| Summary: | Municipal solid waste landfills are major contributors to anthropogenic emissions of methane (CH<sub>4</sub>), which is the major component of natural gas, a potent greenhouse gas, and a precursor for the formation of tropospheric ozone. The development of sensitive, selective, and fast-response instrumentation allows the deployment of mobile measurement platforms for CH<sub>4</sub> measurements at landfills. The objectives of this study are to use mobile monitoring to measure ambient levels of CH<sub>4</sub> at eight large operating landfills in southeast Michigan, USA; to characterize diurnal, daily and spatial variation in CH<sub>4</sub> levels; and to demonstrate the influence of meteorological factors. Elevated CH<sub>4</sub> levels were typically found along the downwind side or corner of the landfill. Levels peaked in the morning, reaching 38 ppm, and dropped to near-baseline levels during midday. Repeat visits showed that concentrations were highly variable. Some variation was attributable to the landfill size, but both mechanistically-based dilution-type models and multivariate models identified that wind speed, boundary layer height, barometric pressure changes, and landfill temperature were key determinants of CH<sub>4</sub> levels. Collectively, these four factors explained most (r<sup>2</sup> = 0.89) of the variation in the maximum CH<sub>4</sub> levels at the landfill visited most frequently. The study demonstrates the ability to assess spatial and temporal variation in CH<sub>4</sub> levels at landfills using mobile monitoring along perimeter roads. Such monitoring can identify the location of leaks and the best locations for long-term emission monitoring using fixed site monitors.
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