Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2011
Publication Date: 11/11/2011
Publication URL: http://handle.nal.usda.gov/10113/59584
Citation: Spokas, K.A., Bogner, J., Chanton, J. 2011. A process-based inventory model for landfill CH4 emissions inclusive of seasonal soil microclimate and CH4 oxidation. Journal of Geophysical Research-Biogeosciences. Available: http://www.agu.org/pubs/crossref/2011/2011JG001741.shtml. Interpretive Summary: CALMIM (CAlifornia Landfill Methane Inventory Model) is a science-based, field-validated methodology for typical annual methane (CH4) emissions from daily, intermediate, and final cover materials at California landfills. This model, a freely-available JAVA tool, is a Tier III IPCC (Intergovernmental Panel on Climate Change) inventory methodology for landfill CH4 emissions which includes a user-friendly template for entering information on site-specific cover materials and the extent of engineered gas extraction, globally-validated USDA climatic and soil temperature/moisture models, and a 1-D diffusional model for annual emissions inclusive of seasonal methane oxidation. Unlike previous inventory models which relied on hypothetical CH4 generation models, CALMIM provides a more realistic emphasis on seasonally-varying transport, oxidation, and emissions processes in specific California landfill cover materials. This methodology is the product of a 3-year project (2007-2010) in cooperation with CalRecycles and the California Air Resources Board (ARB) to develop an improved methodology in the context of the California greenhouse gas (GHG) inventory. Oxidation modeling was based on extensive supporting laboratory studies. Field validation was conducted over two annual cycles at a northern California coastal site (Marina Landfill) and a Los Angeles site (Scholl Canyon). This study has emphasized the need for improved understanding of emissions from all cover types and the incorporation of seasonal as well as spatial variability in landfill monitoring designs and modeling frameworks. This research illustrates the high variability in surface emissions as a function of season, cover thickness and soil type. This research will contribute to improving annual greenhouse gas inventory guidelines where the soil processes need to be estimated for the assessment of the net emissions of methane to the atmosphere. Inclusion of these dynamic relationships coupled with appropriate modeling of soil temperature and moisture conditions will improve annual greenhouse gas inventory assessments.
Technical Abstract: We have developed and field-validated an annual inventory model for California landfill CH4 emissions that incorporates both site-specific soil properties and soil microclimate modeling coupled to 0.5o scale global climatic models. Based on 1-D diffusion, CALMIM (California Landfill Methane Inventory Model) is a freely-available JAVA tool which models a typical annual cycle for CH4 emissions from site-specific daily, intermediate, and final landfill cover designs. Literature over the last decade has emphasized that the major factors controlling emissions in these highly-managed soil systems are the presence or absence of engineered gas extraction, gaseous transport rates as affected by the thickness and physical properties of cover soils, and methanotrophic CH4 oxidation in cover materials as a function of seasonal soil microclimate. Moreover, current IPCC national inventory models for landfill CH4 emissions based on theoretical gas generation have high uncertainties and lack comprehensive field validation. This new approach, which is compliant with IPCC “Tier III” criteria, has been field-validated at two California sites (Monterey County; Los Angeles County), with limited field validation at three additional California sites. CALMIM accurately predicts soil temperature and moisture trends with emission predictions within the same order of magnitude as field measurements, indicating an acceptable initial model comparison in the context of published literature on measured CH4 emissions spanning 7 orders of magnitude. In addition to regional defaults for inventory purposes, CALMIM permits user-selectable parameters and boundary conditions for more rigorous site-specific applications where detailed CH4 emissions, meteorological, and soil microclimate data exist.