Wang Xing Chang, Wang Chuan Kuan, Liu Fan, Zhen Xiao Jie
Center for Ecological Research, Northeast Forestry University, Harbin 150040, China.
Beijing Techno Solutions Limited, Beijing 100089, China.
Ying Yong Sheng Tai Xue Bao. 2017 Mar 18;28(3):983-991. doi: 10.13287/j.1001-9332.201703.023.
The open-path CO/HO analyzer that is currently widely used in the eddy covariance (EC) system possesses a surface heating effect, which may influence the accuracy of the eddy-flux measurements. Taking the open-path EC system in the temperate deciduous forest of the Maoershan Ecosystem Research Station as a case, we assessed the heating effect of the open-path CO/HO ana-lyzer (Li-7500) with fine-wire thermocouples and tested the applicability of Burba's equations. The results indicated that between April 23 and May 28 in 2016, the daytime air temperature in the mid-optical-path of the Li-7500 was on average 0.2 ℃ higher than the ambient air temperature, while the nocturnal temperature was almost equal, with the peak of the heating often occurring in the morning transition period and at noon or in the early afternoon. The diurnal variation in the heating effect on the sensible heat flux (H) estimated from the simple linear-fitting models in the fourth method by Burba (BurbaLF) was overall a rectangle-like wave, whereas that from the multiple regression models (BurbaMR) showed a weak mono-peak shape. Compared with direct mea-surements (K079) with the K-type fine-wire thermocouples (with a 0.079 mm diameter of single bare wire) and the H modeled with two fine-wire thermocouples (K079Model), the estimated H with the BurbaLF and BurbaMR varied little in the daytime with a lower but wider daytime-peak (20 W·m), a sharp transition between daytime and nighttime, and a typical nocturnal va-lue of ＜ 5 W·m. The peak values of H estimated from the K079 and K079Model could be higher than 40 W·m, and the values at night fluctuated around zero. The sensible heat flux in the optical-path of the Li-7500 was increased by 13.6% during the daytime. The value of the daytime heating effect on the CO turbulent flux (F) estimated from the K079 and K079Model was averaged about 0.5 mg CO·m·s, twice of the value estimated previously. Compared with the direct measurements, the Burba equations underestimated the F around the noon due to the inadequate H, but overestimated the F in the morning, the late afternoon and the night because of over-corrections of H, resulting in an overall overestimate of F. These findings validated that both K079 and K079Model methods can be used to estimate the F.
目前在涡度相关（EC）系统中广泛使用的开放路径CO/HO分析仪具有表面加热效应，这可能会影响涡度通量测量的准确性。以帽儿山生态系统研究站温带落叶林中的开放路径EC系统为例，我们用细线热电偶评估了开放路径CO/HO分析仪（Li-7500）的加热效应，并测试了Burba方程的适用性。结果表明，在2016年4月23日至5月28日期间，Li-7500光路中部的白天空气温度平均比周围空气温度高0.2℃，而夜间温度几乎相等，加热峰值通常出现在早晨过渡时段以及中午或下午早些时候。用Burba的第四种方法中的简单线性拟合模型（BurbaLF）估算的感热通量（H）的加热效应日变化总体呈矩形波，而多元回归模型（BurbaMR）的则呈弱单峰形状。与使用K型细线热电偶（单根裸线直径为0.079 mm）的直接测量值（K079）以及用两根细线热电偶建模的H（K079Model）相比，用BurbaLF和BurbaMR估算的H在白天变化较小，但白天峰值较低且更宽（20 W·m），白天和夜间之间有急剧转变，夜间典型值＜5 W·m。从K079和K079Model估算的H峰值可能高于40 W·m，夜间值在零附近波动。Li-7500光路中的感热通量在白天增加了13.6%。从K079和K079Model估算的白天加热效应在CO湍流通量（F）上的值平均约为0.5 mg CO·m·s，是先前估算值的两倍。与直接测量相比，由于H不足，Burba方程在中午左右低估了F，但在早晨、傍晚和夜间因对H的过度校正而高估了F，导致对F的总体高估。这些发现验证了K079和K079Model方法均可用于估算F。
