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16 any other pollutants (e.g., other halogenated compounds). irrespective of the purpose of the inventory. For example, Level 3 is geared toward satisfying scientific needs for a com- whether an inventory is developed for a registry or as part of prehensive accounting of all GHG effects while recognizing a NEPA-like study, the preferred approach should always that Levels 1 and 2 generally capture the bulk of GHG emis- include this ownership and control layer, as it provides sions. Background information on all of these pollutants can clarity in identifying the party/parties that can control those be found in Appendices B through D. emissions. Table 2-2 shows an example of the categorization layer (i.e., "owning entity") in the first two columns (owning entity and source). A third column is provided for informa- Level 1 (only CO2 emissions) is the minimum for a tion purposes to relate these sources to the previously iden- GHG inventory, but Level 2 or higher is preferred. tified Scopes 1, 2, and 3. At an airport, the following three categories relate to ownership and control: Reporting at either Levels 2 or 3 would allow the highest assessment of an airport's carbon footprint and would be the Airport operator owned and controlled, most beneficial to the scientific community. Levels 1 and 2 are Tenant owned and controlled (tenants include airlines, gov- likely to be the most frequently prepared inventory levels. In- ernment, concessionaires, aircraft operators, fixed-based ventory Level 3 would be the most detailed and also the most operators, etc.), and costly for the airport operator to prepare. Levels 2 and 3 serve Public owned and controlled. as loose guidelines since it would also be beneficial to report a set of pollutants (in addition to CO2) that partially cover these levels. For example, an airport may choose to report An ownership and control categorization layer CO2, CH4, and N2O but not have the data and/or resources should be developed for all inventories, irrespec- to report the halogenated compounds (SF6, HFC, and PFC) tive of the purpose of the inventory. in Level 2. In this case, the reporting could be characterized as a partial Level 2. Similarly, partial reporting can be done for Level 3 or a combination of Level 2 and 3 (e.g., reporting For those inventories that will be used to guide and mea- CO2, CH4, and N2O which are common to Levels 2 and 3, sure the success of an emission reduction program, this layer while also reporting NOx from Level 3). provides sufficient transparency of the evolution of emissions over time to the public. As an option, additional layers could be added to point out the level of influence that non-owning To help efficiently use airport resources, a entities have on certain sources. Such additions are at the dis- screening process could be employed to cretion of the airport operator. As explained in Appendix E, identify the major sources. entities that do not own but exert influence on sources can (and often should) be acknowledged for the emissions (or reduc- As part of the data collection and resource issues, a screening tions of emissions) from these sources. As indicated previously, process could be employed to determine the significant sources. the allocation of emissions based on influence is outside the This would be useful for many airports that have limited re- scope of this Guidebook. Therefore, although we note that the sources and need guidance to determine which sources to ex- influence can be acknowledged, there is currently no guid- clude. The protocols from both IPCC and WRI promote the use ance available to help quantify the allocations. of such approaches. Although WRI does not promote quanti- The underlying implication regarding this subject is that tative criteria at this time, IPCC suggests a 95% cutoff point airports can have varying degrees of influence over sources such that sources accounting for 95% of emissions should be they do not own. In general, all airport tenants are affected sufficient to report in an inventory (IPCC 2006). The 95% level by the assets owned and controlled by the airport operator in can be determined from examining prior inventories. CCAR some way, even if loosely through airport policies. As such, (2008) also employs the 95% criterion, referring to 5% as "de the airport operator may influence each source at the airport minimis." This is also supported by TCR (TCRa 2008). How- to varying degrees, and may also be able to claim recognition/ ever, if data are available, and the effort to report the remaining credit for emissions reductions from those sources as well 5% is reasonable, airport operators should consider including (again, to varying degrees). these de minimis sources in the inventory for completeness. In keeping with the guidance provided by WRI and as adopted by a variety of other organizations, as an option, categorization layers for the direct and indirect nature of 2.4 Ownership and emissions and their sources (classified as Scope 1, 2, or 3) Influence Categorizations also could be included. If the inventory is reported to TCR, All airport GHG emissions inventories should have an this categorizing layer would provide consistency with the ownership/control categorization layer. This should be done registry since it also uses these categories.