Measurement of tropospheric HO 2 radical using fluorescence assay by gas expansion with low interferences.

An instrument to detect atmospheric HO radicals using fluorescence assay by gas expansion (FAGE) technique has been developed. HO 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 HO conversion efficiency are also studied. The interferences of RO radicals were investigated by determining the conversion efficiency of RO to OH during the measurement of HO. The dependence of the conversion of HO on NO concentration was investigated, and low HO conversion efficiency was selected to realize the ambient HO measurement, where the conversion efficiency of RO 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 and NO were found in the ambient HO measurement. The detection limits for HO (S/N = 2) are estimated to 4.8 × 10 cm and 1.1 × 10 cm ( [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(OD) was obtained here. The diurnal variation of HOx concentration shows that the OH maximum concentration of those days is about 5.3 × 10 cm appearing around 12:00, while the HO maximum concentration is about 4.2 × 10 cm appearing around 13:30.