The data file includes the following variables:
Column 1: Decimal day of year (local standard time, hourly values)
Column 2: CO2 concentration measured at approximately 100 m using tunable diode laser spectroscopy (micromol/mol)
Column 3: De-spiked CO2 concentration. The raw CO2 concentrations were first filtered for extreme values ([CO2] < 340 micromol/mol or [CO2] > 450 micromol/mol) and then fitted to a model consisting of a quadratic polynomial for the long-term trend of CO2 growth and four-harmonics for the CO2 seasonal cycle [Thoning et al., 1989]. Any data lying outside 5 standard deviations of the residuals between the original data and the model fit were discarded. This procedure was repeated until no outliers were identified.
Column 4: Gap-filled CO2 concentration. A modified Mean Diurnal Variation method [Falge et al., 2001] was used to gap-filled the de-spiked CO2 concentration time series. Gaps were filled by taking the average value at the same time of day (±1 h) within a moving time window of adjacent days. Following an iterative procedure, the moving window includes more days in the vicinity of the gap and the same days of the adjacent year (±1 yr) until there are enough good data points (> 60% of the moving window) for gap filling. The accuracy of the gap-filling method was estimated to be ±1.2 micromol/mol using a Monte Carlo approach.
Reference:
Thoning, K. W., Tans, P. P., Komhyr, W. D. Atmospheric carbon dioxide at Mauna Loa Observatory: 2. Analysis of the NOAA GMCC data, 1974–1985. J. Geophys. Res.-Atmos. 94, 8549-8565 (1989).
Falge, E., Baldocchi, D., Olson, R., Anthoni, P., Aubinet, M., Bernhofer, C., Burba, G., Ceulemans, R., Clement, R., Dolman, H., Granier, A. Gap filling strategies for long term energy flux data sets. Agr. Forest Meteorol. 107, 71-77 (2001).