Global Civil Aviation Black Carbon Emissions - Environmental Science

Article pubs.acs.org/est

Global Civil Aviation Black Carbon Emissions Marc E. J. Stettler,†,‡ Adam M. Boies,† Andreas Petzold,§ and Steven R. H. Barrett‡,* †

Energy Efficient Cities Initiative, Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, United Kingdom ‡ Laboratory for Aviation and the Environment, Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § Forschungszentrum Jülich, Institute of Energy and Climate Research, IEK-8 Troposphere, 52425 Jülich, Germany S Supporting Information *

ABSTRACT: Aircraft black carbon (BC) emissions contribute to climate forcing, but few estimates of BC emitted by aircraft at cruise exist. For the majority of aircraft engines the only BCrelated measurement available is smoke number (SN)a filter based optical method designed to measure near-ground plume visibility, not mass. While the first order approximation (FOA3) technique has been developed to estimate BC mass emissions normalized by fuel burn [EI(BC)] from SN, it is shown that it underestimates EI(BC) by >90% in 35% of directly measured cases (R2 = −0.10). As there are no plans to measure BC emissions from all existing certified engineswhich will be in service for several decadesit is necessary to estimate EI(BC) for existing aircraft on the ground and at cruise. An alternative method, called FOX, that is independent of the SN is developed to estimate BC emissions. Estimates of EI(BC) at ground level are significantly improved (R2 = 0.68), whereas estimates at cruise are within 30% of measurements. Implementing this approach for global civil aviation estimated aircraft BC emissions are revised upward by a factor of ∼3. Direct radiative forcing (RF) due to aviation BC emissions is estimated to be ∼9.5 mW/m2, equivalent to ∼1/3 of the current RF due to aviation CO2 emissions. engine model and thrust setting.13 However, EI(BC) has not been measured for the vast majority of engines currently in service. EI(BC) is typically estimated using the SN for LTO emissions, and no standard method exists for cruise emissions, with researchers typically applying ad hoc estimates of fleetaveraged EI(BC), for example, ref 6. Engine in-service lifetimes can be 20 years or more, while existing certified engines will continue to be produced for many years. As there are no plans to retroactively measure BC emissions from currently certified engines, methods to estimate aircraft EI(BC) will continue to be required for decades and an assessment of their accuracy is essential to quantifying the atmospheric impacts of aviation emissions. In recent impact assessments of aviation,2,6 fleet average EI(BC) estimates based on measurements of EI(BC) at cruise altitude have been used to compute inventories of global aviation BC emissions. However, in order to make these measurements possible the aircraft were operated at reduced engine power settings, reduced flight speeds and reduced aircraft weights compared to typical operations.4 The current

1. INTRODUCTION 1.1. Context. Aircraft gas turbine engines emit carbonaceous particles that warm Earth’s atmosphere,1 thereby contributing to climate change. Black carbon (BC) aerosols emitted by aircraft strongly absorb solar radiation and have a long lifetime relative to near-surface BC emissions, leading to a positive radiative forcing (RF).2,3 They are also thought to have a significant indirect influence by acting as ice nuclei in the formation of contrails4,5 and contributing to aviation-induced cloudiness.1 BC emitted by aircraft also contributes to the degradation of air quality, both globally6 and in the vicinity of airports.7 The current regulations regarding aircraft particulate matter (PM) emissions were originally concerned with plume visibility during the landing and takeoff (LTO) cycle8 (i.e., near the ground). The regulation is applied through a limit on the engine smoke number (SN), the observed change in reflectance of a filter after sampling a given mass of exhaust.9 Current research efforts are focused toward defining a new standard procedure for measurement of BC mass emissions from aircraft at ground level10 for regulatory purposes, and several studies have measured aircraft engine BC emission indices (EI), that is, mass of BC emitted per kg of fuel burned.11−13 These measurements indicate that EI(BC) spans almost 4 orders of magnitude (5 × 10−4 to ∼1 g/kg-fuel) and is a function of © 2013 American Chemical Society

Received: Revised: Accepted: Published: 10397

March 27, 2013 July 4, 2013 July 11, 2013 July 11, 2013 dx.doi.org/10.1021/es401356v | Environ. Sci. Technol. 2013, 47, 10397−10404

Environmental Science & Technology

Article

Q mixed | exhaust[m 3/kg‐fuel] = 0.776(AFR)(1 + β ) + 0.877.

