Development And Application Of A Quick Water-air Gas Exchange Device

The increase of greenhouse gas emissions leads to global warming,which has a serious impact on human survival and the sustainable development of social economy.Methane is a greenhouse gas in the atmosphere,second only to carbon dioxide,but its warming potential is about 28 times that of carbon dioxide.Water ecosystem is an important source of atmospheric methane,but the current estimation of greenhouse gas emissions such as methane in water ecosystem is uncertain.This is mainly caused by the limitations of the water-air interface flux estimation methods,including the limitations of the number of observation points,the limitations of the observation period selection,and the variability of water and atmospheric environmental factors.Direct monitoring of dissolved methane concentration in water can provide an effective solution for such research.However,up to now,the concentration of dissolved methane in water has not been directly observed by the scientific community,only the methane content in gas/gas phase can be monitored,and different techniques or methods based on Henry’s law can be sampled and recalculated.Therefore,it is of great scientific significance to improve the assistant device suitable for monitoring the concentration of dissolved methane in field waters to accurately assess the carbon flux of water ecosystem,especially to understand the temporal and spatial distribution of dissolved methane in water under the conditions of strong spatial heterogeneity of physical and geographical characteristics.Over the past decade,many scholars and businesses have devoted themselves to the development of related technologies or auxiliary devices,but different technologies have their own advantages and disadvantages,and can not be applied to different cleanliness water bodies and real-time continuous monitoring.This paper introduces a self-developed quick water-air gas exchange device and its application.With the help of on-line greenhouse gas analyzer and the fast water-gas exchange device,the on-line continuous observation of dissolved methane concentration in field water can be realized.The device has the characteristics of good stability,short response time and economy.At the same time,because the device does not have a membrane system,it can be used in eutrophic water for a long time without blockage and biological pollution.The main contents are as following.1.A quick water-air gas quick water-air gas exchange device has been developed.The device calculates the concentration of dissolved methane in water according to Henry’s law.The theoretical formulas are as follows:_（6）=0=₄)+（1-）₀)(C_a is the concentration of dissolved methane in water;C_eq is the concentration of methane in the headspace corresponding to the complete equilibrium of water body;C_pi is the concentration of purged gas in air bag;C_pe is the concentration of methane obtained when the device is in equilibrium;is the calibration coefficient of the device).The device consists of a reaction system,an inlet and outlet water system,a purge system,a gas-liquid separation system and a monitoring system.The device extracts water samples to be monitored by a peristaltic pump in the intake system and weakens the intake pressure produced by the pump at the intake device in the intake system.The air pump in the purge system provides purging gas with stable concentration in the air bag for the device,and then purges,exchanges and separates in the gas-liquid separation system.The greenhouse gas analyzer in the monitoring system is used to monitor the gas concentration.Finally,the concentration of dissolved methane in water was calculated according to the theoretical formula.2.The calibration coefficient(=0.99695±5×10^-5)of the device was estimated by purging water samples with known dissolved methane concentration.Repeated experiments used the estimated calibration coefficient formula to calculate the dissolved methane concentration in effluent samples.The results show that the calculated headspace concentration based on the derived formula is very close to the measured headspace concentration of water bags,and the deviation is less than 5%.The accuracy of the calibration coefficients and the feasibility of the device are verified.The concentration of dissolved methane in water samples monitored by the device ranges from 0.02 to 22μmol/l.At the same time,by comparing the headspace concentration of the water sample calculated by the device with that of the actual water sample,the results show that the cleaning efficiency of the device is relatively constant,and about 75%.In addition,the time required for the device to reach equilibrium is observed when the inlet of the device is switched from low concentration to high concentration.The response time of the device is determined to beτ=11±5 s,which is much less than that of other devices,and greatly improves the experimental efficiency of the device.3.The vertical variation of dissolved methane concentration in Lianxin Lake in different seasons was observed by this device.The experimental results show that the dissolved methane concentration from the bottom to the surface of the water body decreases as a whole,which is the same as other research results.At the same time,there are differences in the stratification of methane in different seasons.The vertical stratification of dissolved methane concentration in summer is significant,while there is no obvious stratification in winter.In addition,the concentration of dissolved methane in water is significantly affected by water quality.The vertical distribution of dissolved methane concentration in water is also affected by other environmental factors such as dissolved oxygen.The uncertainties in estimating the methane emission flux at the water-air interface caused by the time heterogeneity of methane concentration in the monitoring point water can be effectively solved by using the fast water-air exchange device to continuously observe the concentration of dissolved methane in water day and night.4.Using this quick water-air gas exchange device,the plane navigation application of Meiziya Reservoir is carried out in a"Z"shape.It takes about 1.5 hours to maintain a uniform speed during the course of sailing.Finally,the plane distribution of dissolved methane concentration in Meiziya Reservoir and other environmental factors can be obtained conveniently and quickly.At the same time,the statistical results of Meiziya Reservoir plane navigation show that the dissolved methane concentration in the surface water of Meiziya Reservoir has significant spatial heterogeneity,which indicates the inaccuracy of the traditional method for estimating the methane flux in the whole reservoir plane at finite points.In addition,the key pathways of methane generation can be determined conveniently and quickly by using this fast water-gas exchange device and stable isotope method,which is helpful to better understand the process of methane generation in water system and to effectively track the spatial and temporal distribution characteristics of carbon in water system.