Formaldehyde, HCHO, is the highest-volume carbonyl in the atmosphere. It absorbs sunlight at wavelengths shorter than 330 nm and photolyses to form H and HCO radicals, which then react with O₂ to form HO₂. Here we show HCHO has an additional HO₂ formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO₂ at low pressures by cavity ring-down spectroscopy and indirectly detect HO₂ at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO₂ to photophysical oxidation (PPO): photoexcited HCHO relaxes non-radiatively to the ground electronic state where the far-from-equilibrium, vibrationally activated HCHO molecules react with thermal O₂. PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O₂ pressure.

甲醛 (HCHO) 是大气中含量最高的羰基化合物。它吸收波长小于330 nm的阳光并光解形成H和HCO自由基，然后与O ₂反应形成HO ₂。在这里，我们证明 HCHO 具有额外的 HO ₂形成途径。在光解能量低于自由基形成的能量阈值时，我们通过腔衰荡光谱直接检测低压下的HO ₂ ，​​并通过傅里叶变换红外光谱最终产物分析间接检测1巴下的HO _{2 。}在电子结构理论和主方程模拟的支持下，我们将这种 HO ₂归因于光物理氧化 (PPO)：光激发的 HCHO 非辐射弛豫到基电子态，其中远离平衡、振动激活的 HCHO 分子与热 O 2 发生_反应。PPO 可能是对流层化学中的一般机制，并且与光解不同，PPO 将随着O ₂压力的增加而增加。