Scope Study for Expanding the Great Lakes Toxic Emission Regional Inventory to include Estimated Emissions from Mobile Sources

Chapter 6 Estimating Air Toxics Emissions from Aircraft

• 6-1. Background
• 6-2. Estimating Toxic Air Emissions from Aircraft by Combining Total Organic Gases and Particulate Matter Emissions with Speciation Profiles
  • 6-2-1. General Procedures
  • 6-2-2. Adjust the Approach and Climbout Time-In-Mode to Represent Local Conditions
  • 6-2-3. Conversion of Hydrocarbon (HC) Emissions to Volatile Organic Compound (VOC) Emissions
  • 6-2-4. Speciation Profiles
• 6-3. Other Emission Sources
• 6-4. References

6-1. Background

In the development of an emission inventory, aircraft can be categorized into three types according to the purpose of their use, use frequency, size, and operating profiles. These categories are:

• Commercial aircraft including those used for scheduled service transporting passengers, freight, or both.
• Civilian aircraft that consist of two subcategories:
  • taxis which include those used for scheduled service carrying passengers and/or freight, but usually are smaller aircraft and operate on a more limited basis than the commercial carriers.
  • aviation which includes most other non-military aircraft used for recreational flying, personal transportation, and various other activities. Business aircraft are classified as general aviation for emission estimation purposes.
• Military aircraft that cover a wide range of sizes, uses, and operating missions.

Among the three aircraft categories, commercial aircraft typically are the largest emission sources because of their size and operating frequency.

There are two major type of engines used to power aircraft: reciprocating piston and gas turbine. In a piston engine, the basic element is a combustion chamber (or cylinder), where mixtures of fuel and air are burned and energy is extracted by a piston and crank mechanism to drive a propeller. The gas turbine engine can be classified by three subcategories: turbofan, turboprop, and turbojet. A gas turbine engine usually consists of a compressor, a combustion chamber, and a turbine. Air enters the forward end of the engine and is compressed by the compressor, then it is heated by burning fuel in the combustion chamber. In turbofan and turboprop engines, the energy in the heated air stream is used in the turbine for aircraft propulsion. However, in turbojet engines, the expanding exhaust stream is used for propulsion and energy from the turbine only drives the compressor. The commercial and military aircraft fleets are dominated by turbine engines while civilian aircraft fleets have a significant proportion of piston engines in addition to turbine engines. It should be noted that different types of engines require different aviation fuels. Piston engines use gasoline, but turbine engines use a kerosene-like "jet fuel". There is a wide variety of engine models in each engine type. Many aircraft use only a single engine model, while others may use two or three different engine models. Pollutant emissions are dependent upon the model of engine on an aircraft.

Estimating aircraft emissions focuses on a mixing zone which is a vertical column of air that begins at the earth’s surface, with a height (mixing height) equal to the inversion layer thickness. Air emissions within this zone are trapped by the inversion layer and ultimately affect ground level pollutant concentrations. When aircraft are above the mixing zone, the emissions tend to disperse and have no ground level effects. The aircraft operations within the mixing zone are defined as the landing and takeoff (LTO) cycle. Each LTO cycle consists of five specific operating modes:

• Approach; the aircraft operates in this mode when it approaches the airport on its descent from the mixing height to actual landing on the runway.
• Taxi/idle-in; the aircraft operates in this mode when it taxis from the runway to the gate.
• Taxi/idle-out; the aircraft taxis from the gate back out to the runway in this mode.
• Takeoff; this mode is characterized primarily by full-throttle operation and typically lasts until the aircraft reaches between 500 and 1000 feet above ground level when the engine power is reduced.
• Climbout; this mode begins right after the takeoff mode and lasts until the aircraft is back up to the mixing height.

The operation time in each of these modes is dependent on the aircraft category, local meteorological conditions, and operational considerations at a given airport. The representative LTO cycle time-in-mode (TIM) are shown in Appendix A-6-1 for typical aircraft categories.

There could be two potential approaches for deriving toxic emissions for aircraft in a manner analogous to that for nonroad sources:

1. use toxic emission factors based on activity level; or
2. combine TOG and PM emissions with speciation profiles.

In concept, toxic air emissions from aircraft can be estimated directly through use of toxic emission factors based on activity level. However, little work has been done to identify the air toxics emissions from aircraft (EPA, 1993a). There are no emission factors available for air toxics in EPA’s current version of the Factor Information Retrieval System (FIRE) (EPA, 1996). Therefore, combining TOG and PM emissions with speciation profiles is a reasonable approach at this time.

The following subsections discuss the general steps for estimating toxic air emissions from aircraft by combining TOG and PM emissions with speciation profiles.

