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.

基于中红外波长调制光谱（WMS）技术开发了一种紧凑型原型设备，用于同时监测城市区域中的一氧化氮（NO）、二氧化氮（NO₂）和氨气（NH₃）。通过时分复用，使用中心频率约为1900.0 cm⁻¹、1600.0 cm⁻¹和1103.4 cm⁻¹的三个量子级联激光器（QCL）分别对NO、NO₂和NH₃进行检测。该仪器采用光程长为60米的开放路径多次反射池，以实现高灵敏度，并将响应时间缩短至不到1秒。该原型设备对所有三种目标气体实现了亚十亿分之一（sub - ppb）的检测限，平均时间约为100秒。该仪器安装在江苏省环境监测中心，对环境空气进行性能测试。连续24小时的测量结果与基于化学发光技术的参考仪器的结果高度吻合。