Modern cars actually clean up the air they burn in terms of pollutants.
The only significant emission is CO2. And water, of course.

So why do they smell?

They don't particularly, although older diesels without particulate traps
don't count.

I remember the days when I was a child (not so long ago), where standing
near urban congestion, you could see a brown fug descend over the cars and
it got hard to breathe. Now in similar circumstances, the air is completely
clear and there is no smell at all, if you avoid any old buses.

Christian.