A new dual-gas sensor has been developed using a quantum cascade laser (QCL) that operates at 1255 cm −1 for fast and simultaneous measurements of nitrous acid (HONO) and methane (CH 4 ). The sensor uses direct absorption spectroscopy to detect HONO and CH 4 in the air, with a multi-pass absorption cell that has a path length of 173m. The sensor can detect HONO and CH 4 with minimum detection limits (1σ) of 198ppt and 2.95ppb, respectively, with an integration time of 1s. These detection limits can be improved to 39ppt and 0.31ppb, respectively, with an optimum integration time of 100s. The sensor can be used to monitor variations in the mixing ratios of HONO and CH 4 in the atmosphere, as well as basic meteorological parameters, to study the sources and sinks of HONO and monitor CH 4 pollution in the air.

A compact spectrometer based on a mid-infrared optical sensor has been developed for high-precision and real-time measurement of water isotope ratios. The instrument uses laser absorption spectroscopy and applies the weighted Kalman filtering method to determine water isotope ratios with high precision and fast time response. The precision of the measurements is 0.41‰ for δ18O and 0.29‰ for δ17O with a 1 s time. This is much faster than the standard running average technique, which takes over 90 s to achieve the same level of precision. The successful development of this compact mid-infrared optical sensor opens up new possibilities for its future applications in atmospheric and breath gas research.

The measurement of the line positions and effective line strengths of the & nu;3 fundamental band of trans-nitrous acid (trans-HONO) near 1280 cm-1 (7.8 & mu;m) by tunable laser absorption spectroscopy (TLAS) utilizing a room temperature continuous-wave quantum cascade laser (cw-QCL) was reported. The effective line strengths of 30 well-resolved trans-HONO absorption lines in the range of 1279.8-1282.2 cm- 1 were determined using the HONO line strength at 1280.3841 cm-1 as a scale. The maximum measurement uncertainty of 7.64% in the line strengths is mainly determined by the uncertainty of the referenced line strength, while the measurement precision of the line positions is better than 5.56 * 10-3 cm- 1. The line positions and strengths of the trans-HONO absorption lines obtained in this work provide a reference for continuous gas monitoring and analysis of the sources and sinks of atmospheric HONO.

Objective 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. Methods 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. Results and Discussions 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) x 10(-6) by a Voigt line shape fitting to the spectral line with a known line intensity of (3. 25 +/- 0. 17) x 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)x10(-9). Conclusions 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 mu m 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)x10(-6), the corresponding minimum detection limit of the system is about (11.15 +/- 0.50) x 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.