Summarizing and Interpreting Aircraft Gaseous and Particulate Emissions Data (2008)

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5This section presents general information on PM emissions with particular attention to the aircraft source. Analytical tools, research activities, and regulatory requirements are described. Much of the general information on PM is paraphrased from U.S. EPA data, information compiled in support of the National Ambient Air Quality Standards (NAAQS) for PM, and ACRP Report 6: Research Needs Associated with Particu- late Emissions at Airports (U.S. EPA Oct 30, 2007; U.S. EPA Mar 6, 2007; U.S. EPA 2005; Webb et al. 2008). 1.1 What Is PM? Particle pollution from fuel combustion is a mixture of microscopic solids, liquid droplets, and particles with solid and liquid components suspended in air. Particles are frequently designated as volatile or non-volatile. Volatile particles are those that may evaporate if their surrounding conditions change—for example, if the temperature is increased. Water droplets are a common example of a volatile particle. Non-volatile particles are those that remain in a condensed phase even when their ambient conditions vary widely. Soot is a common example of a non-volatile particle. Particulate matter emissions are made up of a number of components, including soot or black carbon particles, inorganic acids (and their corresponding salts, such as nitrates and sulfates), organic chemicals from incomplete fuel combustion or from lubrication oil, abraded metals, as well as PM present in the ambient air due to natural sources, such as soil or dust particles, and allergens (such as fragments of pollen or mold spores). The diameters of particles in the ambient atmosphere span five orders of magnitude, ranging from 0.001 μm (or 1 nm) to 100 μm. Dust, soil, or soot particles are often large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope. Particle size is important since smaller particles can be inhaled more deeply into the lungs, with a more significant potential health impact compared to larger particles. Residence time in the air is also dependent on size. Particle size also is a key determi- nant of visibility impacts. Particles smaller than 10 μm (note: in this report, particle size descriptions refer to the aerodynamic diameter; see defi- nition for “classical aerodynamic diameter” in Appendix B, Glossary of Terms) but larger than about 2.5 μm are referred to as coarse particles and typically represent most of the mass included in PM10, the mass of particles smaller than 10 μm. Particles between 2.5 μm and 0.1 μm are referred to as fine particles. A particle 2.5 μm in diameter is approximately one- thirtieth the diameter of a human hair. Particles below 0.1 μm are considered ultrafine particles. Together, fine and ultrafine particles are represented as PM2.5, meaning all particles less than 2.5 μm. 1.2 How and Where Is PM Formed at an Airport? Different particle types tend to have different sources and formation mechanisms. There are many individual PM emis- sion sources at airports. These include the following: • Aircraft engines; • Aircraft auxiliary power units (APU); • Ground support equipment (GSE); • Passenger vehicles; • Tire and brake wear; • Stationary power turbines; • Training fires; • Sand and salt piles; • Construction grading and earth moving; and • Some food preparation ovens (e.g., charbroilers). Particulate matter emissions from each of these sources are different in terms of size, composition, and rate. Coarse C H A P T E R 1 Primer on Particulate Matter Emissions from Aviation

particles are generally primary particles from sources such as wind-blown dust, sea spray, sand or salt storage piles, construction activity, or crushing or grinding operations (most commonly associated with construction activity). Ultrafine particles arise from primary PM produced during combustion (carbon particles), or newly nucleated or con- densed particles formed in the atmosphere and in aircraft plumes from gaseous emissions (sulfuric acid, partially burned fuel, and vaporized lubrication oil). Ultrafine par- ticle sources at airports include the exhaust from various fuel combustion sources such as aircraft, APU, GSE, power turbines, diesel emergency generators, and vehicle traffic in and around the airport, as well as the atmospheric gen- eration of new volatile particles from nucleation. Ultrafine particles grow larger as a result of coagulation and conden- sation onto the particle surfaces in the 0.1 to 0.5 μm range. Diesel particles from GSE and other ground vehicles tend to be larger than aircraft particles and aggregate into chain particles rather than the more spherical particles seen from aircraft engines. Particles emitted directly from a source or formed in the immediate vicinity, are referred to as pri- mary PM. Figure 1 illustrates the range of PM commonly encountered. Figure 2 illustrates the evolution of primary particles. Particle illustrations are not accurate to comparative size; the horizontal axis showing diameter is logarithmic. Secondary particle formation, which results from complex chemical reactions in the atmosphere and/or particle nucle- ation processes, can produce either new particles or add to preexisting particles. Examples of secondary