Thirteen years of atmospheric carbon dioxide measurements at Mt. Cimone station, Italy

Summary The atmospheric carbon dioxide concentration has been continuously measured at Mt. Cimone station from March 1979. In this paper a selection scheme is applied to the measured concentrations in order to obtain data representative of background conditions. Monthly averages, expressed in the WMO X-85 scale are calculated from the selected data and analysed by a technique of time series decomposition until december 1991. Monthly values exibit an increasing long-term trend, mainly due to the fossil fuel combustion: over the whole monitoring period the average CO₂ growth rate is 1.66 p.p.m.v./y. A seasonal cycle, largely due to the biospheric activity of the northern hemisphere is evident. The estimated average peak-to-peak amplitude is 11.32 p.p.m.v. with a maximum occurring in April and a minimum in August. The seasonal amplitude is found to be decreasing with time, by about 1.5 p.p.m.v. over the entire record: no conclusive interpretations are given about this experimental result. Besides the long-term trend and the seasonal cycle, CO₂ interannual variations are observed in the selected record; these correlate negatively with the Southern Oscillation Index (SOI) with a maximum correlation coefficient of 0.6 for a delay of about 7–8 months.