US ethanol fuel consumption has increased exponentially over the last two decades as part of a movement to reduce greenhouse gas emissions and become more fuel independent.  US annual renewable fuel production is ~16 billion gallons and is required to increase to 36 billion according to the renewable fuel standard.  Regardless of the technology or feedstock used to produce renewable fuel, the primary end product will be ethanol. This has spurred extensive debate among a diverse array of US stakeholders about the practicality of the renewable fuel standard. Much of the evidence for both sides of the debate relies on the life cycle analysis of ethanol production and weighing the effects of ethanol production on our economy, environment, and food and water supply.  However, one principal uncertainty not adequately addressed by either side in this debate is “What are the impacts of increasing ethanol fuel consumption and subsequently ethanol and aldehyde emissions on atmospheric composition and chemistry?”  This is despite modeling efforts that predict increases in blended fuel consumption will increase national average ozone concentrations. This is especially true for urban air sheds consisting of blended fuel vehicle emissions that result in elevated concentrations of VOCs with relatively high ozone reactivities (i.e. aldehydes, alcohols). 

The presented work provides evidence of high ethanol atmospheric concentrations associated with densely populated urban areas and ethanol refineries throughout the globe by measuring wet deposition ethanol concentrations. This has also produced the first global deposition flux of ethanol (2.4± 1.6 Tg/yr) based on empirical data and suggests wet deposition removes 6 to 17% of atmospheric ethanol annually. To provide further evidence of the increasing anthropogenic sources leading to higher atmospheric ethanol concentrations, a method to measure the carbon isotopic composition of ethanol at natural abundance levels was developed and carbon isotope signatures (δ¹³C) of vehicle ethanol emission sources are reported for both Brazil (-12.7‰) and the US (-9.8‰).  An isotope mixing model using vehicle and biogenic endmembers was used to estimate ethanol source apportionment in wet deposition and results suggest anthropogenic sources contribute up to 7X more ethanol than previously predicted in modeled ethanol inventories.  With the US renewable fuel standard requiring a substantial increase in the next five years, it is essential that the entities involved in deciding the fate of US fuel consumption have knowledge of all the advantages and disadvantages of increased ethanol consumption, including the atmospheric impacts of ethanol fuel emissions.