particle forma- tion include the following: • Conversion of sulfur oxides (SOx), which are produced by oxidation of the sulfur in fossil fuels, to sulfuric acid (H2SO4) vapor, which then forms droplets as the sulfuric acid nucleates due to its low vapor pressure—the result- ing sulfuric acid aerosol can further react with gaseous ammonia (NH3), for example, in the atmosphere to form various particles of sulfate salts, such as ammonium sulfate (NH4)2SO4; • Conversion of nitrogen dioxide (NO2) to nitric acid (HNO3) vapor that interacts with PM in the atmosphere, and re- acts further with ammonia to form ammonium nitrate (NH4NO3) particles; and • Reactions involving gaseous volatile organic compounds (VOC), yielding condensable organic compounds that also can contribute to atmospheric particles, forming secondary organic aerosol particles. The complex reactions that take place as a result of nucle- ation, condensation, accumulation, and reaction illustrate why measuring PM emissions can be so complex. Aircraft engine emission standards apply at the engine exit, yet PM of concern to regulators and the community is not fully formed at that point. Ultrafine, fine, and coarse particles typically exhibit differ- ent behaviors in the atmosphere since the ambient residence time of particles varies with size. Ultrafine particles are likely 6 0.001 0.010 0.100 1.00 10.0 100.0 aircraft carbon particle nucleation accumulation ultrafine particles large primary particle -like windblown dust or fly ash fine particles coarse particles PM 2.5 PM 10 diesel carbon particle Particle Diameter (micrometers) human hair Gas phase Condensed gas Figure 1. Particle size of airport PM emissions.

to grow into fine particles on the order of minutes to hours, typically traveling less than 10 mi. Fine particles remain suspended in the atmosphere since they do not grow larger and are too small to readily settle out or impact on stationary surfaces. They can be transported thousands of miles and remain in the atmosphere from days to weeks. Coarse parti- cles can settle rapidly from the atmosphere, and have life- times ranging from minutes to hours (occasionally, a few days) depending on their size, atmospheric conditions, and alti- tude. Large coarse particles are generally too large to follow air streams and tend to settle out gravitationally onto sta- tionary surfaces, rarely traveling more than 10 mi. Fine and ultrafine particles suspended in the atmosphere absorb and reflect light, which is the major cause of reduced visibility (haze) in parts of the United States. Sulfates, nitrates, organic matter, and elemental carbon are the primary com- ponents of these small particles. Particles emitted at cruise altitude may contribute to global climate change effects; however, since these particles are emitted beyond the airport environment and were therefore outside of the scope of the tests being summarized in this report, these cruise-level par- ticle emissions are not addressed in this report. 1.3 How Are PM Emissions Quantified? Emissions from airport sources can be quantified by direct measurement using monitoring equipment or estimated using emission inventory methods. Historically, emissions inventory methods have been applied to assess the role of the airport source in determining air quality. Inventory methods generally require information about each source’s population, size, activity rate, and a PM emission factor or emission index. An emission factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. These factors are usually expressed as the weight of the pollut- ant divided by a unit weight, volume, distance, or duration of the activity emitting the pollutant (e.g., milligrams of partic- ulate emitted per kilogram of fuel burned). In some cases, emission factors are simply averages of all available data of acceptable quality, and are generally assumed to be representative of long-term averages for all facilities in the source category (i.e., a population average). The EPA maintains a reference (U.S. EPA 2008) of emission factors for many sources. In other cases, specific emission factors are compiled for each emission source. For example, regulated gaseous emission factors for aircraft engines are included in the International Civil Aviation Organization (ICAO) Aircraft Engine Emissions DataBank (ICAO 2008) but PM are not similarly regulated. Aircraft engine particulate emissions are characterized in the ICAO database using the smoke number, but this is a measurement of visibility and is only weakly cor- related with the mass characteristics relevant to air quality assessments. GSE are commonly the second largest PM source at airports, sometimes comparable to aircraft as a PM source. GSE are mostly powered by diesel engines although smaller percent- ages have gasoline engines, and a still smaller percentage use electric power. The diesel and gasoline engines used by GSE 7 ambient mixing/dilution coated soot particles ~30-100 nm diameter condensation H2O H2SO4HCs accumulation binary collision nucleation nucleation/accumulation mode particles <20 nm diametersoot activation/scavenging aircraft exhaust soot particles chemistry hydrocarbons (HCs) SO3 H2SO4 H2O H2O 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 PM from aircraft engine exhaust.

