Development and optimization of a thermally controlled UV/H₂O₂ reactor for controllable gas-phase hydroxyl radical...

Hydroxyl radicals (•OH) dominate atmospheric oxidation chemistry, yet their controlled and reproducible generation in laboratory systems remains technically challenging. In this work, a gas-phase UV/H₂O₂-based Hydroxyl Radical Generation System (HyRaG) was developed and experimentally optimized to provide stable and tunable •OH production under precisely regulated thermal and flow conditions. The HyRaG integrates three functional modules: a quartz double-cylinder photolysis reactor, a closed-loop dual-channel temperature control unit capable of maintaining outlet temperature variations within ±1°C during continuous operation, and a mass-flow-regulated N₂/H₂O₂ vapor delivery system. An outlet-based sulfur dioxide (SO₂) decay method was employed to quantify •OH generation under carrier gas flow rates of 10-300 mL min^-1 and temperature of 15-50°C, yielding •OH concentrations ranging from 0.41-2.04 × 10¹¹ molec.cm^-3. Single-factor experiments revealed parabolic dependences of •OH generation on both operational parameters, with optimal conditions near 100 mL min^-1 and 40°C. Further optimization using Response Surface Methodology (RSM) based on a Central Composite Design (CCD) produced a robust second-order predictive model (R² = 0.9735), demonstrating significant quadratic effects of both temperature and carrier gas flow rate, while their interaction was statistically insignificant. By decoupling radical generation from the reaction environment and stabilizing thermal conditions, HyRaG enables a controllable and reproducible oxidizing environment for atmospheric simulation studies and provides a scalable platform for mechanistic investigations of radical-driven gas-phase chemistry.