Pressure-dependent calibration of the OH and HO₂ channels of a FAGE HO_x instrument using the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC)

Abstract. The calibration of field instruments used to measure concentrations of OH and HO₂ worldwide has traditionally relied on a single method utilising the photolysis of water vapour in air in a flow tube at atmospheric pressure. Here the calibration of two FAGE (fluorescence assay by gaseous expansion) apparatuses designed for HO_x (OH and HO₂) measurements have been investigated as a function of external pressure using two different laser systems. The conventional method of generating known concentrations of HO_x from H₂O vapour photolysis in a turbulent flow tube impinging just outside the FAGE sample inlet has been used to study instrument sensitivity as a function of internal fluorescence cell pressure (1.8–3.8 mbar). An increase in the calibration constants C_HO and C_HO2 with pressure was observed, and an empirical linear regression of the data was used to describe the trends, with ΔC_HO = (17 ± 11) % and ΔC_HO2 = (31.6 ± 4.4)% increase per millibar air (uncertainties quoted to 2σ). Presented here are the first direct measurements of the FAGE calibration constants as a function of external pressure (440–1000 mbar) in a controlled environment using the University of Leeds HIRAC chamber (Highly Instrumented Reactor for Atmospheric Chemistry). Two methods were used: the temporal decay of hydrocarbons for calibration of OH, and the kinetics of the second-order recombination of HO₂ for HO₂ calibrations. Over comparable conditions for the FAGE cell, the two alternative methods are in good agreement with the conventional method, with the average ratio of calibration factors (conventional : alternative) across the entire pressure range, C_OH(conv)/C_OH(alt) = 1.19 ± 0.26 and C_HO2(conv)/C_HO2(alt) = 0.96 ± 0.18 (2σ). These alternative calibration methods currently have comparable systematic uncertainties to the conventional method: ~ 28% and ~ 41% for the alternative OH and HO₂ calibration methods respectively compared to 35% for the H₂O vapour photolysis method; ways in which these can be reduced in the future are discussed. The good agreement between the very different methods of calibration leads to increased confidence in HO_x field measurements and particularly in aircraft-based HO_x measurements, where there are substantial variations in external pressure, and assumptions are made regarding loss rates on inlets as a function of pressure.