A low-cost in-situ CO 2 sensor based on a membrane and NDIR for long-term measurement in seawater*

Meng LI , Baolu DU , Jinjia GUO ,**, Zhihao ZHANG , Zeyu LU , Rong’er ZHENG

1 College of Information Science and Engineering, Ocean University of China, Qingdao 266100, China

2 R&D Center for Marine Instruments and Apparatuses, Pilot National Laboratory for Marine Science and Technology (Qingdao),Qingdao 266237, China

Abstract The multi-point simultaneous long-term measurement of CO 2 concentration in seawater can provide more-valuable data for further understanding of the spatial and temporal distribution of CO 2.Thus, the requirement for a low-cost sensor with high precision, low power consumption, and a small size is becoming urgent. In this work, an in-situ sensor for CO 2 detection in seawater, based on a permeable membrane and non-dispersive infrared (NDIR) technology, is developed. The sensor has a small size (Ф 66 mm×124 mm), light weight (0.7 kg in air), low power consumption (＜0.9 W), low cost (＜US$1 000),and high-pressure tolerance (＜200 m). After laboratory performance tests, the sensor was found to have a measurement range of (0-2 000)×10 -6, and the gas linear correlation R 2 is 0.99, with a precision of about 0.98% at a sampling rate of 1 s. A comparison measurement was carried out with a commercial sensor in a pool for 7 days, and the results showed a consistent trend. Further, the newly developed sensor was deployed in Qingdao nearshore water for 35 days. The results proved that the sensor could measure the dynamic changes of CO 2 concentration in seawater continuously, and had the potential to carry out long-term observations on an oceanic platform. It is hoped that the sensor could be applied to field ocean observations in near future.

Keyword: in-situ sensor; dissolved CO 2; long-term measurement; permeable membrane; non-dispersive infrared (NDIR); low-cost

1 INTRODUCTION

The ocean is a huge reservoir of carbon and have the capacity for absorbing and retaining CO2(Yin et al., 2006). The oceanic uptake of anthropogenic CO2causes pronounced changes to the marine carbonate system (Clarke et al., 2017). Since the 1980s, 20% to 30% of CO2from human activity has been absorbed by the ocean, which has caused ocean acidification (Bindoffet al., 2019). High quality partial pressure of carbon dioxide (pCO2)measurements with good temporal and spatial coverage are required to monitor the oceanic uptake,identify regions with pronounced carbonate system changes, and observe the effectiveness of CO2emission mitigation strategies (Clarke et al., 2017).Therefore, measuring dynamic changes of CO2in seawater is of great significance to understanding the ocean carbon cycle and ocean acidification.

In the past few decades, underwater in-situ CO2sensors have attracted more and more attention (Clarke et al., 2017). In 2009, the Coastal Technology Alliance(ACT) undertook detailed performance tests on commercial sensors in Hood Canal, Washington, and Kaneohe Bay, Hawaii, for a month, including Contros HydroCTM/CO2, PMEL MAPCO2/Battelle SeaologypCO2monitoring system, Pro-Oceanus Systems Inc.PSI CO2-ProTM, and Sunburst Sensors SAMI-CO2(Schar et al., 2009a, b, c, d). Meanwhile, water samples were collected to measurepCO2in the laboratory by two traditional methods, and in-situpCO2measurements were compared to these references, and estimates of analytical and environmental variability were reported (Schar et al., 2009a, b, c, d). The extensive time-series data provided by these sensors at both test sites revealed patterns inpCO2, and captured a significantly greater dynamic range and temporal resolution than could be obtained from discrete reference samples. Aliasing of water sampling missed some of the extreme and rapid changes inpCO2often observed in these environments (Schar et al., 2009a, b,c, d; Tamburri et al., 2011). The results indicate the feasibility of these sensors for underwater applications,and the importance of continuous in-situ measurements.In addition, some newpCO2sensors have been produced and applied in recent years, including Pro-Oceanus company’s mini CO2sensor (Pro Oceanus,2021a), Solu-Blu CO2probe (Pro Oceanus, 2021b),and Turner-Designs company’s C-Sense probe (Turner Designs, 2021), among others.

