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

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25 This section focuses on collecting and summarizing relevant published articles, results of existing literature reviews, and available documentation that addresses the current issues and informational needs of the airport community on gaseous and particulate emissions at airports. The reader is encouraged to read Chapter 1, Primer on Particulate Matter Emissions from Aviation. Chapter 6 is intended to augment the infor- mation presented in Chapter 1. 6.1 Characteristics of Aircraft PM As discussed in Chapter 1, aircraft PM is categorized either as primary PM or as contributing to secondary PM. Primary PM can be described as either volatile or non-volatile. Non- volatile PM has a size distribution that differs from the volatile PM generated by aircraft gas turbine engines. The di- ameters of non-volatile carbonaceous particles (soot) generated by aircraft gas turbine engines range from approximately 0.02 to 0.06 μm (20 to 60 nm) in diameter. The EPA classifies such PM as PM2.5, which includes particles less than 2.5 μm in aerodynamic diameter. The diameter of volatile PM ranges from approximately 0.001 to 0.015 μm (1 to 15 nm), and also is classified as PM2.5 (Lukachko et al. 2008). Primary volatile PM is initially formed in the near-field plume (<1 min from emission). Volatile PM is composed of a variety of compounds whose emissions indices and relative contributions depend on a number of factors including ambient air conditions, thrust setting, and fuel sulfur content (Anderson et al. 2005). Research suggests volatile PM may be composed of the following compounds: • Sulfuric Acid. Sulfuric acid (H2SO4) resulting from fuel sulfur nucleates as (H2SO4)n•(H2O)m, where n and m are small integers, to form volatile PM (Lukachko et al. 2008). H2SO4 molecules also condense onto preexisting aerosol surfaces (Lukachko et al. 2008). • Hydrocarbons. Hydrocarbons may nucleate as indepen- dent PM sources but may play a more important role in contributing to volatile PM via uptake on existing particles (Wey et al. 2006). • Lubrication Oil. Lubrication oil may also influence volatile PM composition, particularly during transient periods when engine thrust level is switched from one level to the next. APEX3 data indicated that up to 90% of the organic PM emitted by some engines may be lubrication oil (Timko, Onasch et al. 2008). For less efficient engines, lubrication oil makes up as little as about 10% of the total organic PM. In general, lubrication oil is least important in engines with low combustion efficiencies, and under- standing of lubrication oil emissions continues to grow. The total PM reported by EDMS as calculated using the FOA is an estimate of the non-volatile and volatile primary PM. Secondary volatile PM forms on the timescale of minutes to days and may continue to form in air masses moving hundreds of kilometers (or miles) from the source. Nitrogen oxides, sulfur oxides, and HC emissions are important contributors to secondary volatile PM formation. After atmospheric pro- cessing, these species are absorbed into existing particles, some of which are non-volatile particles. Nitric acid (HNO3) is pro- duced by the photochemical processing of NO2. Ammonium nitrate (NH4NO3) found in PM provides evidence that HNO3 contributes to formation of secondary volatile PM. A regional- scale model is needed to calculate the quantities and compo- sition of secondary volatile PM that is formed. Several factors can alter aircraft PM properties. Engine technology influences particle size. Fuel sulfur content also influences primary volatile PM properties since volatile PM concentrations tend to increase with higher fuel sulfur con- tent (Kugele et al. 2005). Organic emissions also contribute to primary volatile PM composition and mass. Secondary PM properties are influenced by coexisting pollutants emitted from other sources (U.S. EPA Jul 2004). Therefore, PM sourced to aircraft can change as emissions from other sources evolve. To date, the studies conducted by Spicer et al. (1992, 1994) have been the primary source of hazardous air pollutant (HAP) C H A P T E R 6 Gaseous and Particulate Matter Emissions Literature Review

emission factors for aircraft. A commercial jet engine and a military jet engine were tested under varying thrust conditions, and the studies identified that formaldehyde and acetaldehyde are the two predominant HAPs contained in jet exhaust. These studies represent a thorough analysis of two jet engines, but also highlight the need for expanded measurements on a broader and more modern range of engine types. 6.2 Literature Reports on Aircraft PM Particulate matter emitted within airport boundaries comes from many sources, such as aircraft engines, aircraft APU, tire and brake wear, GSE, vehicles that travel to and from the airport (ground access vehicles), dust from construction, boilers, and training fires for firefighters. The relative contri- butions of all of these sources are not well characterized, as there are limited data for some PM sources. 