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27 SECTION 5 Emission Factors and Activity Factors Emissions data are used in conjunction with dispersion/ jet engine at four defined operational points. ICAO defines chemistry models in order to predict actual concentrations in these operational points as the fraction of rated thrust from particular locations. Emissions from most pollution sources the engine for the following named conditions: idle (7%), are expressed or calculated as the product of two separate approach (30%), climb-out (85%), and take-off (100%). components: emission factors and activity factors. For exam- The ICAO databank also tabulates the result of a measure- ple, the emission factors for on-road mobile sources are com- ment of exhaust opacity known as the smoke-number, how- monly tabulated as grams per vehicle mile. By combining this ever, reporting only the maximum smoke number is man- emission factor with an activity factor, in this example, miles dated. Smoke-number is frequently omitted for other traveled, the total emission can be estimated. For aircraft operational conditions. All tabulated values have been ei- emissions, emission factors are fuel-based, e.g., g of CO/kg of ther conducted at standard atmospheric pressure, tempera- fuel (also known as an emission index). Off-road emission ture, and relative humidity or corrected to this reference factors are expressed differently, as g/BHP-hr (grams per state. The fuel flow at each power setting is also tabulated in brake-horsepower hour). this databank. For the ICAO species, this database serves as Below is a description of the status of knowledge and the primary source of aviation emissions factor informa- uncertainties associated with the emissions produced from tion. Volatile organic compounds (VOCs) (UHCs) exhibit the following sources: aircraft, airport operations, ground- a different emission profile than does NOx. Figure 6 depicts access vehicles, stationary sources, and de-icing. the average NOx and CO emission indices versus throttle setting. The emission indices of CO are greatest at low throttle and 5.1 Aircraft decrease dramatically with increasing power settings. NOx EIs This category includes emissions from gas turbine engines, start at an average value of 5 g/kg at low thrust and unlike turboprop engines, internal combustion piston engines for for CO, the EI increases with increasing throttle. This depic- fixed-wing aircraft, helicopters, and nonrigid airships such as tion is not quantitatively representative of any single specific blimps. As total fuel consumption at most airports is domi- engine, but is qualitatively true for all jet engines in the sense nated by commercial aircraft activity, the emphasis of this that CO is relatively high at low engine power and NOx is report is on high-bypass turbo-fan equipped commercial relatively high at high engine power. The UHC trend is qual- jets; however, piston-engine aircraft emissions will also be itatively similar to the trend observed in CO emissions. Un- discussed. burned hydrocarbons and VOCs are mostly synonymous in this context. Emissions of VOCs exhibit a similar trend with engine 5.1.1 Commercial Aircraft Emission power as CO emissions do. This is depicted in Figure 7, in Factors which an average VOC emission factor and fuel flow is The ICAO has an established testing protocol for new jet depicted versus the throttle setting. Consideration of engines. The results of this certification process are tabu- the emission factors, fuel flow rates, and times spent in lated in the ICAO databank (www.caa.co.uk). The mass mode indicate that the activity leading to the greatest gas- emissions of NOx, CO, and unburned hydrocarbons (UHC) phase HAP emissions (excluding particulate bound PAH are compiled in units of mass per kilogram of fuel for each species) is idling.
