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Summary:

Methane is a potent greenhouse gas whose atmospheric concentration has more than doubled since industrialization. The United Nations' Intergovernmental Panel on Climate Change (IPCC) data suggest that human activities are now responsible for approximately 70% of global methane emissions [Houghton et al., Climate Change 1995: The Science of Climate Change]. This, coupled with methane's relatively short atmospheric residence life of about 8-12 years [Houghton et al., Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment] makes methane mitigation a particularly desirable goal in any greenhouse gas reduction strategy.

While the national inventories of greenhouse gas emissions, including methane, required by the United Nations' Framework Convention on Climate Change provide general guidelines for assessing mitigation alternatives, significant regional variability exists. Understanding this variability by inventorying emissions at a sub-national or site specific scale should result in more focused and efficient mitigation strategies. The Global Atmospheric Chemistry Group at the UNH Institute for the Study of Earth, Oceans, and Space has completed a county-based methane emissions inventory for the New England region. Data represent annual emissions of methane per county for a typical year in the early 1990s. Emission sources include: wetlands, active landfills, closed landfills, ruminant animals, residential wood combustion, motor vehicles, animal manure, natural gas, wastewater treatment, as well as soil uptake from upland forests. Methane emissions were calculated to be 0.91 Tg per year ( 1 Tg = 1 trillion grams), with wetlands and landfills dominating all other sources. [Read the abstract from Blaha et al Atmospheric Environment 33, 243 - 255].

Emissions estimates that are variables in the New England Methane collection:

• Active Landfills
• All Sources
• Closed Landfills
• Gas Pipelines
• Manure
• Motor Vehicles
• Ruminants
• Upland Soils
• Wastewater Treatment Plants
• Wetlands

Emission Sources and Methodology: (all sources in metric tons methane per year)

Wetlands: Methane emissions from natural wetlands in the region were calculated as a function of habitat-specific unit area flux, area, and emission season and were calculated on a county scale, the smallest scale available for habitat area characterization. Literature flux values reported from wetlands in the northeastern U.S. were grouped and averaged by the wetland habitat classification types used for the National Wetlands Inventory. Estimates of wetland areas were obtained from the six New England states and emissions were calculated, adjusted by the seasonality of methane release observed by researchers at UNH at a long-term flux measurement site in New Hampshire [Frolking and Crill (1994), Global Biogeochem. Cycles 8:385-397; Melloh and Crill (1996), Global Biogeochem. Cycles 10:247-254.]

Active Landfills: Operational parameters for the largest municipal solid waste landfills in each state were obtained from the state environmental agencies and used as inputs to a landfill methane emissions model developed at UNH. Ninety two landfills in New England were surveyed, with a size distribution (in tons per year waste received) from 1500 (Greenville, ME) to 711,000 (Plainville, MA) metric tons. Emissions estimates from landfills with landfill gas recovery systems were modified to reflect the reduction in atmospheric emissions from these sites.

Closed Landfills: Emissions estimates were calculated for those landfills in Connecticut and Massachusetts which were closed between 1960 through 1992. These estimates were then used to derive an average per capita emissions rate for these two states that was then extrapolated to the other New England states using census population statistics to obtain an annual emissions estimate for each of the states from inactive landfills that were closed prior to 1992. These state level estimates cannot be apportioned on a county basis.

Ruminant Animals: Ruminant animal emissions were calculated based on animal populations and published emission factors which include differences in age, sex, and type of animal as well as typical diets. Few cattle are raised in New England for beef, and ruminant populations are primarily dairy cows and replacement heifers. Cattle population data from the 1992 Census of Agriculture were used to map populations and emissions to a county basis.

Residential Wood Combustion: Although wood combustion is a relatively minor methane source in the United States, it is of much greater significance in rural states like Maine and Vermont where it provides an average of 15% of residential heating requirements. In New England, roughly 200,000 homes use wood as the primary heat source and an additional 900,000 homes rely on wood for supplemental heating (Energy Information Administration, Household Energy Consumption and Expenditures 1993). Using emission factors from the Environmental Protection Agency (U.S. EPA Office of Air & Radiation. EPA 430-R-93-003, 1993) and residential energy consumption data from the Energy Information Administration (EIA, 1993), an emissions estimates per household unit for both primary and secondary wood combustion was derived and applied to county data on residential wood use.

