Patterns of diurnal landfill methane flux across different seasons at a landfill in Southeastern US Measurements and Modeling

An eddy covariance (EC) tower was employed in three one-week periods in summer, fall, and winter to investigate diurnal patterns of methane flux from a Southeastern US landfill. Data were analyzed across seasons to understand impacts of atmospheric pressure, temporal variation of atmospheric pressure, wind shear velocity, and air temperature on diurnal methane fluxes. The atmospheric conditions were categorized into unstable, stable and neutral conditions. The EC data collected under stable atmospheric conditions were discarded because of data reliability. Thus, the statistical analyses were separately pursued for neutral and unstable atmospheric conditions. Under unstable atmospheric conditions, air temperature correlates best with methane flux, which is corroborated with an independent dataset of tracer dilution method (TDM) measurements for similar measurement periods. Landfill methane flux varies significantly with shear velocity when the atmosphere is neutral. The EC data revealed that daytime flux can be up to 23 times greater than nighttime fluxes at this landfill. A mathematical model describing atmospheric effects on methane flux from landfills is supported by these data, and this model was incorporated into the Weather Research and Forecasting Model (WRF). WRF-predicted diurnal methane emission variations were consistent with field data: 1.7 to 4.3 (max/min) for representative simulation days in winter, spring and summer. Since most short-term flux measurements (e.g., TDM, airborne + wind modeling, etc.) are conducted during daytime between 12 pm and 6 pm when daily fluxes are highest because of atmospheric effects, these short-term flux measurements could overestimate fluxes by as much as 73% at this site. Therefore, it is necessary to find times of day when the measured fluxes represent diurnal average fluxes. The best times to measure methane flux representative of diurnal average emissions at this landfill are in late morning (~11 am) and in the early evening (~5 pm).