Airborne Methane Flux Quantification and Source Identification Using High Resolution Measurements of Ethane and Methane

Abstract

The work presented herein demonstrates the use of high resolution trace gas measurements of methane CH4and ethane C2H6 to quantify CH4 fluxes and attribute them to specific sources. In The North Slope of Alaska we were able to make measurements of the total CH4 flux from The Prudhoe Bay Oil Field (PBOF) by separating its emissions from the trace gas signature of surrounding methanogenic metabolism within the Arctic permafrost. We found that the total emissions of CH4 were 1500 [1151, 1888] kg CH4 hr-1 and that the available bottom-up estimates were low by a factor of 2-5. Our estimates of CH4 flux from PBOF are the first such estimates made using a top-down measurement approach. After then developing an C2H6 spectrometer capable of making, what we believe to be, one of the world’s most precise airborne C2H6 measurements from a compact laser absorption spectrometer, we deployed our instrument package to a host of urban areas and characterized their CH4 emission profiles. We were able to partition sources of natural gas from the total CH4 enhancement observed downwind of a city using the ratio of CH4 to C2H6 in urban natural gas. We developed methodologies to subtract the signal of upwind CH4 sources from the downwind enhancements and quantify the contribution from only the domains of interest. Our work in the urban sector shows that the total CH4 flux from an individual city can be dominated by sources of both biogenic (landfills, waste water, ect..) and thermogenic (natural gas) origin and that the emission profile has a strong seasonal dependence with thermogenic sources emitting a larger fraction of the total CH4 flux in the winter months. A comparison of our results to currently available inventory products also show that sources within the urban domain are poorly understood and require significant further study.