Atmospheric carbon dioxide at Mauna Loa Observatory: 2. Analysis of the NOAA GMCC data, 1974–1985

The first 12 years (1974–1985) of continuous atmospheric CO 2 measurements from the NOAA GMCC program at the Mauna Loa Observatory in Hawaii are analyzed. Hourly and daily variations in the concentration of CO 2 due to local sources and sinks are described, with subsequent selection of data representing background concentrations. A digital filtering technique using the fast Fourier transform and low‐pass filters was used to smooth the selected data and to separate the seasonal cycle from the long‐term increase in CO 2 . The amplitude of the seasonal cycle was found to be increasing at a rate of 0.05±0.02 ppm yr −1 . The average growth rate of CO 2 was 1.42±0.02 ppm yr −1 , and the fraction of CO 2 remaining in the atmosphere from fossil fuel combustion was 59%. A comparison between the Mauna Loa continuous CO 2 data and the CO 2 flask sample data from the sea level site at Cape Kumukahi, Hawaii, showed that the amplitude of the seasonal cycle at Cape Kumukahi was 23% larger than at Mauna Loa, with the phase of the cycle at Mauna Loa lagging the cycle at Cape Kumukahi by about 1–2 weeks.