Short-term trend and temporal variations in atmospheric methane at an Atlantic coastal site in Southwestern Europe

Abstract

Methane (CH4) was measured at El Arenosillo observatory, in an Atlantic coastal area located in Southwestern Europe, throughout a four-year period (September 2019–December 2023). Short-term trend, temporal variations and patterns were explored. A trend of 12.1 ± 1.1 nmol mol−1 year−1 was obtained based on daily averages. CH4 exhibited its highest mole fraction in cold months and its lowest in summer, and a seasonally varying diurnal cycle was also observed, with peaks in the early morning (7:00–8:00 UTC) and minimum values at 18:00–19:00 UTC. Mitigation strategies should be implemented based on an understanding of CH4 patterns. To achieve this, CH4 variations under an atmosphere governed by two synoptic patterns were studied. The atmospheric dynamic was analysed with the wind fields from ERA5 data reanalysis and the air masses pathways with the back-trajectories computed with the HYSPLIT model. CH4 horizontal distribution was explored with the TROPOMI observations. The CH4 baseline was collected under the influence of Atlantic air, while a rise from 40 to 80 nmol mol−1 was found with continental airflows from the inland Guadalquivir Valley. CH4 under sea-land breezes did not contribute to the increase of CH4. However, XCH4 from TROPOMI showed an accumulation in the marine area of the Gulf of Cadiz. Ongoing CH4 observations will be used to identify potential changes in the trend, understand the contribution of transported CH4 from areas such as the Western Mediterranean Basin, or assess its impact on surface ozone, among other objectives.