Laboratory measurements of the microwave opacity of sulfur dioxide and other cloud-related gases under simulated conditions for the middle atmosphere of Venus

Recent papers attributing the observed microwave opacity of the middle atmosphere of Venus to gaseous sulfur dioxide (SO₂) and other cloud-related gases have motivated laboratory measurements of their microwave absorbing properties under simulated conditions for this region. In the pressure range from 1 to 5 atmospheres and in the temperature range from 297 to 355°K, the absorption of SO₂ in a carbon dioxide (CO₂) atmosphere, at 2.257 and 8.342 GHz, has been found to be approximately 50% larger than that calculated from Van Vleck-Weisskopf theory. The measured absorption is about $\text{25 × 10}{}^6\text{q?}{}^2\text{p}{}^{1.20}\text{T}{}^{\mathrm{?3.1}}\text{(}\text{dB km}{}^{\mathrm{?1}}\text{)}$, where q is the sulfur dioxide number mixing ratio, ? is frequency in gigahertz, p is pressure in atmospheres, and T is temperature in degrees Kelvin. This represents the best-fit expression to the observed pressure dependence, while theoretical values of frequency and temperature dependence are accepted as being consistent with the measurements. Another cloud-related gas, sulfur trioxide (SO₃), was also tested in a CO₂ atmosphere and found to be relatively transparent. These results reduce the amount of SO₂ in the Venus middle atmosphere required to explain the opacity measured by radio occulatation, but this amount still exceeds the abundance measured in situ by atmospheric probes, suggesting that there must be another important source of opacity. Preliminary measurements of the 13-cm absorptivity of gaseous sulfuric acid (H₂SO₄) show it to be a strong microwave absorber, and thus likely to be responsible for a significant and possibly major part of the observed opacity.