Measurement of OH radicals using off-axis integrated output spectroscopy (OA-ICOS) at 2.8 µm

The hydroxyl (OH) free radical plays an important role in atmospheric chemistry due to its high reactivity with volatile organic compounds (VOCs) and trace species (CH4, CO, SO2, etc) [1]. Due to its very short lifetime (~1 s or less) and very low concentration in the atmosphere (in the order of 106 cm-3), in situ and direct measurement of OH concentration in the atmosphere is challenging [2].We report in this paper our recent work on developing a compact spectroscopic instrument based on off-axis integrated cavity output spectroscopy (OA-ICOS) [3] for optical monitoring of OH radicals. In the present work, OH radicals of ~1012 OH radicals/cm3 were generated from continue micro-wave discharge at 2.45 GHz of water vapor at low pressure (0.2-1 mbar), and were used as sample for validation of the developed OA-ICOS approaches. Two experimental approaches are designed for the measurements of OH radicals: (1) OA-ICOS [4] and wavelength modulation enhanced OA-ICOS (WM OA-ICOS) [5]. A distributed feedback (DFB) laser operating at 2.8 µm was employed for probing the Q (1.5e) and Q (1.5f) double-line transitions of the 2Π3/2state at 3568.52382 and 3568.41693 cm-1, respectively. A 1s detection limit of ~2.7×1010 cm-3 was obtained for an averaging time of 125 s using a simple OA-ICOS scheme. This limit of detection is further improved by a factor of 3.4 using a WM OA-ICOS approach.The experimental detail and the preliminary results will be presented and discussed. Acknowledgments. The authors thank the financial supports from the CPER CLIMIBIO program and the Labex CaPPA project (ANR-10-LABX005).References[1] U. Platt, M. Rateike, W. Junkermann, J. Rudolph, and D. H. Ehhalt, New tropospheric OH measurements, J. Geophys. Res. 93 (1988) 5159-5166.[2] D. E. Heard and M. J. Pilling, Measurement of OH and HO2 in the Troposphere, Chem. Rev. 103 (2003) 5163-5198.[3] J. B. Paul, L. Lapson, J. G. Anderson, Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment, Appl. Opt. 40 (2001) 4904-4910.[4] W. Chen, A. A. Kosterev, F. K. Tittel, X. Gao, W. Zhao, "H2S trace concentration measurements using Off-Axis Integrated Cavity Output Spectroscopy in the near-infrared", Appl. Phys. B 90 (2008) 311-315[5] W. Zhao, X. Gao, W. Chen, W. Zhang, T. Huang, T. Wu, H. Cha, Wavelength modulation off-axis integrated cavity output spectroscopy in the near infrared, Appl. Phys. B 86 (2007) 353-359