Assessing climate and air quality benefits of methane mitigation for U.S. landfills

Landfills are an important component of the environment, serving as major sources of methane (CH₄) and reactive trace gases that contribute to radiative forcing and modify atmospheric chemistry, making them a critical target for mitigation. This study employs modeling to investigate the immediate air quality and indirect radiative effects of mitigating landfill CH₄ emissions. The analysis is motivated by the potential of a novel, fuel-flexible combustion technology capable of converting all CH₄ emissions to carbon dioxide. Three scenarios are assessed: (BASE) a control simulation with unchanged landfill CH₄ emissions; (LCC_NAT) a hypothetical national implementation of the combustion system over a summer month; and (LCC_REG) a regional application over Texas. One-month simulations show that, compared to BASE, LCC_NAT yields clear concentration reductions in surface and column-averaged CH₄ (XCH₄) of -3.03 and -1.60 ppb across the contiguous United States, while LCC_REG shows declines of -2.38 and -1.42 ppb over Texas. CH₄ reductions coincide with increases in hydroxyl radical (OH) concentrations under both LCC_NAT and LCC_REG scenarios, indicating an enhanced atmospheric self-cleaning capacity. At the national scale, CH₄ reductions in the LCC_NAT scenario also result in improved air quality, with surface concentrations of carbon monoxide (CO), ozone (O₃), and fine particulate matter (PM_2.5) decreasing by 32.7 ppb, 0.27 ppb, and 0.01 μg m^-3, respectively. Atmospheric cooling is induced within the atmosphere, driven primarily by negative longwave radiative forcing under clear-sky conditions in both LCC_REG (-0.38 W m^-2) and LCC_NAT (-0.04 W m^-2) scenarios. These cooling effects are stronger under all-sky conditions due to cloud radiative effects, as clouds may absorb efficiently thermal radiation emitted from the surface. Furthermore, net radiative forcing under all-sky conditions leads to surface cooling in LCC_NAT but regional warming in LCC_REG, highlighting the spatial variability of climate responses.