Apparent downward CO2 flux observed with open-path eddy covariance over a non-vegetated surface
Apparent downward CO2 flux observed with open-path eddy covariance over a non-vegetated surface
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DOI：
10.1007/s00704-007-0323-3
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发表时间：
2008-05
影响因子：
3.4
通讯作者：
K. Ono;A. Miyata;T. Yamada
中科院分区：
文献类型：
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作者：
K. Ono;A. Miyata;T. Yamada
开放路径涡度相关（EC）方法往往显示不太可能向下的CO2通量在冬末和早春排水稻田，很少有活跃的植物。为了理解为什么，我们进行了密集的测量，在一个裸露的稻田从2003年4月9日至11日，同时使用开放和封闭路径EC方法，空气动力学和动态密闭室的方法也被使用。在此期间，开放路径EC方法显示，即使在应用WPL校正（密度校正）和普通质量控制测试之后，白天的CO2通量也在0至−5.9 µmol m−2s−1之间。由于封闭路径EC和空气动力学方法显示向上的CO2通量，与开放路径EC观察到的向下的CO2通量似乎并不代表真正的CO2运输。白天CO2通量的日变化与白天太阳辐射的增加有关，也与感热通量在弱风条件下的增加有关，但在强风条件下则不然。白天10-Hz的垂直风和CO2混合比的时间序列数据表明，上升气流是CO2贫化的开放路径系统，而相同的上升气流是CO2富集的封闭路径系统。开放和封闭路径EC测量之间的差异的仔细检查显示，从声波风速计和由此产生的显热通量的10赫兹温度信号的幅度太小，以补偿白天观察到的差异。在WPL校正的幅度与未校正的CO2通量本身的幅度一样大的地表和大气条件下，传统应用WPL校正的开路EC方法不一定适合测量小幅度的CO2通量（≤5 µmol m−2s−1）。
The open-path eddy covariance (EC) method often shows unlikely downward CO2fluxes in late winter and early spring over drained paddy fields with few active plants. To understand why, we carried out intensive measurements in a bare paddy field from 9 to 11 April 2003, simultaneously using open- and closed-path EC methods; aerodynamic and dynamic closed-chamber methods were also used. During this period, the open-path EC method showed downward daytime CO2fluxes ranging from 0 to −5.9 µmol m−2s−1, even after application of the WPL correction (density correction) and ordinary quality control tests. Because the closed-path EC and aerodynamic methods showed upward CO2fluxes, the downward CO2fluxes observed with open-path EC appear not to represent true CO2transport. Diurnal variations in the downward daytime CO2fluxes appeared to be correlated with increases in solar radiation during the day, and also with increases in sensible heat flux in weak winds but not under strongly windy conditions. The daytime 10-Hz time series data of vertical wind and CO2mixing ratio demonstrated that updrafts were CO2depleted in the open-path system, whereas the same updrafts were CO2enriched in the closed-path system. Careful examination of the discrepancies between the open- and the closed-path EC measurements revealed that the amplitudes of the 10-Hz temperature signals from the sonic anemometer and the resultant sensible heat fluxes were too small to compensate for the discrepancies observed during the daytime. The open-path EC method with the conventional application of the WPL correction is not necessarily appropriate for measuring small magnitudes of CO2flux (≤5 µmol m−2s−1) under such surface and atmospheric conditions that the magnitude of the WPL correction is as great as that of the uncorrected CO2flux itself.