Simultaneous detection of greenhouse gases CH and CO based on a dual differential photoacoustic spectroscopy system.

In addition to the atmospheric measurement, detection of dissolved carbon oxides and hydrocarbons in a water region is also an important aspect of greenhouse gas monitoring, such as CH and CO. The first step of measuring dissolved gases is the separation process of water and gases. However, slow degassing efficiency is a big challenge which requires the gas detection technology itself with low gas consumption. Photoacoustic spectroscopy (PAS) is a good choice with advantages of high sensitivity, low gas consumption, and zero background, which has been rapidly developed in recent years and is expected to be applied in the field of dissolved gas detection. In this study, a miniaturized differential photoacoustic cell with a volume of 7.9 mL is designed for CH and CO detection, and a dual differential method with four microphones is proposed to enhance the photoacoustic signal. What we believe to be a new method increases photoacoustic signal by 4 times and improves the signal to noise ratio (SNR) over 10 times compared with the conventional single-microphone mode. Two distributed feedback (DFB) lasers at 1651 nm and 2004nm are employed to construct the PAS system for CH and CO detection respectively. Wavelength modulation spectroscopy (WMS) and 2 harmonic demodulation techniques are applied to further improve the SNR. As a result, sensitivity of 0.44 ppm and 7.39 ppm for CH and CO are achieved respectively with an integration time of 10 s. Allan deviation analysis indicates that the sensitivity can be further improved to 42 ppb (NNEA=4.7×10cmWHz) for CH and 0.86 ppm (NNEA=5.3×10cmWHz) for CO when the integration time is extended to 1000 s.