13 YEARS OF ATMOSPHERIC CARBON-DIOXIDE MEASUREMENTS AT MT-CIMONE STATION, ITALY

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 exhibit 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.