Collinear UV–mid-IR cross-validation updates HO 2 self-reaction kinetics and implications for tropospheric HO x and H 2 O 2

The hydroperoxy radical (HO 2 ) self-reaction is an important radical termination pathway under low-NO tropospheric conditions and a major gas-phase source of hydrogen peroxide (H 2 O 2 ), a key oxidant reservoir. Here we determine HO 2 self-reaction kinetics across atmospherically relevant temperatures and pressures and over a range of water vapor and methanol concentrations, using a collinear photolysis–probe geometry with cross-validated ultraviolet and mid-infrared absorption. The resulting rate coefficients are consistently higher than current NASA/JPL recommendations, by ~50% under humid boundary-layer conditions and by up to a factor of two under upper-tropospheric conditions, owing primarily to revised pressure-dependent and water-enhanced contributions. Relative to NASA/JPL, implementation of the updated HO 2 self-reaction kinetics in the global chemistry–transport model STOCHEM-CRI decreases modeled near-surface HO 2 by up to 12% (1.5 ppt) and increases H 2 O 2 by up to 28% (900 ppt). The largest OH response occurs in the upper troposphere, where OH increases by up to 8%. This work provides a comprehensive experimental basis for parameterizing HO 2 self-reaction kinetics in modeling tropospheric HO x and H 2 O 2 , while also supporting further studies of HO 2 -driven radical chemistry.