Method for measuring concentration of CO2 and CH4 in water and calculating flux of CO2 and CH4 on water surface

Abstract

According to the method for measuring the concentration of CO2 and CH4 in water and calculating the flux of CO2 and CH4 on the water surface, a device for the method comprises a water sample measuring bottle, a bottle cap, a gas inlet pipe sleeve, a gas outlet pipe sleeve, a gas inlet pipe, a gas outlet pipe, a drying bottle and a gas detector, and the gas outlet pipe, the drying bottle, the gas detector and the water sample measuring bottle are sequentially connected through hoses. The method comprises the following steps: starting a gas detector to carry out bubbling measurement and record a curve of concentrations of CO2 and CH4 in gas measured when a system is in an equilibrium state, establishing a relationship between the concentrations of CO2 and CH4 in the gas and a ratio k of the concentrations of CO2 and CH4 in the equilibrium state to the concentrations of CO2 and CH4 in liquid, fitting to obtain values of the concentrations of CO2 and CH4 in water, and calculating the flux of CO2 and CH4 on the water surface by utilizing the relationship between the flux and the concentration. According to the measurement method, the water surface CO2 and CH4 flux is calculated, the calculation process is simple, the calculation result is accurate, the water surface CO2 and CH4 flux in the water sample can be continuously monitored and analyzed for a long time, and theoretical data support is provided for subsequent water area analysis, climate change, greenhouse effect treatment and the like.

Description

Technical Field

[0001] The present invention relates to a nuclear radiation detection technology, in particular to a method for measuring the concentrations of CO2 and CH4 in water and calculating the CO2 and CH4 fluxes at the water surface. Background Art

[0002] Carbon dioxide (CO2) and methane (CH4) in water bodies are important sources of greenhouse gases. The greenhouse effect of methane is 21 - 23 times that of carbon dioxide. CO2 and CH4 make significant contributions to the global greenhouse effect. The CO2 and CH4 released from different water surfaces, such as the ocean, freshwater lakes, wetlands, or glaciers, are all different. Therefore, accurately measuring the CO2 and CH4 fluxes of different water surfaces is very important for evaluating the global greenhouse effect, predicting climate change, etc.

[0003] Common methods for measuring the fluxes of CO2 and CH4 include the eddy covariance method, the water - air interface gradient change method, the closed - chamber collection method, and the isotope tracer method, etc. The eddy covariance method is one of the traditional methods for measuring fluxes. This method directly measures the concentrations of CO2 and CH4 gases and the vertical wind speed through high - frequency sensors above 10 Hz and calculates. However, this method has expensive equipment, is only suitable for large - area open water areas, and is greatly affected by the environment. The water - air interface gradient change method calculates by measuring the concentration gradients of CO2 and CH4 in the surface layer of the water body and the atmosphere and combining the transfer rate. This gradient method can only detect the dissolved - state diffusion flux and is greatly affected by parameters such as the temperature and water pressure of the environment, thus affecting the measurement accuracy. The closed - chamber collection method calculates the flux by placing a closed chamber on the water surface and measuring the change in the gas concentration in the chamber. This method is mostly suitable for single - point measurement and requires frequent manual operations, which is not very convenient. The isotope tracer method uses stable isotopes or radioactive isotopes to trace gases. However, this method requires the use of a mass spectrometer, and the measurement cost is relatively high. Therefore, it is very necessary to achieve simple and low - cost measurement of the CO2 and CH4 fluxes at the water surface. Summary of the Invention

[0004] The object of the present invention is to overcome the above - mentioned deficiencies of the prior art and provide a method for measuring the concentrations of CO2 and CH4 in water and calculating the CO2 and CH4 fluxes at the water surface.

[0005] The technical solution of the present invention is: A method for measuring the concentrations of CO2 and CH4 in water and calculating the CO2 and CH4 fluxes at the water surface, including a measurement process and a calculation process. The measurement process is based on a bubbling measurement device. The device includes a water sample measurement bottle, a bottle cap, an air inlet pipe sleeve, an air outlet pipe sleeve, an air inlet pipe, an air outlet pipe, a drying bottle, and a gas detector. The air outlet pipe is connected to the air inlet end of the drying bottle through a hose. The air outlet end of the drying bottle is connected to the air inlet end of the gas detector through a hose. The air outlet end of the gas detector is connected to the air inlet pipe of the water sample measurement bottle through a hose.

