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In-plume spectra

From November 2019, the NovacPPP includes a module for selecting in-plume and out of plume spectra. During normal flux processing, a number of spectra in the plume and added together into an in-plume spectrum. Likewise are a number of spectra are selected out of the plume region and added together into a reference spectrum. The in-plume spectrum, the reference spectrum as well as the dark spectrum from the scan are combined into one .pak file and saved in the generated PlumeSpectra directory created inside the output directory. These files are generated by the class PlumeSpectrumSelector based on the criteria noted below for which scans to use and which spectra. The selection process is based on the process used by F. Dinger 2019.

Selection criteria

Selecting the scan The selection process starts by inspecting the properties of the scan itself and of the sky spectrum for selecting if the scan is to be used at all. The following criteria are checked

• If the scan does not see the plume at all, or has a calculated plume-completeness below 0.7, then the whole scan is rejected.
• If the plume is so close to the horizon (or so wide) that the column does not fall to 50% of the peak value on neither side then the whole scan is rejected.
• If the sky spectrum has an intensity above 88% of maximum intensity at any pixel, the the whole scan is rejected.
• If the sky spectrum has maximum intensity, after dark correction, of less than 12% of the maximum intensity, then the whole scan is rejected.

Selecting spectra in the plume In plume spectra are selected as a range of spectra from the scan which fulfill the criteria below:

• First of all are scan angles outside of the range [-75, +75] degrees rejected since these are often too low spectrum quality.
• Secondly are the plume properties inspected and all spectra outside the range [plumeHalfLow, plumeHalfHigh] rejected. Here plumeHalfLow and plumeHalfHigh are calculated during the plume properties calculations done earlier. The plumeHalfLow corresponds to the highest scan angle which is less than the plume center angle and where the SO2 column has dropped by 50% from the peak value. The plumeHalfHigh likewise corresponds to the lowest scan angle which is higher than the plume center angle and where the SO2 column has dropped by 50% from the peak value.
• The total number of spectra to be selected inside the plume is limited to 10. If the number of selected values are higher than this value then the spectrum to the left or to the right which has the lowest SO2 column value is rejected until the total number of spectra selected equals 10.

Selecting reference spectra The reference spectra (out of plume spectra) are selected as the 10 spectra outside of the plume which fulfills the following criterion:

• The spectrum intensity must nowhere exceed 88% of the full intensity range and must not fall below 12% of the full intensity range after dark correction.