How Does Background Air Pressure Influence the Inner Edge of the Habitable Zone for Tidally Locked Planets in a 3D View?

We examine the effect of varying background N₂ surface pressure (labeled as pN₂) on the inner edge of the habitable zone for 1:1 tidally locked planets around M dwarfs, using the three-dimensional (3D) atmospheric general circulation model (AGCM) ExoCAM. In our experiments, the rotation period is fixed when varying the stellar flux, in order to more clearly isolate the role of pN₂. We find that the stellar flux threshold for the runaway greenhouse is a non-monotonous function of pN₂. This is due to the competing effects of five processes: pressure broadening, heat capacity, lapse rate, relative humidity, and clouds. These competing processes increase the complexity in predicting the location of the inner edge of the habitable zone. For a slow-rotation orbit of 60 Earth days, the critical stellar flux for the runaway greenhouse onset is 1700-1750, 1900-1950, and 1750-1800 W m^-2 under 0.25, 1.0, and 4.0 bar of pN₂, respectively, suggesting that the magnitude of the effect of pN₂ is within ≈13%. For a rapid rotation orbit, the effect of varying pN₂ on the inner edge is smaller, within a range of ≈7%. Moreover, we show that Rayleigh scattering effect as varying pN₂ is unimportant for the inner edge due to the masking effect of cloud scattering and to the strong shortwave absorption by water vapor under hot climates. Future work using AGCMs having different cloud and convection schemes and cloud-resolving models having explicit cloud and convection are Required to revise this problem.