Atmospheric methane in the Mediterranean: Analysis of measurements at the island of Lampedusa during 1995–2005

Measurements of atmospheric methane and other greenhouse gases are routinely carried out at the island of Lampedusa (35.5°N, 12.6°E), in the Mediterranean Sea. The CH4 mixing ratio record obtained during the period June 1995–September 2005 shows a distinct annual cycle characterized by a maximum in March and a minimum in late summer. The cycle peak-to-peak amplitude is about 30 ppb. In the period of investigation the CH4 growth rate (GR) shows two positive peaks, the first in 1998 with 19 ppb yr−1, and the second at the end of 2001 with 14 ppb yr−1. A sharp minimum of −13 ppb yr−1 occurs in April 2000. The GR remains close to zero after mid-2002. The evolution of methane GR and global temperature anomaly appears well correlated (R=0.71). The methane removal by reaction with OH, acting both on a global and on smaller scales, seems to play a large role. Significant differences exist among the GR at stations located within or close to the Mediterranean basin, indicating that regional and smaller scale processes must be taken into account to explain the methane evolution. The weekly measurements have been combined with backward airmass trajectories to study the relationship between CH4 mixing ratio and transport. On average, the methane mixing ratio in airmasses from Eastern Europe is about 8 ppb higher than in airmasses from Western Europe, 15 ppb higher than in airmasses from oceanic regions, and about 20 ppb higher than in airmasses from Africa. Among the airmasses from Africa, those from Northern Algeria display the largest CH4 mixing ratio, possibly because of emissions from gas and oil production and leakage from the gas pipelines. A North-to-South latitude gradient in CH4 mixing ratio is also apparent.