Atmospheric acidity is increasingly determined by carbon dioxide and organic acids^1,2,3. Among the latter, formic acid facilitates the nucleation of cloud droplets⁴ and contributes to the acidity of clouds and rainwater^1,5. At present, chemistry–climate models greatly underestimate the atmospheric burden of formic acid, because key processes related to its sources and sinks remain poorly understood^2,6,7,8,9. Here we present atmospheric chamber experiments that show that formaldehyde is efficiently converted to gaseous formic acid via a multiphase pathway that involves its hydrated form, methanediol. In warm cloud droplets, methanediol undergoes fast outgassing but slow dehydration. Using a chemistry–climate model, we estimate that the gas-phase oxidation of methanediol produces up to four times more formic acid than all other known chemical sources combined. Our findings reconcile model predictions and measurements of formic acid abundance. The additional formic acid burden increases atmospheric acidity by reducing the pH of clouds and rainwater by up to 0.3. The diol mechanism presented here probably applies to other aldehydes and may help to explain the high atmospheric levels of other organic acids that affect aerosol growth and cloud evolution.

大气酸度越来越多地由二氧化碳和有机酸^1,2,3决定。在后者中，甲酸促进云滴的成核⁴并有助于云和雨水^1,5的酸度。目前，化学-气候模型大大低估了甲酸的大气负担，因为与其源和汇相关的关键过程仍然知之甚少^2,6,7,8,9. 在这里，我们展示了大气室实验，这些实验表明甲醛通过涉及其水合形式甲二醇的多相途径有效地转化为气态甲酸。在温暖的云滴中，甲烷二醇经历快速除气但缓慢脱水。使用化学-气候模型，我们估计甲烷二醇的气相氧化产生的甲酸是所有其他已知化学来源加起来的四倍。我们的研究结果协调了甲酸丰度的模型预测和测量。额外的甲酸负荷通过将云和雨水的 pH 值降低多达 0.3 来增加大气酸度。这里介绍的二醇机制可能适用于其他醛类，并可能有助于解释影响气溶胶生长和云演化的其他有机酸的高大气含量。