A comparison of AIRS, MODIS and OMI sulphur dioxide retrievals in volcanic clouds

Abstract

Volcanic degassing is a major contributor to the global sulphur dioxide (SO2) budget, characterized by quiescent emissions in the lower troposphere with sporadic, spatially variable explosive eruptions into the upper troposphere and lower stratosphere (UTLS). The volcanic input of SO2 to the atmosphere can be quantified using a suite of satellite-based instruments with a range of orbits and resolutions, resulting in differing estimates of SO2 extent and concentration from eruptions. We compare near-coincident retrievals of SO2 from the Moderate Resolution Imaging Spectroradiometer (MODIS), Atmospheric Infrared Radiation Sounder (AIRS) and Ozone Monitoring Instrument (OMI) at four eruptive settings. The OMI instrument is the most sensitive, with the ability to detect both low and high altitude clouds, but as an ultraviolet sensor, retrievals are limited to daytime, unlike the infrared sensors. AIRS retrievals are up to an order of magnitude less sensitive than OMI, restricted to water-free clouds in the upper troposphere. MODIS has the lowest sensitivity and is therefore constrained to the largest eruptions. Total tonnages from each sensor reflect these varying sensitivities along with potential calibration discrepancies. Results suggest that by using a number of instruments in synergy a more complete method of eruption detection is achieved.