[Assessing the surface heating effect of the open-path analyzer of an eddy covariance system with fine-wire...

The open-path CO₂/H₂O 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 CO₂/H₂O 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_S,HE) 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_S,HE modeled with two fine-wire thermocouples (K079Model), the estimated H_S,HE with the BurbaLF and BurbaMR varied little in the daytime with a lower but wider daytime-peak (20 W·m^-2), a sharp transition between daytime and nighttime, and a typical nocturnal va-lue of ＜ 5 W·m^-2. The peak values of H_S,HE estimated from the K079 and K079Model could be higher than 40 W·m^-2, 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 CO₂ turbulent flux (F_c,HC) estimated from the K079 and K079Model was averaged about 0.5 mg CO₂·m^-2·s^-1, twice of the value estimated previously. Compared with the direct measurements, the Burba equations underestimated the F_c,HC around the noon due to the inadequate H_S,HE, but overestimated the F_c,HC in the morning, the late afternoon and the night because of over-corrections of H_S,HE, resulting in an overall overestimate of F_c,HC. These findings validated that both K079 and K079Model methods can be used to estimate the F_c,HC.