[0006] The specific steps of the measurement process and the calculation process are as follows: I. Measurement process Take a certain amount of water sample to be measured and pour it into the water sample measurement bottle, then tighten the bottle cap. Set the air outlet of the intake pipe in the water sample to be measured and close to the bottom of the water sample measurement bottle, and the air inlet of the outlet pipe is located above the liquid level of the water sample to be measured.

[0007] Start the pump of the gas detector. Under the action of the pump, the air containing CO2 and CH4 in the water sample bubbles in the water sample measurement bottle. Thus, the air enters the drying bottle through the outlet pipe for drying, and then enters the gas detector. Record the concentration value of the gas detector at this time. Assuming that the gas concentration in the water sample is uniform, when the reading of the gas detector tends to be stable, the concentrations of CO2 and CH4 in the bubbles coming out of the water sample measurement bottle reach equilibrium with the concentrations of CO2 and CH4 in the water in the water sample measurement bottle. Record the concentration curves of CO2 and CH4 in the gas measured by the gas detector from the start to the equilibrium state.

[0008] II. Calculate the concentrations of CO2 and CH4 in water Let the concentrations of CO2 and CH4 in the water in the water sample measurement bottle be C 1 (t) The concentrations of CO2 and CH4 in the gas path of the device for measuring CO2 and CH4 in water by the bubbling method are C 2 (t) The concentrations of CO2 and CH4 in the bubbles coming out of the water sample measurement bottle are C(t) When the concentrations of CO2 and CH4 in the bubbles coming out have reached equilibrium with the concentrations of CO2 and CH4 in the water, considering gas leakage and ignoring the short-term change in water temperature, there is the following calculation expression: (1) Among them, V 1 is the volume of the water sample to be measured in the water sample measurement bottle, with the unit of m 3 ; V 2 is the volume of the gas path, with the unit of m 3 ; L is the flow rate of the pump of the gas detector, with the unit of L / min; l is the leakage rate, with the unit of m / s; k is the concentrations of CO2 and CH4 in the bubbles coming out under the equilibrium state C(t) and the concentrations of CO2 and CH4 in the gas path of the device C 1 (t) The ratio of

[0009] From Henry's constant H kThe definition and the gas partial pressure calculation formula yield the following expression: (2) (3) Wherein, P is the partial pressure of CO2 and CH4 in the mixed gas at the top of the water sample measuring bottle 1; P 总 is the total pressure of the mixed gas at the top of the water sample measuring bottle 1, generally 1 atm.

[0010] Substituting formula (3) into formula (2) and solving gives the following expression: (4) Therefore, from formula (4), the concentrations of CO2 and CH4 coming out of the bubble in the equilibrium state C(t) and the ratio of the concentrations of CO2 and CH4 in the device gas path C2(t) k are calculated by the following formula: (5) Wherein, the Henry's constants of the gases CO2 and CH4 H k is a coefficient related to temperature; let a be the initial concentrations of CO2 and CH4 in the gas path, i.e., C 2 (0) , with the unit of mol / m 3 , b is the initial concentration of CO2 and CH4 in the bubble of the water sample measuring bottle 1, i.e., C 1 (0) , with the unit of mol / m 3 , substituting the two initial values into formula (1), the solution of the equation is: (6) Wherein, , .

[0011] Substituting the value of k calculated by formula (5) into formula (6), and then fitting the measured concentration curves of CO2 and CH4 in the gas using formula (6), the concentrations of CO2 and CH4 in the water are obtained.

[0012] The relationship between the fluxes F of CO2 and CH4 at the water surface and the concentrations of CO2 and CH4 in the water is expressed by the following expression: (7) Wherein,C(H 2 O) are the concentrations of CO2 and CH4 in water, with the unit of mol / m 3 ; A 1 are the rates of CO2 and CH4 in air entering the water surface, with the unit of m / s; A 2 are the rates of CO2 and CH4 in the water surface entering the air, with the unit of m / s; C E are the concentrations of CO2 and CH4 in the environment at the start of the measurement, with the unit of mol / m 3 , since at a specific measurement temperature, A 1 and A 2 are fixed values. Substitute the concentrations of CO2 and CH4 in water obtained by fitting formula (6) into formula (7) to calculate the fluxes F of CO2 and CH4 at the water surface.