6.2.1 Relative Contributions from GSE and Aircraft Brakes/Tires The relative contribution of GSE to total airport emissions depends on many factors, including the size of aircraft served and the length of flight. In addition, the fuel type of the GSE directly affects the PM emissions, especially in the case of electric GSE where the emissions are generated off-site and are therefore not included in the airport inventory. As a result of these factors, the relative contribution of GSE emis- sions is small (less than 20% of the airport total) at some airports and large (greater than 50% of the airport total) at others. An analysis of the mix of GSE equipment and its uti- lization is needed to properly quantify its contribution to emissions at a specific airport. Tire wear rates are calculated both by experiment and by estimation from statistical information. For vehicles, wear rates are typically reported as milligrams per vehicle-kilometer (vkm) traveled (vkm takes into account the four tires on a typical vehicle). Wear rates vary depending on numerous factors, in- cluding the weight of the vehicle, tire composition, and driving conditions. Brake and tire wear rates typically are reported as mg/vkm and can vary greatly depending on braking con- ditions. For light-weight vehicles, the brake wear rates range from 8.8 mg/vkm to 20 mg/vkm (Legret and Pagotto 1999). Aircraft tire and brake emissions are reported on a per LTO basis. Much like vehicles, aircraft tire and break emissions estimates contain large uncertainties and vary depending on the type of aircraft and the landing conditions. For six different air- craft listed in the Project for the Sustainable Development of Heathrow (PSDH) study (UK DfT 2007), the range of emission rates (tire and brake) was measured to be between 110,000 mg per landing (A321) and 780,000 mg per landing (B747-400). These values fall in line with a EUROCONTROL study (Kugele et al. 2005) that estimated the average tire and brake emission rates per LTO as 130,000 mg and 30 mg, respectively. The percentage of emissions from tire and brake wear that become suspended and are classified as PM10 (or PM2.5) is an area of current research. Little data are available for vehicles; none is available for aircraft. For tires, it is believed that less than 10% of emissions become PM10, but studies have shown it can be as high as 30% (Boulter 2005). Nearly all tire wear emissions are larger than PM2.5. For brakes, a study conducted by Sanders et al. (2003) observed that between 50% and 90% of brake emissions become airborne particles (mass mean di- ameter is 6 μm and the number-weighted mean is between 1 to 2 μm). The measurement brackets the United Nations Economic Commission for Europe (UNECE) estimate of 70% of brake lining becoming suspended matter (UK DfT 2007). Since aircraft experience more extreme braking conditions than vehicles do, the PSDH study uses the upper limits of 10% for tire wear and 100% for brake wear for estimates of PM10 emissions for aircraft. 6.3 Modeling PM Using EDMS Researchers use an FAA-developed, EPA-approved tool known as EDMS to estimate PM emissions from aircraft main engines, GSE, on-road vehicles, and stationary sources. The required tool for assessing the changes to local air quality resulting from airport projects is EDMS. The EDMS tool estimates primary PM emissions for ICAO- certified aircraft main engines with a smoke number using FOA 3.0a for U.S. airports and FOA 3.0 for airports outside the United States. The FOA 3.0 method is accepted by the Committee on Aviation Environmental Protection (CAEP), and FOA 3.0a has been approved by the EPA. Together, they represent the latest methods approved by these groups to approximate primary PM emissions from aircraft. The estimate of non-volatile PM emissions is based on smoke number, where the estimates of volatile PM are based on UHC and fuel sulfur content, and—in the case of FOA3a—lubricating oil. For jet and turboprop aircraft without smoke numbers, only the volatile contribution to primary PM is computed. EDMS does not estimate any PM emissions for piston aircraft (CSSI 2008). EDMS uses a standard, single fuel, sulfur level for each aircraft; the level of sulfur can be adjusted for scenarios and aircraft. EDMS models PM from ground support equipment using EPA’s NONROAD model and PM from on-road vehicles using EPA’s MOBILE model. The EPA’s NONROAD model can also be used outside of EDMS to estimate the PM emis- sions from construction equipment engines, but not from other PM sources, such as fugitive dust, that can result from earthmoving activities. Airport modelers must account for these emissions separately. 26

