Dissolved Gases Measurements In Seawater Using High Sensitivity Laser Spectrometer Technology

Understanding the variation and distribution of dissolved gases in seawater is particularly important for studying the biogeochemical cycle of marine organic matter,the natural leakage point of the seafloor or ocean acidification and other environmental change mechanisms.However,intermittent manual or ship-based sampling analysis method cannot well restore the three-dimensional distributions of the dynamic nature,even with high cost,the temperature and pressure conditions of samples collected are altered when brought to the surface.The need for in-situ,rapid and continuous dissolved gas detection technology will provide insights into processes occurring in marine environment.A newly developed laser absorption spectrometry,Tunable diode laser absorption spectroscopy(TDLAS),has the potential to develop a non-intrusive,online gas sensor for long-term continuous measurements due to its high-sensitivity and selectivity,environmental adaptability and absolute quantification ability.Quantitative measurement techniques capable of in situ analyses of dissolved gases in seawater need the miniaturization and long-term,stable validation test.In this paper,the method of gas extraction based on membrane and the principle of measuring gas partial pressure concentration based on TDLAS direct absorption technique are studied,and a compact dissolved gas detection system in the case of underwater deployment is developed.We coupled a polypropylene hollow fiber membrane-based gas extraction system and high-precision long-path laser spectrometer for quantitative measurement of dissolved CH4 and CO2.To avoid the interference of a small amount of water vapor absorption,the absorption spectra of CH4 and CO2 in mid-infrared and near-infrared regions were simulated and analyzed based on HITRAN database.Extracted CH4 and CO2 were determined via their 2927.95 cm-1 and 4990.0 cm-1 optical absorption lines respectively.The CH4 and CO2 gas that was permitted through the hollow-fiber membrane was gathered in a 10 m Herriot gas cell for TDLAS measurements.Robust marine carbon sensors with small size,low power consumption,and high sensitivity provide greater insight into the carbon cycle studies and resolve environmental variability.We report here the development of a diminutively integrated system for in-situ measurement of dissolved CH4 and CO2 concentration in seawater.Two absorption lines of CH4 and CO2 near 6047.0 cm-1 and 4989.97 cm-1 with DFB lasers were selected respectively.With the laser temperature control module integrated,we designed the laser tuning current and locking control circuits for avoiding the laser frequency drift.The inverse algorithm of the dissolved gases measurement system was completed with the embedded platform on the control circuit board.The absorption feature was modeled to a Voigt profiles with the Hui's rational approximations by an Levenberg-Marquardt least-square fitting method.The dissolved gases concentrations in water were calculated and then stored on an SD card.To ensure the repeatability and accuracy of this compact TDLAS-based dissolved gas analyzer,a series of laboratory tests were performed.The fitting results of spectral measurements of the integrated absorbance of CH4 and CO2 show a linear dependence with the various standard concentrations.The Allan variance shows a minimum deviation of 0.68 ppm for CH4 and 4.18 ppm for CO2 were achieved at averaging time of 80 s and 88 s,respectively.The efficiency of dissolved gas extractions was measured under different operating conditions of the flow rate of liquid.The equilibration time was less than 3 min at a liquid flow rate of 3 L/min,and a comparison with the Picarro G2301 analyzer was completed to assess the stability and performance of our TDLAS-based measurements in the same place.The results show a good concordance during the measurement of the dissolved gas components in the tap water.The field test campaign was also accomplished with a hollow fiber membrane contactor,and concentrations of CO2 were quantitatively detected with partial degasification operations.The results clearly show the ability to continuously measure dissolved gases and highlight the potential of the system to help us better understand physical and geochemical processes in a marine environment.