Deventer M Julian, Roman Tyler, Bogoev Ivan, Kolka Randall K, Erickson Matt, Lee Xuhui, Baker John M, Millet Dylan B, Griffis Timothy J
University of Minnesota - Dept. Soil, Water & Climate, St. Paul, MN, USA.
ANECO Institut für Umweltschutz GmbH & Co, Hamburg, Germany.
Agric For Meteorol. 2021 Jan 15;296. doi: 10.1016/j.agrformet.2020.108216. Epub 2020 Nov 20.
Eddy covariance (EC) measurements of ecosystem-atmosphere carbon dioxide (CO) exchange provide the most direct assessment of the terrestrial carbon cycle. Measurement biases for open-path (OP) CO 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 mol m s) persists for all seasons leading to a 110% overestimate of the ecosystem CO sink (cumulative error of 78 gC m). 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 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 CO 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) or to systematic over-correction (by 17-37 gC m). 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 mol m K. This temperature sensitivity causes CO 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 CO measurement errors.
涡度协方差（EC）测量生态系统与大气之间的二氧化碳（CO₂）交换，可对陆地碳循环进行最直接的评估。开放路径（OP）CO₂浓度和通量测量的偏差已被报道30多年，但偏差的来源及合适的校正方法仍未解决。在此，我们量化了OP偏差对亚寒带湿地碳和辐射强迫收支的影响。与参考封闭路径（CP）系统的比较表明，所有季节都存在系统性的OP通量偏差（0.54 mol m⁻² s⁻¹），导致对生态系统CO₂汇的高估达110%（累积误差为78 gC m⁻²）。考虑了两个潜在的OP偏差源：传感器路径热交换（SPHE）和分析仪温度敏感性。我们研究了潜在的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）先前发表的参数化方法；iii）优化算法。测量结果显示，底部传感器表面与大气之间全年平均温度和热通量梯度分别为2.9 °C和16 W m⁻²，表明存在SPHE引起的OP偏差。然而，测量的SPHE与OP和CP CO₂通量之间的观测差异相关性较差。虽然先前提出的名义上通用的SPHE校正减少了累积OP偏差，但它们要么导致系统性的校正不足（38.1 gC m⁻²），要么导致系统性的校正过度（17 - 37 gC m⁻²）。由此产生的收支误差超过了CP随机不确定性，并改变了整体碳和辐射强迫收支的符号。对OP校准残差随温度变化的分析显示，灵敏度为5 mol m⁻² K⁻¹。这种温度敏感性导致CO₂校准误差与采样空气波动成正比，可抵消观测到的生长季通量偏差的50%。因此，我们呼吁建立一个新的OP校正框架，以表征SPHE和温度引起的CO₂测量误差。
