Probabilistic retrieval of volcanic SO 2 layer height and cumulative mass loading using the Cross-track Infrared Sounder (CrIS)

Abstract. During most volcanic eruptions and many periods of volcanic unrest, detectable quantities of sulfur dioxide (SO2) are injected into the atmosphere at a wide range of altitudes, from ground level to the lower stratosphere. Because the fine ash fraction of a volcanic plume is, at times, collocated with SO2 emissions, global tracking of volcanic SO2 is useful in tracking the hazard long after ash detection becomes dominated by noise. Typically, retrievals of SO2 loading have relied heavily on hyperspectral ultraviolet measurements. More recently, infrared sounders have provided additional loading measurements and estimates of the SO2 layer altitude, adding significant value to real-time monitoring of volcanic emissions as well as climatological analyses. These methods leverage the relative simplicity of infrared radiative transfer calculations, providing fast and accurate physics-based retrievals of loading and altitude. In this study, we detail a probabilistic enhancement of an infrared SO2 retrieval method, based on a modified trace-gas retrieval, to estimate SO2 loading and altitude probabilistically using the Cross-track Infrared Sounder (CrIS) on the Joint Polar Satellite System (JPSS) series of satellites. The methodology requires the characterization of real SO2-free spectra aggregated seasonally and spatially. The probabilistic approach replaces loading and altitude estimates with non-parametric probability density functions, fully quantifying the retrieval uncertainty. This framework adds significant value over basic loading and altitude retrieval because it can be readily incorporated into Monte Carlo forecasting of volcanic emission transport. We highlight results including successes and challenges from analysis of several recent significant eruptions including the 22 June 2019 eruption of Raikoke volcano, Kuril Islands; the mid-December 2016 eruption of Bogoslof volcano; and the 26 June 2018 eruption of Sierra Negra volcano, Galapagos Islands. This retrieval method is currently being implemented in the VOLcanic Cloud Analysis Toolkit (VOLCAT), where it will be used to generate additional cloud object properties for real-time detection, characterization, and tracking of volcanic clouds in support of aviation safety.