Commercial CO2sensors play an important role in in-situ measurements based on various underwater platforms. Take the Contros HydroCTM/CO2sensor, for example. In 2011, Fietzek et al. (2011) improved the HydroCTM(CO2/CH4) sensors and successfully deployed them on a variety of fixed and mobile platforms, including water sampler rosette, surface drifter measuring platform, large research Autonomous Underwater Vehicle (AUV), small lander, profile float,ultra-heavy duty remote operated vehicle (ROV), and more, demonstrating the feasibility of the use of this series of sensors on underwater platforms. In 2013,Fiedler et al. (2013) fixed a HydroCTM/CO2sensor equipped with an SBE 5M pump on an Argo-type profiling float, and carried out four consecutive deployments with regularpCO2sensor zeroings near the Cape Verde Ocean Observatory (CVOO) in the eastern tropical North Atlantic. In 2015, Hu et al.(2015) measured in-situ CO2concentrations dissolved in seawater near the hydrothermal vent (within ten meters from the seafloor) in the mid-Okinawa Trough using HydroCTM(CO2) sensors based on the ROV, and the results showed that the maximum values of CO2as high as 12 000×10-6occur near active hydrothermal vents in Iheya North area. In 2020, Totland et al.(2020) carried out submarine CO2leakage detection using the HydroCTM/CO2sensor deployed on an AUV,although the response of the sensor was too slow(about 2 min with the pump) to satisfy the fast-moving measurement requirements of the AUV through the plume (about 10-15 s), so no significant change ofpCO2was directly detected. Apart from the above mentioned, other commercial sensors have also been widely used in in-situ CO2measurements. For example, in 2018, Park and Chung (2018) carried the Pro-Mini CO2sensor on a buoy to study thepCO2dynamics of a stratified reservoir in a temperate zone,and CO2pulse emissions during turnover events.

In addition to commercial sensors, there are also some home-made sensors for use in specific environments. For example, Blackstock et al. (2019)developed a low-cost (US$250-300) Arduino monitoring platform (CO2-LAMP) for recording CO2variability in electronically harsh conditions: humid air, soil, and aquatic environments. A relatively inexpensive CO2gas analyzer was waterproofed using a semi-permeable, expanded polytetrafluoroethylene membrane without additional support and putted in a plastic case housing. The performance and parameters of the CO2-LAMP for detecting the dissolved CO2are shown in Table 1. The CO2-LAMP was deployed at Blowing Springs Cave,and operated alongside a relatively greater-cost CO2monitoring platform. Over the monitoring period,measured values between the two systems covaried linearly (R2=0.99 for cave stream dissolved CO2).Although the CO2-LAMP has a good performance in the field measurement, it can not withstand higher hydrostatic pressure due to its simple packaging, and can not accurately rapidly measure microvariations of the CO2concentration due to its low precision and long response time (Blackstock et al., 2019).

With the development and wide application of new underwater vehicles, such as AUVs, gliders,Argo Floats, and so forth, the acquisition of CO2data with spatial and temporal variability has become more convenient, and new requirements for in-situ CO2sensors have emerged in response. In order to be suitable for these cable-less underwater vehicles, the sensor must fulfill several requirements: (1) low production cost; (2) low power consumption and long-term operation ability; (3) small size; (4) robust against pressure (Fritzsche et al., 2018). Among these requirements, the production cost of the CO2sensor is an important consideration, especially for disposable floats or multi-point simultaneous measurement. The commercial sensors mentioned above have good performances for in-situ CO2measurements, as shown as Table 1, however the price of these commercial sensors is expensive (much more than US$10 000); consequently, it is difficult tobe used as disposable sensors or for multi-point simultaneous measurement. According to the Defense Advanced Research Program Agency(DARPA) Ocean of Things (OoT) program(Waterston et al., 2019), sensors with a small size,low power, and low cost will be the trend in near future. In order to fulfill these requirements for these new platforms and programs, realizing observations of large-scale, long-term measurements of dissolved CO2in seawater, a CO2sensor with low power consumption, a small size, acceptable measurement accuracy, and a price of less than US$1 000 would be a good choice. In this paper, a miniature, low power consumption, low cost in-situ CO2sensor based on a membrane and non-dispersive infrared (NDIR)technology was developed. Both laboratory experiments and field experiments were undertaken for the CO2sensor performance evaluation.

