Background Calculation

The background methane concentration is calculated following the procedure described by ( Sadavarte, et al., 2021 p. 16575):

The ${\mathrm{CH}}_4$ enhancements for each pixel along the transects is defined relative to the background XCH4 which is calculated as the average of 0.5° x 0.5° area centred at a distance of 0.1° upwind from the source. If the number of background observations is less than 20, we use the median XCH4 of all pixels in the domain (20° - 24°S, 146° - 150°E) as background XCH4.

For illustrative purposes, using TROPOMI orbit 09956 processed with version 010302, the calculated upwind box average background concentration is approximately 1812 ($\mathrm{ppb}$), and it satisfies the validity criterion requiring at least 20 contributing pixels.

The alternative background estimation using the median over the full domain yields ${\mathrm{CH}}_4$ values ranging from 1660.22 ($\mathrm{ppb}$) to 1907.36 ($\mathrm{ppb}$), with a domain median concentration of 1806.8 ($\mathrm{ppb}$).