Study Of A Cavity Ring-Down Spectrum Instrument For HO₂ Radical Reaction Kinetics Measurements

Hydroperoxyl radical（HO₂）is an important reaction intermediate in atmospheric chemistry,and a key mediator of primary pollution removal and secondary pollutant generation.It can react with nitric oxide（NO）to form hydroxyl radicals（OH）and nitrogen dioxide（NO₂）,forming the HOx（OH+HO₂）cycle,which impacts the atmospheric oxidation capacity and controls the net synthesis of tropospheric ozone（O₃）at the same time.However,because of the rapid reaction rates and complex mechanisms of HO₂ radicals in the atmosphere,laboratory kinetic studies of HO₂ can help to understand atmospheric photochemical reactions and offer a theoretical basis for atmospheric pollution and climate change.With the development of laser technology,laser absorption spectroscopy has gradually become an efficient means for direct detection of radicals.Continuous-wave cavity ring-down spectroscopy（cw-CRDS）is a highly sensitive laser spectroscopy technique that increases the path of laser interaction with the absorbing material by using a high-finesse optical resonant cavity,which can achieve an effective optical range of kilometers.Moreover,CRDS measures the behavior of the laser intensity in the cavity’s temporal behavior,and is therefore independent of light intensity fluctuations,making it a calibration-free spectroscopic measurement technique.In this paper,a study of HO₂ radical measurement device based on cavity ring-down spectroscopy technique is carried out as follows.Ⅰ.A flash photolysis-cavity ring-down spectroscopy experimental set-up was established using a 266 nm Nd:YAG laser to photolysis a mixture of O₃ and acetylene（C₂H₂）to produce HO₂ radicals,and a near-infrared continuous wave cavity ring-down spectrum was used for direct measurements of the HO₂ decay curve.The mode coupling between the laser and the cavity is achieved by scanning the cavity length of the self-designed optical resonant cavity,and the ring-down time is obtained by fitting the decay curve based on the linear regression summation（LRS）algorithm.The data acquisition system is optimized and the cavity ring-down time is measured for a long time.The minimum detectable absorption of the system of 5.05×10^-11 cm^-1 is achieved with 20.3 km effective optical path and 6.25 s acquiring time.Ⅱ.Based on the flash photolysis-cavity ring-down spectroscopy system,the characteristic absorption of HO₂ radicals at the position 6638.203 cm^-1 was measured,and an absorption spectrum with a resolution of 0.002 cm^-1 was obtained.The absorption cross section σ of HO₂ at 6638.203 cm^-1 was obtained by measuring the decay of HO₂ self-reaction at 20 mbar as 3.3×10^-19 cm²/molecule,corresponding to an absorption spectrum intensity S equal to 6.02×10^-21 cm^-1/（molecule·cm-2）.The measured results are consistent with the values reported in the literature,indicating that this device can be used to carry out HO₂ spectroscopy and kinetic experiments,providing a powerful tool for the study of key chemical reaction processes in HO₂.