are common engine types found in trucks and other industrial vehicles. Particulate matter mass emissions from these engines are well characterized, however, in emission factor references GSE are typically lumped into a diverse set of equipment re- ferred to as “nonroad vehicles.” These also include lawn and garden equipment, agricultural equipment, commercial ma- rine vessels, recreational equipment, and other vehicle types. Although research reports include information about diesel engine emissions, without having an emission factor reference that relates specifically to GSE, it is difficult to compute PM inventories that reflect airport-specific emissions. 1.4 How Is PM Regulated in the United States? The EPA establishes the National Ambient Air Quality Stan- dards (NAAQS), which limit the concentration of select pollut- ants in the outside air. The Clean Air Act requires the EPA to set the NAAQS at levels that protect (1) the public health with an adequate margin of safety (the primary NAAQS); and (2) the public welfare from any known or anticipated adverse effects (the secondary NAAQS). Particulate matter is one of the criteria pollutants regulated through the NAAQS. Particulate matter emissions affect health and visibility and these issues underlie regulation in the United States. Coarse particles can be inhaled but tend to remain in the nasal passage. Smaller particles are more likely to enter the respiratory system. Health studies have shown a significant association between exposure to fine and ultrafine particles and premature death from heart or lung disease. Fine and ultrafine particles can aggravate heart and lung diseases and have been linked to effects such as cardiovascular symptoms, cardiac arrhythmias, heart attacks, respiratory symptoms, asthma attacks, and bronchitis. These effects can result in increased hospital ad- missions, emergency room visits, absences from school or work, and restricted activity days. Individuals that may be particularly sensitive to fine particle exposure include people with heart or lung disease, older adults, and children. Com- prehensive summaries of PM health effects can be found in EPA documentation of their periodic NAAQS review. As of April 1, 2008, these documents were available at http://www. epa.gov/ttn/naaqs/standards/pm/s_pm_2006.html. As a result of health and visibility concerns from PM, EPA set the first NAAQS for PM in 1971. At the time, standards for total suspended particles (TSP) were based on the mass con- centration of particles between 25 and 45 μm, which was the then state of the art for particle samplers. The primary (health- based) standard was set at 260 μg/m3 of ambient air, 24-hr average, not to be exceeded more than once per year and 75 μg/m3 annual average. A secondary (welfare-based) stan- dard of 150 μg/m3, 24-hr average, not to be exceeded more than once per year was also established. The standards were revised in 1987 (moving from TSP to PM10), 1997 (adding PM2.5), and again in 2006. The 2006 standards set levels for PM10 of 150 μg/m3 for 24-hr average and PM2.5 of 35 μg/m3 for 24-hr average and 15 μg/m3 annual average. The welfare- based secondary standards were made the same as the pri- mary standard in 2006. The EPA no longer regulates particles larger than 10 μm (e.g., sand and large dust) since they are not deemed readily inhalable. Recent studies by EPA have shown that PM2.5 cannot be used as a surrogate for ultrafine particles, so future regulatory reviews may emphasize smaller particles, 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- resent metropolitan areas. The local PM concentration is the sum of all regional sources of PM and the regional ambient background. The EPA estimates the annual average back- ground for PM10 ranges from 4 to 8 μg/m3 in the western United States and 5 to 11 μg/m3 in the eastern United States; for PM2.5, estimates range from 1 to 4 μg/m3 in the west to 2 to 5 μg/m3 in the east. Particulate matter emissions from air- ports and other regional sources mix relatively quickly, on the timescale of minutes to hours, with the ambient back- ground PM. The combination of emissions from airports and other regional sources and ambient concentrations of PM result in a combined atmospheric PM loading that depends on complex, nonlinear atmospheric processes, including chemical reactions and pollution transport. This makes it dif- ficult to isolate the contribution of airport activity from all other emissions sources in an area. A wide range of regulatory provisions intended for envi- ronmental purposes apply to mobile sources, including those that operate at an airport. Aircraft engines have certification requirements for smoke emissions; ground access vehicles are subject to tailpipe emission standards; the composition of jet fuel, diesel fuel, and gasoline are regulated; many opera- tional activities and equipment require operating permits; and federal airport actions (such as construction) are subject to the general conformity regulations in locations where the regional air quality does not meet health standards. The EPA sets many such regulatory standards under the Clean Air Act, and many regulatory programs are administered by state agen- cies to which EPA delegates authority. The FAA is responsi- ble for ensuring these regulations do not pose conflicts with safety and other requirements especially for aircraft opera- tions. This regulatory structure has developed over the past several decades. In addition to the NAAQS, other regulations directly or in- directly effect PM emissions from aviation. For example, the ICAO has established aircraft engine certification standards (ICAO 1993) that have been adopted in the United States as federal regulations. The FAA, in turn, monitors and enforces engine certification. 8

