Collinear UV-mid-IR cross-validation updates HO2 self-reaction kinetics and implications for tropospheric HOx and H2O2.

The hydroperoxy radical (HO2) self-reaction is an important radical termination pathway under low-NO tropospheric conditions and a major gas-phase source of hydrogen peroxide (H2O2), a key oxidant reservoir. Here we determine HO2 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 HO2 self-reaction kinetics in the global chemistry-transport model STOCHEM-CRI decreases modeled near-surface HO2 by up to 12% (1.5 ppt) and increases H2O2 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 HO2 self-reaction kinetics in modeling tropospheric HOx and H2O2, while also supporting further studies of HO2-driven radical chemistry.