Solve this at the source
Well, "green" power generation aside: a coal power plant can be fitted with ginormous filters and mechanisms to extract some of the more nasty stuff - and in the civilised world they are required to have these in place today anyway (maybe not to the extent you might want, but still). Those would be quite impractical to fit to a car... He was not talking about CO_2 emissions, although carbon capture and storage can be done easier if your source is stationary and does not require more energy to move your machinery around (I'm not saying anything about the feasibility of the tech... that's a different discussion).