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28 Figure 6. Average NOx and CO emission indices versus Figure 7. Generalized depiction of UHC throttle setting. emission index and fuel flow rate versus engine power. One of the more definitive speciated measurements of hydrocarbons in jet exhaust is the work of Spicer, Holdren et al. (1994). This work reported on a wide variety of individ- summarizes the aircraft engines characterized during these ual hydrocarbons in the exhaust of a TF-39 and a CFM56 campaigns. engine (similar to those found on Boeing 737 airframes) For all of the engines presented in Table 9, the particulate fueled by JP-5 fuel. Whole air canister samples were collected and gas-phase components of the engine exhaust were ana- for off-line analysis by various gas-chromatographic (GC) lyzed by a team of research groups. VOCs were measured by techniques while the engine was running at ground idle and either in-situ instruments (e.g., proton transfer mass spec- at 30% and 80% of rated thrust. The work of Spicer was the trometry [PTRMS], tunable infrared laser diode absorption most authoritative manuscript on this topic for over a decade. spectrometry [TILDAS]) or whole air canister samples for Additional work was performed in the late 1990s by Gerstle subsequent analysis by GC-MS. Additional studies at the and co-workers on behalf of the U.S. Air Force, though these Delta/Atlanta Hartsfield and JETS-APEX2 studies focused on measurements focused on military engines (Gerstle, Virag et the measurement of advected exhaust plumes from in-use al. 1999). Slemr et al. reported hydrocarbon emission factors aircraft. Such measurements are analogous to on-road re- from a Rolls Royce M45H-Mk501 engine measured during mote sensing measurements in that emission factor distribu- flight (Slemr, Giehl et al. 2001). tions can be measured. In the last few years, several emission measurement cam- VOC measurements during these campaigns have both paigns have significantly contributed to the total knowledge confirmed past results and revealed new findings. The main base of VOC emissions from commercial aircraft. Table 9 findings are summarized below. Table 9. Summary of recent aircraft engine exhaust measurement campaigns. Engine Airframe Campaign RB211-535E4 B757 EXCAVATE CFM56-2C2 DC-8 APEX CFM56-3B1 B737 JETS-APEX2, APEX3 CFM56-7B22 B737 JETS-APEX2 RB211-535E4-B B757 APEX3 RB211-535E4-B phase 5 B757 APEX3 PW4158 A300 APEX3 AE3007-A Embraer APEX3 AE3007-A1E Embraer APEX3 CJ6108A NASA Lear Jet APEX3 Notes: EXCAVATE: EXperiment to Characterize Aircraft Volatile Aerosol and Trace-Species Emissions, NASA- Langley, 2002. APEX: Aircraft Particle Emissions eXperiment, Dryden Air Force Base, 2004. JETS APEX2: Oakland International Airport, 2005. APEX3: Cleveland International Airport/NASA-Glenn, 2005.
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29 184.108.40.206 VOC Emission Factors versus Thrust As observed by Spicer and colleagues (1994), VOC meas- urements are a strong function of engine power and are high- est at low thrust; decreasing dramatically at higher powers. Figure 8 shows the formaldehyde (HCHO) emission ratio as a function of fuel flow rate (power) for the CFM56-2C1 en- gine studied during APEX (Yelvington, Herndon et al. 2007). The HCHO emission ratio (parts per billion [ppb] of HCHO per ppm of carbon dioxide [CO2]) is proportional to a fuel- based emission index (g HCHO per kg of fuel). The emission ratio decreases by two orders of magnitude between low thrust (idle) and high thrust (take-off) settings. Furthermore, the decrease in HCHO emission index between 4% rated thrust and 7% is more than a factor of three. The ICAO cer- tification power setting for idle is 7% rated thrust, though lower thrust levels (e.g., 4%) appear to more accurately reflect true operational ground idle. 220.127.116.11 VOC Emissions Follow a Universal Scaling Law The ratio of any two VOCs in engine exhaust is approxi- mately constant and independent of engine, power setting, fuel content, and ambient temperature. Figure 9 displays this feature. Data from one CFM56-3B1, two RB211-535E4-Bs, Credit: Knighton, Herndon et al. 2006. Used with permission. and one PW4158 as measured at APEX3 were used in this plot. The ratio of the emission index of any chosen VOC to Figure 9. Ratios of several VOCs to HCHO. that of formaldehyde is a constant, as indicated by the linear fits through each group of points. Thus speciation profiles separate instrumentation (TILDAS) than the other VOCs that have relied on the work of Spicer, Holdren et al. (1994) (PTRMS). Thus the variation in formaldehyde emission are likely fairly accurate. Formaldehyde is a somewhat ar- index with power and temperature described elsewhere in bitrary choice for the x-axis, though was measured with this section hold true for all VOC species measured. It is un- known whether this universal scaling is affected by extremely low temperatures or engine age/maintenance. 18.104.22.168 VOC Emission Factors Are Highly Dependent on Ambient Temperature Volatile organic compound emissions increase greatly with decreasing temperature. The variation of HCHO emission index with temperature is evident in Figure 8. Figure 10 depicts the measured temperature dependence of HCHO emissions from the APEX-1 campaign. For comparison, the Boeing Fuel Flow Method 2 (BFFM2) temperature depend- ence has been added to the data (Baughcum, Tritz et al. 1996), depicted as the solid lines. These lines have been com- puted by scaling the measured emission rate at 25°C by the ratio of the temperature dependent function to that at 25°C using the following equation: Figure 8. Formaldehyde (HCHO) emission ratio as a function of Emission Rate = BFFM2(T)/BFFM2(25 C) * Measured engine power. HCHO ER.
