Atmospheric carbon dioxide measurements in the Australian region: ten years of aircraft data

Observations of the atmospheric carbon dioxide (CO2) concentration made during the period 1972 through 1981 are presented. The data are based on air samples collected from aircraft throughout the troposphere and lower stratosphere over the Australian-New Zealand region and as far south as Antarctica.A description is given of the degree of natural variability of CO2 concentration at all levels of the troposphere and lower stratosphere using 35 19 individual samples.The average rate of concentration increase in the mid troposphere over the period represented by the data was 1.3 ppmv yr-1, although the rate of increase is shown to vary considerably with time. These variations agree in magnitude and approximately in phase with similar variations observed at the South Pole and at Mauna Loa, Hawaii.The mid-tropospheric data indicate a fossil fuel airborne fraction over the period January 1973 to January 1981 of 0.57 and this compares with an estimate for the past decade of 0.55 based on data from several monitoring stations.The annual variation of the CO2 concentration in the mid troposphere has an amplitude of 1.2 ppmv peak-to-peak with the maximum occurring in October and the minimum in April. The amplitude is seen to increase with altitude to 2 to 3 ppmv in the upper troposphere while the phase of the cycle at higher altitude precedes that in the mid troposphere. This is possibly due to the high altitude meridional transport of northern hemisphere air into the southern hemisphere.Vertical gradients through the middle and upper troposphere are generally of order 0.4 ppmv (10 km)-1 with lower concentrations at lower altitudes throughout most of the year. In the lower troposphere, below 2 km, gradients of up to 4 ppmv km-1 can occur when air has traversed vegetated land. The relatively small effect of a large city (Melbourne) on lower tropospheric CO2 levels is demonstrated.