A stringent upper limit of the PH3 abundance at the cloud top of Venus

Aims: Following the announcement of the detection of phosphine (PH₃) in the cloud deck of Venus at millimeter wavelengths, we searched for other possible signatures of this molecule in the infrared range.
Methods: Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO₂ and H₂O at the cloud top. We identified a spectral interval recorded in March 2015 around 950 cm^-1 where a PH₃ transition is present.
Results: From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-σ upper limit of 5 ppbv for the PH₃ mixing ratio, which is assumed to be constant throughout the atmosphere. This limit is four times lower than the disk-integrated mixing ratio derived at millimeter wavelengths.
Conclusions: Our result places a strong constraint on the maximum PH₃ abundance at the cloud top and in the lower mesosphere of Venus.