The OH radicals produced by HONO photolysis are important oxidants in atmospheric reactions. The sources and mixing ratios of OH radicals are closely related to the level of HONO. The sources of HONO are still unclear under different contaminated conditions, different meteorological conditions, and different reaction conditions. HONO also has a great impact on people's health. To better understand the photochemical cycle of atmospheric HONO and its sources, the levels of HONO need to be accurately measured. N₂O₄ plays an important role in liquid propellants and is also a toxic gas. It is necessary to accurately detect its levels to better understand its reaction mechanism and perform real-time monitoring. Accurate measurement of HONO and N₂O₄ levels requires precise absorption line parameters, such as line positions, line intensities, and spectral line broadening.

In this experiment, high-resolution quantum cascade laser absorption spectroscopy technology is used to measure HONO and N₂O₄ gas samples. A room temperature continuous wave quantum cascade laser (CW-QCL) combined with a 50 m path-length absorption cell are used to measure the absorption line frequencies of the two gases. Using the heterogeneous reactions of NO₂ and H₂O to prepare gas samples of HONO. The overall absorption line frequencies are calibrated by the two H₂O absorption lines at the frequencies of 1280.0475 cm^-1 and 1281.1611 cm^-1. According to the known HONO line intensity at 1280.4 cm^-1, the level of HONO in the gas sample as well as the signal-to-noise ratio and minimum detection limit of the system are calculated by the Beer-Lambert law and Voigt line shape fitting.

The absorption spectra of gas samples in the range of 1279.5-1282.5 cm^-1 are obtained as shown in Fig. 4. The gases that may exist in the absorption cell mainly include three types, i.e., exhaled gases, the gases in the air, and the gases generated by the chemical reaction. The HITRAN database and published papers are used to find the possible gases (CH₄, N₂O, H₂O, CO₂, NO₂, HNO₃, and HONO) in the absorption cell. Among them, CH₄, N₂O, and H₂O are gases in the atmospheric environment. Since the absorption cell has been pumped into a vacuum before the gas sample to be measured is introduced, the absorption characteristics of these three gases will not be displayed in the measured absorption lines theoretically. The CO₂ and H₂O in the exhaled gas will inevitably enter the gas bag. Combining the simulated absorption spectra of NO₂, HNO₃, H₂O, and CO₂ under the same experimental conditions, the interferences of NO₂, HNO₃, and CO₂ can be excluded and the gas species corresponding to each absorption line can be determined. The overall absorption line positions are calibrated by two H₂O absorption lines with frequencies of 1280.0475 cm^-1 and 1281.1611 cm^-1. The specific absorption line positions of HONO and N₂O₄ obtained in this experiment are concluded in Table 1. Due to the instability, solubility, and photolysis of HONO, its absorbance intensities decrease with time in the absorption cell. In order to minimize the measurement error and avoid the reduction of HONO absorbance intensity, the absorption lines will be collected immediately (within 10 s) when the gas sample just entered the absorption cell. Finally, the level of HONO is calculated to be (0.72±0.04)×10^-6 by a Voigt line shape fitting to the spectral line with a known line intensity of (3.25±0.17)×10^-20 cm/(molecule·cm^-2) at wave number of 1280.4 cm^-1. The statistical calculation of the baseline part of Fig. 6 is performed, and the value is used as the noise value N of the output signal. The signal-to-noise ratio of the experimental system is about 64.96 and the minimum detection limit is (11.15±0.50)×10^-9.

Trans-HONO and N₂O₄ gases are continuously measured at the same time, and the specific absorption line frequencies of the two gases in the range of 1279.5-1282.5 cm^-1 are obtained by using a 7.8 μm room-temperature CW-QCL and a long path-length absorption cell. The decay time of HONO in a closed absorption cell made of quartz is obtained by fitting and analyzing the decay curve of HONO. According to the known absorption line intensity of trans-HONO at 1280.4 cm^-1, the level of trans-HONO in the sample to be measured is calculated to be (0.72±0.04)×10^-6, the corresponding minimum detection limit of the system is about (11.15±0.50)×10^-9. As the absorption line intensity of N₂O₄ has not been reported in the spectral database and published articles, the level of N₂O₄ in the sample to be tested has not been calculated. The absorption line frequencies of HONO and N₂O₄ obtained in the experiment provide a reference for real-time continuous gas monitoring, sources and sinks analysis of atmospheric HONO, and analysis of the N₂O₄ chemical reaction process.