of organic and inorganic compounds (e.g., f ormaldehyde, 1 acetaldehyde, benzene, toluene,  acrolein, 1,3-Butadiene, xylene, lead, naphthalene, propionaldehyde). In relation to aviation  sources, such emissions are present in the exhaust of aircraft,  APUs, GSE, and motor vehicle  engines and, to a lesser extent, from boilers, fuel facilities, and other stationary sources.  For a detailed discussion on how to assess  HAPs associated with aviation refer to  Section 6.2  (Hazardous Air Pollutants)  and  Appendix B  ( Emissions Inventory for Hazardous Air Pollutants )  of this  Handbook .  Greenhouse gases (GHGs)  [are pollutants for which there are no NAAQS and]  are emitted  principally from the combustion of fossil fuels, decomposition of was te materials, and  deforestation and are linked to an increase in the earth’s average temperature by means of a  phenomenon called the “greenhouse effect.”  According to the Intergovernmental Panel on Climate Change (IPCC) and Executive Order (EO)  13514 12 , the six main GHGs whose emissions are related to human activities (e.g. combustion of  fossil fuels, agriculture, land use change ) are carbon dioxide (CO$_{2}$), methane (CH$_{4}$), nitrous  oxide (N$_{2}$O), and fluorinated gases such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs)  and sulfur hexafluoride (SF $_{6}$), which are typically reported as carbon dioxide equivalents  (CO$_{2e}$). 13  GHG emissions associated with aviation are principally in the form of CO$_{2 }$and are generated by  aircraft, APUs, GSE, motor vehicles and an assortment of stationary sources. For the most part,  CO$_{2 }$emissions from these sources arise from the combustion of fossil fuels (e.g., jet fuel, Avgas,  diesel, gasoline, compressed natural gas [CNG]) and are emitted as by-products contained in the  engine exhausts. Other GHGs associated with  airport operations include CH$_{4 }$ and N$_{2}$O, water  vapor (H$_{2}$O), soot, and sulfates  - but are emitted by airports  to a far lesser extent  than CO$_{2}$.  Emissions of HFCs, PFCs, and SF 6  are most commonly linked with refrigeration, conditioning, and other coolants.  For a detailed discussion on how to assess  GHG associated with aviation refer to  Section 6.3  (Greenhouse Gases)  and  Appendix C  ( Emissions Inventory for Greenhouse Gases ) of this  Handbook .  Table 3-2  ( Sources of Air Emissions and Pollutants of Concern at Airports)  provides a summary  listing of aviation-related sources of air emissions and the types of pollutants they emit.  12 DOE, Office of Energy Efficiency & Renewable Energy (EERE),  http://energy.gov/eere/femp/articles/executive-order-13514.  13 EPA defines CO$_{2e }$as a metric measure used to compare the emissions from various GHGs based upon their  Global Warming Potential (GWP).  15$_{ }$ 3.2.3.  Greenhouse Gases  Aviation Emissions and Air Quality Handbook 