basis of estimating EI(BC) from SN is a correlation between SN and BC concentration in the aircraft exhaust (CBC). In a companion paper14 we have shown that this correlation depends on the BC particle size distribution and that the correlation endorsed by the International Civil Aviation Organization (ICAO)15 could lead to a factor of 3 underestimation of CBC for a given SN. Furthermore, in a preliminary assessment16 we showed that the current accepted emissions estimation method (FOA317) can underestimate EI(BC) by up to an order of magnitude or more. Thus, estimates of BC emissions during both LTO and cruise may be biased by a significant and unquantified factor. 1.2. Overview of Paper. The accuracy of current methods used to estimate aircraft EI(BC) are evaluated, a new method appropriate to the data limitations is developed, an updated global estimate of aviation BC emissions is provided and the potential climatic importance of this for aviation is discussed. The purpose of the method developed is not to replace future direct measurements of EI(BC), but to “backfill” the majority of the fleet for which emissions measurements are not available and may never be available given current information, and which will continue to be relevant for several decades. Details omitted from the main text of this paper are included in the Supporting Information (SI).

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Engine AFRs are proprietary and in order to apply this methodology to all engines in the fleet, representative AFRs corresponding to four certification thrust settings are defined for use by FOA3. Up to now, a comprehensive assessment of the CBC and Q components has not been published. Cruise. Measurements of EI(BC) at cruise4,23 conducted using a chase plane in the plume of the leading plane suggest that EI(BC) is ∼0.01 g/kg-fuel, although these measurements are representative of engines operating at reduced power (∼20% of maximum engine fuel flow).4,23 Indeed, the measured EI(BC) for the CFM56−3B1 at cruise (0.011 g/kg-fuel)23 is relatively low compared to measured EI(BC) for the same engine model at ground level by Timko et al.13 (0.002−0.360 g/kg-fuel) and is similar to emissions rates at 7 or 30% of maximum engine fuel flow at ground level. We discuss below whether ∼20% fuel flow is representative of conditions at cruise. Döpelheuer and Lecht18 proposed a method of translating known reference values of CBC at ground level to a value at cruise altitude, accounting for changes to internal engine parameters; the equivalence ratio (ϕ), the combustor inlet pressure (p3) and the flame temperature (Tfl). Specifically, the cruise BC concentration is related to ground reference conditions by

2. MATERIALS AND METHODS In this section, existing methods to estimate EI(BC) during LTO and cruise are summarized in order to identify sources of error to be investigated further and the data used to evaluate these methods are described. Methods to estimate EI(BC) independently of SN and quantify global aircraft BC emissions are developed. 2.1. Overview of Current Practice. Landing and Take Off. At the introduction of the SN regulations, experiments to define the standard test procedure correlated SN to BC mass concentration in the engine exhaust (CBC).8 Since then, studies have utilized these correlations to estimate emission rates of BC from aircraft engines.18,19 The most rigorous method to estimate aircraft EI(BC) from the SN to date is the first order approximation version 3.0 (FOA3),17 developed so that emissions from airports could be estimated. The FOA3 method proposes that EI(BC) is equal to the product of the mass concentration of BC, CBC (mg/m3), and the volumetric flow rate, that is, the volume of exhaust gas per kg of fuel burned, Q (m3/kg-fuel), where ⎡ mg ⎤ C BC⎢ 3 ⎥ = 0.0694(SN)1.24 ⎣m ⎦

⎞2.5⎛ p ⎞ ⎟⎟ ⎜⎜ 3 ⎟⎟ p ref ⎠ ⎝ 3,ref ⎠

⎛ ϕ ⎡ mg ⎤ C BC⎢ 3 ⎥ = C BC,ref ⎜⎜ ⎣m ⎦ ⎝ϕ

⎛ e 20000/ Tfl ⎞ ⎜ 20000/ T ⎟ fl,ref ⎠ ⎝e

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where the subscript ref refers to reference values at ground level. (The derivation of this equation is included in the SI.) Applying this method to common aircraft types, Petzold et al.23 estimated a fleet average EI(BC) of 0.038 g/kg-fuel, using the certification SNs to estimate CBC,ref and Hendricks et al.24 estimated an altitudinal variation in EI(BC) based on this estimate. More recently, Lee et al.1 used a fleet average EI(BC) of 0.025 (0.01−0.05) g/kg-fuel, Barrett et al.6 used a fleet average EI(BC) of 0.04 (0.01−0.20) g/kg-fuel and the FAA’s Aviation Environmental Design Tool has been used with a fleet average EI(BC) of 0.035 g/kg-fuel.25 Thus, recent global aviation BC inventories have not accounted for variations in technology and engine operating parameters. Recently, Peck et al.26 proposed a method of estimating EI(BC) at cruise based on an empirical correlation between SN and EI(BC) at ground level using a subset of the data described below. The authors noted discrepancies between certification SN data and recent measurements which we discuss below, and observed that these discrepancies potentially introduce significant and unquantified errors to cruise EI(BC) estimates derived from the certification SN. Comparing their estimates of EI(BC) at cruise to the same chase plane measurements4,23 mentioned above, they assumed an engine thrust setting of 65% of maximum rated thrust at cruise, and estimate EI(BC) with an error between −25 and 55%. 2.2. Measurement Data Sources. Engine Data. The ICAO engine emissions databank27 (EDB) provides certification data for each engine in service, including maximum rated thrust (F00) at International Standard Atmosphere sea level static conditions (288 K, 101325 Pa),9 overall pressure ratio (π00), fuel flow (ṁ f) and SN at four specified thrust settings. More detailed engine operating parameters such as pressure

