I doubt it would affect pollution significantly. It’s not like both ends of the tunnel aren’t open to the air. It would definitely locally displace it so it’s not distributed across the above ground length of the road, but the same amount more or less (minus whatever adheres to walls) is still coming out of either end.
Underground tunnels also have the danger of fires rapidly spiraling out of control and in the past have killed dozens of people, and that was before electric cars became common. I would not want to be in a tunnel when a Tesla’s battery explodes.
I’m not saying this has no advantages, but for the trouble and cost it seems like a train would be better.