Photophysical oxidation of HCHO produces HO 2 radicals.

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 O 2 to form HO 2 . Here we show HCHO has an additional HO 2 formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO 2 at low pressures by cavity ring-down spectroscopy and indirectly detect HO 2 at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO 2 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 O 2 . PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O 2 pressure.