Testing the sensitivity to the pressure knee of the PH₃ vertical distribution. The pressure levels (in mbar) are given by the key in the top left corner, but the differences between these curves is hard to distinguish in the top four panels, showing the difficulty we face in trying to constrain the pressure knee from these data. The χ² / N distributions in the bottom panels show convergence on the 500–650 mbar range. The χ² estimates do not include the spurious emission peaks from the models (see main text and Fig. 7). The best fitting pressure knee, deep mole fraction and fractional scale height are provided in the lower panels.