[0013] A further technical solution of the present invention is that calcium chloride powder is placed in the drying bottle; the gas detector selects the JK40 - M2 - D detector.

[0014] The present invention has the following advantages compared with the prior art: 1. The device for measuring the concentrations of CO2 and CH4 in water provided by the present invention has a simple and portable structure, is easy to operate, and has a lower cost compared with existing measuring instruments, etc.

[0015] 2. Through the measurement method provided by the present invention, the fluxes of CO2 and CH4 in water are calculated by measuring the concentrations of CO2 and CH4 at the water surface. The calculation process is simple, the calculation result is accurate, and long - term continuous monitoring and analysis of CO2 and CH4 in a certain water sample can be carried out.

[0016] The detailed structure of the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. Brief Description of the Drawings

[0017] Figure 1 is a schematic structural diagram of the device for measuring the concentrations of CO2 and CH4 in water of the present invention. Specific Embodiments

[0018] Example 1. A method for measuring the concentrations of CO2 and CH4 in water and calculating the fluxes of CO2 and CH4 at the water surface, including a measurement process and a calculation process. The measurement process is based on an existing bubbling measurement device, such as Figure 1As shown in the figure, it includes a water sample measuring bottle 1, a bottle cap 2, an air inlet pipe sleeve 3, an air outlet pipe sleeve 4, an air inlet pipe 5, an air outlet pipe 6, a drying bottle 7, and a gas detector 8. Substances for drying gas, such as calcium chloride powder, etc., are placed in the drying bottle 7. The gas detector 8 selects a JK40 - M2 - D instrument. The air inlet pipe sleeve 3 and the air outlet pipe sleeve 4 are respectively installed on the bottle cap 2. The air inlet pipe 5 is inserted into the air inlet pipe sleeve 3, and the air outlet pipe 6 is inserted into the air outlet pipe sleeve 4. The air outlet pipe 6 is connected to the air inlet end of the drying bottle 7 through a hose. The air outlet end of the drying bottle 7 is connected to the air inlet end of the gas detector 8 through a hose. The air outlet end of the gas detector 8 is connected to the air inlet pipe 5 of the water sample measuring bottle 1 through a hose.

[0019] The specific steps of the measurement process and the calculation process are as follows: I. Measurement process Pour a certain amount of the water sample to be measured into the water sample measuring bottle 1, and tighten the bottle cap 2. Set the air outlet of the air inlet pipe 5 inside the water sample to be measured and close to the bottom of the water sample measuring bottle 1, and the air inlet of the air outlet pipe 6 is located above the liquid level of the water sample to be measured.

[0020] Start the pump of the gas detector 8. Under the action of the pump, the air containing CO2 and CH4 in the water sample bubbles in the water sample measuring bottle 1. Thus, the air enters the drying bottle 7 through the air outlet pipe 6 for drying, and then enters the gas detector 8. Record the concentration value of the gas detector 8 at this time. Assuming that the gas concentration in the water sample is uniform, when the reading of the gas detector 8 tends to be stable, the concentrations of CO2 and CH4 in the bubbles coming out of the water sample measuring bottle 1 reach equilibrium with the concentrations of CO2 and CH4 in the water of the water sample measuring bottle 1. Record the concentration curves of CO2 and CH4 in the gas measured by the gas detector 8 from the start to the equilibrium state.

[0021] II. Calculate the concentrations of CO2 and CH4 in water Let the concentrations of CO2 and CH4 in the water of the water sample measuring bottle 1 be C 1 (t) , the concentrations of CO2 and CH4 in the gas path of the device for measuring CO2 and CH4 in water by the bubbling method be C 2 (t) , and the concentrations of CO2 and CH4 in the bubbles coming out of the water sample measuring bottle 1 be C(t) . When the concentrations of CO2 and CH4 in the bubbles coming out have reached equilibrium with the concentrations of CO2 and CH4 in the water, considering gas leakage and ignoring the short - time change in water temperature, there are the following calculation expressions: (1) Among them, V 1 is the volume of the water sample to be measured in the water sample measuring bottle 1, with the unit of m 3 ; V2 is the volume of the gas path, with the unit of m 3 ; L is the flow rate of the pump of the gas detector 8, fixed at 0.5 L / min; l is the leakage rate, with the unit of m / s; k is the concentrations of CO2 and CH4 coming out of the bubble under the equilibrium state C(t) and the concentrations of CO2 and CH4 in the gas path of the device C 1 (t) The ratio of, which is a dimensionless quantity.