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This SN-CBC correlation is based on earlier studies8,20−22 and is recommended for use where the SN is less than 30. Q is estimated by assuming an air-to-fuel ratio (AFR) (derivation shown in the SI). The SN is measured using an arrangement of probes that are placed close to the engine exhaust nozzle exit plane. For some engines the exhaust from the engine core is mixed with the bypass flow, air that is drawn through the fan and around the engine core that is not involved in combustion. For these mixed turbofan engines, the bypass flow dilutes the concentration of BC in the core flow and is accounted for using the engine bypass ratio (β), the ratio of the bypass air mass flow rate to the core air mass flow rate, that is, Q core | exhaust[m 3/kg‐fuel] = 0.776(AFR) + 0.877

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dx.doi.org/10.1021/es401356v | Environ. Sci. Technol. 2013, 47, 10397−10404


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computational fluid dynamics with detailed kinetic chemistry and soot microphysics. Recently, a 0D model with detailed chemistry and soot microphysics showed capability of predicting a range of pollutants including soot.36 However, it requires calibration against measurements to find appropriate values for certain input parameters and has so far only been applied to one engine at idle. There have been previous efforts to use Arrhenius equation forms to model gas turbine engine soot emissions.35,37 These preceded the aforementioned measurement campaigns which have greatly increased the range of EI(BC) measurements over different engine models. Wen et al.38 modeled soot formation in turbulent kerosene/air jet diffusion flames using Arrhenius equations to model soot inception, coagulation, growth, and oxidation. They found that a polycyclic aromatic hydrocarbon (PAH) inception model provided better estimates of soot volume fraction than an acetylene inception model. Given the uncertain correlation between SN and CBC an empirical method, independent of SN, is developed to obtain for EI(BC) for all engines in the fleet using only data available in the ICAO EDB representing the physical mechanisms (with significant simplification) by which soot is formed and oxidized. Soot inception and oxygen oxidation terms from Wen et al.38 with corresponding activation temperatures are used and the dependence on the concentration of fuel and air are simplified. CBC, as measured at the exit of the core, is estimated to be proportional to the difference between the formation and oxidation terms,

and temperature at different stages within the engine are proprietary and/or unavailable. EI(BC) Measurements. Measurements are compiled from the Aircraft Particle Emissions eXperiment (APEX 1−3),13,28 the Delta-Atlanta Hartsfield study (Delta-ATL)29,30 and a study conducted by Agrawal et al.,31 during which BC mass concentration was measured using gravimetric or optical methods.10,13,30−32 A recent intercomparison of these measurement methods suggests that current optical, thermal and gravimetric methods agree within 10% for BC emitted from gas turbines,32 enabling our comparison across different campaigns. However a standard measurement method is currently being developed and method improvements are likely to be reccommended.10 Measurements for the same engine model are combined where multiple engines of the same model were measured and are averaged over periods during which the engine thrust setting is held constant in order to obtain EI(BC) as a function of engine thrust setting. Air to Fuel Ratios. Data from the APEX 1−3 campaign is used to calculate the AFR for four different engines for comparison with the fleet average values used in FOA3 and development of a relationship between AFR and engine thrust setting. AFRs are calculated according to the method outlined in the SI using reported concentrations of CO2 in the exhaust plume at specified engine thrust settings. Smoke Number. SN measurements were taken alongside EI(BC) measurements during the APEX 1−3 campaign for a range of engines at various engine thrust settings.26 SN measurements are compared to the certification SN values in the EDB where recent measurements were taken at the certification engine thrust settings. This enables an evaluation of the correlation relating SN to CBC. Flight Data Recorder. Data recorded by the flight data recorder (FDR) of three aircraft types over multiple flights is used to observe how engine parameters, ṁ f, and combustor inlet temperature (T3) and pressure (p3), change during different phases of the flight including the LTO and cruise. The data enables the validation of thermodynamic relationships that are used to estimate otherwise proprietary data. 2.3. Alternative SN-CBC Correlations. In a companion paper,14 we have shown that the correlation between SN and CBC is dependent on the size distribution of the BC particles. It was experimentally determined that for BC particles with geometric mean diameter (GMD) in the range 20−30 nm (of similar size to typical aircraft BC33) ⎡ mg ⎤ C BC⎢ 3 ⎥ = 0.236(SN)1.126 ⎣m ⎦

