Anyone who's visited Birmingham will be familiar with the process of pollutants sticking to tunnel walls. They'll also likely be familiar with the air you end up breathing when made to share an underground space with a number of combustion engines.
I reckon it's largely a red herring as far as air quality goes, but there's logic to burying the cars in order to reclaim the space divided by an impenetrable trunk road.
(I thought this topic was going to be about Elon Musk's Boring Company. He seems to be under the impression that building roads reduces congestion, or at least playing to those who do. Nevertheless, he has form for getting results by throwing the Californian IT startup approach at tough engineering problems, and if they manage to make tunnelling an order of magnitude cheaper, it could have other interesting consequences.)