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7 chemistry H2 O binary H2 O nucleation accumulation collision H2SO4 SO3 nucleation/accumulation hydrocarbons mode particles (HCs) soot <20 nm diameter activation/scavenging aircraft exhaust soot particles HCs H2O H2SO4 condensation coated soot particles ~30-100 nm diameter ambient mixing/dilution Aircraft engines emit a mixture of soot and volatile gases. As pictured above, these gases cool to ambient temperature by mixing with ambient air and convert to the particle phase by condensation and nucleation/growth. The nucleation/growth mode particles and soot coatings are complex mixtures of sulfuric acid, water, partially burned hydrocarbons, and engine oil. Figure 2. Evolution of PM from aircraft engine exhaust. to grow into fine particles on the order of minutes to hours, airport source in determining air quality. Inventory methods typically traveling less than 10 mi. Fine particles remain generally require information about each source's population, suspended in the atmosphere since they do not grow larger size, activity rate, and a PM emission factor or emission index. and are too small to readily settle out or impact on stationary An emission factor is a representative value that attempts to surfaces. They can be transported thousands of miles and relate the quantity of a pollutant released to the atmosphere remain in the atmosphere from days to weeks. Coarse parti- with an activity associated with the release of that pollutant. cles can settle rapidly from the atmosphere, and have life- These factors are usually expressed as the weight of the pollut- times ranging from minutes to hours (occasionally, a few days) ant divided by a unit weight, volume, distance, or duration of depending on their size, atmospheric conditions, and alti- the activity emitting the pollutant (e.g., milligrams of partic- tude. Large coarse particles are generally too large to follow ulate emitted per kilogram of fuel burned). air streams and tend to settle out gravitationally onto sta- In some cases, emission factors are simply averages of all tionary surfaces, rarely traveling more than 10 mi. available data of acceptable quality, and are generally assumed Fine and ultrafine particles suspended in the atmosphere to be representative of long-term averages for all facilities in absorb and reflect light, which is the major cause of reduced the source category (i.e., a population average). The EPA visibility (haze) in parts of the United States. Sulfates, nitrates, maintains a reference (U.S. EPA 2008) of emission factors for organic matter, and elemental carbon are the primary com- many sources. In other cases, specific emission factors are ponents of these small particles. Particles emitted at cruise compiled for each emission source. For example, regulated altitude may contribute to global climate change effects; gaseous emission factors for aircraft engines are included in however, since these particles are emitted beyond the airport the International Civil Aviation Organization (ICAO) Aircraft environment and were therefore outside of the scope of the Engine Emissions DataBank (ICAO 2008) but PM are not tests being summarized in this report, these cruise-level par- similarly regulated. Aircraft engine particulate emissions are ticle emissions are not addressed in this report. characterized in the ICAO database using the smoke number, but this is a measurement of visibility and is only weakly cor- related with the mass characteristics relevant to air quality 1.3 How Are PM Emissions assessments. Quantified? GSE are commonly the second largest PM source at airports, Emissions from airport sources can be quantified by direct sometimes comparable to aircraft as a PM source. GSE are measurement using monitoring equipment or estimated mostly powered by diesel engines although smaller percent- using emission inventory methods. Historically, emissions ages have gasoline engines, and a still smaller percentage use inventory methods have been applied to assess the role of the electric power. The diesel and gasoline engines used by GSE