Measurements of carbon monoxide mixing ratios in Houston using a compact high-power CW DFB-QCL-based QEPAS sensor Przemyslaw Stefan ´ski • Rafal Lewicki • Nancy P. Sanchez • Jan Tarka • Robert J. Griffin • Manijeh Razeghi • Frank K. Tittel Received: 12 March 2014 / Accepted: 19 May 2014 / Published online: 3 June 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract Measurements of carbon monoxide (CO) mixing ratios in Houston, Texas, during the period from May 16, 2013 to May 28, 2013 were performed using a sensitive, selective, compact, and portable quartz-enhanced photoacoustic spec- troscopy (QEPAS)-based CO sensor employing a high-power continuous wave (CW) distributed feedback quantum cascade laser (DFB-QCL). The minimum detectable CO concentra- tion was 3 ppbv for the strong, interference-free R(6) absorption line at 2,169.2 cm -1 and a 5 s data acquisition time. The average CO concentration during the measurement period was 299.1 ± 81.4 ppb with observed minimum and maximum values of 210.5 and 4,307.9 ppb, respectively. A commercially available electrochemical sensor was employed in-line for simultaneous measurements to confirm the response of the CW DFB-QCL-based QEPAS sensor to variations of the CO mixing ratios. Moderate agreement (R 2 = 0.7) was found between both sets of CO measurements. 1 Introduction Carbon monoxide is a major global pollutant. The main source of its production and emission to the atmosphere is the partial oxidation of carbon-containing compounds associated with combustion processes typically as a result of human activities. Natural sources for atmospheric CO include oxidation reactions of methane and other volatile organic compounds [1–4]. CO, even at low concentration levels, is hazardous to human health and therefore must be accurately and precisely measured. CO should be moni- tored for healthcare because exposure affects the human respiratory system and can result in dizziness. CO is also an effective biomarker for oxidative stress and anemia [5]. In homes, CO concentrations can be elevated as it is produced by gas and water heaters, stoves and other gasoline pow- ered equipment used in households. Typically, CO levels of 0.5–5 ppm are expected in homes in the absence of high-efficiency heaters and stoves. In spaces where gas stoves/heaters are operated, the CO concentration can increase up to 30 ppm [6]. In this work, a trace gas detection system based on QEPAS [7] was employed to selectively detect and pre- cisely monitor atmospheric CO concentration levels. In a QEPAS-based trace gas sensor, a resonant quartz tuning fork (QTF) is used as the acoustic energy-accumulating element. The QEPAS-based sensor technique benefits from a large quality factor (Q *8,000 at atmospheric pressure) and resonant frequency (typically f res = 2 15 = 32,768 Hz) compared to conventional photoacoustic cells, where typ- ical values for the Q and f res are reported in the range of 40–200 and f res *1 k–4 kHz, respectively [7, 8]. In QEPAS, the energy of the modulated light is first absorbed in a gas sample and is then released as heat to create local temperature variations, which subsequently induce acoustic P. Stefan ´ski Á R. Lewicki Á J. Tarka Á F. K. Tittel (&) Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA e-mail: fkt@rice.edu P. Stefan ´ski e-mail: pps2@rice.edu P. Stefan ´ski Á J. Tarka Laser and Fiber Electronics Group, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland N. P. Sanchez Á R. J. Griffin Department of Civil and Environmental Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA M. Razeghi Department of Electrical Engineering and Computer Science, Center for Quantum Devices, Northwestern University, 633 Clark Street, Evanston, IL, USA 123 Appl. Phys. B (2014) 117:519–526 DOI 10.1007/s00340-014-5863-5