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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 H2 O 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 particulate matter from aircraft engine exhaust. How Does PM Affect Health? for ensuring these regulations do not pose conflicts with safety and other requirements especially for aircraft operations. This Coarse particles can be inhaled but tend to remain in the regulatory structure has developed over the past several nasal passage. Smaller particles are more likely to enter the res- decades. piratory system. Health studies have shown a significant asso- As a result of health and visibility concerns from PM, EPA ciation between exposure to fine and ultrafine particles and set the first NAAQS for PM in 1971. At the time, standards for premature death from heart or lung disease. Fine and ultrafine "total suspended particles" (TSP) were based on the mass- particles can aggravate heart and lung diseases and have been based concentration of particles between 25 and 45 m, which linked to effects such as cardiovascular symptoms, cardiac then was the state-of-the-art for particle samplers. The primary arrhythmias, heart attacks, respiratory symptoms, asthma (health-based) standard was set at 260 g per cubic meter of attacks, and bronchitis. These effects can result in increased ambient air, 24-hour average, not to be exceeded more than hospital admissions, emergency room visits, absences from once per year and 75 g/m3 annual average. A secondary school or work, and restricted activity days. Individuals (welfare-based) standard of 150 µg/m3, 24-hour average, not to that may be particularly sensitive to fine particle exposure be exceeded more than once per year was also established. The include people with heart or lung disease, older adults, and standards were revised in 1987 (moving from TSP to PM10), children. 1997 (adding PM2.5), and again in 2006. The 2006 standards set levels for PM10 of 150 µg/m3 for 24-hour average and for PM2.5 of 35 µg/m3 for 24-hour average and 15 µg/m3 annual average. How is PM Regulated The welfare-based secondary standards were made the same as in the United States? the primary standard in 2006. EPA no longer regulates parti- A wide range of regulatory provisions intended for cles larger than 10 m (e.g., sand and large dust) since they are environmental purposes applies to airport activity and equip- not deemed readily inhalable. Recent studies by EPA have ment. Aircraft engines have certification requirements for shown that PM2.5 cannot be used as a surrogate for ultrafine smoke emissions; ground access vehicles are subject to tailpipe particles, so future regulatory reviews may emphasize smaller emission standards; the composition of jet fuel, diesel fuel, and particles, possibly using PM1.0 as the regulatory standard. gasoline is regulated to limit harmful emissions; many op- The regulatory approach of the EPA sets standards for erational activities and equipment require operating permits; ambient air quality in geographic regions that generally rep- and airport construction and expansion plans are subject to resent metropolitan areas. The local PM concentration is the constraints where the regional air quality does not meet sum of all regional sources of PM and the regional ambient healthy standards. EPA sets most regulatory standards and background. The EPA estimates the annual average back- many are administered by state agencies. FAA is responsible ground for PM10 ranges from 4 to 8 µg/m3 in the western