Assessing daytime variations in methane emissions from landfills and impact on whole-landfill emission estimates.

Methane (CH4) emissions from landfills may vary diurnally and seasonally, influenced by cover type, atmospheric conditions, and the presence/absence of gas collection and control systems (GCCS). To evaluate how these factors influence CH4 flux, high-frequency eddy covariance (EC) data were collected from portions of three U.S. landfills - Southeastern US Landfill (SE), California Landfill 1 (CA 1), and California Landfill 2 (CA 2). Fluxes for each landfill cover for each measurement day were normalized by th e mean daytime flux (9am-9 pm). For all cover types and landfills, emissions varied with time of day, with peak fluxes between 12 PM - 6 PM. At SE, intermediate covers without GCCS showed average normalized peak fluxes (-) of 1.45 (2012) and 1.39 (2013), i.e., fluxes were 39-45% higher than the daily average during the most emissive period, with peak emissions between 12-3 PM on 49% of days. Similar results occurred at CA 1 and CA 2 landfills for landfill covers with or without GCCS. At the CA 1 landfill, EC measurements from the landfill covers were used to estimate the whole-landfill emissions between 9am and 9 pm. Between 12 PM and 6 PM, whole-landfill emissions were ∼ 20% higher than the daytime mean. At the SE landfill, where data were available to assess emissions over 24 h, measurements during peak emission periods could overestimate daily emissions by up to 70%. These findings underscore the need for diurnally resolved emission data, especially from uncovered or poorly controlled landfill sites, to support accurate emission inventories and optimal mitigation strategies.