Nitrous oxides (NOx) are greenhouse gases and contribute to poor air quality and smog. While all bio-fuels produce less NOx than fossil fuels, some are quite a bit better than others.
Gasoline produces 40% more NOx than corn and soy biofuels, 85% more than reed canary grass and 115% more than switch grass. Another, but somewhat underdeveloped source of biofuels, fast growing poplar trees, also hit the 115% mark.
So the lesson learned here is that we're just at the beginning of this journey. As cellulosic sources of ethanol become more economically viable, biofuels will become even more environmentally friendly.