Compound-Specific Carbon Isotopic Composition of Ethanol in Brazil and US Vehicle Emissions and Wet Deposition

Global atmospheric ethanol budget models include large uncertainties in the magnitude of ethanol emission sources and sinks. To apply stable isotope techniques to constrain ethanol emission sources, a headspace solid phase microextraction gas chromatograph-combustion-isotope ratio mass spectrometry method (HS-SPME-GC-C-IRMS) was developed to measure the carbon isotopic composition of aqueous phase ethanol at natural abundance levels (1–30 μM) with a precision of 0.4‰. The method was applied to determine the carbon isotope signatures (δ¹³C) of vehicle ethanol emission sources in Brazil (−12.8 ± 2.4‰) and the US (−9.8 ± 2.5‰), and to measure the carbon isotope composition of ethanol in wet deposition (−22.6 to −12.7‰). A two end-member isotope mixing model was developed using anthropogenic and biogenic end members and fractionation scenarios to estimate ethanol source contributions to wet deposition collected in Brazil and US. Mixing model results indicate anthropogenic sources contribute two and a half to four times more ethanol to the atmosphere than previously predicted in modeled global ethanol inventories. As established and developing countries continue to rapidly increase ethanol fuel consumption and subsequent emissions, understanding the magnitude of all ethanol sources and sinks will be essential for modeling future atmospheric chemistry and air quality impacts.