C BC ∝ [fuel]e(6390/ T ) − [air]e(−19778/ T ) ⎛ [air] (−19778/ T )⎞ e ∝[fuel]⎜e(6390/ T ) − ⎟, ⎝ ⎠ [fuel]

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where [Fuel] is proportional to the ṁ f and [air]/[fuel] is proportional to the AFR as measured at the engine exhaust, i.e. ⎡ mg ⎤ C BC⎢ 3 ⎥ ≈ ṁ f (A form e(6390/ Tfl) − AFRAox e(19778/ Tfl)) ⎣m ⎦

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where Aform and Aox are assumed to be constant and have units of mg s/kg-fuel m3, Tfl is the flame temperature (K) and ṁ f is the fuel flow rate (kg/s). EI(BC) is given by the product of CBC and Q, from eq 2; as eq 7 estimates the BC mass concentration in the core exhaust, the bypass ratio is no longer relevant. This is referred to as the Formation OXidation (FOX) method henceforth. This method is currently appropriate for engines burning conventional jet fuel since engine measurements from tests with conventional fuels are used to calibrate the model. However, it could be developed to capture the reductions in EI(BC) observed as a result of burning synthetic or other alternative fuels39 which contain fewer PAH compounds.40 In order to apply eq 7 methods to estimate Tfl are developed and FDR data is used to validate intermediate steps. First, during the LTO p3 is approximately linearly dependent on engine thrust setting; at zero thrust, p3 is approximately equal to atmospheric pressure, while at full thrust p3 is defined by π00. Linear interpolation between these two points leads to the following equation, which is validated in the SI,

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This updated correlation provides estimates of CBC that are 2.5−3 times greater than predicted by the correlation currently used in FOA3 (eq 1) for SN < 15. The precise correlation to use for any single SN measurement depends on both the particle size distributions and experimental details which can be in compliance with ranges specified by the standard test procedure34 and affect the measured SN for a given CBC. These experimental details may vary between engine manufacturers and are not disclosed in the EDB. We estimated that a ± 25% bound accounts for variation in GMD on the range 20−30 nm and uncertainty in the exact sampling set up of each manufacturer, specifically with regards to filter diameter. 2.4. Modeling EI(BC)The FOX Method. Existing models of soot emissions from aircraft gas turbine engines vary in complexity and detail,35 from empirical models to

⎛ F ⎞ p3 [atm] ≈ (π00 − 1)⎜ ⎟ + 1 ⎝ F00 ⎠ 10399

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airport directional pair is simulated and fuel burn along flight segments is calculated from BADA look-up tables and corrected for altitude and installation effects using Boeing’s Fuel Flow Method 2 (BFFM2). The LTO portion of each flight is modeled according to the methodology outlined by Stettler et al.,16 and the aircraft-engine assignments are also taken from that reference. Four BC estimation procedures are used to estimate global aviation emissions of BC in 2005: (i) the FOX method for LTO and cruise; (ii) FOA3 for the LTO and eq 1 to calculate CBC,ref using certification SN for cruise; (iii) as (ii) but with the updated SN-CBC correlation for LTO and cruise (eq 5); and (iv) current practice, comprising FOA3 for the LTO and the fleet average EI(BC) with altitude variation from Hendricks et al.24

T3 is estimated using the definition of the compressor polytropic efficiency,41 that is, ⎛ p ⎞γ − 1/ γηp T3 = ⎜⎜ 3 ⎟⎟ T2 ⎝ p2 ⎠

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where γ is the heat capacity ratio (γ = 1.4), ηp is the polytropic efficiency (assumed to be 90%45) and T2 is the engine inlet temperature. For the LTO atmospheric pressure p2 = 101 325 Pa and ambient temperature T2 = 293 K are assumed, and FDR data verifies this relation as shown in the SI. Finally, the dependence of the adiabatic flame temperature, Tfl, on T3 is calculated assuming stoichiometry and tabulated enthalpies, and is approximated to be linearly dependent on T3 (see SI), Tfl[K] = 0.9T3 + 2120.

