Atmospheric CO₂ concentration variations recorded at mace head, Ireland, from 1992 to 1994

Atmospheric CO₂ has been monitored continuously at the Irish station of Mace-Head (MHD) since July 1992. The mean rate of increase is 1.85 ppmv yr^-1 with a peak-to-peak amplitude of 15.1±1.1 ppmv. The record also shows important short-term variations from the smoothed curve due to both European and oceanic influences and amplified by day/night local effects. Two different baseline conditions are defined in order to remove local perturbations. Restricted Baseline Conditions (RBC) keep data only representing atmospheric background conditions from the West. Extended Baseline Conditions (EBC) add stable air masses coming from all angular sectors to RBC selection. The contrast between western and eastern air masses coming respectively from North-Atlantic ocean and Europe is studied. For air masses arriving from the West, a systematic depletion in 1992-93 is found relative to all surrounding stations within the North-Atlantic basin. The partition of this depletion between European biospheric uptake and the North-Atlantic ocean cannot be performed as long as other tracers are not sampled regularly, or during intensive campaigns at Mace Head (¹³C/¹²C, Radon-222 and O₂/N₂). In contrast, Eastern air masses present strong positive anomalies in CO₂ that reflect European sources.