6-2. Estimating Toxic Air Emissions from Aircraft by Combining Total Organic Gases and Particulate Matter Emissions with Speciation Profiles

The following equations explain the basis for estimating toxic air emissions from aircraft by combining total organic gases and particulate matter emissions with speciation profiles.

Where:	Ej,l,HC or PM	=	Hydrocarbon (HC) or PM emissions produced by engine type j used on aircraft category l, in pounds
	TIMk,l,m	=	Time in mode for mode k of aircraft model m within aircraft category l, in minutes
	FFj,k,n	=	Fuel flowrate in mode k for engine model n within engine type j, in pounds per minute
	EFj,k,n,HC or PM	=	Emission factor for HC or PM in mode k for engine model n within engine type j, in pounds per one thousand pounds of fuel
	NEj,l,m,n	=	Number of model n engines within engine type j used on aircraft model m within aircraft category l
	NLTOl,m	=	Number of LTO cycles for aircraft model m with aircraft category l
	Ei,j,l	=	Emissions of toxic pollutant i produced by engine type j used on aircraft category l, in pounds
	Cj,k,HC to TOG	=	Conversion factor from HC emissions to TOG emissions for type j engine used on aircraft category l
	%Wi,j,l	=	Weight percent of toxic pollutant i in the TOG or PM emissions for type j engine used on aircraft category l
	Aircraft category l	=	Commercial, civilian, or military
	Aircraft model m	=	Such as Boeing B-737-200C, Airbus A-300-B4, and McDonnell Doug DC10-10
	Engine type j	=	Piston or turbine
	Engine model n	=	Such as J79, TF33-P3, and CTM-56

More detailed discussions on estimating toxic air emissions from aircraft are presented in the following sub-sections.

6-2-1. General Procedures

The general steps illustrated below for estimating emissions are the same for all categories of aircraft.

1. Identify all airports to be included in each county
2. Determine the mixing height to be applied to the LTO cycle
3. Define the fleet make-up for aircraft category using each airport
4. Determine airport activity as to the number of LTO cycles for each aircraft category
5. Select HC and PM emission factors for each operation mode for each engine model within each aircraft model
6. Estimate a time-in-mode (TIM) for each aircraft model at each airport
7. Calculate HC and PM emissions based on the airport activity, TIM, and aircraft emission factors for each model of engine at each county
8. Aggregate the HC and PM emissions to each engine type and each aircraft category
9. Convert HC emissions to TOG emissions
10. Speciate the TOG and PM emissions to individual toxic air pollutants.

In general, the fleet make-up is more difficult to obtain for the civilian aircraft than for the commercial aircraft because the FAA only tracks operations by aircraft model for the commercial aircraft category. Therefore, as an alternative, the fleet-average approach is recommended for the civilian aircraft category. In this approach, fleet average HC emission factors are assumed as 0.394 lb/LTO and 1.234 lb/LTO for general aviation aircraft and air taxis, respectively (Webb, 1991).

HC and PM emission factors can be obtained from the Federal Aviation Administration (FAA) Aircraft Engine Emission Database (FAEED) which contains emission factors for various aircraft engines and data correlating engines to specific aircraft. In addition, this database contains an application program that is capable of computing the total HC and PM emissions produced by a specific fleet. These emission factors and emission calculations have been verified and reviewed extensively (FAEED, 1995). The FAA will update the FAEED when new emission factors become available. Therefore, use of the FAEED and its program is the appropriate approach to estimate HC and PM emissions from aircraft. The estimated HC emissions then can be converted to TOG emissions with proper conversion factors.

For an aircraft with several potential engine models, where no emissions data are available for one engine, reallocate the market share among the engines for which emission data are available. If emission rate information (fuel consumption and emission factor) for an engine model still cannot be located, then the engine manufacturer should be contacted directly.

It should be noted that very few emissions factors are available for PM emissions from certain aircraft engines. These available emission factors may be used for other aircraft engines. However, as pointed out by engine manufacturers, the existing emission factors tend to overestimate PM emissions so that they may represent the worst case estimations (Wilcox, 1996). In addition, PM emission factors for diesel trucks may be extrapolated to aircraft. This is because the majority of commercial and military aircraft use turbine engines and burn kerosene-like jet fuel which has a chemical composition similar to diesel fuel (Wilcox, 1996). The information on PM emission factors may also be obtained from the engine manufacturers and the FAA Office of Environment and Energy.

Table 6-1 summarizes the information required for estimating aircraft HC and PM emissions and the sources for the information.

6-2-2. Adjust the Approach and Climbout Time-In-Mode to Represent Local Conditions

The time-in-approach mode and climbout mode is directly affected by the mixing height. The FAEED sets a default mixing height of 3000 feet. Equations 6-3 and 6-4 provide the adjustments of the approach and climbout TIM to represent the local mixing height. The climbout mode is assumed to begin with the transition from takeoff to climbout at 500 feet and to continue until the aircraft exits the mixing zone.

