Improving retrieval of volcanic sulfur dioxide from backscattered UV satellite observations

Existing algorithms that use satellite measurements of solar backscattered ultraviolet (BUV) radiances to retrieve sulfur dioxide (SO 2 ) vertical columns underestimate the large SO 2 amounts encountered in fresh volcanic eruption clouds. To eliminate this underestimation we have developed a new technique, named the Iterative Spectral Fitting (ISF) algorithm, for accurate retrieval of SO 2 vertical columns in the full range of volcanic emissions. The ISF algorithm is applied to Ozone Monitoring Instrument (OMI) BUV measurements of the Sierra Negra eruption (Galàpagos Islands, Ecuador) in October 2005. The results represent major improvements over the operational OMI SO 2 products. Based on the ISF data, we report the largest SO 2 vertical column amount (>1000 Dobson Units (DU), where 1 DU = 2.69 × 10 16 molecules/cm 2 ) ever observed by a space borne instrument, implying that very high concentrations of SO 2 can occur in the lower troposphere during effusive eruptions.