Contribution of Oil and Gas Production to atmospheric CH4 in the South-Central United States: Reconciling Bottom-up and Top-down Estimates

Estimates of anthropogenic CH₄ emissions in the United States have been largely inconsistent, particularly for oil and gas production (OGP) in the South-Central United States. We have quantified the contribution of OGP to the South-Central US (TX/OK/KS) CH₄ budget through atmospheric regional transport modeling with the Community Multi-scale Air Quality model (CMAQ) model driven by a new process-based, spatially resolved OGP CH₄ emissions inventory sponsored by US EPA. We applied Bayesian inference to calibrate CMAQ emissions inputs using continuous CH₄ measurements at the DOE Southern Great Plains (SGP) central facility. It is found that OGP emissions are the largest source of CH₄ observed at the DOE SGP site and the largest source of CH₄ in TX/OK/KS, constituting 2.7 (+0.45/-0.32) Tg a^-1 (~45%) of total CH₄ emission in the region, or about one half (47%) of national total OGP emissions. Other sources, such as livestock, are relatively less important. Using continuous measurements of CH₄, we found evidence of rapid nocturnal transport by the Great Plains low-level jet (LLJ) and sporadic oil and gas emissions. Our study demonstrates the importance of using improved knowledge of the spatial and temporal features of oil and gas emissions in top-down inversion studies that seek to constrain the CH₄ budget at regional and national scales.