Direct Measurement of the OH + HO₂ Cross Reaction Using Infrared Kinetic Spectroscopy

The cross reaction between the free radicals OH and HO₂ plays a critical role in a wide range of environments, including combustion and the atmospheres of Earth and Mars. In the middle atmosphere on Earth, it is the ultimate terminating step of odd-hydrogen chemistry, which has significant implications for the abundance of ozone. Recent laboratory measurements have brought established rate coefficients into question, introducing additional uncertainties into chemical models of the upper stratosphere and mesosphere. In this work, we present measurements of the rate coefficient from 265–350 K using high-resolution infrared and ultraviolet kinetic spectroscopy. A scheme was developed which almost entirely avoids interfering reactions, providing a direct measurement of the decay of OH due to HO₂ and reduces uncertainties associated with absolute calibration of OH and HO₂ concentrations. We find a room temperature rate coefficient of k(295) = (9.0 ± 1.9) × 10^–11 cm³ molecule^–1 s^–1 (1σ error) and a slight temperature dependence over 265–330 K with an E/R of −155 ± 50 K.