Detecting Volcanic CO2 from Space: Target Mode Observations with GOSAT

Volcanoes are highly variable but continuous CO₂ emitters, distributed globally, and emissions often occur at high altitudes. We seek to measure volcanic CO₂ emissions from space for three reasons. (a) to improve the currently very poor global CO₂ source strength estimates for volcanoes; (b) to utilize volcanic CO₂ emissions for high altitude strong point source emission and dispersion studies; (c) to develop new volcano monitoring techniques, because volcanic CO₂ emissions before eruptions are potentially the earliest available indicators of unrest. In attempting to detect strong point sources of CO₂ from space, several obstacles have to be faced: Orographic clouds, unknown dispersion behavior, a high CO₂ background in the troposphere, and sparse data coverage from existing satellite sensors. These obstacles can be overcome by a small field of view, enhanced spectral resolving power, and by employing target mode observation strategies. The Japanese GOSAT instrument has been operational since January 2009, producing CO₂ total column measurements with a repeat cycle of 3 days and a field of view of 10km. GOSAT thus has the potential to provide spatially integrated data for entire volcanic edifices, especially in target mode. Since summer 2010 we have conducted repeated target mode observations on over 20 selected global volcanoes including Etna (Italy), Erta Ale (Ethiopia), and Ambrym (Vanuatu), using L2 GOSAT FTS SWIR data. Where available, we compare these to SO₂ data from the Ozone Monitoring Instrument (OMI) and ground-based SO₂ measurements. First results are promising, indicating a possible correlation between ground based SO₂ and GOSAT data, for several volcanoes. We also compare NIES L2 products with alternative processing by NASA, where available in volcanic areas.