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3. RESULTS AND DISCUSSION 3.1. Air to Fuel Ratios. As shown in Figure 1, the FOA3 suggested AFRs are within the range of measured values at the

In the Results and Discussion section, a relationship between AFR and engine thrust setting using measurements described in Section 2.2 is determined, and the constants Aform and Aox are obtained by minimizing the squared residuals between estimated and measured values of EI(BC) at ground level. 2.5. Estimating EI(BC) at Cruise. To estimate EI(BC) at cruise, we follow a similar approach to Petzold et al.;23 first a ground level reference CBC,ref is calculated for 100% F/F00 (i.e., maximum rated thrust) at ground level using eq 7. Equation 4 is then used to correct for conditions at altitude requiring methods to estimate p3 and Tfl at cruise. FDR data suggests that eq 8 also applies for engines operating at cruise altitude (see SI). At cruise the compressor inlet pressure and temperature, p2 and T2, are equal to the total (stagnation) pressure and temperature, respectively. These are obtained from the ambient (static) pressure and temperature (ps,Ts) and Mach number (Ma) from ⎛ γ − 1 2⎞γ / γ− 1 p2 [Pa] = ps ⎜1 + Ma ⎟ ⎝ ⎠ 2

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and ⎛ γ − 1 2⎞ T2[K] = Ts⎜1 + Ma ⎟ ⎝ ⎠ 2

Figure 1. AFRs for five different engines measured during the APEX 1−3 campaigns. Error bars represent 1σ variability in measurement data. The blue circles represent the suggested fleet-average AFRs from FOA3 and the solid red line represents eq 14.

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T3 is calculated by eq 9 and Tfl by eq 10. Finally, the equivalence ratio is inversely proportional to the AFR so that AFR ref ϕ = ϕref AFR

ICAO certification thrust settings for engines measured during the APEX 1−3 campaigns. For all engines, the AFR decreases with increasing thrust setting. There is significant spread in the measured AFRs at 7% F/F00 (90−115 kg-air/kg-fuel). At higher thrust, measured AFRs lie in the range 50−60. Fitting a linear equation to the inverse of the AFR (i.e., the fuel to air ratio) leads to the following relationship relating engine exhaust AFR to engine thrust setting,

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and AFRs at reference and cruise conditions are calculated using the relationship derived in Section 3. Fuel flow rate as a proportion of the maximum fuel flow rate at ground level is assumed to be representative of the engine thrust setting as a proportion of maximum;42 in the SI this assumption is shown to hold generally for certified engines. A summary of the FOX method and a worked example are also shown in the SI. 2.6. Global Aircraft Emissions Inventory. The Aviation Emissions Inventory Code (AEIC) developed by Simone et al.43 is used to estimate global civil aviation BC emissions in 2005. AEIC uses EUROCONTROL’s Base of Aircraft Data (BADA) version 3.9 and flight schedule data for 2005 to capture emissions associated with 99% of passenger movements. It has been shown to generate estimates of total global fuel burn within 4% of other published estimates for 2004 and 200625 and flight-level fuel burn with a mean bias of less than ±10% in general for different aircraft.43 Each unique aircraft-

⎞−1 ⎡ kg‐air ⎤ ⎛ ⎛ F ⎞ AFR⎢ ⎥ = ⎜⎜0.0121⎜ ⎟ + 0.008⎟⎟ ⎣ kg‐fuel ⎦ ⎝ ⎝ F00 ⎠ ⎠

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The majority of measurements fall within a ± 30% uncertainty on this function and this translates to a ± 30% uncertainty on Q estimated using eqs 2 or 3. Combined with the uncertainty on the SN-CBC correlation that we estimated in our companion paper14 (±25%), the uncertainty on estimates of EI(BC) derived from the SN are in the order of ±40%. 3.2. Aircraft SN Discrepancies. A comparison between SN data from the APEX 1−3 campaign and certification values 10400



