Monitoring passively degassing volcanoes from space: A comparison between ASTER and OMI retrievals of lower tropospheric SO2

Passively degassing volcanoes contribute a climatologically significant quantity of sulfur dioxide (SO2) to the atmosphere. Both the Advanced Thermal Emission and Reflection Radiometer (ASTER) and the Ozone Monitoring Instrument (OMI) are capable of detecting emissions from volcanoes in a non-eruptive state. There are fundamental differences between the sensors that affect their sensitivity to SO2. OMI operates in the ultraviolet with a 13x24 km nadir footprint and a 2600 km swath width, providing daily global coverage and retrievals of SO2 at all altitudes from the planetary boundary layer to the stratosphere (Carn et al, 2008). In contrast, ASTER operates in the infrared (specifically the 8.6 µm region of the thermal infrared for SO2 detection) with 90 m spatial resolution and a 60 km swath width. Hence the temporal resolution and geographic coverage of ASTER is somewhat less than OMI, with one ASTER scene acquired every 5-16 days for a given location. However, the higher spatial resolution of ASTER provides more information on the structure of tropospheric SO2 plumes. Six volcanoes were selected based on their differing climates, altitudes and SO2 emission rates: Mt Etna, Sicily; Pacaya, Guatemala; Masaya, Nicaragua; Popocatepetl, Mexico; Nyiragongo, DR Congo; and Kilauea, Hawaii. These volcanoes are continually active and typically emit in excess of 1000 metric tons per day of SO2. ASTER and OMI data were acquired for each volcano and processed in order to compare the satellite SO2 retrievals under different conditions. Our goal is to determine the optimum conditions for lower tropospheric SO2 retrievals using each instrument, and constrain the lower limit of volcanic SO2 emission rate that can be detected and monitored from space.