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, the influence factors of HO₂ conversion efficiency are also studied. The interferences of RO₂ radicals 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 less than 6% in the atmosphere. 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 ( [Formula: see text] = 20%) under night and noon conditions, with 60 sec signal integration time. The instrument was successfully deployed during STORM-2018 field campaign at Shenzhen graduate school of Peking University. The concentration of atmospheric HOx radical and the good correlation of OH with j(O¹D) was obtained here. The diurnal variation of HOx concentration shows that the OH maximum concentration of those days is about 5.3 × 10⁶ cm^-3 appearing around 12:00, while the HO₂ maximum concentration is about 4.2 × 10⁸ cm^-3 appearing around 13:30.