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3.4. EI(BC) Measurements. The EI(BC) data from the three aircraft engine emissions measurement campaigns are included in the SI. For the majority of engines with conventional combustor technology, EI(BC) tends to increase with engine thrust setting. For some, EI(BC) increases at low thrust settings 65% F/F00) EI(BC) spans almost 4 orders of magnitude (1− 781 mg/kg-fuel) across different engines. 3.5. Current Practice Estimates of EI(BC). To implement FOA3, certification SNs from the EDB are used to estimate EI(BC) as is recommended by ICAO.15 The above results suggest that any discrepancies between FOA3 estimates and measurements will arise from (i) ∼30% uncertainty in fleet average AFRs; (ii) underestimation of EI(BC) arising from using the SN-CBC correlation used by FOA3 (factor ∼2−3); and (iii) changes to the SN over the lifetime of the engine or changes due to measurement details. In Figure 3 a comparison

contained in the EDB highlights significant discrepancies (figure and data in the SI). At low thrust settings (7 and 30% F/F00) the SNs measured during APEX 1−3 are 30−70% lower than the certification SN on average for the same engine at the same engine thrust setting. Conversely, at high thrust settings (85 and 100% F/F00) the APEX 1−3 SNs are on average a factor of 2 greater than the certification SN. Thus, for this small data set (N = 16), the certification SN is not representative of more recent SN measurements and this highlights that estimating EI(BC) from certification SNs is contingent upon the reliability of that data. We suggest that the SN is measured at the certification thrust settings as part of future measurement campaigns, in order to assess whether the discrepancy in SN is due to measurement details, engine degradation or engine maintenance cycles. 3.3. SN-CBC Correlation. The FOA3 methodology is used to estimate EI(BC) using SN measurements that were taken concurrently with EI(BC) measurements during APEX 1−3 (N = 61) (data in the SI). The volumetric exhaust flow is calculated using eq 2 or 3 depending on the engine type and AFRs are estimated using eq 14. For the APEX 1−3 engines the current FOA3 methodology systematically underestimates EI(BC) and the coefficient of determination (R2 = 0.10) indicates poor model performance. The alternative correlation from our companion paper14 (eq 5) leads to significantly improved EI(BC) estimates, all else equal (R2 = 0.79). This is consistent with reported measurements of the GMD for these engines33 which showed that for the majority of engine thrust settings, the GMD was on the range 10−20 nm, and supports the conclusion from our companion paper,14 which were that the correlation currently used in FOA3 is not suitable for aircraft BC as it was empirically determined for BC particles with larger GMD than reported for aircraft emissions. The majority of estimates using eq 5 are within ±50% of measured EI(BC), which provides an approximate uncertainty bound on this approach when the SN is known with confidence.

Figure 3. Comparison between estimated EI(BC) and measured EI(BC) derived from ICAO EDB SN data using FOA3 (black open box), the FOA3 approach with the updated SN-CI correlation from Stettler et al.14 (blue triangle), and estimates obtained from the FOX method (red circle) that are not dependent on SN. Error bars represent 1σ variability in measured EI(BC) and ±50% uncertainty in estimated EI(BC) using eq 5.

between modeled and measured values of EI(BC) at the ICAO certification test points (i.e., 7, 30, 85, and 100% F/F00) using the FOA3 method is shown. Data for the AE3007E and AE3007A1E at 8.4% F/F00 is included as representative of the idle test point. For the PW4158, only the maximum SN is contained in the EDB so the SNs at the other certification points are estimated using the method suggested by ICAO.15,44 FOA3 leads to a systematic underestimation of EI(BC), that engines for which the certification SN is zero have nonzero EIs and that the coefficient of determination is negative R2 = −0.10 (N = 37). The negative R2 values indicates that FOA3 performs worse than a mean EI(BC) value. EI(BC) is underestimated by a factor of 10 or more for 13 out of the 37 data points and all estimates at 85 or 100% F/F00 are underestimates. The magnitude of the observed underestimation eliminates the AFR uncertainty as a primary factor. In order to assess sensitivity to SN-CBC correlation, eq 5 is used in place of eq 1.

Figure 2. Comparison between estimated EI(BC) using SN measurements taken concurrently with EI(BC) measurements during the APEX 1−3 campaign. Estimates using the alternative SN-CBC correlation from Stettler et al.14 (eq 5) (red) and the correlation suggested in FOA3 (gray) are compared. Vertical error bars represent a ± 50% uncertainty in estimated EI(BC) and the shaded gray area represents a ± 50% error bound. 10401



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Table 1. Comparison between Measured and Estimated EI(BC) at Cruise Following the Proposed Methodologies

a

aircraft

A310−300

B737−300

A340

B707

engine

CF6−80C2A2

CFM56−3B1

CFM56−5C4

JT3D-3B

ṁ f/ṁ f,max (%) EI(BC) measured (g/kg-fuel) EI(BC) FOX CBC,ref (g/kg-fuel) EI(BC) FOA3 CBC,ref (g/kg-fuel) EI(BC) eq 5 CBC,ref (g/kg-fuel)