[0022] From the definition of the Henry constant H k and the gas partial pressure calculation formula, the following expression is obtained: (2) (3) Among them, P is the partial pressure of CO2 and CH4 in the mixed gas at the top of the water sample measurement bottle 1; P 总 is the total pressure of the mixed gas at the top of the water sample measurement bottle 1, generally 1 atm.

[0023] Substitute formula (3) into formula (2) and solve to obtain the following expression: (4) Therefore, from formula (4), it is known that the concentrations of CO2 and CH4 coming out of the bubble under the equilibrium state C(t) and the ratio of the concentrations of CO2 and CH4 in the gas path of the device C2(t) k , is calculated by the following formula: (5) Among them, the Henry constants of the gases CO2 and CH4 H k is a coefficient related to temperature, and specific data can be obtained by referring to relevant literature. For example, at room temperature of 25°C, the theoretical value of the Henry constant of carbon dioxide is 0.0345~0.0399 M atm -1 , substitute the Henry constant related to temperature into formula (5) to solve for k value.

[0024] Let a be the initial concentrations of CO2 and CH4 in the gas path, that is, C2(0), with the unit of mol / m 3 , and is obtained by reading the data through the connected gas detector 8, that is, the data of the gas detector 8 recorded after startup; b is the initial concentration of CO2 and CH4 in the bubble of the water sample measurement bottle 1, that isC 1 (0) , with the unit of mol / m 3 . Substitute the two initial values into formula (1), and the solutions of the equation are obtained as follows: (6) Wherein, , .

[0025] Substitute the k value calculated by formula (5) into formula (6), and then fit the measured CO2 and CH4 concentration curves in the gas by using formula (6), and the initial concentrations of CO2 and CH4 in the water sample measuring bottle can be obtained b . The fluxes of CO2 and CH4 at the water surface F are related to the CO2 and CH4 concentrations in the water and are expressed by the following expression: (7) Wherein, C 1 (0) is the CO2 and CH4 concentrations in the water, with the unit of mol / m 3 ; A 1 is the rate of CO2 and CH4 in the air entering the water surface, with the unit of m / s; A 2 is the rate of CO2 and CH4 at the water surface entering the air, with the unit of m / s; C E is the CO2 and CH4 concentrations in the environment at the start of the measurement, with the unit of mol / m 3 , which is directly measured by the gas detector 8 at the start of the measurement. Since A1 and A2 are fixed values at a specific measurement temperature, they are obtained through theoretical calculation or experimental measurement. Substitute the CO2 and CH4 concentrations in the water obtained by fitting formula (6) into formula (7) to calculate the fluxes F of CO2 and CH4 at the water surface.

