Stringent upper limits of minor species at the cloud top of Venus: PH3, HCN, and NH3

Aims. Following several reports announcing the detection or non-detection of minor species above the clouds of Venus, we have searched for other possible signatures of PH3, HCN, and NH3 in the infrared range. Methods. Since 2012, we have performed ground-based observations of Venus in the thermal infrared at various wavelengths to monitor the behavior of SO2 and H2O at the cloud top. We have identified spectral intervals where transitions of PH3 (around 955 cm(-1)), HCN (around 747 cm(-1)), and NH3 (around 951cm(-1)) are present. Results. From the absence of any feature at these frequencies, we derive, on the disk-integrated spectrum, a 3-sigma upper limit of 3 ppbv for the PH3 mixing ratio, 0.5 ppbv for HCN, and 0.3 ppbv for NH3, assuming that these species have a constant mixing ratio throughout the atmosphere. Maps of the Venus disk recorded at the center position of the lines show that there is no evidence for local detection anywhere over the Venus disk. Conclusions. Our results bring new constraints on the maximum abundance of these species at the cloud top and in the lower mesosphere of Venus.