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8 are common engine types found in trucks and other industrial revised in 1987 (moving from TSP to PM10), 1997 (adding vehicles. Particulate matter mass emissions from these engines PM2.5), and again in 2006. The 2006 standards set levels for are well characterized, however, in emission factor references PM10 of 150 g/m3 for 24-hr average and PM2.5 of 35 g/m3 GSE are typically lumped into a diverse set of equipment re- for 24-hr average and 15 g/m3 annual average. The welfare- ferred to as "nonroad vehicles." These also include lawn and based secondary standards were made the same as the pri- garden equipment, agricultural equipment, commercial ma- mary standard in 2006. The EPA no longer regulates particles rine vessels, recreational equipment, and other vehicle types. larger than 10 m (e.g., sand and large dust) since they are not Although research reports include information about diesel deemed readily inhalable. Recent studies by EPA have shown engine emissions, without having an emission factor reference that PM2.5 cannot be used as a surrogate for ultrafine particles, that relates specifically to GSE, it is difficult to compute PM so future regulatory reviews may emphasize smaller particles, inventories that reflect airport-specific emissions. possibly using PM1.0 or PM0.1 as the regulatory standard. The regulatory approach of the EPA sets standards for ambient air quality in geographic regions that generally rep- 1.4 How Is PM Regulated resent metropolitan areas. The local PM concentration is the in the United States? sum of all regional sources of PM and the regional ambient The EPA establishes the National Ambient Air Quality Stan- background. The EPA estimates the annual average back- dards (NAAQS), which limit the concentration of select pollut- ground for PM10 ranges from 4 to 8 g/m3 in the western ants in the outside air. The Clean Air Act requires the EPA to United States and 5 to 11 g/m3 in the eastern United States; set the NAAQS at levels that protect (1) the public health with for PM2.5, estimates range from 1 to 4 g/m3 in the west to 2 to an adequate margin of safety (the primary NAAQS); and 5 g/m3 in the east. Particulate matter emissions from air- (2) the public welfare from any known or anticipated adverse ports and other regional sources mix relatively quickly, on effects (the secondary NAAQS). Particulate matter is one of the timescale of minutes to hours, with the ambient back- the criteria pollutants regulated through the NAAQS. ground PM. The combination of emissions from airports Particulate matter emissions affect health and visibility and and other regional sources and ambient concentrations of PM these issues underlie regulation in the United States. Coarse result in a combined atmospheric PM loading that depends particles can be inhaled but tend to remain in the nasal passage. on complex, nonlinear atmospheric processes, including Smaller particles are more likely to enter the respiratory system. chemical reactions and pollution transport. This makes it dif- Health studies have shown a significant association between ficult to isolate the contribution of airport activity from all exposure to fine and ultrafine particles and premature death other emissions sources in an area. from heart or lung disease. Fine and ultrafine particles can A wide range of regulatory provisions intended for envi- aggravate heart and lung diseases and have been linked to ronmental purposes apply to mobile sources, including those effects such as cardiovascular symptoms, cardiac arrhythmias, that operate at an airport. Aircraft engines have certification heart attacks, respiratory symptoms, asthma attacks, and requirements for smoke emissions; ground access vehicles are bronchitis. These effects can result in increased hospital ad- subject to tailpipe emission standards; the composition of missions, emergency room visits, absences from school or jet fuel, diesel fuel, and gasoline are regulated; many opera- work, and restricted activity days. Individuals that may be tional activities and equipment require operating permits; particularly sensitive to fine particle exposure include people and federal airport actions (such as construction) are subject with heart or lung disease, older adults, and children. Com- to the general conformity regulations in locations where the prehensive summaries of PM health effects can be found in regional air quality does not meet health standards. The EPA EPA documentation of their periodic NAAQS review. As of sets many such regulatory standards under the Clean Air Act, April 1, 2008, these documents were available at http://www. and many regulatory programs are administered by state agen- cies to which EPA delegates authority. The FAA is responsi- As a result of health and visibility concerns from PM, EPA ble for ensuring these regulations do not pose conflicts with set the first NAAQS for PM in 1971. At the time, standards for safety and other requirements especially for aircraft opera- total suspended particles (TSP) were based on the mass con- tions. This regulatory structure has developed over the past centration of particles between 25 and 45 m, which was the several decades. then state of the art for particle samplers. The primary (health- In addition to the NAAQS, other regulations directly or in- based) standard was set at 260 g/m3 of ambient air, 24-hr directly effect PM emissions from aviation. For example, the average, not to be exceeded more than once per year and ICAO has established aircraft engine certification standards 75 g/m3 annual average. A secondary (welfare-based) stan- (ICAO 1993) that have been adopted in the United States as dard of 150 g/m3, 24-hr average, not to be exceeded more federal regulations. The FAA, in turn, monitors and enforces than once per year was also established. The standards were engine certification.