I have just read this in answer to another question:
"And the notion that Venus is hot because the pressure is high is absolute nonsense. Yes, air is hot when it comes out of a compressor, but the same air quickly cools down if it remains in the tank. If somehow, a huge amount of gas were suddenly dropped on a planet, and for simplicity, let's say the gas is nitrogen, which is not a greenhouse gas, the planet would get very warm. And then it would be in a state of radiative disequilibruim, until it cools back down."
How would it cool back down?
My understanding is that N2 does not radiate because its emissivity is so low. The air in the tank on earth can conduct and convect. The N2 on Venus cannot do that.