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9 inventory methods. Historically for airport sources, emis- as an approach to estimate emissions based on smoke num- sions inventory methods have been most prevalent. These ber, a measure of soot obscuration in aircraft plumes. More methods generally require information about each source's recently, FAA, NASA, EPA and others funded a series of air- population, size, activity rate, and a PM emission factor or craft engine emission measurement programs known as emission index. An emissions factor is a representative value APEX (Aircraft Particle Emissions eXperiment). The infor- that attempts to relate the quantity of a pollutant released to mation from the first APEX1 tests, initially published in the atmosphere with an activity associated with the release of 2006, is basic, fundamental data on the quantity and char- that pollutant. These factors are usually expressed as the acteristics of PM from a single engine type. The JETS- weight of pollutant divided by a unit weight, volume, dis- APEX2 study, from which a report has been released by the tance, or duration of the activity emitting the pollutant (e.g., California Air Resources Board (CARB), and APEX3, from milligrams of particulate emitted per kilogram of fuel which a report is to be released soon, cover a range of burned). Such factors make it easier to estimate emissions commercial engines, but the data are still limited relative to from various sources of air pollution. the entire fleet. In some cases, these factors are simply averages of all avail- Another initiative organized to help close the knowledge able data of acceptable quality, and are generally assumed to gap on aviation PM emissions is the National PM Roadmap be representative of long-term averages for all facilities in the for Aviation. It is a PM research collaboration among federal source category (i.e., a population average). EPA maintains a agencies (e.g., FAA, NASA, DOT, DOD, and EPA), universi- reference10 of emission factors for many sources. In other ties, aircraft and engine manufacturers, airports, airlines, and cases, specific emission factors are compiled for each emis- other stakeholders. It was organized in 2004 to coordinate sion source. For example, gaseous emission factors specifi- aviation PM research and leverage limited resources to focus cally for aircraft are included in the ICAO Aircraft Engine on the most important research needs. Emissions Data Bank.11 Unfortunately, PM emission factors Recently, the ACRP funded this study of aviation PM for aircraft, the largest PM source at airports, are not included emissions and a second study (ACRP 02-04A), which is an in the Emissions Data Bank. Aircraft engine particulate emis- assessment of the data from the APEX tests. ACRP initiatives sions have not been well studied or characterized in the past should help bring needed focus to airport-specific PM emis- and are only now being tested. Smoke number data are in the sion concerns. ICAO databank, but are only surrogates for PM emissions via the First Order Approximation (FOA) (see below). Why are Aviation-Related PM Issues The second largest PM source at airports is commonly GSE, so Important to Airport Operators? sometimes comparable to aircraft as a PM source. Ground sup- port equipment is mostly powered by diesel engines although Airports today are faced with community, employee, and a smaller percentage have gasoline engines and a smaller per- regulatory concerns about PM emissions, yet they have very centage still use electric power. The diesel and gasoline engines limited data on PM emissions from aircraft engines and used by GSE are common engine types found in trucks and APUs, data on other sources varies in quality and availability, other industrial vehicles. Particulate matter emissions from and only limited data are available on ambient PM around these engines are well characterized for mass of emissions; airports. Newly tightened ambient air quality standards and however, in emission factor references, GSE is typically lumped greater health and environmental concerns present hurdles into a diverse set of equipment referred to as nonroad vehicles. for airports as they need to modernize and expand to meet These also include lawn and garden equipment, agricultural the increasing demand for air transportation. Yet airports equipment, commercial marine vessels, recreational equip- represent only one PM emission source category among ment, and other vehicle types. This makes it difficult to com- many in a region. pute PM inventories that reflect airport-specific emissions. In addition to complying with general conformity require- ments and assisting states in complying with national ambi- What are the Most Recent Aviation ent air quality standards, airports must address complaints PM Research Efforts? from communities and employees who are concerned about health impacts resulting from exposure to airport emissions. To remedy the lack of information about PM emissions Many airports also receive complaints about deposits of soot, from aircraft, several initiatives have been pursued in the grit, and the oily residue that airport neighbors find on their last few years. The FAA developed the FOA, initially in 2002, cars and outdoor furniture, which the complainants believe must come from airport activity. 10AP-42, 11 ICAO Aircraft Engine Emissions Data Bank Several airports have conducted particle deposition stud- default.aspx? catid=702&pagetype=90. ies in nearby and adjacent communities to evaluate whether