Measurement of tropospheric HO2 radical using fluorescence assay by gas expansion with low interferences

    An instrument to detect atmospheric HO₂ radicals using fluorescence assay by gas expansion (FAGE) technique has been developed. HO₂ is measured by reaction with NO to form OH and subsequent detection of OH by laser-induced fluorescence at low pressure. The system performance has been improved by optimizing the expansion distance and pressure, and the influence factors of HO₂ conversion efficiency are also studied. The interferences of RO₂ radicals produced from OH plus some typical organic compounds were investigated by determining the conversion efficiency of RO₂ to OH during the measurement of HO₂. The dependence of the conversion of HO₂ on NO concentration was investigated, and low HO₂ conversion efficiency was selected to realize the ambient HO₂ measurement, where the conversion efficiency of RO₂ derived by propane, ethene, isoprene and methanol to OH has been reduced to no more than 6%. Furthermore, no significant interferences from PM_2.5 and NO were found in the ambient HO₂ measurement. The detection limits for HO₂ (S/N=2) are estimated to 4.8×10⁵ cm^-3 and 1.1×10⁶ cm^-3 (the conversion efficiency of HO₂ to OH, =20%) under night and noon conditions, with 60s signal integration time. The instrument was successfully deployed during STORM-2018 field campaign at Shenzhen graduate school of Peking University. The diurnal variation of HOx concentration shows that the OH maximum concentration of those days is about 5.5×10⁶ cm^-3 appearing around 12:00, while the HO₂ maximum concentration is about 5.0×10⁸ cm^-3 appearing around 13:30.