Greenhouse Gas Measurements Over a 144 km Open Path in the Canary Islands

A new technique for the satellite remote sensing of greenhouse gases in the atmosphere via the absorption of short-wave infrared laser signals transmitted between counter-rotating satellites in low Earth orbit has recently been proposed; this would enable the acquisition of a longterm, stable, global set of altitude-resolved concentration measurements. We present the first ground-based experimental demonstration of this new infrared-laser occultation method, in which the atmospheric absorption of CO2 near 2.1 μm was measured over a ∼ 144 km path length between two peaks in the Canary Islands (at an altitude of ∼ 2.4 km), using relatively low power diode lasers (∼ 4 to 10 mW). The retrieved CO2 volume mixing ratio of 400 ppm (± 15 ppm) is consistent within experimental uncertainty with simultaneously recorded in situ validation measurements. We conclude that the new method has a sound basis for monitoring CO2 in the free atmosphere; other greenhouse gases such as methane, nitrous oxide and water vapour can be monitored in the same way.