We use an ensemble of aircraft measurements with the GEOS-Chem chemical transport model to constrain present-day North American ethanol sources, and gauge potential long-range impacts of increased ethanol fuel use. We find that current ethanol emissions are underestimated by 50% in Western North America, and overestimated by a factor of two in the east. Our best estimate for year- 2005 North American ethanol emissions is 670 GgC/y, with 440 GgC/y from the continental US. We apply these optimized source estimates to investigate two scenarios for increased ethanol fuel use in the US: one that assumes a complete transition from gasoline to E85 fuel, and one tied to the biofuel requirements of the US Energy Indepence and Security Act (EISA). For both scenarios, increased  ethanol emissions lead to higher atmospheric acetaldehyde concentrations (by up to 14% during winter for All-E85 scenario) and an associated shift in reactive nitrogen partitioning reflected by an increase in the PAN:NOy ration.