Simultaneous measurement of NO and NO2 by dual-channel cavity photoacoustic spectroscopy technique

The levels of nitric oxide (NO) and nitrogen oxide (NO2) are trace gases in the order of ppb, which are the main products of fossil fuel, anthropogenic combustion, natural lightning and microbial emissions, and are important for further analysis of air quality. Photoacoustic spectroscopy (PAS) technique, which is based on absorption at the wavelength of 403.65 nm, has been used for NO2 measurements. In this study, a dual-channel PAS system for NO2 and NOx detection is reported. The PAS method is capable of measuring species with good sensitivity and accuracy. The calibrated slopes of photoacoustic cavity A and B are 0.040 mu v/ppb and 0.039 mu v/ppb, respectively. And the detection limits of the developed PAS system for NO2 and NOx measurements are estimated to be about 1.47 ppb (30, 60 s) and 1.50 ppb (30, 60 s), respectively. In order to reduce the influence of local noise, the photoacoustic cavity with multiple buffer partitions is designed and the corresponding photoacoustic frequency of the photoacoustic pool is analyzed. The sound pressure value and resonance frequency of the photoacoustic pool gradually decrease with the increase of the number of buffer partitions, but the amplitude is small. The relationship between the gas absorption cross section, volume fraction, laser power and photoacoustic excitation signal is derived. After deducting the influence of local noise, the photoacoustic response gradients of dual -channel cavities are obtained, the linear fitting of the response gradient of both cavities is good, the correlation R2 is greater than 0.99, and the fitting slope is 0.040 and 0.039 respectively. The influence of O3 generated by photolysis of the mercury lamp is considered under different NO level conditions. An intercomparison be-tween the system and a cavity ring-down system (CRDS) independently developed by our research group showed a good agreement for measuring the ambient atmosphere NO2 gas. The results of NO2 showed a linear correction factor (R2) of 0.943 in a slope of 0.944 +/- 0.002, with an offset of (0.636 & PLUSMN; 0.021) ppb. The accuracy of NO2 measurement by the photoacoustic spectroscopy system was verified. In addition, the simultaneous measurement of NO and NO2 by dual-channel cavity photoacoustic spectroscopy technique was deployed. These observations indicate that NO2 and NO concentrations can be effectively observed with the dual-channel PAS instrument.