Table 1 Parameters of some in-situ CO 2 sensors

2 MATERIAL AND METHOD

2.1 Material

Due to the particularity of in-situ detection of dissolved CO2in seawater, it is necessary to consider the sensor as a whole in order to improve its adaptability. The configuration of the newly developed CO2sensor is shown in Fig.1. The sensor includes three parts: gas-liquid separation, gas detection, and electronics. The CO2detection part and electronic part are packaged in a pressure vessel. A permeable membrane for gas-liquid separation is installed in the front end cap of the vessel, and an 8-pin connecting port is installed in the rear end cap of the vessel.

2.1.1 Gas detection

In order to achieve the accuracy of the long-term measurement data, a high-precision CO2detector and a temperature, humidity, and pressure sensor were selected. The CO2detector (NE Sensor Technologies,Ltd, 7NE/CO2), based on NDIR technology with a 2 000×10-6full scale detection range and 1×10-6resolution, has good selectivity and no oxygen dependence. It has an inner optical cavity with multiple reflection structures and dual-channel detectors. This cavity can achieve spatial dual optical path reference compensation, leading to a stable performance and small fluctuations for CO2detection.In addition, the CO2detector is compensated by temperature (0-50 °C). The temperature of seawater ranges from 0 to 30 °C approximately. In practical applications, the heat inside the in-situ CO2sensor is constantly exchanged with the heat in the seawater surrounding the sensor. Considering the heat dissipation of the devices, the temperature inside the sensor is approximately 5-35 °C, within the temperature compensation range, so the selected CO2detector is suitable for our application requirement,and does not need extra temperature correction theoretically. In addition, the detector measures the CO2absorption band at 4.3 μm, while water vapor has no absorption at 4.3 μm, so it is not affected by humidity theoretically. The high-precision temperature, humidity, and pressure sensor (BOSCH,BME680) was used to monitor the condition inside the in-situ sensor and correct the data from the CO2detector. Its temperature measurement range is 0-65 °C, with an accuracy of ±1 °C and a resolution of 0.01 °C; the humidity measurement range is 20%-80% relative humidity (RH), with an accuracy of±3%RH and a resolution of 0.008%RH; the pressure measurement range is 300 to 1 100 hPa, with an accuracy of 0.6 hPa and a resolution of 0.18 Pa.Furthermore, the compact structure and size of the CO2detector and temperature, humidity, and pressure sensor are suitable for underwater sensor encapsulation, to maximize the utilization of space inside the sensor.

Fig.1 Structure diagram (a) and photograph (b) of the newly developed CO 2 sensor

2.1.2 Electronics

Fig.2 Connection diagram of each module inside the sensor

In order to obtain data with a high spatial and temporal resolution, a high sampling frequency can be set as 1 Hz (1 s). However, for uncabled platforms such as buoys, it is difficult to send data in real time to a shore-based system, so a data storage module is essential. The connection of each module inside the sensor is shown in Fig.2. The STM32 module, as the main controller of the sensor, records the time from the real time clock (RTC) module, environmental parameters (temperature, humidity, and pressure),and CO2concentration into the trans flash (TF) card for storage through the serial peripheral interface(SPI) bus. The communication module converts transistor-transistor logic (TTL) to the RS232 to obtain more stable and reliable data. Each data will be recorded and saved as the format of “xxxx/xx/xx xx:xx:xx xx.xx degC xxxxxx.xx Pa xx.xx%RH xxxx ppm” with the capacity of 66 bytes. Thus, it can be calculated that if the sensor works continuously for 1 year with the sampling frequency of 1 s, the data will just take up 1.94 GB of storage space. The TF card selected here has a data reading speed of up to 100 Mb/s and a total capacity of 16 Gb, which fully meets the requirements of high-frequency continuous long-term observation. The power conversion module was used to avoid the situation where the sensor would not work normally due to an excessive cable pressure drop. As a result, the CO2sensor has two working modes: interactive mode and automatic mode. When working in the interactive mode, the obtained data is directly stored and displayed in the deck computer via a waterproof cable, and the data is also stored inside as a backup. When working in the automatic mode, an additional pressure vessel with 12-V batteries inside was used for the power supply,and the CO2sensor operates intermittently accordingto the initial setup. Considering the integration of these modules above, the electronic part with a 50-mm long by 50-mm diameter was developed.