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30 tor) agreed to within 10% at low powers. At low power (<10% thrust), the sum of compounds measured by TILDAS and PTR-MS was ~10% higher than the GC-FID measure- ment, quite possibly due to the latter technique's lack of sen- sitivity to formaldehyde. At higher powers, the total VOC concentrations were lower than the detection limit for the TILDAS and PTR-MS as deployed during those campaigns (Yelvington, Herndon et al. 2007). 22.214.171.124 Absolute VOC EIs Vary by More Than a Factor of 10 Between Different Engines This is reflected in both the individually-measured VOC emission indices as well as the ICAO certification data for total hydrocarbon (HC) emissions. Table 10 displays the HCHO emission index (EIHCHO) of the engines observed at the APEX campaigns and the corresponding ICAO certifica- tion HC emission index, both at 7% power (ICAO idle). The Figure 10. APEX-1 measured temperature CF6 engine is included in the table for comparison. dependence of formaldehyde (HCHO) emissions. The variation in the HCICAO/HCHOAPEXx ratio illustrates the difficulty in comparing these quantities due to the large In this way, only the temperature dependence of the variation in VOC emissions with power and temperature. BFFM2 modeling function is compared to the data. An The variations should not be interpreted as actual variations absolute comparison is not directly possible, as the procedure in the HCHO contribution to total HC. Comparison of indi- was developed to scale reference ICAO UHC. vidual VOC measurements to the total HC measurement by Though the absolute magnitude of the reference value may GC-FID has not been done for JETS-APEX2 and APEX3 yet. be arbitrary, the temperature dependence is evidently slightly steeper than predicted. It should be noted that the developers 126.96.36.199 Emission Factors: of the BFFM2 protocol have more certainty in its ability to True Idle versus 7% (ICAO) predict NOx emission at take-off and during cruise aloft. In the description of the method, it acknowledges challenges The category labeled "idle" in the ICAO databank of emis- associated with using 30% and 7% as the only model inputs sions is a standardized thrust setting. The certification setting for CO and HC emissions. (7% of rated maximum engine thrust) is somewhat greater There are no measurements below 15°C as such knowledge than the setting at which many modern high-bypass ratio of HAP emissions in cold temperatures is extremely limited. engines actually idle in today's fleet. Although it is not for- mally correct, numerous data plots and representations 188.8.131.52 Negligible Variation of VOC Emission attribute "true ground idle" to be approximately 4% of rated Factors with Fuel Composition thrust. Figure 10 portrays the temperature dependence of actual HCHO emissions measured during APEX1 along with Variations in aromatic content and sulfur content were the predicted temperature dependence of Boeing Fuel Flow found to have a minimal effect on VOC emissions during Method-2. This figure also shows the increase in HCHO APEX and are minor compared to variation with engine emission rate between 7% and 4% (ground-idle). power and ambient temperature (Anderson, Chen et al. 2006; A current problem in emission inventory modeling (such Knighton, Rogers et al. 2007; Yelvington, Herndon et al. as EDMS) involves the treatment of how the taxi phase of the 2007). Note that while the "aromatic" content of the fuels LTO is treated. This discussion is tightly coupled to the dif- tested varied from 18% to 22%, the C/H ratio was not ference in VOC emission rates between 7% and ground-idle. affected. Clearly the full variation in hydrocarbon matrix of In each of the studies where the engine has been measured at the fuel was not fully explored. both 7% and "ground idle," the CO and VOC species are con- sistently greater at the lower effective thrust setting. Though the 184.108.40.206 Agreement Between GC-FID number of aircraft engines sampled through the EXCAVATE and Individually Measured VOCs and APEX missions has grown, a systematic characterization At APEX, the sum of individual VOC measurements and a of emissions resulting from the range of idle levels used (3% total HC measurement by GC-FID (flame ionization detec- to 10%) is not available.
