Challenges to the retrieval of PH3 from ALMA & JCMT observations

Phosphine (PH3) has been recently reported to be present in the atmosphere of Venus (Greaves et at. 2020), as determined from radio observations with the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA). We find that the spectral feature identified with JCMT and ascribed to PH3 could instead be explained employing plausible mesospheric SO2 abundances (~100 ppbv), while the identification of PH3 in the ALMA spectra as presented was affected by severe spectral passband calibration issues. We have independently calibrated and analyzed the raw ALMA data using different interferometric analysis tools, and in all cases, we do not detect the PH3 spectral line. Furthermore, our radiative transfer models show that for a PH3 signature to be observable in either the ALMA or JCMT spectra, phosphine needs to be present in the atmosphere of Venus at altitudes above 70 km, in contrast with the suggested altitudes of 55-65 km from the reported photochemical network. From our analysis of these data, we ultimately determine sensitive upper-limits for the mesospheric abundance of PH3 in Venus.