You're missing the point on NOx, as bio-diesel is a renewable resource any NOx is used up by the next crop of canola, sunflower, etc used to make bio. Unless of course one is huffing it straight out of the exhaust! When viewed in the full cycle, virtuous circle Bio-diesels are actually zero emissions unlike even the cleanest burning diesels which are cumulative emitters.
EDIT: To clarify, I might be incorrect that canola, etc absorb NOx, the only studies I can find are on trees absorbing such, so bio-diesel can be proven to be carbon neutral, but not NOx neutral thus far.
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