Infrared satellite observations of hydrogen sulfide in the volcanic plume of the August 2008 Kasatochi eruption

Hydrogen sulphide (H 2 S) is one of the main trace gases released from volcanoes with yearly global emissions estimated between 1 and 37 Tg. With sulfur dioxide (SO 2 , 15–21 Tg/year), it dominates the volcanic sulfur budget, and the emission ratio H 2 S:SO 2 is an important geochemical probe for studying source conditions, sulfur chemistry and magma‐water interactions. Contrary to SO 2 , measurements of H 2 S are sparse and difficult. Here we report the first measurements of a large H 2 S plume from space. Observations were made with the infrared sounder IASI of the volcanic plume released after the 7–8 August 2008 eruption of Kasatochi volcano. The eruption was characterized by 5 consecutive explosive events. The first events were phreatomagmatic producing a plume rich in water vapor and poor in ash and SO 2 . We show that the observed H 2 S plume, calculated at 29 ± 10 kT with integrated columns exceeding 140 ± 25 Dobson Units (DU), is likely associated with these first explosions. H 2 S:SO 2 ratios with maximum values of 12 ± 2 are found, representative of redox conditions in the hydrothermal envelop. With a detection threshold of 25 DU, future space observations of H 2 S plumes are certain. These will be important for improving the atmospheric sulfur budget and characterizing the H 2 S:SO 2 fingerprint of different eruptions.