Aviation Emissions and Air Quality Handbook  It is again noteworthy that the HAPs emission inventory results are not compared to the NAAQS  or any other significance criteria. Rather, the information is provided for informational purposes  as a means of disclosing the project’s potential effects on HAPs.  Greenhouse gases (GHGs) are another category of pollutants for which there are no NAAQS but  are of concern because of their climate-changing potential.  Section 3  ( Sources and Types of Air  Emissions ) identifies and generally describes t hese emissions as they pertain to aviation.  These  include gases such as CO$_{2}$, CH$_{4}$, and N$_{2}$O as by-products of fuel combustion in aircraft, APU,  GSE, and motor vehicle engines as well as emissions of HFCs, PFCs, and SF 6  linked with  refrigeration and air conditioning.  This section addresses the available guidance and approaches for assessing these emissions in  connection with FAA projects/actions.  Appendix C  ( Emissions Inventory for Greenhouse  Gases ) provides additional information on these aviation- related GHGs including emission  inventory methods, data sources and other supporting materials.  There is presently a  broad scientific consensus that human activities, primarily in the form of  GHGs, are contributing to changes in the earth’s atmosphere. These GHGs, brought principally by the combustion of fossil fuels, decomposition of waste materials , and  deforestation, cause an increase in the earth’s average temperature  – a phenomenon which is  referred to as the “greenhouse effect” or “global climate change.”  As a result, the scientific community is continuing  its efforts to better understand the impact of  aviation emissions on the global atmosphere. In particular, the FAA is leading and participating  in a number of initiatives intended to clarify the role that aviation plays in GHG emissions and  climate. For example, the FAA, with support from the U.S. Global Change Research Program  (GCRP) 58  and its participating federal agencies ( i.e., NASA, NOAA, EPA, and DOE) 59 , has  developed the Aviation Climate Change Research Initiative (ACCRI) 60  in an effort to advance  scientific understanding of regional and global climate impacts of aircraft emissions, quantified uncertainties for current and projected aviation scenarios under changing atmospheric  58  U.S. Global Change Research Program,  http://www.globalchange.gov/about.  59  National Aeronautics and Space Administration (NASA) at http://www.nasa.gov/, National Oceanic and  Atmospheric Administration (NOAA) at http://www.noaa.gov/, and Department of Energy (DOE) at  http://energy.gov/.  60  FAA,  Aviation Climate Change Research Initiative ,  http://www.faa.gov/about/office_org/headquarters_offices/apl/research/science_integrated_modeling/accri/.  49$_{ }$ 6.3.1.  Introduction & Background  6.3. Greenhouse Gases  Phenol (carbolic acid)  1.54  1.81  1.54  1.75  1.48  1.80  Propionaldehyde  1.62  1.90  1.63  1.84  1.57  1.89  Styrene  0.68  0.79  0.68  0.77  0.65  0.79  Toluene  3.32  3.64  3.33  3.60  3.28  3.64 