A Stable Column ΔXCH₄/ΔXCO₂ Ratio over Los Angeles (2011–2025): An Inventory-Independent Test of Urban Methane Policy
Abstract
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.
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BibTeX
@misc{yang2026stable,
title = {{A Stable Column ΔXCH₄/ΔXCO₂ Ratio over Los Angeles (2011–2025): An Inventory-Independent Test of Urban Methane Policy}},
author = {Jiani Yang and Xianlei San and Ellin Qiao and Zhao-Cheng Zeng and Thomas Pongetti and Yangcheng Luo and Carl Percival and Christian Frankenberg},
howpublished = {EarthArXiv},
year = {2026},
month = oct,
doi = {10.31223/x57v4q},
url = {https://eartharxiv.org/repository/view/15362/},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - A Stable Column ΔXCH₄/ΔXCO₂ Ratio over Los Angeles (2011–2025): An Inventory-Independent Test of Urban Methane Policy
AU - Jiani Yang
AU - Xianlei San
AU - Ellin Qiao
AU - Zhao-Cheng Zeng
AU - Thomas Pongetti
AU - Yangcheng Luo
AU - Carl Percival
AU - Christian Frankenberg
PY - 2026
DA - 2026/10/05
PB - EarthArXiv
DO - 10.31223/x57v4q
UR - https://eartharxiv.org/repository/view/15362/
AB - 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.
ER -