GHG emissions associated with the  onsite generation of electricity using coal, oil, natural gas,  etc. may be included in an aviation-related GHG emissions inventory. Typically, these values are  based on aviation-related electrical usage and emiss ion factors from local utilities or from the  EPA eGRID 67  system.  The GHG emissions associated with refrigeration and heating and air -conditioning systems as  well as the GHG emissions from recycling of solid waste associated with an airport are likewise  occasionally included. These values are based mostly on product -specific material-balancing  methods and emission factors which take into account the use and disposal of these materials.  Notably, GHG emissions associated with the  entire “supply chains” or full “life cycles” (e.g.,  production, consumption and/or disposal of goods and materials such as paper, plastic and waste  products, foodstuffs, building materials, etc.) by either airport facilities or its tenants are typically  not included in an aviation-related GHG emissions inventory.  Moreover, the storage of fuel (e.g., jet fuel, Avgas, gasoline, and diesel) is a potential source of  evaporative hydrocarbon emissions, but does not produce the type of HCs that contribute directly  to global climate change.  Due to the fact that CO $_{2}$, CH$_{4}$, and N$_{2}$O are by-products of fuel combustion, they are also the  predominant GHGs associated with most airports. Emissions of HFCs, PFCs, and SF 6  linked  with refrigeration, air conditioning, and other coolants also occur, but at  far lesser amounts.  However, in all cases, the estimated GHGs should be converted to CO 2e  values using Global  Warming Potentials (GWPs). $_{68}$,$_{69 }$ Table 6-6  ( GHG Emission Inventory Methods ) provides a summary listing and  Appendix C  ( Emissions Inventory for Greenhouse Gases ) provides further guidance on the computation of  GHG emissions associated with aviation including the emission inventory methods, data sources,  and other supporting materials.  Source  Method  Aircraft Engines  Fuel use rates within the study area based on EDMS/AEDT  combined with fuel-specific emission factors.  APUs  Fuel use rates from EDMS/AEDT combined with fuel-specific  emission factors.  GSE  Fuel use or fuel use rates from NONROAD combined with  fuel-specific model emission factors.  Ground Access Vehicles  Fuel use or VMT combined with emission factors from  MOVES model.  Stationary Sources  Fuel use combined with fuel-specific emission factors.  Electric Usage  Kilowatt hours combined with emission factors from local  utility and/or eGRID system.  Refrigeration, Heating, Air  Material balancing accounting for charging, use and disposal  Table 6-6. GHG Emission Inventory Methods  67  EPA,  Emissions & Generation Resource Integrated Database (eGRID) ,  http://www.epa.gov/cleanenergy/energy-resources/egrid/index.html.  68  GWPs are a relative measure of how much heat a GHG traps in the atmosphere when compared CO$_{2}$.  69  EPA,  Global Warming Potentials of Ozone-Depleting Substances (ODS) Substitutes ,  http://www.epa.gov/ozone/geninfo/gwps.html.  54$_{ }$ Aviation Emissions and Air Quality Handbook 