Hm, what I've read elsewhere indicates there's in inverse relationship between tuning the spark for HC/CO emissions vs. for performance, so what's good for HC/CO emissions is bad for performance and vice-versa, which corresponds to heat being wasted combustion energy that isn't going towards useful work (explosive pressure) -- i.e., a large gap and less advanced ign. timing produces a longer, softer, later burn that produces more total heat to more fully consume the HC in the intake charge, but less explosive pressure and later in the cycle when it's less useful, whereas a small gap and more advanced timing produces a more abrupt, explosive, earlier burn that produces more useful pressure earlier but less heat and thus consumes less of the HC.

NOx is different, as that's a byproduct of heat fusing the oxygen and nitrogen in the air, so more heat produces more NOx. Fortunately, they don't test for NOx around here.

Why retarding spark advance reduces HC emissions

"With the maximum spark plug gap, the engine produces minimum hydrocarbon emissions..."

"Decreasing the gap can make it vulnerable to incomplete burn or misfire."