We report bidirectional exchange of volatile acids, including isocyanic and alkanoic acids, over a pine forest across multiple seasons. The exchange velocity of these acids is well correlated with dew point depression, suggesting an equilibrium‐driven continuum of flux. Wetness on forest surfaces impacts the vertical exchange of gases, and we suggest that water films and droplets drive equilibrium partitioning, with acids being solvated in surface wetness and released through evaporation. Despite their volatility, these acids partition into neutral‐to‐alkaline aqueous films, consistent with reported dew pH. This relationship between exchange velocity and dew point depression holds for a wetter mixed forest, but not a very dry orchard. Dew point depression is an excellent indicator of acid fluxes so long as the canopy is occasionally wetted. Plain Language Summary Volatile, water‐soluble organic acids are exchanged between ecosystems and the atmosphere, but the processes underlying that exchange are poorly understood. We present evidence that water films and droplets on ecosystem surfaces uptake atmospheric organic acids by an equilibrium solvation process. As solar radiation warms ecosystem surfaces, surface wetness evaporates releasing absorbed acids into the gas phase. Uptake by surface wetness can cause net canopy‐scale organic acid deposition. Organic acids released by evaporating surface wetness can cause net canopy‐scale emissions.

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地表湿度对森林挥发性有机酸交换的意外控制

我们报告了跨多个季节在松树林中的挥发性酸（包括异氰酸和链烷酸）的双向交换。这些酸的交换速度与露点降低密切相关，表明存在平衡驱动的连续通量。森林表面的湿度会影响气体的垂直交换，我们建议水膜和水滴驱动平衡分配，酸在表面湿度中被溶剂化并通过蒸发释放。尽管它们具有挥发性，但这些酸会分配成中性至碱性的水膜，与报道的露水 pH 值一致。交换速度和露点降低之间的这种关系适用于较湿的混交林，但不是非常干燥的果园。露点下降是酸通量的极好指标，只要树冠偶尔被弄湿。简单语言总结 挥发性、水溶性有机酸在生态系统和大气之间进行交换，但人们对这种交换的潜在过程知之甚少。我们提出证据表明生态系统表面上的水膜和水滴通过平衡溶剂化过程吸收大气有机酸。随着太阳辐射使生态系统表面变暖，表面水分蒸发，将吸收的酸释放到气相中。地表湿气的吸收会导致净冠层有机酸沉积。蒸发表面湿度释放的有机酸会导致净冠层排放。我们提出证据表明生态系统表面上的水膜和水滴通过平衡溶剂化过程吸收大气有机酸。随着太阳辐射使生态系统表面变暖，表面水分蒸发，将吸收的酸释放到气相中。地表湿气的吸收会导致净冠层有机酸沉积。蒸发表面湿度释放的有机酸会导致净冠层排放。我们提出证据表明生态系统表面上的水膜和水滴通过平衡溶剂化过程吸收大气有机酸。随着太阳辐射使生态系统表面变暖，表面水分蒸发，将吸收的酸释放到气相中。地表湿气的吸收会导致净冠层有机酸沉积。蒸发表面湿度释放的有机酸会导致冠层净排放。