Table 2 Design results of the pressure cabin

2.1.3 Gas-liquid separation and pressure vessel

The response time of the sensor is an important parameter for underwater in-situ measurement.Although the change of CO2concentration is a slow process which will be no sudden change in a short time for fixed-point long-term measurement in seawater. In order to measure the CO2concentration in real time and accurately, the response time should be as short as possible without affecting other parameters and performance. The response time of the sensor depends on several factors, including the gas-liquid separation efficiency of the membrane, the time for gas to fill the chamber, and the response time of the CO2detector. In order to improve the efficiency of gas-liquid separation, the effective area of the permeable membrane should be enlarged as much as possible. To realize the measurement of dissolved CO2in water, a 70-μm thickness Teflon AF2400 membrane with good permeability to CO2was selected (Biogeneral, 2021). Its high mechanical strength and slight pressure effect make it very suitable for measuring dissolved CO2in seawater.Teflon amorphous f luoropolymer (AF) membrane has good compressive resistance, the hydrostatic pressure on the outside of the membrane has little effect on the pressure on the inside (Chua et al., 2016), so the CO2detector does not need pressure correction. However,the larger the effective area of the permeable membrane, it is the easier to rupture because of the influence of external liquid pressure underwater, so the effective area of the permeable membrane should be suitable, and a sintered stainless steel plate was included to support the membrane. Considering the size of the CO2detector and a shorter response time,the effective diameter of the membrane is designed to be 34 mm, which is consistent with the diameter of the internal CO2detector.

In addition, the aperture and thickness of the sintered metal plate will not only affect the time for gas penetration, but also affect its compression resistance. The aperture of the sintered metal plate is usually 0.22-100 μm. The larger the aperture is, the rougher the surface of the sintered metal plate is, and the more easily the membrane is damaged. The smaller the aperture is, the longer the time for the gas to pass through the metal plate, and the slower the overall sensor response. Therefore, a sintered stainless steel plate with a moderate diameter of 50 μm was selected.

According to the parameters such as design pressure, inner diameter of the pressure vessel and tensile strength of the material, the size of the corresponding pressure vessel and the thickness of the sintered metal plate can be designed. The wall thicknessδof the pressure vessel can be calculated by Eq.1, and the thickness of the end capδ1and the thickness of the sintered metal plateδ1′ can be calculated by Eq.2 (Cheng, 2008).

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In the equayion,δis the wall thickness of the pressure vessel (mm),δ1is the thickness of the end cap (mm),Pis the design pressure (MPa),Dis the inner diameter of the pressure vessel (mm),σbis the tensile strength (MPa), andnis the safety coefficient.

It should be noted thatδandδ1depends on the size of the electronic module (D=50 mm), whileδ1′depends on the diameter of the CO2detector(D=34 mm). As shown in Table 2, if the material is polyoxymethylene (POM) whose tensile strength is 70 MPa, and the stress resistance of pressure vessel is 2 MPa (water depth is ～200 m), the wall thicknessδshould be no less than 3.6 mm, and the thickness of the end capδ1should be no less than 8.2 mm under 5 times the safety factor by formula calculation. To facilitate the fixing of the end cap and the pressure vessel, the thickness of the pressure vesselδis thickened to 8.0 mm, thus the diameter of the pressure vessel is 66 mm. To fit the waterproof connector, the thickness of the end capδ1is thickened to 20.0 mm, as same as the screw thread length of the waterproof connector. Since the tensile strength of the permeable membrane which material is Teflon and sintered metal plate are unknown, it is impossible to accurately calculate the specific correspondence between the effective diameter of the membrane and the thickness of the sintered metal plate through the formula. As a result, we use half of the tensile strength of 316L stainless steel (500 MPa) to estimate the tensile strength of the sintered stainless steel plate (250 MPa).According to Eq.2, the thickness of the sintered stainless steel plateδ1′ should be no less than 2.9 mm under 5 times the safety factor, so it was designed as 3.0 mm. Considering the length of the inner devices,the total length of the pressure cabin is 124 mm. Then,a corresponding pressure cabin was made, and the success of the pressure test proved that it could withstand underwater pressure of 2 MPa.

