Quantifying methane emissions from United States landfills

Abstract

Methane emissions from solid waste may represent a substantial fraction of the global anthropogenic budget, but few comprehensive studies exist to assess inventory assumptions. We quantified emissions at hundreds of large landfills across 18 states in the United States between 2016 and 2022 using airborne imaging spectrometers. Spanning 20% of open United States landfills, this represents the most systematic measurement-based study of methane point sources of the waste sector. We detected significant point source emissions at a majority (52%) of these sites, many with emissions persisting over multiple revisits (weeks to years). We compared these against independent contemporaneous in situ airborne observations at 15 landfills and established good agreement. Our findings indicate a need for long-term, synoptic-scale monitoring of landfill emissions in the context of climate change mitigation policy.

Fig. 1.. Landfills flown between 2016 and 2022 using the AVIRIS-NG or the GAO.

(A) Spatial extent of US surveys. Blue dots represent all landfills with flyovers, and red dots represent landfills where point sources were detected on at least one overpass. (B) Total number of large (>20,000 MtCO₂e reported to GHGRP) landfills surveyed by state (light blue) and the number of landfills where we detected point sources in at least one overpass (dark blue). (C) Average and SD of detection frequency, also known as persistence (number of detections/number of overpasses), across all surveyed landfills as a function of the number of overpasses. Dashed lines represent the average persistence for facilities flown at least three times on three different days.

Fig. 2.. Example of multiple persistent point sources at a landfill.

(A) At least two plumes can be seen detected from a single overflight in May 2021. (B) The plume emanating from the blue circled region potentially corresponds to a vent or unlit flare. (C to E) Emission from this vent persisted across all other overflights between May 2021 and June 2022. Visible basemaps are provided by Google Earth.

Fig. 3.. Comparison of emission rates at landfills derived using the SA mass-balance approach and the GAO imaging spectrometer.

(A) Fifteen landfill overpasses where comparison between emissions was possible. The plus symbol indicates simultaneous data acquisition. The asterisk indicates asynchronous but same-day observations within 2 hours of one another. Error bars represent 1 SD uncertainties on emission rates. (B and C) CH₄ observations at LF06 and LF12, respectively, from (A). The white arrow indicates wind direction.

Fig. 4.. Comparison of aerial emission rates with EPA GHGRP for landfills where point sources were detected at least once.

(A) Mean CH₄ emission rates across all aerial overpasses compared with average GHGRP emissions. The gray line represents the one-to-one line. The size of the dots corresponds to number of overflights. Two red-colored dots in (A) correspond to landfills with 5+ years of observations in which the observed emission trends are significant (P < 0.05). (B) Trends for these landfills. Black squares represent GHGRP reporting for that year. Error bars represent 1 SD uncertainties on emission rates.