Adiabatic Lapse Rate and Static Stability in the Venus Atmosphere calculated from Real Gas Mixture Models

It is known that the ideal gas equation of state is not valid in the lower atmosphere of Venus where surface pressures reach 9 MPa and surface temperatures approach 750K. Moreover, the presence of a small amount of nitrogen slightly complicates the calculation of thermodynamic properties of the real gas mixture present in the atmosphere. Previous calculations of the adiabatic lapse rate in the Venus atmosphere have used approximations to estimate the adiabatic lapse rate. Here, we calculate the adiabatic lapse rate more accurately by using multi-parameter mixture models formulated in reduced Helmholtz free energy to account for the real gas mixture effects. Our results show small differences from the Seiff et al. (1980) values for the adiabatic lapse rate which may be significant where the Venus atmosphere is close to being neutral. For accurate knowledge of the static stability for atmosphere circulation, a local value of the adiabatic lapse rate is necessary.