A Stable Column ΔXCH₄/ΔXCO₂ Ratio over Los Angeles (2011–2025): An Inventory-Independent Test of Urban Methane Policy

JIANI YANG, Xianlei San, Ellin Qiao, Zhao-Cheng Zeng, Thomas Pongetti, Yangcheng Luo, Carl Percival, Christian Frankenberg

We extend the column-averaged CH₄ and CO₂ (XCH₄, XCO₂) record from the California Laboratory for Atmospheric Remote Sensing (CLARS) through July 2025, spanning 15 years over the Los Angeles megacity. CLARS retrievals show persistent enhancements of both gases above the free-tropospheric background, with a strong excess correlation (R = 0.93) and an emission ratio ΔXCH₄/ΔXCO₂ ≈ 5.1 ppb ppm⁻¹ that has no significant trend over 2011–2025 (−0.005 ± 0.006 yr⁻¹, p = 0.84). Because the ratio is self-normalizing and requires no emission inventory, it provides an observation-only metric that is robust to transport and boundary-layer variability. The ratio peaks in winter and is lowest in summer, and the relative pattern of methane hotspots in the retrieved field is essentially unchanged, co-located with landfills and energy infrastructure. Methane co-varies most strongly with residential natural-gas consumption (R² up to 0.43) and negligibly with other sectors. The stable ratio does not imply that absolute methane emissions were constant: the basin CH₄ and CO₂ signals declined by comparable relative amounts and largely cancel in the ratio. What 15 years of observations do show is that methane has fallen no faster than CO₂-correlated activity, so methane-targeted regulation has left no detectable signature on the basin’s CH₄-to-CO₂ balance, and mitigation aimed specifically at fugitive gas and landfill sources remains necessary.