18.6 0.019 ± 0.010 0.017 0.001 0.002

22.5 0.011 ± 0.005 0.015 0.001 0.002

20.0a 0.010 ± 0.003 0.011 0.002 0.006

40.0a 0.500 ± 0.100 0.096 0.063 0.150

Fuel flow for A340 and B707 approximately 20% and 40% of maximum respectively.45

EI(BC) estimates are improved (R2 =0.34, N = 37) and systematic underestimation at higher thrust settings is reduced, however the majority of estimates do not agree with measurements within the measurement (1σ) and estimation uncertainty (±50%) suggesting that the remaining discrepancy is due to changes in the engine SN over time or inconsistency in measurement of SN. 3.6. FOX Estimates of EI(BC) During the LTO. The soot formation and oxidation constants, Aform and Aox, in the FOX model are found by minimizing the sum of the squared residuals between estimates and ground level measurements of EI(BC), leading to Aform = 356 mg s/kg-fuel m3 and Aox = 608 mg s/kg-fuel m3 (R2 = 0.68, N = 60). The modeled estimates are significantly improved over FOA3 at the ICAO certification thrust settings (R2 = 0.86, N = 37, cf. R2 = −0.10, N = 37 for FOA3). For thrust settings F/F00 ≥85% (N = 14) more than 90% of the FOX estimates agree to within ±50% of the measurements and this is taken as a lower bound on the uncertainty on the FOX method. The FOX estimates are compared to measured values in Figure 3 along with estimates using FOA3. If the activation temperatures are also allowed to vary, recalibration of the model does not lead to improvement in R2 and the optimized activation temperatures are within 10% and 1% of the values from Wen et al.38 for the formation and oxidation terms, respectively, which is indicative of the robustness of the activation temperatures applied here. If the EI is multiplied by engine fuel flow to obtain an emissions rate (mg/s), the FOX model provides the most accurate estimates compared to measurements and R2 values are 0.19, 0.65, and 0.80 for FOA3, FOA3 with eq 5 and FOX, respectively. This is discussed further in the SI. 3.7. FOX Estimates of EI(BC) during Cruise. Estimates of cruise EI(BC) using the FOX method, eq 1 and eq 5 are shown in Table 1 for four aircraft measured during the SULFUR 1−7 experiments.4,23 For the FOX method, estimates are within −11%, 27%, and 10% for the CF6−80C2A2, CFM56−3B1, and CFM56−5C4 respectively, within the reported measurement uncertainties. These errors are improved over the method proposed by Peck et al.26 and account for the reported engine fuel flow. If the certification SN is used to obtain CBC,ref using eq 1, FOA3, and eq 4 to relate ground to cruise, the estimates of EI(BC) at cruise are approximately an order of magnitude smaller than the measurements. Using eq 5 instead of eq 1 leads to underestimation by a factor of 2−9.5, indicating again that uncertainties associated with the certification SN data is likely a primary cause of underestimation by these methods. For the JT3D-3B, an older low-bypass ratio engine that first entered service in early 1970s, the measured EI(BC) at cruise is an order of magnitude greater than for the other more modern engines (post 1983, bypass ratio >5). The FOX estimate is a

factor of 5 low, reflecting the model calibration to modern engine data, and estimates based on the SN under-predict by a factor of 3.3 and 8 using eq 5 (i.e., FOA3 with the updated SNCBC correlation) and FOA3, respectively. AEIC, using BADA tables, predicts fuel flows at cruise altitude to be in the range 30−60% of maximum rated fuel flow (see SI). Moreover, FDR data shows cruise fuel flow to be in the range 20−50% of maximum rated fuel flow (see SI). Since EI(BC) is highly dependent on engine fuel flow, the SULFUR measurements4,23 of engines at reduced engine power are likely to be lower bounds of EI(BC) at cruise. In order to demonstrate the effect of cruise fuel flow, EI(BC) at cruise using the FOX method and assuming 60% of maximum fuel flow for the engines in Table 1 are recalculated. This results in 0.23, 0.13, and 0.14 g/kg-fuel for the CFM56−80C2A2, CFM56−3B1, and CFM56−5C4 respectivelyan order of magnitude higher than the EI(BC) measurements at cruise at 20% of maximum fuel flow (see SI for intermediate data). In the SI, the intermediate data used to derive these cruise EI(BC) estimates is included for each method discussed here. 3.8. Global Aircraft BC Emissions. An updated global inventory of aircraft BC emissions is obtained by implementing the FOX method combined with eq 4 within AEIC; this is compared to existing methods in Table 2. Using the FOX Table 2. Comparison of Global Aviation BC Estimates total BC emissions (Gg/year) FOX method FOA3 SN-CBC Stettler et al. SN-CBC (eq 5) standard practice Lee at al.1(2004)

average EI(BC) (g/kgfuel)

