half the acidity in rainfall over the US in summer does indeed come from volatile compounds given off by plants – just not the compounds Reagan was thinking of.

Formic acid is produced when we burn fossil fuels and biomass, and when plant compounds called terpenoids are oxidised by sunlight. It contributes to acid rain in remote regions, but more of it ends up in the atmosphere than we could trace back to a source. Until now.