Noise-Immune Cavity-Enhanced Optical Heterodyne Detection of HO₂ in the Near-Infrared Range

Abstract

Accurate measurements of the absolute concentrations of radical species present in the atmosphere are invaluable for better understanding atmospheric processes and their impact on Earth systems. One of the most interesting species is HO(2), the hydroperoxyl radical, whose atmospheric daytime levels are on the order of 10 ppt and whose observation therefore requires very sensitive detection techniques. In this work, we demonstrate the first steps toward the application of external-cavity diode-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) to the detection of the HO(2) radical in the near-infrared range. Measurements of stable species and of HO(2) were made in a laboratory setting, and the possibilities of extending the sensitivity of the technique to atmospheric conditions are discussed.

HO2 reaction kinetics in an atmospheric pressure plasma jet determined by cavity ring-down spectroscopy

“…To measure HO 2 , we employ CRDS using a diode laser at 6638.2 cm −1 as the light source to probe transitions in the first vibrational overtone of the O − H stretch, i.e. the 2v 1 band (centred at 6649 cm −1 ) [14,15,16,17,18]. We used CRDS instead of OF-CEAS in the current study for a number of reasons.…”

Detection of HO2in an atmospheric pressure plasma jet using optical feedback cavity-enhanced absorption spectroscopy

“…A wide class of cavity-enhanced spectroscopy techniques have been developed such as cavity ring-down spectroscopy (CRDS), off-axis cavityenhanced absorption spectroscopy (CEAS), optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) [17,18]. Of these variants, CRDS has been the method of choice for the detection of HO 2 utilizing absorption bands in the near-infrared (NIR) range [3,[19][20][21], although its detection using NICE-OHMS was also reported [22]. Similarly, CRDS has been employed for the detection of species in plasmas.…”

RES-Q-Trace: A Mobile CEAS-Based Demonstrator for Multi-Component Trace Gas Detection in the MIR

“…Because of a distorted baseline by a sinusoidal structure (being visible in Figure 11 ), a background spectrum was measured and stored for the same number of scans and used for the calculation of α according to Equation (2) [ 22 ]. Such sinusoidal structured baselines are often observed in CEAS and CRDS when optical interference with parasitic reflections in the setup could not be minimized, e.g., [ 45 , 46 ]. In Figure 12 , the mean value for α min is plotted against the acquisition time τ whereas its standard deviation serves as an uncertainty for this value.…”