The hydroxyl radical (OH) sets the oxidative capacity of the atmosphere and, thus, profoundly affects the removal rate of pollutants and reactive greenhouse gases. While observationally derived constraints exist for global annual mean present-day OH abundances and interannual variability, OH estimates for past and future periods rely primarily on global atmospheric chemistry models. These models disagree ± 30% in mean OH and in its changes from the preindustrial to late 21st century, even when forced with identical anthropogenic emissions. A simple steady-state relationship that accounts for ozone photolysis frequencies, water vapor, and the ratio of reactive nitrogen to carbon emissions explains temporal variability within most models, but not intermodel differences. Here, we show that departure from the expected relationship reflects the treatment of reactive oxidized nitrogen species (NO_y) and the fraction of emitted carbon that reacts within each chemical mechanism, which remain poorly known due to a lack of observational data. Our findings imply a need for additional observational constraints on NO_y partitioning and lifetime, especially in the remote free troposphere, as well as the fate of carbon-containing reaction intermediates to test models, thereby reducing uncertainties in projections of OH and, hence, lifetimes of pollutants and greenhouse gases.

羟基自由基 (OH) 决定了大气的氧化能力，因此深刻影响污染物和反应性温室气体的去除率。虽然观测得出的全球平均现在 OH 丰度和年际变化存在限制，但过去和未来时期的 OH 估计主要依赖于全球大气化学模型。这些模型在平均 OH 及其从工业化前到 21 世纪后期的变化方面存在 ± 30% 的差异，即使在强制使用相同的人为排放的情况下也是如此。考虑臭氧光解频率、水蒸气和活性氮与碳排放的比率的简单稳态关系解释了大多数模型中的时间变化，但不能解释模型间的差异。这里，_y ) 以及在每种化学机制中发生反应的排放碳的比例，由于缺乏观测数据，这仍然鲜为人知。我们的研究结果意味着需要对 NO _y分配和寿命进行额外的观测限制，特别是在偏远的自由对流层中，以及含碳反应中间体的命运以测试模型，从而减少 OH 预测的不确定性，从而减少寿命污染物和温室气体。