The size and weight of each part are shown in Table 3. The total weight is 0.7 kg in air and 0.25 kg in water. The power consumption of the sensor is below 0.9 W. Although the membrane material with high permeability and the CO2detector with high precision were chosen, the cost of the newly developed in-situ sensor was kept under US$1 000, about a twentieth to thirtieth of the price of similar commercial sensors shown in Table 1 (except MAPCO2).

2.2 Concentration calculation method

While measuring the concentration of dissolved CO2in seawater, it is necessary that convert the concentration from the gas-phase to the aqueousphase. For the special case of this sensor, the gasphase concentration (xCO2, ×10-6) in the gas cell could be expressed in terms of partial pressure in the gas-phase (pCO2, μatm) whilst under equilibrium state using the Eq.3 or Eq.3′ (Weiss, 1974; Takahashi et al., 2009; Wu et al., 2021).

wherexCO2is the CO2mole fraction in dry gas that equilibrated with water sample and the barometric pressure (Pb,in, μatm) in gas cell after correcting for the vapor pressure (PH2O,in, μatm) at 100% relative humidity (Wu et al., 2021).wCO2is the CO2mole fraction in wet gas, can be obtained through the CO2detector encapsulated in the sensor. In addition, the value of the vapor partial pressure is calculated by Eq.4 at in-situ temperature (Tw, K) and salinity (S)(Weiss and Price, 1980). Finally, the concentration of CO2dissolved in the seawater (CO2(aq)) can be acquired by Eq.5 (Johnson, 1999; Pro Oceanus, 2019;Zhang et al., 2021).

Table 3 The size and weight of each part, and the assembled sensor

In summary, to calculate the concentration of dissolved CO2in seawater, the solubility and partial pressure of the gas are required to be known. The gas solubility can be calculated by Eq.6, seawater temperatureTwandS, and the partial pressure of CO2can be calculated by Eq.3′, measured value of the CO2detectorwCO2and measured value of the pressure sensorPwet. The concentration of dissolved CO2in seawater can be calculated by the Eq.7.

3 RESULT AND DISCUSSION

Fig.3 Calibration results of the newly developed CO 2 sensor with standard gases of different concentrations

To test the long-term measuring ability of the newly developed in-situ CO2sensor, in the first place the performances of the CO2detector based on NDIR technology were evaluated in the laboratory, including the experiments ofits accuracy, linearity, response time and precision by different concentration of CO2standard gas, evaluation ofits temperature compensation effect, and verification of the issue that if the changes in humidity will affect its measured values. Then the in-situ CO2sensor and similar commercial instruments were placed in the pool for comparison to verify the overall measurement accuracy and precision of the sensor. Finally, a longterm nearshore experiment was carried out, and the data of the in-situ CO2sensor were analyzed reasonably through the changes of seawater temperature and tide,so as to verify the actual long-term measurement ability of the newly developed in-situ CO2sensor. The following content will introduce the experiment process and analyze the results one by one.

3.1 Calibration experiment

3.2 Precision experiment

Fig.4 Continuous monitoring of 528.28×10 -6 CO 2 with a duration of 60 min

To evaluate the measurement precision of the CO2sensor, one-hour continuous measurements were performed with a 528×10-6CO2standard gas. The standard gas was f lushed into the gas chamber with flow of 400 mL/min by a mass flow controller for 5 min, and then two valves on the gas chamber were closed to keep the concentration of CO2gas in chamber constantly. The precision experimental results are shown in Fig.4. For clarity, the scatter plots have been converted into a frequency distribution histogram, which is fitted using a Gaussian function.From Fig.4a, we can see that the concentration values are mainly distributed in the range of (528.28±5.16)×10-6. From Fig.4b, we can see that the frequency distribution of concentration value shows a roughly normal distribution. Taking the ratio of the half width at half maximum (HWHM) to the average concentration value as the precision, we obtain a precision of 0.98% for the CO2sensor at a sampling rate of 1 s.

