These are the co-added JCMT (green) and ALMA (purple) spectra before removal of a final polynomial baseline. The ALMA spectrum has been scaled up by a factor of three, the estimated loss for spatial filtering (compare the first two l:c ratio entries in Table 1). Vertical red bars connect the JCMT and ALMA data (bin centres agree in velocity within ±0.2 km s−1). A line feature is considered to be real where this dispersion (red bar) is low; only the candidate PH3 feature around v = 0 km s−1 meets this criterion. Other ‘dips’ across the band have high dispersion (they occur in only one dataset) or cover only a few contiguous bins (much less than the line width expected for Venus’s upper-atmosphere absorption).