The Canadian constitution says Ottawa has no jurisdiction over the regulation of air pollutants.

That's indeed the key point, as I understand it. I'll be interested to see if it's made by the MSM. What it means is that Canada can't do anything unilateral in this field. Consultation with the provinces and consultation with industry have to produce voluntary agreements in order for federal leadership to have any tangible benefits.

I suppose I should read the link!  

BC is good example of this statement.  The province is currently looking at implementing California auto emissions standards for all vehicles sold.
While Harper yesterday announced Canada will abide by US federal standards.
I'm hoping that this will get ugly, but without much faith.
.
California gets away with it because their population (~34 million) is close to the size of Canada's and, these days, because Maine (1.3m), Massachusets (6.4m), New York (19m)and Vermont (0.6m) have jumped on California's bandwagon.  

Some new vehicles sold in the rest of the USA (and Canada) can't be sold in these 5 states, though I have no idea how that works out in practice. Adding BC (4.6m) to the list could have more of an impact on the Canadian auto industry than on air quality. Would that be good policy?

It should go without saying that the federal-state division of powers is very different from what we have in Canada, but TOD Canada may have a few non-Canadian readers. Emissions from new vehicles in Canada are regulated under Canada's Environmental Protection Act and, incidentally, have been harmonized with US standards since 2004. The MSM seems to have missed this point too. Would separate provincial regulations be likely to stand a constitutional challenge?