Flux retrieval from TROPOMI satellite data using PlumeTraj |
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The spectral radiances acquired by passive satellite remote sensing instruments such as TROPOMI are strongly dependent on plume height, but lack altitude information. Thus, the L2 data products used for this study provide images containing SO2 vertical column densities (VCD) and assume a box of fixed height (altitude level) within which the SO2 is located. In total, there are three different altitude levels (lower troposphere: surface to 1 km, mid-troposphere: 6.5 to 7.5 km (Figure 1b) and upper troposphere/lower stratosphere: 14.5 to 15.5 km), corresponding to three images. To retrieve accurate volcanic plume SO2 masses from these images the actual volcanic plume height and vertical extension have to be retrieved first. To that end a pixel-by-pixel numerical procedure called PlumeTraj is used, which is fully detailed in Pardini et al. (2017) and Pardini et al. (2018) .
PlumeTraj integrates the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) using custom-built routines written in the Python programming language. The scheme selects those pixels, which are associated with trajectories emerging from the location of the volcanic vent (plume pixels). For each of these pixels PlumeTraj computes the height (above sea level, asl) at which the SO2 is located at satellite measurement time instant (plume height), the height (asl) at which the prevailing atmospheric winds starts to disperse the gas into the atmosphere (injection height) and the time when the SO2 reaches the injection height (injection time). The retrieved plume height is used to correct the VCDs of each pixel of the satellite images. From the three quantities and the corrected VCDs the SO2 load is computed for each plume pixel. To compute a flux time series used in the comparison, time bins of 30 minutes length are defined. Pixels with injection times matching a given bin time are combined. The integrated SO2 load (sum of SO2 mass of all pixels per bin) divided by the bin length yields the flux for a given time bin. The uncertainty of the SO2 flux then results from the error in SO2 mass and injection time. More details can be found here: Queißer et al. (2019) |