My limited understanding of emissions is that running lean (in an engine not specifically designed with a lean-burn mode) increases smog-forming NOx in the exhaust; while running rich increases unburnt hydrocarbon and CO output.
Exactly! too much O2 will combine with the Nitrogen instead of the proper amount of Carbon and Hydrogen (hydrocarbons = fuel) - smell the exhost . . . rich smells sweet, lean burns and stings your nose.