Comparative evaluation of high-resolution seawater CO2 partial pressure measurement methods and their application in coastal monitoring networks
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摘要: 高稳定性的表层海水二氧化碳分压（pCO2）监测是评估长时间尺度和大空间尺度上海-气二氧化碳通量的关键基础，而水-气平衡法和膜平衡法是目前国内外常用的表层海水pCO2测定方法。本研究利用基于水-气平衡法的LI-5405A与基于膜平衡法的CONTROS HydroC® CO2，在乐清湾和瓯江口海域开展了走航连续观测，并在扁鳗屿进行了定点连续观测，系统对比了两者的性能差异与适用场景。结果表明，两种方法在各场景下测得的表层海水pCO2数据趋势一致，具有较高的相关性，整个观测期间两种方法所得数据的差值平均为0.76 ± 4.46 μatm，但其各自优势存在差异：膜平衡法凭借低功耗特性与一体化水密设计，适合在原位或供电及空间受限的情境下进行长期连续监测；水-气平衡法则具备快速响应和高精度特点，更适用于高动态环境下的高分辨率观测。因此，构建“膜平衡法基线监测+水-气平衡法校准巡检”的协同观测体系，可实现时间尺度与空间维度的互补，显著提升海-气二氧化碳通量监测的时空覆盖度与数据可靠性，为近岸海水碳源汇评估提供多维度技术支撑。Abstract: High-stability monitoring of air-sea CO2 partial pressure (pCO2) is fundamental for assessing air-sea CO2 fluxes across long temporal and broad spatial scales. Among the commonly used methods worldwide, the air-water equilibration technique and the membrane-based diffusion technique are widely applied for measuring pCO2. In this study, we conducted underway continuous observations in Yueqing Bay and a 50-hour fixed-point deployment near Bianmanyu Island using the LI-5405A (air-water equilibration method) and the CONTROS HydroC® CO2 sensor (membrane equilibration method). The performance characteristics and application scenarios of the two approaches were systematically compared. The average difference between the two methods was 0.76 ± 4.46 μatm during the entire observation period, but both approaches produced consistent temporal trends with strong correlation. These two approaches exhibited distinct advantages: the membrane equilibrium method features low power consumption and an integrated watertight design, making it suitable for long-term continuous monitoring in in-situ scenarios or where power supply and space are limited. The air-water equilibration method, featuring rapid response and high precision, is more applicable for high-resolution measurements in dynamic environments. We propose a synergistic observational framework combining baseline monitoring using the membrane equilibration technique with periodic calibration via the air-water equilibration method. This integrated approach enhances both temporal and spatial resolution of air-sea CO2 flux observations, thereby improving data reliability and providing robust technical support for nearshore carbon source-sink assessments.
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图 1 观测路线图
其中，蓝色路径为乐清湾大面走航观测路径，粉色三角形指示扁鳗屿定点观测位置
Fig. 1 Trajectory map
The blue line represents the underway trajectory in Yueqing Bay and Oujiang estuary, and the pink triangle represents the fixed observation station.
图 2 调零校准过程数据图
其中，A区域为调零校准前数据点，虚线为A区域数据均值；B区域为调零校准程序运行时数据点，使用平稳后的B2区域数据进行1.1中公式③及以后步骤处理；C区域为冲洗程序运行时数据点；D区域为冲洗程序完成后，直到pCO2测定数值与调零前数据均值的差值在误差范围内的数据点；E区域为pCO2测定数值与调零前数据均值的差值达到误差范围内及以后的数据点；粉色区域为实际保留讨论的观测数据点。
Fig. 2 Course of zeroing calibration
Zone A: Pre-zeroing data points (Dashed lines indicate mean value). Zone B: Data during zeroing procedure (stabilized B2 sub-zone data used for Equation ③ in Section 1.1). Zone C: System flushing period. Zone D: Post-flushing data until pCO2 deviation from pre-zeroing mean falls within error margin. Zone E: Data meeting error criterion (partial pressure of CO2 deviation ≤ threshold). Pink zone: Valid observational data retained for analysis.
图 3 水-气平衡法二氧化碳标准气体测定结果
Fig. 3 Results of CO2 standard gas via water-air equilibrium method
图 4 水-气平衡法二氧化碳标准气体测定标准曲线
Fig. 4 Calibration curve for CO2 standard gas using water-Air equilibrium method
图 6 定点观测一个潮周期潮位与表层海水pCO2数据图
Fig. 6 Tidal elevation versus pCO2 during a tidal cycle at fixed station
图 7 比测数据对比图
其中，蓝色底为大面走航观测时间段数据，粉色底为定点观测时间段数据
Fig. 7 Comparative measurement data
Blue background: Underway transect data; Pink background: Fixed-point observations.
图 8 比测结果相关性
圆形点为同一时刻水气平衡法和膜平衡法测得的数据点，灰色阴影区域表示由精度更低的膜平衡法不确定性给出的1:1线附近的允许误差范围，黑色虚线是线性回归模型的最佳拟合。
Fig. 8 Correlation of Comparative Results
Circular points: Simultaneous measurements by air-water equilibration and membrane equilibration. Gray shaded area: Uncertainty band around 1:1 line derived from lower-precision membrane permeation method. Black dashed line: Best-fit linear regression model.
图 10 水气平衡法与膜平衡法测定值差值影响因素相关性热图
Fig. 10 Correlation heat map of factors affecting the difference between the determination pCO2 values of the air-water equilibration technique and the membrane-based diffusion technique
图 11 表层海水pCO2空间变异性较大区域比对数据
Fig. 11 Comparative data from regions with high pCO2 spatial variability
表 1 比测数据分析表
Tab. 1 Analysis of comparative measurement data
测定区域 差值ΔpCO2=pCO2，水气平衡法−pCO2，膜平衡法 最大值/μatm 最小值/μatm 平均/μatm 标准偏差/μatm 大面
走航
观测11月26日 18.46 −1.69 3.37 3.23 11月27日 17.67 −2.43 3.67 3.10 11月28日 13.58 −9.84 2.08 4.14 定点
连续
观测12月2日 4.04 −9.37 −3.96 2.74 12月3日 4.58 −8.69 −3.66 1.85 12月4日 10.28 0 3.84 1.95 全程 18.46 −9.84 0.76 4.46 
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表 2 技术参数对比
Tab. 2 Comparison of Technical Specifications
CONTROS HydroC® CO2 LI-5405A 体积 π*8.9 cm*8.9 cm*50 cm 69 cm*51 cm*41 cm（主机）
51 cm*33 cm*18 cm（检测器）净重 4.5 kg（空气中）
2.2 kg（水中）43 kg（主机）
10.5 kg（检测器）工作电压 12-30 V 110 V或220 V 功率 5.9 W ＞10 W（主机）*
22 W（检测器）
370 W（外接水泵）**
30 W（外置数据处理存储装置）响应时间（τ99%） 6-15分钟*** 2分钟 备注：*公开资料未提供仪器功率。**370W为本次比测中使用的潜水泵功率。外接水泵将原位表层海水抽送至传感器，且传感器所监测到的水流速需达到2.5升每分钟。***6分钟为本次比测数据结果，15分钟为Saderne等（2013）报道结果。
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