The development of a fast in situ optical sensor based on a continuous-wave (cw) quantum cascade laser (QCL) operating at 8 μm for measurement of environmental gaseous nitrous acid (HONO) is reported. A strong absorption line at 1252.2109 cm $^{-1}$ is found to be free of interferences and used for the first time for ambient HONO detection. Loss of HONO on the wall of the gas sample absorption cell is evaluated. The resulting 1-s detection limit for HONO is 110 pptv with a folded path length of 172 m. Continuous monitoring of HONO and NO2 mixing ratios variations along with the basic meteorological parameters were performed during a field campaign of several days. Typical photochemical reactions of HONO have been observed on sunny and snowy days. Atmospheric HONO mixing ratios observed in this study indicate that direct emission from traffic exhausts and local heterogeneous reactions are potentially important sources of HONO.

In this study, a method for double-beam quantum cascade laser absorption spectroscopy (DB-QCLAS) was developed. Two mid-infrared distributed feedback quantum cascade laser beams were coupled in an optical cavity for the monitoring of NO and NO₂ (NO at 5.26 μm; NO₂ at 6.13 μm). Appropriate lines in the absorption spectra were selected, and the influence of common gases in the atmosphere, such as H₂O and CO₂, was avoided. By analyzing the spectral lines under different pressure conditions, the appropriate measurement pressure of 111 mbar was selected. Under this pressure, the interference between adjacent spectral lines could be effectively distinguished. The experimental results show that the standard deviations for NO and NO₂ were 1.57 ppm and 2.67 ppm, respectively. Moreover, in order to improve the feasibility of this technology for detecting chemical reactions between NO and O₂, the standard gases of NO and O₂ were used to fill the cavity. A chemical reaction instantaneously began, and the concentrations of the two gases were immediately changed. Through this experiment, we hope to develop new ideas for the accurate and rapid analysis of the process of NOx conversion and to lay a foundation for a deeper understanding of the chemical changes in atmospheric environments.