Intercomparison of peroxy radical measurements obtained at atmospheric conditions by laser-induced fluorescence and electron spin resonance spectroscopy

Abstract. Measurements of hydroperoxy radical (HO₂) and organic peroxy radical (RO₂) concentrations were performed by two different techniques in the atmospheric simulation chamber SAPHIR in Jülich, Germany. The first technique was the well-established Matrix Isolation Electron Spin Resonance (MIESR), which provides absolute measurements with a time resolution of 30 min and high accuracy (10%, 2 σ). The other technique, ROxLIF, has been newly developed. It is based on the selective chemical conversion of RO_x radicals (HO₂ and RO₂) to OH, which is detected with high sensitivity by laser-induced fluorescence (LIF). ROxLIF is calibrated by quantitative photolysis of water vapor at 185 nm and provides ambient measurements at a temporal resolution of 1 min and accuracy of 20% (2 σ). The measurements of HO₂ and RO₂ obtained by the two techniques were compared for two types of atmospheric simulation experiments. In one experiment, HO₂ and CH₃O₂ radicals were produced by photooxidation of methane in air at tropospheric conditions. In the second experiment, HO₂ and C₂H₅O₂ were produced by ozonolysis of 1-butene in air at dark conditions. The radical concentrations were within the range of 16 to 100 pptv for HO₂ and 12 to 45 pptv for RO₂. Good agreement was found in the comparison of the ROxLIF and MIESR measurements within their combined experimental uncertainties. Linear regressions to the combined data set yield slopes of 1.02±0.13 (1 σ) for RO₂ and 0.98±0.08 (1 σ) for HO₂ without significant offsets. The results confirm the calibration of the ROxLIF instrument and demonstrate that it can be applied with good accuracy for measurements of atmospheric peroxy radical concentrations.