Example fit to a position 9 spectrum for which the lowest upper limits were retrieved from the entire position 9 dataset. In blue is shown the fit to the measured spectrum taking only CO₂ absorption and instrumental parameters into account, with the synthetic spectrum showing the additional contribution of 60 ppbv of SO₂, corresponding to the perceived 3σ level, superimposed in magenta. Estimated spectral uncertainty is shaded in grey. All spectra are normalised to the retrieved transmission baseline at unity for clarity. Panels a and b: spectral fit when the transmission baseline is retrieved only for the initial fitting of the CO₂ lines with zero SO₂ abundance. Panels c and d: spectral fit when the transmission baseline is re-retrieved following the addition of 60 ppbv of SO₂ into the forward model.