Volcanic Ash and SO2 retrievals using synthetic MODIS TIR data: comparison between inversion procedures and sensitivity analysis

In this work the volcanic ash and SO2 retrievals obtained by applying three different procedures (LUT - Look Up Table, NN - Neural Network and VPR - Volcanic Plume Removal) on MODIS Thermal InfraRed (TIR)
synthetic measurements have been compared. The synthetic measurements are generated using MODTRAN Radiative Transfer Model (RTM) for defined volcanic cloud configurations. The results, presented as the percentage
difference between the retrieved ash and SO2 total masses and the true values used for the synthetic data generation, indicate maximum differences of +/- 15% and +/- 10% for all the procedures and for ash and
SO2 retrievals respectively. A sensitivity analysis has been also realized to investigate the influence of volcanic
cloud altitude and water vapour profile on SO2 retrievals at 7.3 and 8.6 mm. Results confirm the high sensitivity of the 7.3 mm retrieval to the volcanic cloud altitude and show that the SO2 total masses estimated at
7.3 and 8.6 mm separately can be used to improve the information on the plume height. Finally, the water vapour profile is used to compute the minimum altitude over which the 7.3 mm retrieval is effective.