TROPOMI CSF Software Algorithm

The algorithm can be decomposed into a sequence of discrete steps, which are outlined below in pseudocode form.

1. Filter suitable orbits
   1. Domain filter - Identify and retain only those orbits that intersect the predefined analysis domain.
   2. Pixel quality filter - From the orbits passing the previous Step, discard any orbit containing fewer than 500 high quality pixels (quality assurance flag qa = 1) within the domain. Pixel quality information is provided in the TROPOMI Level 2 data products.
   3. Correlation filter - For the orbits determined in the previous Step, remove orbits exhibiting strong correlations within the domain, defined as (0.5 < abs(R)) between
      • methane mixing ratio and surface albedo, or
      • methane mixing ratio and aerosol optical thickness.
   4. Wind-speed filter - For orbits determined in the previous step, reject any orbit for which the pressure averaged boundary layer wind speed is less than 2 ($\frac{m}{s}$).
2. For each orbit passing the filtering criteria described in Step 1, the following processing steps are applied.
   1. Background Concentration Determination Sadavarte, et al., 2021
      1. Define the upwind box - Construct the box with dimensions of 0.5° × 0.5° located 0.5° upwind from the source. Calculate average value and pixel count
      2. Compute upwind statistics
         • Calculate the mean ${\mathrm{CH}}_4$ concentration within the upwind box.
         • Count the number of valid pixels contained within the upwind box.
         If the upwind box contains more than 20 pixels, set the background value equal to the upwind box mean concentration.
      3. Else the background value is the domain median concentration.
   2. Downwind Box Determination
      1. Direction adjustment
         1. Initialise the downwind box with:
            • Length = 0.4°
            • Width = 0.2°
            • Orientation aligned with the pressure averaged boundary layer wind direction (rotation = 0°).
         2. Rotate downwind box by 5° increments over the range -40° to +40°
         3. For each rotation: compute the Methane enhancement (${\mathrm{CH}}_4$ concentration above background) within the box.
         4. Select the rotation that yields the highest average enhancement.
      2. Length adjustment
         1. Initialise a while loop using the selected rotation with the default box dimensions.
         2. Iteratively:
            • Increase the box length by 0.1°.
            • Recalculate the total enhancement within the extended box.
         3. Acceptance criterion:
            • If the enhancement increases by more than 5 ($\mathrm{ppb}$), accept the new length and continue the loop.
            • Otherwise, reject the extension and terminate the loop.
      3. Width adjustment
         1. Initialize the while loop with enhancement value for previously determined direction and length.
         2. Iteratively:
            • Increase the box width by 0.05° on each side.
            • Recalculate the total enhancement within the expanded box.
         3. Acceptance criterion:
            • If the enhancement increases by more than 5 ($\mathrm{ppb}$), accept the new length and continue the loop.
            • Otherwise, reject the extension and terminate the loop.
   3. Transect Construction and Filtering
      1. Create 15 equally spaced transects across the final downwind box. Ignore the first 3 in the further analysis.
      2. For each transect (box model) - Determine the fraction of intersected pixels that are valid.
      3. Reject transects for which fewer than 75 % of intersected pixels are valid.
      4. If fewer than three transects remain valid, discard the entire orbit from further analysis.
   4. Emission Rate Calculation
      1. For each valid transect: Compute the excess ${\mathrm{CH}}_4$ mass relative to the background.
      2. Convert excess mass to an emission rate using the prescribed CSF formulation.
      3. Compute the final emission rate for the orbit as the arithmetic mean of emission rates across all valid transects.