Quantifying urban and landfill methane emissions in the United States using TROPOMI satellite data

TL;DR

This study uses TROPOMI satellite data to accurately quantify and attribute methane emissions in 12 US urban areas, revealing significant underestimations in official inventories mainly due to landfill emissions.

Contribution

It introduces a satellite-based inversion method to improve urban methane emission estimates and highlights discrepancies in landfill gas collection efficiencies.

Findings

EPA GHGI underestimates urban methane emissions by 80% on average.

Landfills are the main source of underestimation in urban methane emissions.

Landfill gas collection efficiencies are overestimated in official reports, especially outside Los Angeles.

Abstract

Urban areas are major sources of methane due to population needs for landfills, natural gas distribution, wastewater treatment, and residential combustion. Here we apply an inversion of TROPOMI satellite observations of atmospheric methane to quantify and attribute annual methane emissions at 12x12 km2 resolution for 12 major US urban areas in 2022. The US Environmental Protection Agency Greenhouse Gas Inventory (EPA GHGI) is used as prior estimate. Our results indicate that the GHGI underestimates methane emissions by 80% on average for the 12 urban areas, with 22%-290% underestimations in most urban areas, except Los Angeles and Cincinnati where emissions are overestimated by 32%-37%. This is corroborated by independent surface-based observations in the Northeast Corridor and Los Angeles. Landfills are the principal cause of urban emission underestimates, with downstream gas activities contributing to a lesser extent than previously found. Examination of individual landfills other than in Los Angeles shows that emissions reported by facilities with gas collection and control systems to the Greenhouse Gas Reporting Program (GHGRP) and used in the GHGI are too low by a factor of 4 when using the prevailing recovery-first reporting method. This is because GHGRP-estimated gas collection efficiencies (average 70%, range 40-87%) are much higher than inferred from our work (average 38%, range 5-90%). Los Angeles landfills have much higher collection efficiencies (average 78% in GHGRP; 85% in our work) than elsewhere in the US, suggesting that operational practices there could help inform methane mitigation in other urban areas.