The sulphur thing has been addressed, although ULSD (D15) is not universally available/required yet. CA is ahead of the curve, but it is not so elsewhere. Now the big nuts are particulates and NOx (especially NOx). There are means to deal with both, but the physical size of the equipment is a show-stopper for a car as small as a smart. Probably as much mass as the engine itself and all to be concentrated in the rear, unlike larger cars where it can be spread out from front to rear.
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