Acid production in clouds

Formic acid contributes to atmospheric acidity, but known sources cannot fully account for how much of it is present in the atmosphere. This paper finds that the conversion of formaldehyde via a multiphase pathway in cloud droplets is the dominant source of formic acid, producing up to four times more formic acid than all other known sources combined. The study also finds that this source mechanism can increase the acidity of clouds and rainwater globally.