Example ACS MIR spectra and upper limits for PH₃. Panel a: normalized spectra for diffraction order 143 extracted from a single detector frame observed near 13 km tangent height. The lines in various shades of grey are the individual normalized spectra, the black line is a mean spectrum, and the dashed and solid purple lines are best-fit lines to the mean spectrum with two fixed quantities of PH₃, 5 and 10 ppbv, respectively. Panel b: 1σ detection limits derived from non-normalized spectra (shades of grey) and from the mean spectrum (black). The standard error computed from all 12 sets of spectra is also shown, as in Fig. 4. The detection limit derived near 13 km is 0.5 ppbv for the entire occultation and 1.5–2.5 ppbv for the individual extracted detector rows.