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 . 

## 3.2.3. Greenhouse Gases 

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. 

