Impacts of meteorological parameters on short-term methane emissions from landfills
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Landfills are an important anthropogenic source of greenhouse gas emissions, accounting for about 18% of anthropogenic methane emissions. With growing regulatory interest in greenhouse gas emissions and the development of fast and accurate gas sampling tools, an increasing number of landfill methane emissions measurements have been reported. Data from these studies indicate significant short-term variability, the cause of which is unknown. It is hypothesized that short-term (within a day) variations of landfill emissions are associated with temporal fluctuations of some atmospheric parameters. Atmospheric modeling supports the hypothesis that short-term emission variations would lead to short-term variations of downwind measurements. ☐ Because methane emissions may vary within short periods (~ a few hours), understanding the mechanisms driving emission variations will help to understand their temporal variability. A long-term data set of eddy covariance (EC) measurements from a Southeastern US landfill was examined to determine the influence of atmospheric conditions on methane emissions and their diurnal variation. The results indicate that surface methane flux significantly varies with wind shear velocity when the atmosphere is neutral. Under unstable atmospheric conditions, air temperature has the best correlation with methane flux, which was corroborated with an independent dataset of tracer dilution method (TDM) measurements for similar measurement periods. These field data support the formulation of interface flux employed recently in the Weather Research Forecasting model for landfill gas emissions. The EC data display diurnal flux variations for this landfill (daytime fluxes up to 23 times greater than nighttime fluxes). Flux measurements from this landfill between 12 pm and 6 pm might overestimate diurnal-average fluxes up to 73%. Therefore, the best measurement times representing diurnally averaged fluxes for this landfill are before noon (~11 am) and in the evening (~5 pm). ☐ Among current methane emission measurement methods, the TDM is believed best for quantifying the whole-landfill methane emissions. However, not all the collected data are reliable. The level of data reliability depends on landfill topography, properties of downwind measurements (e.g. distance to downwind roads and alignment compared to landfill), gas tracer setup (number of tracer bottles and their locations compared to high methane emission spots), and meteorological conditions (wind direction, wind speed, and atmospheric stability). Therefore, data filtering techniques have been proposed for removing unreliable data. Statistical analyses were conducted between previously established data filtering criteria and meteorological parameters to determine the meteorological conditions and time of day when the most accurate TDM data may be collected. Using the data collected from the Southeastern US landfill as a test case, results indicate that the rate of passing the TDM filtering criteria increases as wind speed increases and as the circular standard deviation of the wind direction decreases. Comparison between the mobile and stationary TDM approaches indicates that the stationary approach is more sensitive to the atmospheric conditions, with 64% of stationary data passing the filtering criteria when wind speed is greater than 5 m/s and the standard deviation of circular wind direction is less than 0.4. About 65% of the data obtained by the mobile approach pass these filtering criteria when circular wind direction standard deviations are less than 0.5 with no wind speed limitation. This study showed that measurements during early morning and late evening are more susceptible to error because of atmospheric stability conditions at this landfill. Moreover, the measurements during colder seasons were more accurate than results in warmer seasons for this landfill, because of higher frequency of unstable atmosphere during warmer seasons. At the Southeastern US landfill, the best times of day to collect the most accurate and reliable data are ~ 11 am and ~ 4 pm. Interestingly, conducting TDM measurements at these times leads not only to high data accuracy, but also data that are most representative of daily emissions.
Description
Keywords
Climate change, Diurnal emissions, Landfills, Methane, Solid waste, Tracer dispersion method