Increased Propane Emissions from the United States over the Last Decade

Propane (C₃H₈) is the second most abundant non-methane hydrocarbon in the atmosphere. It contributes to photochemical air pollution, including ozone and aerosol formation in the troposphere. It is also commonly used as a tracer for distinguishing fossil fuel emissions of methane from natural and other anthropogenic sources. Global atmospheric observations of C₃H₈ suggest a recent reversal of its atmospheric trend that is largely due to U.S. oil and natural gas production, after gradual decreases of atmospheric C₃H₈ had been observed a decade ago [Helmig et al., 2016]. We analyzed atmospheric C₃H₈ measurements from the continental U.S. as well as those from the remote atmosphere. Measured C₃H₈ mole fractions over the continental U.S. are up to three orders of magnitude larger than those measured in the remote northern hemisphere, and they show a clear increasing trend, especially near oil and gas production regions. We then performed inverse modeling analyses of C₃H₈ with and without consideration of the photochemical losses of C₃H₈ to reaction with OH. The estimated diurnal-average atmospheric lifetime of C₃H₈ in July is 5.5 days [Miller et al., 2012], which is comparable to the transit time of air parcels across the model domain. When chemical losses are considered, C₃H₈ mole fraction enhancements over the continental U.S. are substantially larger and display a larger increasing trend over time than when chemical losses are not considered. Inverse-modeled emissions of C₃H₈ averaged over 2008 to 2015 are a few factors larger than reported by inventories (i.e., MACCity; POET; RETRO). They further confirm an increase of U.S. C₃H₈ emissions over the 2008 to 2015 period and these emissions are primarily from oil and natural gas production regions. The emission increase derived here from the atmospheric observations is comparable to the relative increase in reported U.S. C₃H₈ production over the same time period.