The flux divergence method works best with long temporal averages; for TROPOMI analyses, annual or multi-year averages are used. We adapted the method for the current project to handle GCAS data from 27 individual scenes spanning 10 days. We found that the method worked best when the GCAS data was oversampled to the 444 × 444 m 2 CAMx grid. Only pixels with an aircraft roll angle below 0.5° were used.