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31 Table 10. Comparison of formaldehyde and ICAO HC emission indices for several engines. Engine EIHCHO (7%, g/kg) ICAO EIHC (g/kg) HC ICAO/HCHO(APEXx) CFM56-2C1 0.43 NA NA CFM56-3B1 0.33 2.3 7.0 CFM56-7B22 0.31 2.5 8.1 RB211-535E4B 0.05 0.14 2.8 RB211-535E4-B phase 5 0.22 0.28 1.3 PW4158 1.00 1.8 1.8 AE3007-A 0.41 2.5 6.1 AE3007-A1E 0.54 3.5 6.5 CF6-6D NA 21 NA Notes: EIHCHO formaldehyde emission index g/kg grams per kilogram ICAO International Civil Aviation Organization EIHC hydrocarbon emission index APEX Aircraft Particle Emissions eXperiment NA not applicable Measurements of in-use aircraft at Oakland Airport dur- side of the circle represents the conservative underestimated ing JETs-APEX2 have helped to characterize the real-world amount, assuming a temperature of 18°C. emissions at true operational idle. Although use of 7% thrust was known to be too high by aircraft operators, the use of 7% 220.127.116.11 Reactive Aldehyde Emission Factors idle in engine calculations has remained common in numer- ous studies (e.g., Pison and Menut 2004; Unal, Hu et al. Due to the importance that this report and others assign to 2005). acrolein as an air toxic, a few details regarding its emission in- Figure 11 depicts the underestimate of 1,3-butadiene emis- dices are included here. The emissions of acrolein from air- sions from PHL resulting from the assumption of 7% for the craft engines are greater than anticipated by simple extrapo- thrust value used during the idle phase. The shaded area out- lations of other combustion sources. The ratio of acrolein to formaldehyde (HCHO) in gasoline "engine out" (pre- catalytic converter) is 0.4% by mass (Schauer, Kleeman et al. 2002). This same ratio in diesel truck exhaust is slightly greater, 15% by mass (Schauer, Kleeman et al. 1999). In the Spicer, Holdren et al. (1994) determination of VOC content in the exhaust of the CFM-56, the measured ratio of acrolein to HCHO is 29%, by mass. Preliminary analysis of wind-advected plumes at the taxi- way of Oakland International Airport using the fast re- sponse online instrumentation during the JETS/APEX2 campaign supports the observations of elevated acrolein emissions from aircraft engine. One such event is depicted Figure 11. Depiction of the in Figure 12, where the exhaust from an in-use CF-6 engine emissions of 1,3-butadiene was sampled. The time series show high correlation between from aircraft, GSE, GAV, and the sum of the butene isomers and acrolein, HCHO, and stationary sources at PHL. The CO2. The PTR-MS instrument used to measure acrolein + shaded area outside of the butene was not using a recently developed scrubber system circle represents the extra which can distinguish the two compounds independently. emissions from aircraft when calculated assuming that the As a result, this analysis must rely on the Spicer et al. frac- power settings used during tion of acrolein to total acrolein + butene isomers (0.64). idle/taxi are equal amounts of The Spicer, Holdren et al. fraction is very similar to that ob- time spent at 7% thrust and served in diesel truck exhaust (Schauer, Kleeman et al. 4% thrust (versus the standard 1999). The advected plume data in Figure 12 suggest the 7% assumption). ratio of acrolein to HCHO is 56%. This further underscores