3.3 Influence evaluations of temperature and humidity

As what mentioned before, although the adopted CO2detector theoretically does not need temperature and humidity correction, some experiments were still carried out for evaluation and verification. The data from the CO2detector with temperature change were measured firstly, to evaluate the temperature compensation effect. The CO2detector was placed in the climate chamber (Vötschtechnik, VC37034). The humidity in the chamber was set at a constant value of 70%RH, and the temperature was set to decrease gradually from 40 °C to 10 °C, to observe if the data from the CO2detector change. The temperature and the data from CO2detector in the chamber are shown in the Fig.5. The blanks in the temperature and CO2data were caused by an accidental power failure. It can be seen from Fig.5 that with the increase of temperature, the data from the CO2detector almost have no change. Therefore, it is proved that the adopted CO2detector has excellent temperature compensation effect, and does not need extra temperature correction practically.

Fig.5 The evaluation of the temperature compensation effect of the CO 2 detector

Since long-term measurements in seawater will inevitably lead to an increase in humidity inside the in-situ CO2sensor, we conducted simulation tests in the laboratory. The in-situ CO2sensor was placed in a sealed tank filled with water to test the humidity change inside the sensor. After determining the range of humidity variation, the influence of humidity on the data from the CO2detector was evaluated. The CO2detector was placed in the climate chamber mentioned above. The temperature in the chamber was set at a constant value, and the humidity was set according to the range of humidity variation in the last test, to observe the changes in the data from the CO2detector.The test of the humidity change inside the sensor lasted for about 5 days with the sampling frequency of 1 s, and the results are shown in Fig.6a from which we can see that the humidity inside the CO2sensor shows an exponential growth trend, and it can be predicted that the humidity will stabilize at 68.17%RH through exponential fitting. Next, the temperature in the chamber was set at 10 °C constantly, and the humidity was set at 10%RH, 30%RH, 50%RH, and 70%RH,respectively, to observe the changes in the data from the CO2detector. The humidity and the data from CO2detector in the chamber are shown in the Fig.6b. It can be seen that with the increase of humidity, the data from the CO2detector fluctuated within the range of(517.55±4.02)×10-6, without significant change.Therefore, it can be considered that humidity has no effect on the CO2detector.

Fig.6 The humidity change inside the sensor for a long-term test (a) and the influence of humidity on the data from the CO 2 detector (b)

Fig.7 The 7-day comparison results between the newly developed CO 2 sensor and the commercial CO 2 sensor in the pool

3.4 Stability measurement in the pool

After evaluating the basic performances of our CO2sensor with standard gases, a 7-day stability measurement was carried out in a pool. A commercial CO2sensor (Pro Oceanus, Mini CO2) was used simultaneously for comparison. The data of the newly developed sensor were recorded per second, and the data of commercial sensor were recorded per two seconds. Because the newly developed sensor and the commercial sensor have different sampling frequencies, to facilitate the comparison, we averaged the raw data from two sensors to one value per minute.The 7-day comparison results of the commercial sensor and the newly developed CO2sensor are shown in Fig.7, from which we can see the two sensors’results have good consistency, withR2of 0.87. With the same NDIR principle, our CO2sensor shows better precision compared with the commercial sensor. The results indicate our CO2sensor has good stability for dissolved CO2measurements in water.

3.5 Field experimental results at the Qingdao nearshore

Fig.8 The 35-day measurement results of the newly developed CO 2 sensor, tidal heights data from the website, and seawater temperature data from the commercial sensor

Fig.9 Changes of the CO 2 concentration, tidal heights, and seawater temperature within a week

Field experiments were carried out at a depth of～1 m in Qingdao nearshore waters from May 17,2019 to June 21, 2019. The continuously 35-day CO2concentration measurement results were obtained.Meanwhile, in order to make a reasonable explanation for the CO2measurement results, the seawater temperature was detected by a commercial multiparameter water quality Sonde (YSI, EXO2). The 35-day CO2concentration measurement results are shown in Fig.8, and the tidal heights and seawater temperature data are also given. The tidal heights data observed at the Qingdao Station were downloaded from the China Maritime Services Network. Several blanks in the CO2and temperature data were caused by accidental power failures, and equipment maintenance, especially biofouling checking and cleaning regularly to ensure the accuracy of the CO2measurement results.