Certification standards that limit smoke emissions, as measured by smoke number, indirectly influence aircraft PM emissions since smoke is a component of total PM. Limits for oxides of nitrogen (NOx) from jet engines have also been established. These limit the amount of NOx emitted, which can produce nitrates that condense in the atmosphere hours to days after emissions contributing to secondary volatile particles. Sulfur emissions are directly related to the sulfur content of the fuel. Internationally accepted standards for Jet A (ASTM D 1655-04a May 2005), which is the commercial aviation fuel used in the United States, limit fuel sulfur content to 0.30% weight maximum. In practice, however, Jet A sulfur content ranges between 0.04 and 0.06% weight (Penner et al. 1999), although lower sulfur jet fuels are now sometimes being seen as diesel fuel sulfur levels drop. Nonroad diesel equipment, such as GSE, are not required to have emission controls like diesel vehicles licensed for on- road use. Under new national regulations, EPA requires diesel fuel suppliers for nonroad equipment to reduce fuel sulfur content, eventually to the same ultra-low sulfur limits re- quired for on-road diesel. This will influence the introduction of advanced emission control technologies for nonroad equip- ment, which may be a requirement for these vehicles in the future. Requirements for diesel fuel sulfur limits and engine emission standards are being phased in between now and 2014. Reducing the fuel sulfur content and adding emission controls is expected to reduce PM emissions from nonroad equipment by 90% (PM is emitted during electricity genera- tion at the power plant, however, utility power production is well controlled compared to internal combustion engines and the net result is fewer PM emissions). GSE using alternative fuels, such as compressed natural gas, propane, or electricity (U.S. EPA Jul 2004), have very little or no PM emissions. Stationary emission sources at airports include various facilities and equipment like boilers, emergency generators, incinerators, fire training facilities, fuel storage tanks, and food preparation. Many of these equipment types require specific operating permits with PM emission limits. Stationary sources typically represent about 1% of PM emissions at airports. The National Environmental Policy Act of 1969 (NEPA) established a policy to protect the quality of the human envi- ronment and requires careful scrutiny of the environmental impacts of federal actions, which could include grants, loans, leases, permits, and other decisions or actions requiring federal review or approval. For airports, NEPA applies to most major construction projects as a result of FAA funding or approval. Required by NEPA is the consideration of emissions associ- ated with a project and identification of the project-related effects as being significant if the project would result in an exceedance of the ambient air quality standards. When un- dertaking a federal action, before the federal agency can approve the action, it must first be shown to conform with the state implementation plan, the state’s plan for assuring compliance with the health-based air quality standards. Con- formity requires the federal agency to show that the project will not create a new exceedance of the standard or exacerbate any existing exceedances. The FAA must demonstrate con- formity at airports located in a maintenance or nonattainment area for PM10 or PM2.5. 1.5 What Are the Most Recent Aviation PM Research Efforts? To remedy the lack of information about PM emissions from aircraft, several initiatives have been pursued in the last few years. The FAA developed the First Order Approximation (FOA), initially in 2002, as an approach to estimate emissions based on smoke number, a measure of soot obscuration in aircraft plumes. Recently, NASA, EPA, FAA, California Air Resources Board (CARB), and others funded a series of aircraft engine emission measurement programs known as APEX (Aircraft Particle Emissions eXperiment). The information from the first APEX1 tests, initially published in 2006, is basic, fundamental data on the quantity and characteristics of PM from a single engine type. The JETS-APEX2 study, from which a report has been released by CARB, and APEX3, from which a report is to be released soon, cover a range of commercial engines, but the data are still limited, relative to the entire fleet. This report describes the findings of these emission measurement programs in detail. Another initiative organized to help close the knowledge gap on aviation PM emissions is the National PM Roadmap for Aviation, a research collaboration among federal agencies (e.g., FAA, NASA, U.S.DOT, the Department of Defense [DOD], and EPA), universities, aircraft and engine manufac- turers, airports, airlines, and other stakeholders, that organ- ized in 2004 to coordinate aviation PM research and leverage limited resources. Recently, the scope of this initiative has been expanded to include other emissions and has been re- named Aircraft Emissions Characterization (AEC) Roadmap. Recently, ACRP published a study entitled ACRP Report 6: Research Needs Associated with Particulate Emissions at Air- ports and, now, this report. Such ACRP initiatives help bring needed focus to airport-specific PM emission concerns. 