The monthly data files contain 6 columns. These files are based on daily data which is combined and averaged to yield a monthly mean centered on the middle of the month. Column 1 is the date in fractional years. Column 2 is the relevant concentration (e.g. CO2 in ppm, O2/N2 and APO in per meg units). Column 3 gives the data after a seasonal adjustment to remove the quasi-regular seasonal cycle. The adjustment involves subtracting from the data a 4-harmonic fit to the seasonal cycle. Column 4 is a smoothed version of the data generated from a stiff cubic spline function plus the 4-harmonic function. Column 5 is the same smoothed version with the seasonal cycle removed. Column 6 indicates the number of sampling dates that are incorporated into the monthly average. Missing values are set to zero.