From the 35-day data, we can see an interesting phenomenon: the CO2concentration showed a“double peak” distribution within a day, like a halfday tide. There is an obvious negative correlation between the CO2concentration and the tidal heights.Figure 9 shows zoomed data in the week from June 4 to 11. Due to the field experiment location being in a dock in Qingdao, which is close to the city, the measured concentration of dissolved CO2in coastal seawater is affected by hydrological (Takahashi et al.,1993; Wanninkhof et al., 2019), biological (Millero,1995), surface runoff, and terrestrial input factors(Zhai et al., 2005). Therefore, it is very difficult to comprehensively explain the CO2concentration data obtained from fixed-point observations in the Qingdao nearshore. The obvious correlation between the CO2concentration and the tides, and the seawater temperature, needs to be explored further.

We speculate that the correlation between the CO2concentration and the tidal height may be related to submarine groundwater discharge (SGD) because in coastal zones, SGD is an important pathway for terrestrial materials to be delivered into the sea(Moore, 1996; Burnett et al., 2006; Zhang et al.,2020). Dissolved inorganic carbon concentrations in groundwater are often much higher than those in surface waters, leading groundwater seepage plays a significant role in carbon budgets in aquatic ecosystems (Charette, 2007; Santos et al., 2012,2019). SGD fluxes usually show an inversely correlated pattern with the tides (Burnett and Dulaiova, 2003); we therefore speculate the semidiurnal pattern of CO2we observed was possibly caused by the SGD process in the studied coastal zone. As to the relative correlation between the CO2concentration and the seawater temperature, we speculate that this phenomenon is related to the solubility of CO2in seawater. With the increase (or decrease) of the seawater temperature, the solubility of CO2decreases (or increases), leading to a decrease(or increase) of the CO2concentration in seawater. It also can be affected by the weather, because the sensors were located close to the sea surface. For example, on June 6, there was a heavy rain/shower accompanied by a southeast wind of magnitude 6-7.The rain brought CO2in the air into the sea water. As the CO2concentration in the air is usually lower than that in the sea water, and the strong wind accelerated the mixing of air and the sea surface, the intraday CO2concentration on the sea surface showed an overall downward trend. In addition, rainfall will enrich the groundwater and promote the discharge of groundwater into the sea. However, this process takes a period of time, so the CO2concentration on the sea surface showed an upward trend during the period after the rain stopped (June 7-8).

The 35-day field experiment proved the performance of the newly developed CO2sensor. It can be seen that our sensor measured the dynamic changes of the CO2concentration in seawater continuously, and had the potential to carry out longterm observations on an oceanic platform.

4 CONCLUSION

In order to realize the miniaturization, low power consumption, and low cost of in-situ CO2sensors in the ocean, we developed a CO2sensor based on a permeable membrane and NDIR technology in this paper. The sensor has small dimensions (Ф66 mm×124 mm), low power consumption (＜0.9 W), a light weight (0.7 kg in air and 0.25 kg in water), low cost(＜US$1 000), and high pressure tolerance (＜200 m).It is suitable for a variety of offshore platforms and mobile platforms in the sea. After laboratory performance tests, the sensor showed a measurement range of (0-2 000)×10-6, and the gas linear correlationR2was 0.99, with a precision of about 0.98%. To evaluate the performance of the newly developed sensor, a comparison measurement was carried out with a commercial sensor in a pool for seven days.The experimental results showed consistent trends,and our CO2sensor showed better precision compared with the commercial sensor. The newly developed sensor was also deployed in seawater at a depth of～1 m in the Qingdao nearshore for 35 days. Some interesting phenomena were found from the results of the field experiment, and some reasonable explanations for these were given. The experiment proved that the newly developed sensor could measure the dynamic changes of CO2concentration in seawater continuously, and had the potential to carry out longterm observations on an oceanic platform. It is hoped that the sensor could be applied to field ocean observations in near future.

5 DATA AVAILABILITY STATEMENT

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

6 ACKNOWLEDGMENT

The authors would like to thank Wangquan YE and Ning LI for their helpful discussion of the experiments,and thank the Institute of Oceanographic Institution,Shandong Academy of Sciences for providing the sea trial platform.