6.4 Current Model Limitations Particle diameter is influenced by throttle setting, but all operating modes produce particles less than 2.5 μm in diam- eter (PM2.5). Particle chemical composition also varies with thrust setting (Lobo, Whitefield et al. 2007). Accurately estimating throttle setting is important to account for changes in engine conditions that influence PM emission indices. For certification, throttle setting is specified for the LTO cycle describing aircraft operation to a height of approximately 900 m (2,953 ft) by regulation. Over the LTO cycle, ICAO specifies generic time in mode and thrust as- sumptions for aircraft engine certification with four discrete settings: taxi/idle (26.0 min, 7% throttle), takeoff (0.7 min, 100% throttle), climb (2.2 min, 85% throttle), and approach (4.0 min, 30% throttle) (ICAO 1993). The certification prescribed LTO cycle is not necessarily representative of actual flight procedures. This affects result- ing estimates of total PM emissions (Fleuti and Polymeris 2004). This question was addressed by APEX1, which looked at PM emissions at 11 thrust settings: 4, 5.5, 7, 15, 30, 40, 60, 65, 70, 85, and 100% to understand trends at intermediate thrusts and below the prescribed idle setting. Particulate matter emissions trends below 7% vary by PM component. In general, non-volatile PM (black carbon) emissions are rel- atively small at low thrust settings. However, volatile PM components exhibit more complex behavior as precursor gases condense downstream (Wey et al. 2006). Typically, when modeling airport activity using EDMS, the user assumes that once an aircraft has pushed back from the gate, the APU is turned off and the main engines are used to provide power to the aircraft. In reality, however, anticipated delays prompt pilots to shut off main engines and run the APU to conserve fuel. Recommended warm-up and cool- down times are dependent on design parameters for each spe- cific engine type, and influence a pilot’s decision to shut off main engines (ICAO 2000). Although airlines have individual operating procedures, the ultimate decision rests with the pilot (ICAO 2000). Assuming that the aircraft main engines remain operating at 7% thrust throughout the taxi/queue portion of the LTO is conservative, but may not accurately represent the actual operation of the aircraft. 6.5 Mitigation Although there are many sources of PM emissions at air- ports, only a few of the sources are under the direct control of the airport. Stationary sources, GSE, and some aircraft oper- ational characteristics are the most likely to be influenced through airport policy. Examples of mitigation for PM from each of those sources is as follows: • For stationary sources like emergency generators, inciner- ators, power turbines, and oil-fired boilers, particle traps can be installed on exhaust stacks to control PM emissions. • Ground support equipment PM emissions can be miti- gated by using an ultra-low sulfur diesel fuel. Keeping the engines properly maintained and tuned is important for minimizing particle emissions as well. Working with ten- ants to promote the use of alternative fuels can also be beneficial. This might include supplying alternative, lower emission fuels. Working with airlines to install chargers in the ramp area for electric GSE may encourage greater use of zero-emission electric vehicles. • Installing electrical power and preconditioned air at each gate can provide the airlines with the power and ventilation they need without running APU. • Many airports have changed from using Jet A or diesel fuel to propane or other cleaner burning fuels in their fire- fighter training. This change in fuels reduces smoke and soot emissions from about 1,000 lbs/1,000 gal for jet fuel to about 120 lbs/1,000 gal for propane (FAA and USAF 1997). • Controlling ground operations to minimize delays reduces aircraft emissions. Establishing airport policies to promote fuel conservation practices among airlines and other tenants can reduce airport emissions. Such a policy might recom- mend single-engine taxiing, de-rated takeoff, enhancing GSE maintenance, and using ultra-low sulfur diesel in GSE and other vehicles. • On the landside, airports do not have any regulatory au- thority over passenger vehicles. However, many airports have worked with local taxi companies to encourage, or even mandate, use of low-emission taxis as a requirement for serving the airport. For example, all taxicabs permitted to pick up passengers at Seattle Tacoma International Air- port are required to use compressed natural gas. Through fees and licenses, some airports have taken strides to reduce the frequency of circulating through the airport by hotel, parking, and car rental vans. Similarly, the use of cell phone waiting areas allows vehicles to remain nearby with their engines off until passengers are ready to be picked up. • Providing ultra-low sulfur diesel or other reduced-sulfur fuel for use in GSE, boilers, emergency generators, etc., can reduce particulate emissions. A new generation of alterna- tive fuels, known as synthetic paraffinic kerosene, which in their pure form contain no sulfur, show promise in reduc- ing PM emissions from the aforementioned sources and turbine-powered aircraft (Hileman et al. 2008). 27