Detection of HO₂ by laser-induced fluorescence: calibration and interferences from RO₂ radicals

Abstract. HO₂ concentration measurements are widely accomplished by chemical conversion of HO₂ to OH including reaction with NO and subsequent detection of OH by laser-induced fluorescence. RO₂ radicals can be converted to OH via a similar radical reaction sequence including reaction with NO, so that they are potential interferences for HO₂ measurements. Here, the conversion efficiency of various RO₂ species to HO₂ is investigated. Experiments were conducted with a radical source that produces OH and HO₂ by water photolysis at 185 nm, which is frequently used for calibration of LIF instruments. The ratio of HO₂ and the sum of OH and HO₂ concentrations provided by the radical source was investigated and was found to be 0.50 ± 0.02. RO₂ radicals are produced by the reaction of various organic compounds with OH in the radical source. Interferences via chemical conversion from RO₂ radicals produced by the reaction of OH with methane and ethane (H-atom abstraction) are negligible consistent with measurements in the past. However, RO₂ radicals from OH plus alkene- and aromatic-precursors including isoprene (mainly OH-addition) are detected with a relative sensitivity larger than 80 % with respect to that for HO₂ for the configuration of the instrument with which it was operated during field campaigns. Also RO₂ from OH plus methyl vinyl ketone and methacrolein exhibit a relative detection sensitivity of 60 %. Thus, previous measurements of HO₂ radical concentrations with this instrument were biased in the presence of high RO₂ radical concentrations from isoprene, alkenes or aromatics, but were not affected by interferences in remote clean environment with no significant emissions of biogenic VOCs, when the OH reactivity was dominated by small alkanes. By reducing the NO concentration and/or the transport time between NO addition and OH detection, interference from these RO₂ species are suppressed to values below 20 % relative to the HO₂ detection sensitivity. The HO₂ conversion efficiency is also smaller by a factor of four, but this is still sufficient for atmospheric HO₂ concentration measurements for a wide range of conditions.