Claims

1. Method for measuring CO2 and CH4 concentrations in water and calculating CO2 and CH4 fluxes at the water surface, including a measurement process and a calculation process. The measurement process is based on a bubbling measurement device, which includes a water sample measurement bottle, a bottle cap, an air inlet pipe sleeve, an air outlet pipe sleeve, an air inlet pipe, an air outlet pipe, a drying bottle, and a gas detector. The air outlet pipe is connected to the air inlet end of the drying bottle through a hose. The air outlet end of the drying bottle is connected to the air inlet end of the gas detector through a hose. The air outlet end of the gas detector is connected to the air inlet pipe of the water sample measurement bottle through a hose; It is characterized in that: The specific steps of the measurement process and the calculation process are as follows: I. Measurement process Take a certain amount of the water sample to be measured and pour it into the water sample measurement bottle, and tighten the bottle cap. Set the air outlet of the air inlet pipe in the water sample to be measured and close to the bottom of the water sample measurement bottle, and the air inlet of the air outlet pipe is located above the liquid level of the water sample to be measured; Start the pump of the gas detector. Under the action of the pump, the air containing CO2 and CH4 in the water sample bubbles in the water sample measurement bottle. Thus, the air enters the drying bottle through the air outlet pipe for drying, and then enters the gas detector. Record the concentration value of the gas detector at this time. Assume that the gas concentration in the water sample is uniform. When the reading of the gas detector tends to be stable, the CO2 and CH4 concentrations in the bubbles coming out of the water sample measurement bottle reach equilibrium with the CO2 and CH4 concentrations in the water in the water sample measurement bottle. Record the CO2 and CH4 concentration curves of the gas measured by the gas detector from the start to the equilibrium state; II. Calculate the CO2 and CH4 concentrations in water Let the concentrations of CO2 and CH4 in the water in the water sample measuring bottle be C 1 (t) , and the concentrations of CO2 and CH4 in the gas path of the device for measuring CO2 and CH4 in water by the bubbling method be C 2 (t) , and the concentrations of CO2 and CH4 coming out of the bubbling in the water sample measuring bottle be C(t) , when the concentrations of CO2 and CH4 coming out of the bubbling have reached equilibrium with the concentrations of CO2 and CH4 in the water, considering gas leakage and ignoring the water temperature change in a short time, there is the following calculation expression: （1） Among them, V 1 is the volume of the water sample to be measured in the water sample measuring bottle, with the unit of m 3 ; V 2 is the volume of the gas path, with the unit of m 3 ; L is the flow rate of the gas detector pump, with the unit of L / min; l is the leakage rate, with the unit of m / s; k is the concentrations of CO2 and CH4 coming out of the bubble in the equilibrium state C(t) and the concentrations of CO2 and CH4 in the device gas path C 1 (t) the ratio of, which is a dimensionless quantity; From Henry's constant H k The following expression is obtained from the definition of the gas partial pressure calculation formula: （2） （3） Among them, P is the partial pressure of CO2 and CH4 gases in the mixed gas at the top of the water sample measurement bottle; P 总 is the total pressure of the mixed gas at the top of the water sample measurement bottle 1, generally 1 atm; Substitute formula (3) into formula (2) and solve to obtain the following expression: （4） Therefore, from Equation (4), the concentrations of CO2 and CH4 emerging from the bubble under the equilibrium state are known C(t) and the concentrations of CO2 and CH4 in the gas circuit of the device C 2 (t) The ratio of k , is calculated by the following formula: （5） Among them, the Henry's constants of the gases CO2 and CH4 H k are coefficients related to temperature; assume a is the initial concentration of CO2 and CH4 in the gas path, that is C 2 (0) , with the unit of mol / m 3 , b is the initial concentration of CO2 and CH4 in the bubbling of the water sample measuring bottle 1, that is C 1 (0) , with the unit of mol / m 3 . Substituting the two initial values into formula (1), the solution of the equation is obtained as follows: （6） Among them, , ； Substitute the k value calculated by formula (5) into formula (6), and then fit the measured CO2 and CH4 concentration curves of the gas using formula (6), that is, obtain the CO2 and CH4 concentrations in water; Water surface CO2 and CH4 fluxes F The relationship with the CO2 and CH4 concentrations in water is expressed by the following formula: （7） wherein, C 1 (0) are the concentrations of CO2 and CH4 in water, with the unit of mol / m 3 ; A 1 are the rates of CO2 and CH4 in air entering the water surface, with the unit of m / s; A 2 are the rates of CO2 and CH4 in the water surface entering the air, with the unit of m / s; C E are the concentrations of CO2 and CH4 in the environment at the start of the measurement, with the unit of mol / m 3 , since A1 and A2 are constant values at a specific measurement temperature, substituting the concentrations of CO2 and CH4 in water obtained by fitting formula (6) into formula (7) can calculate the fluxes F of CO2 and CH4 at the water surface.

2. The method for measuring the concentrations of CO2 and CH4 in water and calculating the CO2 and CH4 fluxes at the water surface according to claim 1, wherein: Calcium chloride powder is placed in the drying bottle; the gas detector selects the JK40-M2-D detector.

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