El Arenosillo is located in Mazagón, Huelva, about 700 m from the Atlantic coast and close to Doñana National Park. It is operated by the National Institute for Aerospace Technology (INTA). Its coastal setting receives air masses of marine and continental origin, providing a complementary perspective to the inland and high-mountain atmospheric stations of ICOS-Spain.
Continuous in situ measurements of CO₂, CH₄ and CO use a Picarro G2401 cavity ring-down spectrometer (CRDS) housed in a laboratory at the base of a 100 m tower. Air is sampled at 10, 50 and 100 m, with meteorological observations at the same levels. CO₂ and CH₄ are greenhouse gases; CO is used as a tracer of atmospheric processes and sources.
The greenhouse gas measurement programme began in September 2019. In November 2021, the analyser was moved to the tower laboratory and sampling at three heights began. The sampling system was designed and installed to meet ICOS requirements. El Arenosillo was labelled as an ICOS Class 2 atmospheric station on 26 November 2025.
The station supports studies of atmospheric composition and air-mass transport between the Atlantic, the Mediterranean and the Iberian Peninsula. Measurements at different heights help characterise vertical variability and the influence of regional sources, strengthening the coastal coverage of the Spanish atmospheric network.