Motor Vehicles: Department of Motor Vehicle registration records from each state were obtained to estimate the number of passenger vehicles, light and heavy trucks, and motorcycles in each county. Department of Transportation highway statistics were used to calculate the average annual vehicle miles traveled by vehicle type in each of the six New England states. Methane emissions were then calculated using emission factors, by vehicle type, from the U.S. Environmental Protection Agency (U.S. EPA Office of Air & Radiation. EPA 430-R-93-003, 1993).

Animal Manure: Methane production and release from animal manure is dependent on animal population sizes, their diets, manure management systems, and local climate. Emissions estimates for the New England region are based largely on the waste management systems and emission factors reported by Safley et al. (U.S. EPA Office of Air & Radiation. EPA 400/1-91/048, 1992) and then updated by the U.S. EPA (U.S. EPA Office of Air & Radiation. EPA 430-R-93-003, 1993) in conjunction with 1992 Census of Agriculture data on cattle and cow population.

Natural Gas Transmission Pipelines: Methane is the major constituent of natural gas. Emissions estimates were derived using published EPA emission factors (U.S. EPA Office of Air & Radiation. EPA 430-R-93-003, 1993) in combination with a pipeline atlas of transmission pipelines in the region.

Wastewater Treatment: Two hundred of the largest wastewater treatment plants in New England were surveyed to obtain their operating values that were then used as inputs to a wastewater treatment emissions model developed by UNH (Czepiel et al. (1993), Environmental Science and Technology 27: 2472-2477.) Methane emissions were estimated from both liquid treatment processes and the anaerobic digestion of sludge. Fugitive emissions of biogas from anaerobic digestors were not considered.

Soil Uptake from Upland Forests: Data on county-level forest areas were obtained from the U.S. Forestry Service Forest Inventory and Analysis (FIA) project. To address concern about including areas of forested swamp as both sources and sinks, the areas of forest growing on very poorly drained land was subtracted from total forested areas. This should not be problematic since methane consumption occurs only in relatively well-drained soils. Agricultural land use data (cropland, pasture, rangeland, orchards) were obtained from the 1992 Census of Agriculture. Consumption rates from these soils are reduced due to the effects of soil compaction and fertilizer application: it was assumed that rates were only 20% (cropland) or 30% (pasture, rangeland, and orchards) of that of undisturbed forests. Since it does not appear that forest type (deciduous, evergreen) significantly affects uptake rates, a simple average for data reported in New England was obtained and applied to all forests.

Although soil methane consumption occurs during the warm season, it appears to be largely controlled by rates of methane diffusion into the soil rather than temperature, except during a relatively brief period in the spring (Crill, 1991, Global Biogeochem. Cycles 5:319-334). Using a multi-year data set from New Hampshire reported in Crill (1991) to determine the period over which uptake takes place, it was assumed that methane consumption occurred at a "warm season" constant rate over this period. This season, the time when the NOAA air temperature network 30 year monthly average temperature rises above freezing through the time when the average monthly minimum falls below freezing, varied from 214 days (northern Vermont, New Hampshire, and Maine) to 306 days (Barnstable, Dukes, and Nantucket counties, MA).

Reference:

D. Blaha, K. Bartlett, P. Czepiel, R. Harriss, and P. Crill (1999). Natural and Anthropogenic Methane Sources in New England. Atmospheric Environment 33:243 - 255.

Data Providers:

Denise Blaha, Karen Bartlett, Peter Czepiel, Robert Harriss and Patrick Crill. Complex Systems Research Center, Institute for the Study of Earth, Oceans, and Space, Morse Hall, University of New Hampshire, Durham, New Hampshire, USA. Ph: 603.862.1792, Fax: 603.862.0188, Email: denise.blaha@unh.edu.

Last Data Update:

3/07/2000

Last Doc. Updated:

10/10/2001

Doc. Updated By:

Denise Blaha

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