The Four Corners Methane Hotspot: Implications for Emission Inventories (Invited Presentation)

Methane is a potent greenhouse gas and ozone precursor whose control could mitigate climate change and air pollution. However, it has many natural and anthropogenic sources of biogenic, pyrogenic and thermogenic origins that are ill quantified. We report observations from space (SCIAMACHY, 2003-2009) and ground (TCCON, 2011-2013) that demonstrate that the largest anomalous methane levels over the continental U. S. are located in the Four Corners region. Simulations performed with the EDGAR v4.2 emissions inventory under-predict the observed methane and its daily variation by a factor of 3.5. The persistence of this high methane since 2003 suggests that the source is likely from established gas, coal, and coal-bed methane mining and processing. Furthermore, collocated observations of ethane and methane from our ground site in the vicinity of 2 large coal plants and coal mines in southern Four Corners are highly correlated with a ratio of 1.4% consistent with a coal related source. Inverse modeling of recent global satellite data (GOSAT, 2009-2011) shows that the EDGAR v4.2 methane inventory is biased low by a factor of 3-9. We show that space-borne observations can identify methane hotspots and how these hotspots can then be interrogated with high fidelity suborbital observations from surface stations and aircraft. Research in the Four Corners region has shown how this tiered approach can be used to evaluate emission inventories and prioritize airborne and ground field campaigns to locate and attribute methane sources.