Measurement of OH radicals using off-axis integrated output spectroscopy (OA-ICOS) at 2.8 &#181;m
Authors: Ngo Minh N. , Ba Tong N. , Petitprez Denis , Cazier Fabrice , Zhao Weixiong , Chen Weidong 
Published: March 2021

&lt;p&gt;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&lt;sub&gt;4, &lt;/sub&gt;CO, SO&lt;sub&gt;2&lt;/sub&gt;, etc) [1]. Due to its very short lifetime (~1 s or less) and very low concentration in the atmosphere (in the order of 10&lt;sup&gt;6&lt;/sup&gt; cm&lt;sup&gt;-&lt;/sup&gt;&lt;sup&gt;3&lt;/sup&gt;), in situ and direct measurement of OH concentration in the atmosphere is challenging [2].&lt;/p&gt;&lt;p&gt;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 ~10&lt;sup&gt;12&lt;/sup&gt; OH radicals/cm&lt;sup&gt;3&lt;/sup&gt; 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 &amp;#181;m was employed for probing the Q (1.5e) and Q (1.5f) double-line transitions of the &lt;sup&gt;2&lt;/sup&gt;&amp;#928;&lt;sub&gt;3/2&lt;/sub&gt;&lt;sub&gt;&lt;/sub&gt;state at 3568.52382 and 3568.41693 cm&lt;sup&gt;-&lt;/sup&gt;&lt;sup&gt;1&lt;/sup&gt;, respectively. A 1s detection limit of ~2.7&amp;#215;10&lt;sup&gt;10&lt;/sup&gt; cm&lt;sup&gt;-3&lt;/sup&gt; &amp;#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.&lt;/p&gt;&lt;p&gt;The experimental detail and the preliminary results will be presented and discussed.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;strong&gt;Acknowledgments. &lt;/strong&gt;The authors thank the financial supports from the CPER CLIMIBIO program and the Labex CaPPA project (ANR-10-LABX005).&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[1]&amp;#160; U. Platt, M. Rateike, W. Junkermann, J. Rudolph, and D. H. Ehhalt, New tropospheric OH measurements, J. Geophys. Res. &lt;strong&gt;93&lt;/strong&gt; (1988) 5159-5166.&lt;/p&gt;&lt;p&gt;[2]&amp;#160; D. E. Heard and M. J. Pilling, Measurement of OH and HO&lt;sub&gt;2&lt;/sub&gt; in the Troposphere, Chem. Rev. &lt;strong&gt;103&lt;/strong&gt; (2003) 5163-5198.&lt;/p&gt;&lt;p&gt;[3]&amp;#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.&lt;/p&gt;&lt;p&gt;[4]&amp;#160; W. Chen, A. A. Kosterev, F. K. Tittel, X. Gao, W. Zhao, &quot;H&lt;sub&gt;2&lt;/sub&gt;S trace concentration measurements using Off-Axis Integrated Cavity Output Spectroscopy in the near-infrared&quot;, Appl. Phys. B 90 (2008) 311-315&lt;/p&gt;&lt;p&gt;[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&lt;/p&gt;