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32 flame is established, the rotation speed and the fuel flow are increased together to the nominal ground idle operating point. Over the course of 30 to 90 sec, the temperature of the engine reaches a stable condition and the emission indices of hydrocarbons and CO stabilize. As it pertains to total emissions accounting, any fuel that is pushed through the combustor before ignition is "un- burned." The nature of these emissions will have the same speciation as evaporative emissions from the fuel stock. This pre-ignition contribution will have a very different speciation profile than the engine-on profile of HAP emissions, and they are much less toxic, since they consist mainly of alkanes. There are a few different approaches to assessing the mag- nitude of start-up emissions relative to the total emissions during an LTO. Direct determinations, based on fuel flow measurements and analytical UHC instruments, are chal- lenging because the unburned fuel can foul sampling lines, Figure 12. Acrolein sampled from CF-6 engine and standard methods using combustion reference gases can- exhaust. not be used prior to the initiation of the flame in the com- bustor. Alternately, the combustion efficiency can be esti- the need to characterize the acrolein emission index with mated from engine exit plane temperature measurements. If robust measurements. assumptions about the HC profile are associated with com- bustion efficiency, the start-up emissions can be estimated. 18.104.22.168 Aircraft Emissions at Engine Start Such calculations using unpublished test data yield esti- At many airports, aircraft can plug-in to an electrical ser- mates that pre-ignition emissions of unburned fuel are be- vice at the gate, which provides basic power to the aircraft tween 30 and 90 g of fuel (hydrocarbons). The range is largely during preparation for flight and deplaning, and allows the a result of the uncertainty in the ignition timing. The compo- APU to be turned off. At other airports the APU is used to sition of this emission source should resemble evaporated fuel generate power for the aircraft while at the gate and, in either and be less similar to the exhaust HC profile. Pre-ignition case, the APU is used to start the main engines. In starting the emissions are estimated in Table 11. This estimate uses a fuel engine, the turbo-machinery is first accelerated to a nominal flow rate (kg/s) that is half of the ICAO 7% value for a CFM56- starting rotational speed, at which point, fuel is supplied to 7B22 engine during the initial act of starting the engine. the combustor and a fuel spray is generated. With the initia- The next line involves estimating the emissions at the crit- tion of the fuel flow, the spark ignition system is activated, ical point of ignition. The contribution of the near-ignition and a flame is quickly established in the combustor. Once the phase is small compared to the total LTO; however, there is Table 11. Single engine emissions of hazardous air pollutants at start-up. Emission Emission Emission Index Fuel Flow UHC ~HAP Condition (g kg-1) Time (kg s-1) (g) (g) Pre-ignition 1000 0.3-0.5 s 0.05 15-25 - Ignition 1000-10 0.01-10 ms <1 Warm-Up 36 30 90 s 0.105 10 50 10 50 Taxiway 2.5 24 min 0.105 410 410 Activity Pre-ignition--The time and fuel flow values have been estimated using an unpublished draft of a working paper on start-up emissions (Will Dodds, GE, personal communication). Ignition--These large ranges are meant to indicate the vast uncertainty in this transition state in the combustor. Warm-up--The approximate "doubling" of the emission index during warm-up as well as the estimate of how long the machinery of the engine requires to come to a stable temperature is based on an unpublished analysis of an RB211-535-E4-B engine during APEX3. Taxiway activity--As the template for these estimates, the emission index and fuel flow rate for a CFM56- 7B22 have been drawn from the engine certification value.
