Abstract Eddy covariance (EC) measurements of ecosystem-atmosphere carbon dioxide (CO2) exchange provide the most direct assessment of the terrestrial carbon cycle. Measurement biases for open-path (OP) CO2 concentration and flux measurements have been reported for over 30 years, but their origin and appropriate correction approach remain unresolved. Here, we quantify the impacts of OP biases on carbon and radiative forcing budgets for a sub-boreal wetland. Comparison with a reference closed-path (CP) system indicates that a systematic OP flux bias (0.54 μ mol m − 2 s − 1 ) persists for all seasons leading to a 110% overestimate of the ecosystem CO2 sink (cumulative error of 78 gC m−2). Two potential OP bias sources are considered: Sensor-path heat exchange (SPHE) and analyzer temperature sensitivity. We examined potential OP correction approaches including: i) Fast temperature measurements within the measurement path and sensor surfaces; ii) Previously published parameterizations; and iii) Optimization algorithms. The measurements revealed year-round average temperature and heat flux gradients of 2.9 °C and 16 W m−2 between the bottom sensor surfaces and atmosphere, indicating SPHE-induced OP bias. However, measured SPHE correlated poorly with the observed differences between OP and CP CO2 fluxes. While previously proposed nominally universal corrections for SPHE reduced the cumulative OP bias, they led to either systematic under-correction (by 38.1 gC m−2) or to systematic over-correction (by 17-37 gC m−2). The resulting budget errors exceeded CP random uncertainty and change the sign of the overall carbon and radiative forcing budgets. Analysis of OP calibration residuals as a function of temperature revealed a sensitivity of 5 μ mol m − 3 K − 1 . This temperature sensitivity causes CO2 calibration errors proportional to sample air fluctuations that can offset the observed growing season flux bias by 50%. Consequently, we call for a new OP correction framework that characterizes SPHE- and temperature-induced CO2 measurement errors.

中文翻译：

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开路二氧化碳通量测量中的偏差：仪器表面热交换和分析仪温度灵敏度的作用

摘要 生态系统-大气二氧化碳 (CO2) 交换的涡度协方差 (EC) 测量提供了对陆地碳循环的最直接评估。开放路径 (OP) CO2 浓度和通量测量的测量偏差已报告了 30 多年，但其起源和适当的校正方法仍未解决。在这里，我们量化了 OP 偏差对亚寒带湿地碳和辐射强迫预算的影响。与参考闭路 (CP) 系统的比较表明，系统的 OP 通量偏差 (0.54 μ mol m - 2 s - 1 ) 在所有季节都持续存在，导致生态系统 CO2 汇高估 110%（累积误差为 78 gC m-2)。考虑了两个潜在的 OP 偏置源：传感器路径热交换 (SPHE) 和分析仪温度灵敏度。我们检查了潜在的 OP 校正方法，包括：i) 测量路径和传感器表面内的快速温度测量；ii) 先前发布的参数化；iii) 优化算法。测量结果显示底部传感器表面和大气之间的全年平均温度和热通量梯度分别为 2.9 °C 和 16 W m-2，表明 SPHE 引起的 OP 偏差。然而，测量的 SPHE 与观察到的 OP 和 CP CO2 通量之间的差异相关性较差。虽然之前提出的 SPHE 名义上的通用校正减少了累积 OP 偏差，但它们导致系统性欠校正（38.1 gC m-2）或系统性过度校正（17-37 gC m-2）。由此产生的预算误差超过了 CP 随机不确定性，并改变了总体碳和辐射强迫预算的符号。对作为温度函数的 OP 校准残差的分析揭示了 5 μ mol m - 3 K - 1 的灵敏度。这种温度敏感性会导致与样品空气波动成比例的 CO2 校准误差，这可以将观察到的生长季节通量偏差抵消 50%。因此，我们需要一个新的 OP 校正框架来表征 SPHE 和温度引起的 CO2 测量误差。