Abstract
A sensitive nitrogen dioxide (NO2) sensor system based on Faraday Rotation Spectroscopy (FRS) was developed. The advantages of FRS include high detection sensitivity, zero background and free from the influence of diamagnetic species, such as humidity and carbon dioxide. A widely tunable external cavity quantum cascade laser (EC-QCL) was used as the excitation source. The tunable EC-QCL operates mode-hop free between 1600cm−1 and 1650cm−1 and allows targeting the optimum 441<−440 Q-branch transition of NO2 at 1613.2cm−1 with an optical power of ∼135mW. A custom made 22.47cm long multipass gas cell (MPGC) was implemented in which the EC-QCL radiation was passed 45 times and provided an effective optical path length of 10.1m in order to design a compact sensor system with high detection sensitivity. An air core solenoid surrounding the MPGC was used for generating a magnetic field of 200G. Performance details of a FRS based sensor and its application for long time continuous NO2 measurements are reported. The long time stability of the sensor system was evaluated. A NO2 detection sensitivity based on an Allan deviation plot of ∼95ppt was obtained by averaging the associated data for 300s.
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