Volcanic ash and SO2 in the Kasatochi and Okmok eruptions: Interference and impact on quantitative retrievals

Abstract

Ash and SO2 were detected in the eruptions of Kasatochi and Okmok by infrared and UV sensors, including MODIS, AIRS and OMI. The simultaneous presence of ash and SO2 in these clouds presents a problem for retrieving SO2 column abundance because ash absorbs strongly near the 8.7 mm SO2 absorption feature. This leads to a overestimation of the MODIS SO2 abundance, unless the effects of ash are taken into account. Here, we use a new radiative transfer scheme to correct for the effects of ash on SO2 retrieval at 8.7 mm and also account for the effects of ash on SO2 retrieval at 7.3 mm, although this is much less severe, as we will demonstrate. The retrievals provide an improved quantitative description of the ash and SO2 in volcanic eruptive clouds and will assist in developing warnings for aviation. A comparison between MODIS retrievals and contemporaneous OMI SO2 retrievals and AIRS ash and SO2 retrievals has been also presented.