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In addition to various spatial scales, both top-down and bottom-up approaches have varying temporal scales. At the same time, the total facility emissions might be used to represent emissions from other similar facilities, like a bottom-up assessment. At these intermediate scales, emissions from multiple sources or components within a facility may be aggregated like a top-down assessment.
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At spatial scales in between an individual source and a source region (e.g., total emissions from a large complex facility such as a natural gas processing plant, an animal feeding operation, or a large regional landfill), emission estimation might be considered either top-down or bottom-up or both. These bottom-up assessments are intended to be representative of broader categories of emissions. At smaller spatial scales, measurements from single processes, individual sources, or components within a facility are extrapolated to larger scales (regional, national, global).
These emission estimates, which aggregate emissions from multiple sources, are defined by the Committee as top-down assessments. At larger spatial scales (e.g., global, continental, and regional), atmospheric methane concentrations can be transformed, using a variety of modeling tools, to estimate methane emissions from broad geographic areas. Methane measurements and emission estimates occur along a spectrum of spatial and temporal scales ( Figure 3.1), from large-scale global assessments of annual emissions to small-scale measurements of emissions from individual sources over short timescales (e.g., instantaneous). Furthermore, atmospheric monitoring of methane concentrations is also needed to detect regional trends in emissions and enable rigorous comparisons to bottom-up approaches. Field measurement of emissions from various sectoral sources can provide improved understanding of processes that lead to emissions, which contributes to the development of process-based emission models as well as regional- and urban-scale mitigation strategies. Measurements of emissions and monitoring of methane are essential for the development of robust emission inventories as described in Chapter 2. CHAPTER THREE Methane Emission Measurement and Monitoring Methods