Highly-sensitive NO, NO₂, and NH₃ measurements with an open-multipass cell based on mid-infrared wavelength modulation spectroscopy

A compact prototype based on mid-infrared wavelength modulation spectroscopy (WMS) is developed for the simultaneous monitoring of NO, NO₂, and NH₃ in the urban area. Three quantum cascade lasers (QCLs) with central frequencies around 1900.0 cm^-1, 1600.0 cm^-1, and 1103.4 cm^-1 are used for NO, NO₂, and NH₃ detections, respectively, by time-division multiplex. An open-path multi-pass cell of 60-m optical path length is applied to the instrument for high sensitivity and reducing the response time to less than 1 s. The prototype achieves a sub-ppb detection limit for all the three target gases with an average time of about 100 s. The instrument is installed in the Jiangsu environmental monitoring center to conduct performance tests on ambient air. Continuous 24-hour measurements show good agreement with the results of a reference instrument based on the chemiluminescence technique.