Located on the west coast of Ireland, the Atmospheric Research Station at Mace Head, Carna, County Galway is unique in Europe, offering westerly exposure to the North Atlantic ocean (clean sector, 180 degrees through west to 300 degrees) and the opportunity to study atmospheric composition under Northern Hemispheric background conditions as well as European continental emissions when the winds favor transport from that region.

The site location (53°20’ N, 9°54’ W) is in the path of the mid-latitude cyclones which frequently traverse the North Atlantic. Mace Head ~57km West of Galway city (~88km by road) which is the nearest major conurbation and population is 75,414 according to the 2011 census.

The main Atlantic shipping routes are over 150 km away, while the transatlantic air corridors are over 80 km away. There are three small islands offshore which are within the clean sector, but these are uninhabited and do not appear to influence any of the measurements made at the site. The meteorological records show that on average, over 60% of the air masses arrive at the station via the clean sector. These air masses are ideal for carrying out background aerosol and trace gas measurements. Significant pollution events also occur at the site when European continental air masses, generally originating from an easterly direction, reach Mace Head. The Mace Head research station is uniquely positioned for resolving these different air masses and for comparative studies of their constituents and characteristics.

Two Picarro instruments are running in parallel at Mace Head station to monitor ambient CO2 and CH4 concentrations in the air. One is a G1301 series and is run by the Irish EPA, the other is a new G2301 series and is run by LSCE. The two instruments are calibrated against the same CO2/CH4 scale and we use the same Target gas to check for the short term stability of both units. The G2301 instrument measures CO2/CH4 concentrations in ambient air that is previously dried using a cryogenic trap (-50°C). For the G1301 unit, we don't operate any air drying but we use the correction formula that are provided by Picarro to take into account the water vapour effect on CO2 and CH4 concentrations.