Isotopic Composition of Net Ecosystem CO2 Exchange at Harvard Forest EMS Tower since 2011

This archive features long-term measurements of the eddy and storage fluxes of 16O12C16O, 16O13C16O, and 18O12C16O at the Harvard Forest EMS flux tower. Provided are the individual isotopologue fluxes, the total CO2 flux, and the δ13C and δ18O isofluxes. The eddy (iso)fluxes were measured by eddy covariance (EC), with a 30-minute integration period on a 40-minute duty cycle. The storage fluxes were measured as the increase in storage below 29 m during the EC integration period, based on vertical integrations over 7 air sampling heights on the tower (0.2, 1.0, 7.5, 12.7, 18.1, 24.1, 29.0 m) prior to July 3, 2012, and over 6 air sampling heights on the tower (0.2, 1.0, 7.5, 12.7, 18.1, 29.0 m) after July 3, 2012, once every 40 minutes. Corrected and uncorrected versions of the eddy (iso)fluxes are provided; the corrections account for high-frequency signal attenuation, and were made by comparing w-CO2 and w-T cospectra. The method is novel and complex and is described, along with all further details of the measurements, in an upcoming publication. The original, uncorrected cospectra are provided as well, for those wishing to employ an alternate correction method. These are not Fourier cospectra but rather multiresolution cospectra, as described in the following reference: Howell, J., Mahrt, L., 1997. Multiresolution flux decomposition. Boundary-Layer Meteorology 83, 117-137. The covariance is the simple sum (i.e. not the discretized integral) of all 15 points in the corresponding multiresolution cospectrum. Finally, this archive provides measurements of the total molar mixing ratio of CO2 to dry air and of the isotope ratios δ13C and δ18O in CO2, at each of the 7 heights. For the lower six heights (or the lower 5 heights after July 3, 2012), these measurements are provided only as 40-second averages of the original 4 Hz data (which are too copious to archive here), made once every 40 minutes. For the 29 m inlet (the main inlet, used for EC), these measurements are provided as 2-minute averages of the 4 Hz data, made continuously during the 30-minute EC integration periods. The last 40-seconds of each EC integration period is included also as the 7th height (6th height after July 3, 2012) in the vertical profile record.本数据集存档包含哈佛大学森林EMS通量塔上针对三种二氧化碳同位素异构体（isotopologue）：¹⁶O¹²C¹⁶O、¹⁶O¹³C¹⁶O及¹⁸O¹²C¹⁶O的涡动通量与储存通量长期观测数据。数据集提供各同位素异构体通量、总CO₂通量，以及δ¹³C与δ¹⁸O同位素通量（isofluxes）。涡动（同位素）通量采用涡动协方差（eddy covariance, EC）方法观测，积分时长为30分钟，占空比为40分钟。储存通量通过计算EC积分时段内29米以下大气储存量的增量得到，观测依托塔架上的垂直采样高度剖面：2012年7月3日前采用7个采样高度（0.2、1.0、7.5、12.7、18.1、24.1、29.0米），2012年7月3日后调整为6个采样高度（0.2、1.0、7.5、12.7、18.1、29.0米），观测频率为每40分钟一次。本数据集同时提供涡动（同位素）通量的校正与未校正版本，校正过程针对高频信号衰减问题，通过对比w-CO₂与w-T协谱（cospectrum）完成。该校正方法新颖且复杂，相关细节与观测全流程说明将发表于即将出版的学术论文中。此外，数据集还提供原始未校正协谱，以供需要使用其他校正方法的使用者参考。此处协谱并非傅里叶协谱（Fourier cospectrum），而是多分辨率协谱（multiresolution cospectrum），具体说明可参考如下文献：Howell J, Mahrt L. 1997. 多分辨率通量分解. 《边界层气象学》, 83: 117-137。协方差值为对应多分辨率协谱全部15个测点的直接求和（而非离散积分结果）。最后，本数据集还提供各采样高度上CO₂与干空气的总摩尔混合比（molar mixing ratio），以及CO₂中δ¹³C与δ¹⁸O同位素比值的观测数据。针对前6个采样高度（2012年7月3日后为前5个），上述观测数据仅提供原始4Hz数据的40秒平均值——原始4Hz数据量过大无法在此存档，平均值生成频率为每40分钟一次。而29米采样口（涡动协方差观测的主采样口）的观测数据，则提供原始4Hz数据的2分钟平均值，该平均值在30分钟的EC积分时段内连续生成。每个EC积分时段的最后40秒数据，同时作为垂直剖面记录中的第7个高度（2012年7月3日后为第6个高度）纳入存档。