An intercomparison of HO<sub>2</sub> measurements by fluorescence assay by gas expansion and cavity ring-down spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)

Abstract

Abstract. The HO2 radical was monitored simultaneously using two independent techniques in the Leeds HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) atmospheric simulation chamber at room temperature and total pressures of 150 and 1000 mbar of synthetic air. In the first method, HO2 was measured indirectly following sampling through a pinhole expansion to 3 mbar when sampling from 1000 mbar and to 1 mbar when sampling from 150 mbar. Subsequent addition of NO converted it to OH, which was detected via laser-induced fluorescence spectroscopy using the FAGE (fluorescence assay by gas expansion) technique. The FAGE method is used widely to measure HO2 concentrations in the field and was calibrated using the 185 nm photolysis of water vapour in synthetic air with a limit of detection at 1000 mbar of 1.6 × 106 molecule cm−3 for an averaging time of 30 s. In the second method, HO2 was measured directly and absolutely without the need for calibration using cavity ring-down spectroscopy (CRDS), with the optical path across the entire ∼ 1.4 m width of the chamber, with excitation of the first O-H overtone at 1506.43 nm using a diode laser and with a sensitivity determined from Allan deviation plots of 3.0 × 108 and 1.5 × 109 molecule cm−3 at 150 and 1000 mbar respectively, for an averaging period of 30 s. HO2 was generated in HIRAC by the photolysis of Cl2 using black lamps in the presence of methanol in synthetic air and was monitored by FAGE and CRDS for ∼ 5–10 min periods with the lamps on and also during the HO2 decay after the lamps were switched off. At 1000 mbar total pressure the correlation plot of [HO2]FAGE versus [HO2]CRDS gave an average gradient of 0.84 ± 0.08 for HO2 concentrations in the range ∼ 4–100 × 109 molecule cm−3, while at 150 mbar total pressure the corresponding gradient was 0.90 ± 0.12 on average for HO2 concentrations in the range ∼ 6–750 × 108 molecule cm−3.For the period after the lamps were switched off, the second-order decay of the HO2 FAGE signal via its self-reaction was used to calculate the FAGE calibration constant for both 150 and 1000 mbar total pressure. This enabled a calibration of the FAGE method at 150 mbar, an independent measurement of the FAGE calibration at 1000 mbar and an independent determination of the HO2 cross section at 1506.43 nm, σHO2, at both pressures. For CRDS, the HO2 concentration obtained using σHO2, determined using previous reported spectral data for HO2, and the kinetic decay of HO2 method agreed to within 20 and 12 % at 150 and 1000 mbar respectively. For the FAGE method a very good agreement (difference within 8 %) has been obtained at 1000 mbar between the water vapour calibration method and the kinetic decay of the HO2 fluorescence signal method. This is the first intercomparison of HO2 between the FAGE and CRDS methods, and the good agreement between HO2 concentrations measured using the indirect FAGE method and the direct CRDS method provides validation for the FAGE method, which is used widely for field measurements of HO2 in the atmosphere.

Peroxy radical measurements by ethane – nitric oxide chemical amplification and laser-induced fluorescence during the IRRONIC field campaign in a forest in Indiana

“…where S R i O 2 /S HO 2 is the sensitivity of ECHAMP to individual RO 2 compounds relative to that of HO 2 , L i is the fractional sampling transmission of an individual organic peroxy species R i O 2 through the short inlet into the reaction chambers (relative to that of HO 2 ), Y is the alkyl nitrate yield (Y = R8b/(R8a + R8b)), and the remaining terms in parentheses account for alkyl nitrite (RONO) formation. Alkyl nitrate yields increase with the carbon backbone number, from less than 0.1 % for CH 3 O 2 to 8 % for isoprene and to over 25 % for C10 and larger alkyl peroxy radicals (Lockwood et al, 2010;Orlando and Tyndall, 2012). Alkyl nitrite (RONO) formation accounts for less than a 4 % loss for most organic peroxy radicals and is likely negligible for alkene-derived peroxy radicals due to the rapid decomposition of β-hydroxy alkoxy radicals (Atkinson, 1997) but can sequester a calculated 10 % loss of CH 3 O 2 (Wood et al, 2017).…”

The Absorption Spectrum and Absolute Absorption Cross Sections of Acetylperoxy Radicals, CH3C(O)O2 in the near IR

“…Therefore, a series of experiments was conducted to quantify the initial amount of radicals. This was achieved by quantitatively converting all Cl-atoms to HO 2 radicals, which can be reliably measured by cw-CRDS on a strong absorption line at 6638.205 cm −1 [23,[26][27][28][29] . The measurement of the HO 2 absorption cross section is based on the measurement of HO 2 decays during self-reaction, which allows for retrieval of the initial concentration, and thus the absorption cross section, if the rate constant is known.…”

Sensitive detection of HO2 radicals produced in an atmospheric pressure plasma using Faraday rotation cavity ring-down spectroscopy

“…We have not quantified these losses, but with an estimated residence time of 1 s in the cavity, an initial concentration of 10 11 cm −3 would reduce by 26% within 1 s, based on a self-reaction rate of 1.79 × 10 −12 cm 3 s −1 . 21 spectrometer allows for retrieval of absorption coefficient, α, and θ values from the same data set based on (A33) and 2, respectively. Ring-down measurements were acquired across the spectral region with a step size of 0.0036 cm −1 with 200 RDTs averaged (integration time: ∼2 s) for each spectral data point.…”

“…20 Onel et al demonstrated a sensitivity of 3.0 × 10 8 cm −3 for HO 2 at a total sample pressure of 150 mbar using CRDS at 1506 nm. 21 In the context of radical detection, HO 2 also possesses a nonzero magnetic moment and is therefore susceptible to the Faraday effect, i.e., the polarization of a linearly polarized light wave propagating in the same direction as an applied magnetic field will be rotated when it travels through the paramagnetic sample. The Faraday effect is a consequence of magnetic circular birefringence (MCB) induced by the Zeeman effect, in which left-hand circularly polarized (LHCP) and right-hand circularly polarized (RHCP) waves experience different refractive indices.…”