Research Needs Associated with Particulate Emissions at Airports (2008)

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19 In addition to the airplane engines, aircraft can also emit PM from other on-board sources. These other aircraft sources include auxiliary power units (APUs) and aircraft brake and tire emissions during touchdown. Beyond the airplanes themselves, other sources also abound at any given airport, including ground support equipment (GSE) and a wide range of airport facility equipment for power generation, cooking, international waste incineration, construction, and so forth. Vehicles bringing passengers, freight, and equipment and supplies can also contribute to emissions associated with airport operations, and thus the sources associated with all of the roadside transport ac- tivity must also be accounted for when quantifying PM emissions. Knowledge • Particles in GSE exhaust are well characterized, especially with respect to mass, since the engines used in GSE are the same as those found in vehicles in construction, industrial, and commercial applications as well as in on-road vehicles. The GSE equipment market is too small for purpose-built engines so production engines from manufacturers are used. Particulate matter testing, measurement, and charac- terization conducted by EPA and others effectively describe the particles found in GSE exhaust. • The EPA’s emission standards for nonroad equipment apply to GSE. Emission factors for these vehicles are estimated by using EPA’s NONROAD model. • Federal regulations requiring the use of ultra-low sulfur diesel in nonroad equipment are being phased in and will apply to GSE. These regulations will significantly reduce PM emissions from GSE. • Diesel particulate filters are available for use on equipment fueled with ultra-low sulfur diesel which, when combined with the fuel sulfur reduction, can result in a 90% reduc- tion in PM emissions. • The EPA’s standards for on-road vehicles apply to ground access vehicles. Emission factors from on-road vehicles are estimated by using EPA’s MOBILE 6 model. Fleet charac- teristics such as vehicle mix and age, as well as operating cycles, are well defined and embedded in the MOBILE model. • Most ground access vehicles are fueled with gasoline, which produces very little PM. • For ground access vehicles fueled with diesel, ultra-low sulfur diesel fuel is being phased in and diesel particulate filters will be required for on-road use. This will substan- tially reduce PM emissions from diesel-fueled ground access vehicles. • Airport facility equipment emissions are often defined by EPA standards. Emission factors for stationary sources can be found in a variety of sources, most commonly from AP-4222 and Air Quality Procedures for Civilian Airports and Air Force Bases (The Air Quality Handbook).23 Emission factors for most common stationary equipment at airports are contained in EDMS. Gaps • GSE operational practices are not well characterized. GSE spend much of their operational time idling and only lim- ited periods under load. For example, a baggage tug may be idling while awaiting the arrival of an aircraft at a gate. It is then positioned adjacent to the aircraft where it sits at idle. Once bags are loaded on or unloaded from the air- craft, the tug moves back to the terminal area and may sit at idle. Operating practices vary considerably from airport to airport. C H A P T E R 6 Particulate Matter From Other Airport Sources 22 Environmental Protection Agency, AP 42, Compilation of Air Pollutant Emis- sion Factors, http://www.epa.gov/otaq/ap42.htm. 23 Federal Aviation Administration, United States Air Force, Air Quality Proce- dures for Civilian Airports & Air Force Bases, April 1997.

• Populations and age, which are needed to reliably quantify GSE emissions, are often difficult to determine. Many air- port environmental studies use estimates of number of ve- hicles and engine run time needed to service an aircraft; however, these estimates are not well documented and again vary considerably from airport to airport. This lack of consistent, reliable GSE emission estimation procedures is reflected in the variation seen in airport emission inven- tories, where GSE PM emissions can vary from less than 15% to more than 50% of the inventory total. • Minimal PM data are available for APU emissions. APUs are essentially small jet engines and it is expected their emissions would be similar to those of the aircraft main engines; how- ever, APU have different operating cycles. They typically operate at three power settings: no load, environmental control condition,24 and engine start condition.25 • Fuel consumption for APUs is unknown, however, they use much less fuel than the main aircraft engines, and their emissions are expected to be proportionately less. • For the purpose of developing PM inventories, lack of knowledge on APU usage (duration and rate), even once PM emissions data might become available, remains a gap. • All gaps for aircraft engine PM, both nonvolatile and volatile, also apply to APUs since very little PM measure- ment work has been performed on APUs. • The magnitude of the contribution to airport PM invento- ries from APU’s use is currently highly uncertain although expected to be relatively small. • There is currently no data on PM number, size, and mass for brake and tire emissions from landing aircraft. LIDAR data collected in the UK suggest that this could be a signif- icant PM source at airports. While it is evident that aircraft leave a great deal of rubber on the runway in the vicinity of touchdown, it is also evident that a puff of smoke or par- ticulate matter is created during touchdown. • The relationships between aircraft brake and tire emissions and brake and tire emissions from roadside vehicles (and GSE) are not well defined; however, their use would not be expected to cause similar brake and tire wear compared to landing aircraft. The materials and usage patterns are also quite different for aircraft and road vehicles. • The contribution to airport PM inventories from aircraft brakes and tires is highly uncertain at present. • The level of detail on PM emissions in non–aircraft-engine emission sources is not commensurate with that being acquired for aircraft. While number, size, mass, and com- position as a function of engine operating condition is being acquired for aircraft engines, often average numbers are used for other emissions sources. Understanding num- ber, size, and composition may be less important for compiling emission inventories than for understanding the health effects of these PM sources. • Due to various data limitations, we currently lack a means to estimate the relative contributions from aircraft and other PM sources to airport PM inventories that reflect actual, source-specific PM data and source operational procedures. Where airport PM inventories have been reported, it is not clear that consistent, reliable methodologies, equipment counts, and appropriate operating cycles were used for each emissions source. 20 24 This is an intermediate power level needed to maintain the aircraft cabin’s envi- ronmental control system including the air conditioning and ventilation system. 25 This is essentially maximum power needed to start the main aircraft engines.