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5 CHAPTER 1 Primer on Particulate Matter Emissions from Aviation This section presents general information on PM emissions impact compared to larger particles. Residence time in the air with particular attention to the aircraft source. Analytical tools, is also dependent on size. Particle size also is a key determi- research activities, and regulatory requirements are described. nant of visibility impacts. Much of the general information on PM is paraphrased from Particles smaller than 10 m (note: in this report, particle U.S. EPA data, information compiled in support of the size descriptions refer to the aerodynamic diameter; see defi- National Ambient Air Quality Standards (NAAQS) for PM, nition for "classical aerodynamic diameter" in Appendix B, and ACRP Report 6: Research Needs Associated with Particu- Glossary of Terms) but larger than about 2.5 m are referred late Emissions at Airports (U.S. EPA Oct 30, 2007; U.S. EPA to as coarse particles and typically represent most of the mass Mar 6, 2007; U.S. EPA 2005; Webb et al. 2008). included in PM10, the mass of 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 diameter is approximately one- 1.1 What Is PM? thirtieth the diameter of a human hair. Particles below 0.1 m Particle pollution from fuel combustion is a mixture of are considered ultrafine particles. Together, fine and ultrafine microscopic solids, liquid droplets, and particles with solid and particles are represented as PM2.5, meaning all particles less liquid components suspended in air. Particles are frequently than 2.5 m. designated as volatile or non-volatile. Volatile particles are those that may evaporate if their surrounding conditions 1.2 How and Where Is PM Formed change--for example, if the temperature is increased. Water at an Airport? droplets are a common example of a volatile particle. Non-volatile particles are those that remain in a condensed Different particle types tend to have different sources and phase even when their ambient conditions vary widely. Soot formation mechanisms. There are many individual PM emis- is a common example of a non-volatile particle. Particulate sion sources at airports. These include the following: matter emissions are made up of a number of components, including soot or black carbon particles, inorganic acids · Aircraft engines; (and their corresponding salts, such as nitrates and sulfates), · Aircraft auxiliary power units (APU); organic chemicals from incomplete fuel combustion or from · Ground support equipment (GSE); lubrication oil, abraded metals, as well as PM present in the · Passenger vehicles; ambient air due to natural sources, such as soil or dust particles, · Tire and brake wear; and allergens (such as fragments of pollen or mold spores). · Stationary power turbines; The diameters of particles in the ambient atmosphere span · Training fires; five orders of magnitude, ranging from 0.001 m (or 1 nm) · Sand and salt piles; to 100 m. Dust, soil, or soot particles are often large or dark · Construction grading and earth moving; and enough to be seen with the naked eye. Others are so small they · Some food preparation ovens (e.g., charbroilers). can only be detected using an electron microscope. Particle size is important since smaller particles can be inhaled more Particulate matter emissions from each of these sources deeply into the lungs, with a more significant potential health are different in terms of size, composition, and rate. Coarse
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6 particles are generally primary particles from sources such preexisting particles. Examples of secondary particle forma- as wind-blown dust, sea spray, sand or salt storage piles, tion include the following: construction activity, or crushing or grinding operations (most commonly associated with construction activity). · Conversion of sulfur oxides (SOx), which are produced by Ultrafine particles arise from primary PM produced during oxidation of the sulfur in fossil fuels, to sulfuric acid combustion (carbon particles), or newly nucleated or con- (H2SO4) vapor, which then forms droplets as the sulfuric densed particles formed in the atmosphere and in aircraft acid nucleates due to its low vapor pressure--the result- plumes from gaseous emissions (sulfuric acid, partially ing sulfuric acid aerosol can further react with gaseous burned fuel, and vaporized lubrication oil). Ultrafine par- ammonia (NH3), for example, in the atmosphere to form ticle sources at airports include the exhaust from various various particles of sulfate salts, such as ammonium sulfate fuel combustion sources such as aircraft, APU, GSE, power (NH4)2SO4; turbines, diesel emergency generators, and vehicle traffic · Conversion of nitrogen dioxide (NO2) to nitric acid (HNO3) in and around the airport, as well as the atmospheric gen- vapor that interacts with PM in the atmosphere, and re- eration of new volatile particles from nucleation. Ultrafine acts further with ammonia to form ammonium nitrate particles grow larger as a result of coagulation and conden- (NH4NO3) particles; and sation onto the particle surfaces in the 0.1 to 0.5 m range. · Reactions involving gaseous volatile organic compounds Diesel particles from GSE and other ground vehicles tend (VOC), yielding condensable organic compounds that also to be larger than aircraft particles and aggregate into chain can contribute to atmospheric particles, forming secondary particles rather than the more spherical particles seen from organic aerosol particles. aircraft engines. Particles emitted directly from a source or formed in the immediate vicinity, are referred to as pri- The complex reactions that take place as a result of nucle- mary PM. Figure 1 illustrates the range of PM commonly ation, condensation, accumulation, and reaction illustrate encountered. why measuring PM emissions can be so complex. Aircraft Figure 2 illustrates the evolution of primary particles. engine emission standards apply at the engine exit, yet PM of Particle illustrations are not accurate to comparative size; the concern to regulators and the community is not fully formed horizontal axis showing diameter is logarithmic. at that point. Secondary particle formation, which results from complex Ultrafine, fine, and coarse particles typically exhibit differ- chemical reactions in the atmosphere and/or particle nucle- ent behaviors in the atmosphere since the ambient residence ation processes, can produce either new particles or add to time of particles varies with size. Ultrafine particles are likely 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 10.0 100.0 Particle Diameter (micrometers) Figure 1. Particle size of airport PM emissions.