<p>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 (CH_4,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].</p><p>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 &#181;m was employed for probing the Q (1.5e) and Q (1.5f) double-line transitions of the ²&#928;_3/2state at 3568.52382 and 3568.41693 cm^-¹, respectively. A 1s detection limit of ~2.7&#215;10¹⁰ cm^-3 &#160;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.</p><p>The experimental detail and the preliminary results will be presented and discussed.</p><p><strong>&#160;</strong><strong>Acknowledgments. </strong>The authors thank the financial supports from the CPER CLIMIBIO program and the Labex CaPPA project (ANR-10-LABX005).</p><p><strong>References</strong></p><p>[1]&#160; U. Platt, M. Rateike, W. Junkermann, J. Rudolph, and D. H. Ehhalt, New tropospheric OH measurements, J. Geophys. Res. <strong>93</strong> (1988) 5159-5166.</p><p>[2]&#160; D. E. Heard and M. J. Pilling, Measurement of OH and HO₂ in the Troposphere, Chem. Rev. <strong>103</strong> (2003) 5163-5198.</p><p>[3]&#160; 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.</p><p>[4]&#160; 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</p><p>[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</p>