Upper limit of H₂S from position 5 spectra. Panel a: fit to the spectrum with the lowest retrieved H₂S upper limit in the position 5 dataset, with a 3σ value equivalent to 45 ppbv. In grey are the observed spectra at each row of the detector array normalised to the retrieved unity baseline (black, dashed): the darker the grey colour, the greater the weighting used for the calculation of μ^* and ς^*. The red and blue lines respectively show the contributions of each of the interfering gas species (in this case, CO₂ and H₂O) to the fit to the row of the detector array with the greatest S/N, while the brown line shows the synthetic spectrum following the addition of 3σ of H₂S. Panel b: residuals of each of the gas contributions compared with the observed spectrum with the greatest S/N, where the shaded grey region represents the estimated noise level.