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33 no certainty as to the compositional make up of these emis- emission factors are quantitatively related to the lead fuel con- sions. There is no known measurement of in-use engine tent and total lead emissions can be calculated to a high degree emissions at the point of ignition, however these estimates in- of certainty. For example, annual AvGas throughput at FLL in dicate it will be small relative to the emissions during an LTO. 2005 was 6x106 gallons (higher than most other commercial An unpublished analysis of an engine test from APEX3 airports). The FLL environmental impact statement (Lan- indicates that the emissions stabilize about 40 sec after start. drum and Brown 2007) used a fuel-based emission factor of Other anecdotal material suggests this is a reasonable esti- 0.15 g Pb/L AvGas, which resulted in total Pb emissions (at the mate of a typical warm-up time. Preliminary analysis of airport and aloft) of 3.7 tons, of which 14% (0.53 tons/year) APEX3 data indicates that the initial magnitude of the HC appears on FLL's speciated HAP emissions inventory. emission indices was approximately twice as high as idle Very little is known regarding the emission of hydrocar- emission indices. For example, the initial CO emission index bons from general aviation, though a recent study has been was measured to be 70 g kg-1 and this settled to just less released by the Swiss Federal Office of Civil Aviation than 35 g kg-1 after warm up. The direct measurements of (Rindlisbacher 2007). This lack of knowledge regarding HAP formaldehyde and benzene saw very similar trends. emissions from piston engines represents a large informa- The total impact of the warm-up period relative to the taxi- tion gap. way portion is fairly straightforward to estimate. If the emis- sion rate is approximately double for the first minute, then 5.1.3 Activity Factors for Jet Engines the warm-up phase "adds 1 min" to the idle phase. If the real time spent in the taxiway mode is 20 min, then the start-up The ICAO databank also defines a standard landing/take- emissions account for 5% of HC emissions that stem from the off cycle (LTO) by specifying the time spent in each mode. As combined start-up + idle emissions. Due to the uncertainty illustrated in Figure 13, when the emissions vary by large fac- in the composition of the emissions during the ignition tors with the operational state, knowledge of the time spent phase, and the possibility that the aircraft analyzed in APEX3 in each mode is crucial for accurately calculating the total is not representative of the fleet emission rate behavior, a emissions. A factor that often complicates the time-in-mode more conservative estimate of the HAP emissions associated estimate involves the approach and climb-out phases. In the with "start-up" is < 10%. Further, it is important to note that context of assessing airport-related HAP emissions, the the preceding analysis is based on comparing start-up emis- climb-out and landing phases of an LTO are mostly minor sion indices to those at "ground idle." If the ICAO 7% idle is since VOC emissions are dominated by the idle phase. Fur- used as a reference point, correspondingly different ratios thermore, approach emissions are spread out over a very would result. large distance. In contrast, assessment of NOx emissions re- quires knowledge of the height of the mixing (boundary) layer and time in all LTO modes since NOx emissions increase 5.1.2 General Aviation with engine thrust. The ICAO certification data sheets use The general aviation category comprises aircraft that use 3000 feet as a nominal mixing height, and defines the climb- piston engines, turbojet engines, and low bypass turbofan en- out period as aircraft movement from a height of 500 ft up to gines such as business jets. The VOC emissions of all of these 3000 ft and lasting 2.2 min. The true mixing height will vary are unregulated, and as a result are mostly unknown. Piston with season, time of day, and daily meteorology, and there- engine aircraft activity accounts for a small fraction of total fore the true amount of time spent in climb-out phase will fuel consumption at U.S. airports, however piston engines are vary accordingly. Such accounting for time spent in climb- the biggest source of airborne lead (Pb) from airports due to out and approach is not very relevant for assessing VOC/HAP the continued use of leaded aviation gasoline ("AvGas"). For emissions since the idle phase dominates the emissions of example, total throughput of AvGas at PHL in 2003 was these species. 144,000 gallons, compared to 401,000,000 gallons of Jet Fuel The fraction of total HAP emissions for an LTO is domi- A. Thus at most airports the contribution to total VOC emis- nated by consideration of the "idle" mode (>90%). This is pre- sions is minor. There are many general aviation airports in the sented in Figure 13, in which the unburned hydrocarbon U.S. at which piston engine aircraft are the most common type (UHC) emission rate (g/s) is plotted versus time during an of aircraft, however, and so they are considered here. LTO cycle. The total emission (g/LTO) from each LTO phase Although technically a criteria pollutant, lead is often included is equal to the area of the relevant "block." Evident is the pre- as a hazardous air pollutant as well. The most common grade dominance of the idle phase over the other phases as well as of leaded AvGas is "100 LL," which has a maximum lead con- APU operation. Small deviations from the 7% ICAO thrust tent 0.56 g Pb/L (Chevron 2006). Between 75% and 95% of level at idle have large effects on the total HAP emissions dur- the lead in the fuel is emitted in the exhaust; therefore lead ing the idle mode. Evidence to date indicates that true ground