Sources of variation in delta C-13 of fossil fuel emissions in Salt Lake City, USA

The isotopic composition of fossil fuels is an important component of many studies of C sources and sinks based on atmospheric measurements of CO₂. In C budget studies, the isotopic composition of crude petroleum and CH₄ are often used as a proxy for the isotopic composition of CO₂ emissions from combustion. In this study, the C isotope composition (d13C) of exhaust from the major fossil fuel emission sources in Salt Lake City, USA, was characterized with 159 measurements of vehicle exhaust of various types and eight measurements of residential furnace exhaust. These two sources were found to be isotopically distinct, and differed from global-scale estimates based on average values for crude petroleum and CH₄. Vehicle-specific factors such as engine load and operation time had no effect on d13C of vehicle exhaust. A small difference was found between the mean d13C of vehicle exhaust collected randomly from different vehicles and the mean d13C of gasoline collected from multiple fueling stations representing major gasoline distributors in Salt Lake City and the surrounding area. However, a paired comparison of d13C of exhaust and gasoline for six different vehicles did not show any consistent C isotope fractionation during vehicle combustion. The mean d13C of crude petroleum processed for local distribution differed slightly from refined gasoline collected at multiple fueling stations, but time lags between processing and transportation cannot be ruled out as an uncontrollable contributing factor. Measured isotope ratios were then combined with fuel consumption statistics to predict the annual cycle of d13C of fossil fuel emissions for the Salt Lake City metropolitan area. The results showed that the isotopic composition of CO₂ emissions from fossil fuel combustion varied by almost 3& over the course of the 2002 calendar year. This study illustrates that on a regional scale, the isotopic composition of fossil fuel emissions shows a high degree of both spatial and temporal variability that may influence characterization of C sources and sinks with atmospheric measurements.