Multiyear average characteristics of CO2 variations in the free atmosphere over Colorado (40° N, 104° W)

A statistical analysis is made of vertical changes in CO2 mole fraction and its seasonal variations in the free troposphere from the data of flask aircraft measurements over Briggsdale and Carr, Colorado, USA (∼40° N, ∼104° W) during years 1992–2011. Polynomials give good approximations for the general CO2 19-year grows rates at different altitudes in the troposphere. The averaged over altitudes 4–8 km 19-year mean CO2 mole fraction related to year 2002 is 372.1 ± 0.1 ppm, its mean growth rate is 1.97 ± 0.02 ppm yr−1 and acceleration of the grows is 0.019 ± 0.01 ppm yr−2. Observed CO2 seasonal cycles, also amplitudes and phases of their spectral components are less variable in the troposphere above altitude 4–5 km than below. This may reflect better mixing and larger influence of atmospheric circulation there, than at lower altitudes. Annual and semiannual components could prevail in average CO2 seasonal cycle in the troposphere above altitude 4–5 km, while shorter period components are more important at lower altitudes. The amplitude of the semiannual component grows in time faster than the amplitude of annual component. In the lower part of the troposphere, transitions from low-altitude CO2 characteristics (partly influenced by local sources) to more homogeneous ones in the upper troposphere are observed.