TIMapp = TIMapp 3000 * (H/3000)

(6 - 3)

TIMclm = TIMclm 3000 * [(H - 500)/2500]

(6 - 4)

Where:	TIMapp	=	Time in the approach mode for mixing height H
	TIMapp 3000	=	Time in the approach mode for mixing height of 3000 feet in Appendix 6-A-1
	TIMclm	=	Time in the climbout mode for mixing height H
	TIMclm 3000	=	Time in the climbout mode for mixing height of 3000 feet in Appendix 6-A-1.

6-2-3. Conversion of Hydrocarbon (HC) Emissions to Volatile Organic Compound (VOC) Emissions

The emission factors for exhaust HC in the FAEED represent total hydrocarbon (THC). Equation 6-5 illustrates the method that is recommended by EPA for converting HC to VOC emissions (EPA, 1992).

EVOC = EHC * CHC-VOC

(6 - 5)

Where:	EVOC	=	VOC emissions
	EHC	=	HC emissions
	CHC-VOC	=	Conversion factor from HC to VOC

Table 6-2 shows the EPA recommended conversion factors for each type of aircraft.

The appropriate conversion factors from VOC to TOG should be determined before speciating the emissions to individual toxic air pollutants.

6-2-4. Speciation Profiles

Speciation profiles for aircraft can be obtained from the same information sources as identified in Section 4-4-2-3. Tables 6-3 and 6-4 show the existing EPA and the California Air Resources Board (CARB) TOG and PM speciation profiles for aircraft (EPA, 1993b; CARB, 1991).

In addition, speciation information may be obtained through a literature search. EPA searched available literature for toxic emissions from aircraft engines in 1993 (EPA, 1993b). This search provides recent available information sources for speciated hydrocarbon emissions and toxics from jet turbine engine exhaust.

6-3. Other Emission Sources

An auxiliary power unit (APU) is a typical part of large aircraft. The APU provides power and preconditioned air to maintain the aircraft’s operability when the aircraft is on the ground with its engines shut down and ground-based power and air source are unavailable. The APUs are small jet engines which burn jet fuel and generate exhaust emissions. The FAEED does not have the emission information for APUs. Therefore, APUs, corresponding aircraft models, and some HC emission factors for these APUs are shown in Appendix 6-A-2 and 6-A-3. If emission factors are not available for a specific APU, the emission factors for a similar horsepower APU can be used. The operation time of the APUs can be obtained from airlines or military base commanders. All other procedures in estimating air toxic emissions are the same as those for aircraft.

Besides exhaust emissions discussed in this chapter, toxic air emissions also occur from refueling and spillage evaporation, preflight checks of the aircraft, and diurnal temperature cycles that cause the fuel tank to vent. The refueling and spillage emissions are included in the airports’ total emission estimates which are considered in point source emission inventory. The preflight and diurnal emissions are being evaluated by EPA. They should be considered when further information becomes available.

6-4. References

CARB, Speciation Manual, Volume 1: Identification of Volatile Organic Compound Species Profiles, Volume 2: Identification of Particulate Matter Species Profiles, 2nd Edition, State of California Air Resource Board, August 1991.

EPA, Factor Information Retrieval System (FIRE) Version 5.1a, U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC, May 1996.

EPA, Procedures for Emission Inventory Preparation: Volume IV: Mobile Sources, U.S. Environmental Protection Agency, Office of Mobile Sources, Ann Arbor, MI and Office of Air Quality Planning and Standards, Research Triangle Park., NC, EPA-450/4-81-026d (Revised), 1992.

EPA, Toxic Air Emissions from Aircraft Engines: A Search of Available Literature, Emission Inventory Branch (MD-14), Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC, EPA-453/R-93-028, July 1993a.

EPA, Volatile Organic Compounds (VOC)/Particulate Matter (PM) Speciation Data System (SPECIATE), Version 1.5, Emission Inventory Branch (MD-14), Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. 1993b.

FAEED, FAA Aircraft Engine Emission User Guide and Database, U.S. Environmental Protection Agency Technology Transfer Network, Office of Mobile Sources (OMS) Bulletin Board System, 1995.

Webb, S., Memorandum to Wilcox, R., Office of Mobile Sources, U.S. Environmental Protection Agency, Ann Arbor, MI, General Aviation Generalized Emission Indexes, June, 10, 1991.

Wilcox, R., Office of Mobile Sources, U.S. Environmental Protection Agency, Ann Arbor, MI, Personal Conversation, August 13, 1996.