An Unexpected Seasonal Cycle in U.S. Oil and Gas Methane Emissions.

Accurate quantification of methane (CH4) emissions is essential for understanding changes in its atmospheric abundance. Atmospheric observations can supply independent emission information that complements and strengthens inventory-based estimates. In this study, we quantified annual and monthly U.S. CH4 emissions in 2008-2021 using inverse modeling of ground and airborne measurements at sites across the U.S. with 10-12 km atmospheric transport simulations. While the magnitude, spatial distribution, and trend of the estimated CH4 emissions align with some previous studies, our results reveal an unexpected seasonal cycle in CH4 emissions from the oil and gas sector, where wintertime emissions are about 40 (20-50, 2σ) % higher than summertime. This seasonality is supported by methane and propane measurements at these same sites, as well as methane isotope measurements made from an independent aircraft campaign over the U.S. Although the exact cause of this emission seasonality is unclear, its spatial distribution indicates that the enhanced CH4 emissions are primarily from natural gas production regions, and to a lesser extent, from natural gas consumption in winter.