1.6 Why Are Aviation-Related PM Issues Important to Airport Operators? In addition to complying with general conformity require- ments and assisting states in complying with NAAQS, airports must address complaints from communities and employees 9

who are concerned about health impacts resulting from exposure to airport emissions. Many airports also receive complaints about deposits of soot, grit, and the oily residue that airport neighbors find on their cars and outdoor furni- ture, which the complainants believe must come from airport activity. However, airports have very limited data on PM emissions from aircraft engines and APUs. Data for other airport sources varies in quality and availability, and only limited data are available on ambient PM around airports. Several airports have conducted particle deposition studies in nearby and adjacent communities near Los Angeles Inter- national Airport (Barbosa et al. 1999; Barbosa et al. 2001; Eden et al. 2000; Venkatesan 1998), Rhode Island’s T.F. Green Airport (VHB, Inc. 2006), Boston Logan International Airport (Hoffnagle 1996; KM Chng 1996), Charlotte/Douglas Inter- national Airport (Goldman 2005; KM Chng 1998), John Wayne-Orange County Airport (Stolzenbach 2001), Seattle- Tacoma International Airport (Port of Seattle 1995), Fort Lauderdale Hollywood International Airport (Suarez et al. 2004; Webb 2006), and Chicago O’Hare International Airport (Goldman 2005; KM Chng 1999). None of these studies have shown a definitive link between the airports and the deposited material. These studies commonly find the deposits are typical of the material found throughout urban areas that come from diesel trucks, construction activity, wind-blown dust, pollen, and mold. This is perhaps not unexpected since it was the results of the APEX studies that first clearly indicated that PM from aircraft is comprised of fine or ultrafine particles, which are too small to settle gravitationally or to be deposited on stationary surfaces and, thus, remain suspended in the atmosphere. The studies prior to APEX are not conclusive, however, since they used different methodologies and many only sampled dry deposition and did not collect material de- posited through rainfall, which is a primary mechanism for scrubbing suspended particles from the atmosphere. Future deposition studies will be able to build both on these findings and on new information coming from aircraft PM research to improve our understanding of the contribution of airport emissions to deposited PM. Subsequent sections of this re- port discuss the movement of PM off the airport and gaseous emissions from aircraft engines. As a result of federally funded research programs, PM emissions from a few engine types have been partially char- acterized, but most engine models in the fleet remain untested. Research results are still being analyzed to better understand PM formation in aircraft engines and its evolution in the plume. More testing will be required to acquire data needed to develop emission factors related to engine operating con- ditions with the same level of confidence as those available for gaseous emissions. With regard to GSE, EPA has taken steps to reduce PM emissions from nonroad vehicles. In response to national environmental regulations, refiners will begin producing low- sulfur diesel fuel for use in locomotives, ships, and nonroad equipment, including GSE. Low-sulfur diesel fuel must meet a 500 parts per million (ppm) sulfur maximum. This is the first step of EPA’s nonroad diesel rule, with an eventual goal of reducing the sulfur level of fuel for these engines to meet an ultra-low standard (15 ppm) to encourage the introduction of new advanced emission-control technologies for engines used in locomotives, ships, and other nonroad equipment. These most recent nonroad engine and fuel regulations com- plement similarly stringent regulations for diesel highway trucks and buses and highway diesel fuel for 2007. Beginning June 1, 2006, refiners began producing clean ultra-low sulfur diesel fuel, with a sulfur level at or below 15 ppm, for use in highway diesel engines. Low-sulfur (500 ppm) diesel fuel for nonroad diesel engines was required in 2007, followed by ultra-low sulfur diesel fuel for these vehicles in 2010 (U.S. EPA May 2004b). Stringent emissions standards for new GSE will be phased in between 2008 and 2014 as part of this rule. Whether—and when—similar re- ductions in fuel sulfur content will occur in aviation jet fuel has not been determined. 10