Volcanic ash and SO2 in the 2008 Kasatochi eruption: Retrievals comparison from different IR satellite sensors

The Kasatochi 2008 eruption was detected by several infrared satellite sensors including Moderate Resolution Imaging Spectroradiometer (MODIS), Advanced Very High Resolution Radiometer (AVHRR), and Atmospheric Infrared Sounder (AIRS). In this work a comparison between the volcanic cloud SO2 and ash retrievals derived from these instruments has been undertaken. The SO2 retrieval is carried out by using both the 7.3 and 8.7 mu m absorption features while ash retrieval exploits the 10-12 mu m atmospheric window. A radiative transfer scheme is also used to correct the volcanic ash effect on the 8.7 mu m SO2 signature. As test cases, three near-contemporary images for each sensor, collected during the first days of the eruption, have been analyzed. The results show that the volcanic SO2 and ash are simultaneously present and generally collocated. The MODIS and AVHRR total ash mass loadings are in good agreement and estimated to be about 0.5 Tg, while the AIRS retrievals are slightly lower and equal to about 0.3 Tg. The AIRS and MODIS 7.3 mu m SO2 mass loadings are also in good agreement and vary between 0.3 and 1.2 Tg, while the MODIS ash corrected 8.7 mu m SO2 masses vary between 0.4 and 2.7 Tg. The mass increase with time confirms the continuous SO2 injection in the atmosphere after the main explosive episodes. Moreover the difference between the 7.3 and 8.7 mu m retrievals suggests a vertical stratification of the volcanic cloud. The results also confirm the importance of the ash correction; the corrected 8.7 mu m SO2 total masses are less than 30-40% of the uncorrected values.