Many have thought for years that trees are to blame for acid rain and now we have evidence that backs it up. Up to half of the acidity in rainfall over the US in summer does come from compounds given off by plants.
Here is how it works. Formic acid is produced from burning fossil fuels and biomass, also when plant compounds called terpenoids are oxidized by sunlight. The latter contributes to acid rain in remote regions and most of it ends up in the atmosphere, compared to what we could trace back to the source. Until now that is.
Thanks to satellite data we now know that more than 100 million tons of formic acid is produced naturally every year. No one had any idea that it could be so much. That number is 10 times the total from all known sources combined.
Interestingly, the highest densities are found above tropical and northern boreal forests during the spring growing season. It was once thought that ants might be a missing source, but the cause was narrowed down to terpenoids using lab studies and modeling.