Relationships between Atmospheric Circulation Patterns and CO2 Greenhouse-Gas Concentration Levels in the Alpine Troposphere

Middle tropospheric CO2 air concentrations, measured during a four year observation period at an Alpine Sta-tion (Plateau Rosa , Italian North-West Alps, 3480 m a.s.l.), have been correlated with the relevant synoptic air tra?jectories crossing the observation site Meaningful relationships have been found among average curvature, altitude and potential temperature of clus?ters of homogeneous trajectories and their related CO2 concentrations measured at Plateau Rosa, allowing an objec?tive identification of weather conditions giving rise to fully mixed air masses corresponding to background atmos?pheric CO2 levels. Air trajectories were calculated by using the wind speed fields provided by the ECMWF objective analysis. As during the analysis period a change of the ECMWF model resolution occurred (from T106 to T213), the study was al?so carried out in the sub-periods respectively preceding and following this change (in September 1991). Even if some features of the wind field turned out to be statistically different in the two sub-periods, nevertheless the differences on the trajectory patterns were small enough to keep almost unchanged all conclusions drawn for the whole four-year period.