Integration of bottom-up and top-down methods to evaluate national methane emission inventories

Abstract

Individual countries estimate emissions of methane, a potent greenhouse gas, in national inventories. These inventories are used to report emissions to the United Nations Framework Convention on Climate Change (UNFCCC) and are used as the basis for national climate policy including the establishment of methane mitigation targets. The national inventories use ‘bottom-up’ methods that apply an emission factor to source activity data to estimate emissions but these estimates have large uncertainties, especially for fossil fuels. ‘Top-down’ information from observations of atmospheric methane can help to evaluate and improve the bottom-up national inventories through inverse modeling. This requires spatial information on emissions but national inventories typical only provide emissions on the national scale. We present 0.1° x 0.1° resolution gridded inventories that spatially represent the national emissions reported to the UNFCCC. This includes a global gridded inventory of methane emissions from oil, gas, and coal exploitation and national gridded inventories of anthropogenic methane emissions from Canada and Mexico. For the global inventory an array of databases is used to spatially allocate national emissions to infrastructure including wells, pipelines, oil refineries, gas processing plants, gas compressor stations, gas storage facilities, and coal mines. The national inventories are constructed in collaboration with national agencies to ensure agreement with policy-relevant national inventories. Spatial allocation is done using an ensemble of national datasets for methane-emitting activities resolving individual municipalities and point sources for both countries. For Mexico, we identify 16 hotspots on the 0.1° x 0.1° grid with individual emissions higher than 20 Gg a-1 (2.3 tons h-1) including large landfills, offshore oil Thesis advisor: Professor Daniel Jacob Tia Scarpelli iv production, coal mines in northern Mexico, a gas processing complex, and a cattle processing facility. We find that emissions in Canada are more widely distributed, with only 8 hotspots emitting more than 1 ton h-1 on the 0.1º x 0.1º grid. We use the detailed emissions information and geospatial data embedded in the bottom-up inventories to interpret the results from inversions of atmospheric observations. Our interpretation of differences between top-down and bottom-up estimates can be used to improve understanding of national emissions.