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3 Massachusetts Environmental Policy Act (MEPA) requires with the use of refrigeration and fire extinguishers, but these preparation of a CO2 emissions inventory, and in King County emissions are not well documented. Of the three remaining Washington, under Washington's State Environmental Pol- gases, emissions of CO2 at an airport tend to be better under- icy Act (SEPA), county-based projects are required to prepare stood than N2O and CH4. a GHG inventory. In addition to the direct emissions, consideration can often There have been emerging discussions about federal GHG be given to the other following pollutants that have the po- regulation following the recent U.S. Supreme Court case of tential to exert climate change effects: water vapor (H2O), par- Massachusetts v. USEPA. On April 2, 2007, the Supreme Court ticulate matter (PM), sulfur oxides (SOx), oxides of nitrogen ruled on a 5-to-4 vote that USEPA has the authority to regu- (NOx), carbon monoxide (CO), and nonmethane volatile or- late GHG emissions and that USEPA must reevaluate its stance ganic compounds (NMVOC). These pollutants can produce in not choosing to do so thus far. Other efforts include the some direct effects, but their main contributions are as pre- 2008 lawsuit by California to regulate GHG emissions from cursors for indirect effects. mobile sources and California's petition of USEPA to regu- The direct effects that H2O exert tend to be dominated by the late industrial GHG emissions. As such, the USEPA has been normal, natural hydrologic cycle (rainfall, evaporation, etc.). under increasing pressure to regulate GHG emissions under However, water vapor still may have an important effect, es- the Clean Air Act (CAA). pecially for direct emissions into the stratosphere as occur This Guideline has been prepared to aid airports that wish for some aircraft flights. Similarly, the effects produced by to voluntarily prepare airport-specific inventories, as well as PM species (i.e., black carbon or soot and sulfate aerosols) those that may be required by existing and future mandates. can be important. SOx adds to this effect since it can react in the atmosphere and form sulfate aerosols. Both H2O and PM also have indirect effects through contrail formation. Ozone 1.3 Overview of Greenhouse (O3) also has a climate change effect but is not directly emit- Gas Emissions ted. Rather, O3 is produced in the troposphere through reac- This Guidebook focuses on the development of inventories tions involving NOx or CO and NMVOCs. In the stratosphere, for the following GHGs: it is produced through a reaction involving oxygen molecules (O2) and ultraviolet (UV) radiation. Since O3 is not directly 1. Carbon dioxide (CO2), emitted, it cannot be included in an airport emissions inven- 2. Methane (CH4), tory. However, its precursors, NOx, CO, and NMVOC can be 3. Nitrous oxide (N2O), included. NOx can also produce nitrate aerosols, thus further 4. Sulfur hexafluoride (SF6), complicating the assessment of indirect effects. 5. Hydrofluorocarbons (HFC), and Although the indirect effects are generally considered im- 6. Perfluorocarbons (PFC). portant, they also have the largest uncertainties associated with their climate impacts. Inclusion of these precursor emis- sions within a GHG inventory arguably helps to comprehen- Chapter 2 discusses different levels of evaluation sively capture all of the emissions related to climate change, based on the pollutants considered. This Guide- consistent with the general guidelines specified by the IPCC book recommends that airport GHG inventories consider the six Kyoto pollutants (Level 2). See in promoting the need to quantify even indirect emissions Section 2.3. as part of the overall GHG inventory (IPCC 2006). However, since there are technical issues for these precursors, such as no well-established CO2 equivalencies for these precursors, This list mirrors the gases regulated under the Kyoto Pro- these emissions cannot be directly compared to each other at tocol. These gases are typically covered in most GHG emis- this time using simple multipliers. More complex climate sions reporting protocols including the guidelines from the models are required for this purpose. Intergovernmental Panel on Climate Change (IPCC) and the recent protocol from TCR (IPCC 1999 and TCR 2008a). For 1.4 Overview of Reasons for U.S. economic sectors as a whole, these gases generally repre- Preparing Greenhouse Gas sent the most notable GHGs based on a combination of the Emissions Inventories quantity of pollutant emitted and potential for exerting cli- mate change effects. For aviation, emissions of the fluorinated Each year, USEPA prepares a GHG inventory for the United compounds (including HFC and PFC) are less significant be- States (USEPAb 2008). Even though the inventory is devel- cause these compounds are generally emitted from industrial oped using a "bottom-up" approach (i.e., it reflects the assess- activities. They can be emitted from airport activities associated ment of individual sectors), the sectoral data are large-scale