Every time I have my car smogged, I laugh at the ridiculous futility of that gesture.
While I am waiting through the process, semi-trucks and buses rumble past spewing fumes and black, stinky smoke so thick it is nauseating. I'm sure any one of those vehicles causes more pollution in a couple days than my well-maintained vehicle creates in an entire year.
If our government is serous about diminishing air pollutants, enforce emission standards for trucks, buses and any other vehicles using diesel fuel. Aside from the visible black smoke they emit, the stink they put into the air is intolerable.
I'm sure there is a lobby group somewhere claiming it would be too expensive to equip such vehicles with smog controls. Possibly true, but let's decide the priorities. Are we preserving profits for trucking companies, or preserving breathable air for the citizens of our world? Once that point is decided, the course of action will be obvious.