There's no way running an Espar all night is cleaner then idling a new gasoline engine for 15 minutes (especially if the catalytic converter is warm). My Espar uses almost a gallon of diesel overnight. Idling a gas engine for 15 minutes uses less than an ounce. The huge difference is the cat - because of the cat, the air coming out of a modern car with a gasoline engine, after the engine warms up, is cleaner than the air going into it in a major urban area. Smog from newer cars comes almost entirely at startup.
Here in CA EVERYTHING has a catalytic converter. Water heaters are about $100 more here, and you can't import one from a non smog area, because the ones sold here have a cat. My guess is CA figured having an Espar run all night is cleaner than having the existing truck fleet idle all night. As trucks get cleaner, I'd expect CA to eventually require a catalytic converter on Espars too, until, of course, they eventually ban them outright as they strive for zero fossil fuel use.
PS I can stand behind any new gasoline car and not smell anything. However, my Espar exhaust stinks like diesel fumes. You don't need an exhaust gas analyzer to tell my brand new Espar S2 (installed exactly to the Esapr specifications and with about 40 hours on it) isn't clean compared to a properly operating new car. I do love it though.