Methane Source Attribution in California during 2012 using NASA SARP Airborne Measurements and Back Trajectory Modeling

Abstract:

Methane is a greenhouse gas emitted from both natural and anthropogenic sources. In California, much of the methane emissions are related to the agriculture and the oil/natural gas industries. In this study, Whole Air Samples (WAS) were taken from the NASA DC8 flying laboratory aircraft from 2009 to 2019 throughout California as part of the NASA Student Airborne Research Program (SARP). These samples were analyzed for over 100 trace gas species in the UC Irvine Rowland-Blake lab using gas chromatography.

Total regional enhancement, or local addition to methane, was calculated on each flight using troposphere concentrations of methane as the background. Hydrocarbons, such as propane and ethane, were used to indicate the source of the methane to be thermogenic as opposed to biogenic. We compared regional methane observations to the modeled atmospheric enhancement from an EPA inventory combined with a transport model. The transport model used was the Stochastic Time-Inverted Lagrangian Transport (STILT) model generated using meteorological data from the North American Mesoscale Forecast System sigmoidal (NAMs) meteorological drivers. When combined with the EPA methane inventory, the model predicts enhancements to the background levels from local sources.

The results from this study show that the EPA inventories underestimate the methane by as much as 60% in many of the samples, but accurately predicts agricultural sources. There are thermogenic methane sources underrepresented in the EPA inventory. The results from this study indicate methane emissions are being underestimated which is critical for state and city governments to know if they want to meet their own emission reduction goals.