Any tunnel needs lots of ventilation. That air has to go somewhere, and that's above ground. Unless they employ (expensive) measures to filter and clean the air, most of the pollution has to be released. So it's concentrating the pollution to the few spots where the ventilation systems stand instead of having low but mostly equal levels along the entire road. So the health of anyone who lives far away from those systems improves, and anyone who lives close to them faces more pollution with corresponding health consequences.
Unfortunately they decided not to filter the air at the egress points. This has been the subject of much debate and several law suits by people living near the tunnel ends.
Yes, smog is usually heavier than air (oxygen+nitrogen mix). If you do not blow it up there on purpose you can filter it and get it outside of the city.
