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.