New Methane Emission Estimation Technique: Step by Step Methodology

The text below outlines the recommended methodology for determining methane emission rates from a MSW landfill.

Step 1a: Formulate Receptor Sub-Model

Obtain ambient air VOC measurements at numerous locations within the landfill, as required by 40 CFR Part 60, Subparts WWW. If the landfill does not require quarterly ambient air VOC measurements, a walking survey with instruments (a portable FID and a GPS) is required to obtain the ambient concentrations and to link each concentration with a location.

Also, the local wind speed and direction, and stability class must be determined during the time of the walking survey.

q = argmin F ⋅ q − c_measuredI⋅ q ≥ 0

2

the number of receptors. Locations of these sources should be chosen generally upwind of receptors, not too close nor too far away. Note that this somewhat arbitrary locating of sources will influence the final answers for individual source strengths, and could influence the final estimate of overall emissions using this approach.

Step 1c: Formulate Matrix Dispersion Model

Solve the standard Gaussian dispersion equations by matrix inversion methods, as previously described, to determine the methane emission rates throughout the MSW landfill.

This can be done easily in MATLAB, which specializes in numerical computing and allows easy matrix manipulation. A simple scenario test case was devised by the author, and is described in APPENDIX B. Based on the finding in APPENDIX B, it is concluded that the matrix inversion methods can perform extremely accurately.

Step 1d: Perform Sensitivity Studies

Perform sensitivity studies to estimate the sensitivity of results to our techniques.

Step 1e: Formulate Air Dispersion Model to Test Validity

After determining the best representation of methane point-source emission rates from the MSW landfill, run those emission rates through the steady-state Gaussian air dispersion model, ISC, to determine the ambient air VOC concentrations. ISC is utilized instead of the current EPA steady-state Gaussian air dispersion model, AERMOD, because the ISC model allows one to specify one particular hour of meteorological data, such as local wind speed, wind direction, ambient temperature, and stability class. Compare ISC’s modeled receptor

concentrations with the initial set of ambient air VOC measurements (Step 1.1) taken within the landfill to test the validity of the methane emission rates.

3.2 Step 2: Air Dispersion Modeling

The methane emission rates are applied to the SCL using the air dispersion model AERMOD. AERMOD (American Meteorological Society/Environmental Protection Agency Regulatory Model) is EPA’s official dispersion model; it is a steady-state Gaussian plume model which 1) is based on planetary boundary layer turbulence structure and scaling concepts, 2) treats both surface and elevated sources, and 3) incorporates both simple and complex terrain.

3.2.1 AERMOD: Theory

AERMOD is intended for use on transport distances up to 50 km, and is applicable to rural and urban areas, flat and complex terrain, surface and elevated releases, and multiple sources (point, area, and volume sources). The AERMOD modeling system consists of two pre-processors, AERMET and AERMAP, and the dispersion model, AERMOD. AERMET is the meteorological preprocessor and AERMAP is the terrain pre-processor. Figure 4 presents the flow of data into each pre-processor and into AERMOD.

Figure 4: AERMOD Flow Data

AERMET uses meteorological data and surface characteristics to calculate boundary layer parameters needed for AERMOD. The meteorological inputs into AERMET include wind speed, wind direction, temperature, and cloud cover. The surface characteristic inputs into AERMET include albedo, surface roughness, and the Bowen ratio. AERMET then calculates parameters such as friction velocity, Monin-Obukhov length, convective velocity scale, temperature scale, mixing height, and surface heat flux; these parameters are used in AERMOD to calculate vertical profiles of wind speed, lateral and vertical turbulent fluctuations, potential temperature gradient, potential temperature, and the horizontal Lagrangian time scale.

AERMAP is utilized to characterize the terrain of interests and generate receptor grids and terrain-influence elevations needed for AERMOD. Gridded data must be in the format of the Digital Elevation Mapping (DEM) data.

3.2.2 AERMOD: Step by Step Methodology

The methodology below outlines the recommended methodology for determining methane concentrations using AERMOD.

NWS Surface

Upper Air

On-Site

DEM File

Surface File Profile

File

AER MET

AER MAP

AER MOD

Out put

Step 2a: Run AERMET Preprocessor

To process meteorological data, AERMET requires three input files. The first input file will extract/QA NWS hourly surface data & upper air data. The second input file will merge extracted/QA surface & upper air files, and the third input file will develop boundary layer parameters. The final outputs from AERMET include processed surface and upper air files which become inputs to AERMOD.

Step 2b: Run AERMAP Preprocessor

To process terrain data, AERMAP requires an input file with a control pathway, a source pathway, a receptor pathway, and an output pathway. The DEM file is the most critical data file required when processing AERMAP. The final outputs from AERMAP include generated source and receptor terrain data which become inputs to AERMOD.

Step 2c: Run AERMOD

To run AERMOD, a control pathway, source pathway, receptor pathway, meteorology pathway, and output pathway are required. The control pathway will specify dispersion options, pollutant of interest, and the averaging time that will be modeled. The source pathway will specify the source type (point, area, volume), the emission rate for each source, and the AERMAP processed terrain data for each source. The receptor pathway will specify the type of receptor grid system (Cartesian, polar) and the AERMAP processed terrain data for each

that will be modeled. The output pathway will specify the desired tabular printed output and threshold violation files.