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5 CHAPTER 3 Primer on Particulate Matter Emissions From Aviation This section presents basic information on particulate the lungs, with a more significant potential health impact matter (PM) emissions in general and aviation emissions compared to larger particles. Residence time in the air is also specifically. Research activities are described, as are regulatory dependent on size. Particle size also is a key determinant of requirements. Analytical tools that are used to analyze these visibility impacts. emissions are also described. Much of the general informa- Larger particles, those smaller than 10 m4 but larger than tion on particulate matter is adapted from U.S. EPA data and about 2.5 m, are referred to as coarse particles and typically information compiled in support of the National Ambient represent most of the mass included in PM10, the mass of Air Quality Standards (NAAQS) for particulate matter.1,2,3 particles smaller than 10 m. Particles between 2.5 m and 0.1 m are referred to as fine particles. A particle 2.5 m in di- ameter is approximately one-thirtieth the diameter of a human What is PM? hair. Particles below 0.1 m are considered ultrafine particles. Together, fine and ultrafine particles are represented as PM2.5, Particle pollution from fuel combustion is a mixture of mi- meaning all particles less than 2.5 m. croscopic solids, liquid droplets, and particles with solid and liquid components suspended in air. Solid particles are referred to as nonvolatile particles and liquid droplets are referred to as How is PM Formed? volatile particles. This pollution, also known as particulate matter, is made up of a number of components, including soot Different particle types tend to have different sources and or black carbon particles, inorganic acids (and their corre- formation mechanisms. Coarse particles around airports are sponding salts, such as nitrates and sulfates), organic chemicals generally primary particles from sources such as wind-blown from incomplete fuel combustion or from lubrication oil, dust, sea spray, sand or salt storage piles, construction activ- abraded metals, as well as PM present in the ambient air due to ity, or crushing or grinding operations (most commonly natural sources, such as soil or dust particles, and allergens associated with construction activity). Ultrafine particles can (such as fragments of pollen or mold spores). arise from a number of sources as well, including primary PM The diameters of particles in the ambient atmosphere span produced during combustion or newly nucleated (e.g., con- five orders of magnitude, ranging from 0.001 m (or 1 m) densed) particles formed in the atmosphere or in aircraft to 100 m. Larger particles, such as dust, soil, or soot, are plumes from condensable gases. Ultrafine particle emission often large or dark enough to be seen with the naked eye. sources at airports include various fuel combustion sources Others are so small they can only be detected using an elec- such as aircraft, auxiliary power units (APU), ground support tron microscope. Particle size is critical to the health effects it equipment (GSE), power turbines, diesel emergency genera- poses since smaller particles can be inhaled more deeply into tors, and vehicle traffic in and around the airport, as well as the atmospheric generation of new volatile particles from 1 Fine Particle (PM 2.5) Designations, Basic Information http://www.epa.gov/ condensation. Ultrafine particles in aircraft exhaust include a pmdesignations/basicinfo.htm. variety of particle types ranging from those that form in the 2 Particulate Matter, Basic Information http://www.epa.gov/oar/particlepollution/ combustor (carbon particles), to those that nucleate from basic.html. 3 Review of the National Ambient Air Quality Standards for Particulate Matter: 4In this paper, particle size descriptions refer to the aerodynamic diameter (see Policy Assessment of Scientific and Technical Information, December 2005, http://www.eap.gov/ttn/naaqu/standards/pm/data/pmstaffpaper_20051221.pdf. definition for "classical aerodynamic diameter" in glossary).
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6 condensable gases (sulfuric acid, partially burned fuel, and particles. The complex reactions that take place as a result of vaporized lubrication oil) and grow larger as a result of coag- nucleation, condensation, accumulation, and reaction illus- ulation and condensation onto the particle surfaces in the 0.1 trate why measuring PM emissions can be so complex. to 0.5 m range. Diesel particles from GSE and other ground Aircraft engine emission standards apply at the engine exit, vehicles tend to be larger than aircraft particles and aggregate yet PM of concern to regulators and the community is not into chain particles rather than the more spherical particles fully formed at that point. Figure 2 illustrates the evolution of seen from aircraft engines. The particles described here, primary and secondary particles. which are emitted directly from a source or form in the Ultrafine, fine, and coarse particles typically exhibit differ- immediate vicinity of the source, are referred to as primary ent behaviors in the atmosphere as the ambient residence particles or primary PM. Figure 1 illustrates the range of PM time of particles varies with size. Ultrafine particles have a commonly encountered. relatively short life, on the order of minutes to hours, and Secondary particle formation, which results from complex generally travel from less than a mile to less than 10 mi since chemical reactions in the atmosphere and/or particle nucle- they are likely to grow larger into fine particles. Fine particles ation processes, can produce either new particles or add to remain suspended longer in the atmosphere since they do not pre-existing particles. Examples of secondary particle forma- grow larger and are too small to readily settle out or impact tion include: (1) the conversion of sulfur oxides (SOx), which on stationary surfaces. They can be transported thousands of are produced by oxidation of the sulfur in fossil fuels, to miles and remain in the atmosphere from days to weeks. sulfuric acid (H2SO4) vapor, which then forms droplets as the Coarse particles can settle rapidly from the atmosphere with sulfuric acid condenses due to its low vapor pressure. The lifetimes ranging from minutes to hours (occasionally a few resulting sulfuric acid aerosol can further react with gaseous days) depending on their size, atmospheric conditions, and ammonia (NH3) in the atmosphere, for example, to form altitude. Large coarse particles are generally too large to various particles of sulfate salts (e.g., ammonium sulfate follow air streams and tend to settle out gravitationally and (NH4)2SO4); (2) the conversion of nitrogen dioxide (NO2) to by impacting onto stationary surfaces, rarely traveling more nitric acid (HNO3) vapor that interacts with PM in the than 10 mi. atmosphere, and reacts further with ammonia to form Fine and ultrafine particles suspended in the atmosphere ammonium nitrate (NH4NO3) particles; and (3) reactions absorb and reflect light, which is the major cause of reduced involving gaseous volatile organic compounds (VOC), yield- visibility (haze) in parts of the United States. Sulfates, ing condensable organic compounds that can also contribute nitrates, organic matter, and elemental carbon are primary to atmospheric particles, forming secondary organic aerosol components of these small particles. PM 10 PM 2.5 ultrafine particles fine particles coarse particles human hair nucleation accumulation large primary particle -like windblown dust or fly ash Gas diesel carbon particle phase Condensed gas aircraft carbon particle 0.001 0.010 0.100 1.00 10.0 100. Particle Diameter (micrometers) Figure 1. Particle size of airport PM emission.