Pressure-dependent calibration of the OH and HO<sub>2</sub> channels of a FAGE HO<sub>x</sub> instrument using the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC)

Abstract

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 HOx (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 HOx 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 CHO and CHO2 with pressure was observed, and an empirical linear regression of the data was used to describe the trends, with ΔCHO = (17 ± 11) % and ΔCHO2 = (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, COH(conv)/COH(alt) = 1.19 ± 0.26 and CHO2(conv)/CHO2(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 HOx field measurements and particularly in aircraft-based HOx measurements, where there are substantial variations in external pressure, and assumptions are made regarding loss rates on inlets as a function of pressure.

Measurement of OH reactivity by laser flash photolysis coupled with laser-induced fluorescence spectroscopy

“…and 343 K and pressures up to 760 Torr (Glowacki et al, 2007;Malkin et al, 2010;Winiberg et al, 2015).…”

Heterogeneous reaction of HO<sub>2</sub> with airborne TiO<sub>2</sub> particles and its implication for climate change mitigation strategies

“…HO 2 was sampled by a 0.7 mm diameter pinhole at the end of the flow tube, and after chemical conversion to OH by addition of excess NO (50 sccm, BOC, 99.5 %) just inside the pinhole, laser-induced fluorescence (LIF) at low-pressure (the fluorescence assay by gas expansion (FAGE) technique; Heard and Pilling, 2003) was used to measure OH. The relative LIF signal from converted HO 2 was calibrated using an established method (Winiberg et al, 2015) developed for field measurements of OH and HO 2 radicals. Hence the experiment is able to measure the absolute concentration of HO 2 during passage from the injector to the sampling inlet.…”

Direct measurements of OH and other product yields from the HO<sub>2</sub> + CH<sub>3</sub>C(O)O<sub>2</sub> reaction

“…Analysis of sample FTIR spectra was conducted at ∼ 2000 cm −1 for CH 3 OH and 1600-1800 cm −1 for all other detectable species. Quantitative analysis was aided by a custom written iterative non-linear least-squares fitting algorithm (Winiberg, 2014). Supporting online measurements of CH 3 OH and CH 3 CHO were conducted using gas chromatography with flame ionisation detection (GC-FID), using an evacuated sampling loop into which gas from the chamber was expanded.…”

“…The instrument was used as described previously in the literature (Glowacki et al, 2007a;Malkin et al, 2010;Winiberg et al, 2015). LIF with 4026 F. A. F. Winiberg et al: Direct measurements of OH and other product yields excitation at 308 nm (A 2 + (ν = 0) ← X 2 i (ν = 0) transition) was used to probe the OH radicals directly, and the resulting fluorescence was collected via a (305 ± 5 nm) nm interference filter.…”

“…The laser power entering each fluorescence cell was typically 7-10 and 3-5 mW for the OH and HO 2 cells, respectively. The FAGE instrument was calibrated using the H 2 O vapour photolysis method detailed in Winiberg et al (2015), and was shown to have a typical uncertainty of 38 % (2σ ) and a limit of detection of 1.6 × 10 6 molecule cm −3 at 60 s averaging and for a signalto-noise ratio of unity.…”