LTO

cruise

total

LTO

cruise

total

2.4 0.6 1.5

14.5 1.4 3.5

16.9 2.0 5.0

0.147 0.037 0.094

0.088 0.009 0.021

0.093 0.011 0.028

0.6

5.7

6.3 6

0.037

0.035

0.035 0.025

method it is estimated that in 2005, total BC emissions were 16.9 Gg/year, with a fleet average EI(BC) of 0.093 g/kg-fuel. These are a factor of ∼2.7 higher than estimates obtained using standard practicecomprising FOA3 for the LTO and the fleet average EI(BC) with altitudinal variation from Hendricks et al.,24 which leads to a total of 6.3 Gg/year and 0.04 g/kg-fuel. Using certification SNs and the SN-CBC correlation from FOA3 (eq 1) to estimate EI(BC) during the LTO and cruise (with altitude corrected by eq 4), leads to an estimate of global aviation BC emissions that is an order of magnitude lower than that from the FOX method. Using the updated SN-CBC correlation from Stettler et al.14 (eq 5) leads to a factor 2.5 increase compared to FOA3, yet this is a factor of ∼3.4 lower than the FOX estimate and 20% less than the standard practice estimate. The primary reason the at the FOX estimate is higher 10402



Article

∼20% fuel flow) would be extremely valuable and could be used to validate the FOX method and the updated global inventory of aviation BC emissions.

than previous estimates is that engine fuel flow at cruise has been accounted for, resulting in higher EI(BC) as discussed in the previous section. Using the FOX method, ∼9% of total fuel consumption and ∼14% of total BC emissions occur during the LTO cycle, reflecting the higher EI(BC) during high thrust engine operations (i.e., takeoff and climb). The fleet average EI(BC) for LTO is ∼70% higher than that for cruise. LTO emissions totals for the U.S. and U.K. using the different methods are included in the SI as specific examples. The spatial distribution of aviation BC emissions in 2005 using the FOX method is shown in Figure 4. The highest

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: [email protected]; e-mail: 617-452-2550. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was from UK EPSRC as part of the Energy Efficient Cities Initiative and MIT. The authors are grateful to MS&T and Aerodyne Research Inc., for supplying aircraft emissions data. We thank Nick Simone and our other colleagues at the MIT Laboratory for Aviation and the Environment for technical assistance.

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Figure 4. Global distribution of aviation BC emissions in 2005.

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density of BC emissions generally occurs where there is the highest aviation activity, that is, in North America, Europe, and East Asia. The locations of busy airports are apparent as a result of relatively higher EI(BC) during the LTO. EI(BC) at cruise typically lies on the range 0.050−0.100 g/kg-fuel (average 0.088 g/kg-fuel), consistent with our above analysis of cruise fuel flow. Charts showing the spatial distribution of aviation BC emissions by latitude and longitude are included in the SI and a gridded inventory is available at http://lae.mit.edu. In the most recent assessment of the climate impact of aviation BC, Lee et al.1 used a fleet average EI(BC) of 0.025 g/ kg-fuel with an estimated total fuel burn of 224 Tg in 2004, leading to 6 Gg of emitted BC. This total fuel burn is 24% greater than the 181 Tg predicted by AEIC for 2005 meaning that differences in average EI(BC) between our estimates and that of Lee et al. are not directly translated to differences in total aviation BC emissions. Thus, while the FOX method leads to a factor ∼3.7 increase in average EI(BC) over Lee et al., our total aviation BC estimate is a factor ∼2.8 higher. Using their estimate of aviation BC emissions, Lee et al. estimated the direct RF due to aviation BC to be ∼0.0034 W/m2. Applying a linear scaling to account for the increased estimate of BC emissions from the FOX method, the direct RF due to aviation BC is estimated to be ∼0.0095 W/m2, equivalent to ∼1/3 of the RF due to CO2 (0.0280 W/m2) and ∼70% of the net RF due to aviation NOx emissions (0.0138 W/m2). If indirect RF due to aircraft BC (i.e., contrails and induced cirrus cloudiness) were taken into account, the impact of the updated BC emissions inventory is likely to be even more significant. However, we note that there remains significant uncertainty in aviation direct BC RF estimates even given a specific BC emissions estimate. In conclusion, the FOX method provides an updated estimate of global aviation BC emissions. While it has been optimized to estimate EI(BC) during the LTO and shown to estimate EI(BC) at cruise with better accuracy than existing methods, further measurements of EI(BC) at cruise for flights that are representative of real operations (i.e., greater than 10403



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