Particle and Gaseous Emissions from Commercial Aircraft at Each

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Environ. Sci. Technol. 2009, 43, 441–446

Particle and Gaseous Emissions from Commercial Aircraft at Each Stage of the Landing and Takeoff Cycle M. MAZAHERI, G. R. JOHNSON, AND L. MORAWSKA* International Laboratory for Air Quality and Health, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia

Received May 21, 2008. Revised manuscript received September 25, 2008. Accepted November 9, 2008.

A novel technique was used to measure emission factors for commonly used commercial aircraft including a range of Boeing and Airbus airframes under real world conditions. Engine exhaust emission factors for particles in terms of particle number and mass (PM2.5), along with those for CO2 and NOx, were measured for over 280 individual aircraft during the various modes of landing/takeoff (LTO) cycle. Results from this study show that particle number, and NOx emission factors are dependent on aircraft engine thrust level. Minimum and maximum emissions factors for particle number, PM2.5, and NOx emissions were found to be in the range of 4.16 × 1015-5.42 × 1016 kg-1, 0.03-0.72 g.kg-1, and 3.25-37.94 g.kg-1, respectively, for all measured airframes and LTO cycle modes. Number size distributions of emitted particles for the naturally diluted aircraft plumes in each mode of LTO cycle showed that particles were predominantly in the range of 4-100 nm in diameter in all cases. In general, size distributions exhibit similar modality during all phases of the LTO cycle. A very distinct nucleation mode was observed in all particle size distributions, except for taxiing and landing of A320 aircraft. Accumulation modes were also observed in all particle size distributions. Analysis of aircraft engine emissions during LTO cycle showed that aircraft thrust level is considerably higher during taxiing than idling suggesting that International Civil Aviation Organization (ICAO) standards need to be modified as the thrust levels for taxi and idle are considered to be the same (7% of total thrust) (Environmental Protection, Annex 16, Vol. II, Aircraft Engine Emissions, 2nd ed.; ICAOsInternational Civil Aviation Organization: Montreal, 1993).

1. Introduction It is well recognized internationally that the existence and operation of an airport has a potentially significant impact on the environment and health of people living or working in its vicinity in terms of related air pollutant emissions (2). The potential impact of airport operations on local air quality is mainly due to aircraft activities on the ground, categorized as modes within the landing and takeoff (LTO) cycle by the International Civil Aviation Organization (ICAO) (1, 3-5). Each location and airport facility has unique attributes that * Corresponding author phone: +61 7 3138 2616; e-mail: [email protected]. 10.1021/es8013985 CCC: $40.75

Published on Web 12/15/2008

 2009 American Chemical Society

affect the magnitude of the emissions during the LTO cycle. The rate of pollutant emission from an aircraft engine is directly related to the engine specifications, mode of operation, and airport layout which determines LTO cycle duration. Combustion of aviation fuel (kerosene) results primarily in the release of CO2 and water, and their exact proportions depend on the specific fuel carbon-hydrogen ratio. A range of other emissions are also produced such as particulate matter, NOx or SO2. As discussed in the Intergovernmental Panel on Climate Change (IPCC) report 1999, CO2, H2O, and SOx emissions are directly related to the engines fuel consumption in its different flight phases. CO and hydrocarbons (HC) are produced in greater proportion during lowpower engine operating conditions, while occurring during high power operation (3, 6). Particle and NOx emissions have a nearly linear relationship with engine thrust levels and therefore with combustion temperature (4, 7, 8). A small fraction of sulfur may be found in the additives currently used in jet fuel, and the fuel sulfur content is limited to 0.3% (6). The aim of this research was to quantify commercial aircraft particle and gas emission factors during different modes of LTO operation under real world conditions. Measurements were conducted at Brisbane Airport, Australia, at varying distances downwind from the aircraft using a novel sampling technique. This paper analyses particle (number and mass) and gas (CO2, and NOx,) emissions in engine exhaust plumes from a range of Boeing and Airbus airframes under real-world conditions.

2. Experimental Section The novel mobile measurement system used in these measurements is the Plume Capture and Analysis System (PCAS), developed and calibrated in International Laboratory for Air Quality and Health (ILAQH) at Queensland University of Technology (QUT), Australia. The PCAS and its application to aircraft emission measurements is described in detail elsewhere (9, 10). A description of the instrumentations used for the measurements, as well as the description of the measurement conditions and map of Brisbane airport is provided in the Supporting Information. 2.1. Data Analysis. A total of 283 individual aircraft were tested during LTO cycle, including idle, taxi, landing, and takeoff. The total number of measured aircraft in each aircraft family is shown in Table 1.

3. Results and Discussion All emissions referred to in this discussion are those of the aircraft engines attached to the airframe. Each combination of airframe and engine is referred to as an “aircraft” with type designation given as that of the airframe. This approach was taken because the specific engine type is not usually available for a given aircraft although each airframe type is typically equipped with a common engine type which is known. Wherever we refer to aircraft emissions, it is the engines which are assumed to be the emission source under discussion. 3.1. Concentrations of Emitted Pollutants. Figure 1 shows average concentrations of pollutants from B737 and B767 aircraft engine plumes during different modes of aircraft LTO cycle; measured at a distance of 80 m from the aircraft. The majority of the measurements used to calculate emission factors were also conducted at this distance. Measured background concentrations throughout the campaign were VOL. 43, NO. 2, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Total Number of Measured Airframes in Brisbane Domestic Airport airframe type

engine typea

Boeing 737 (B737) CFM56-7 Boeing 767 (B767) CF6 80C Boeing 777 (B777) PW40 Airbus-A320 (A320) CFM56-5 or IAE V2500 Airbus-A330 (A330) CF6-80E1 total number of measured aircraft a

taxi

takeoff

landing

total

19 5

88 10

2

6 2

57 14 1 9 4

43 11 3 9

207 40 4 26 6 283

Boeing and Airbus Web sites, refs 11, 12.

FIGURE 1. Average particle number, particle mass (PM2.5), CO2, and NOx concentrations measured at a distance of 80 m from B737, and B767 aircraft and at different phases of LTO cycle. at least 2 orders of magnitude lower than the concentrations measured in the aircraft engine plumes; however, subtraction of the background concentration was nevertheless performed. Despite inevitable variations in wind conditions and plume capture efficiency, the exhaust CO2 concentrations show a statistically significant correlation with engine power setting, and higher engine power settings correspond to higher concentrations. This trend may alternatively be a result of differences in the effective source geometry. For example the aircraft during takeoff resembles a line source but during idling is better approximated as a point source. Furthermore differences in engine technology may also result in differences in concentrations of CO2 and other emissions leaving the engine at a given level of thrust. As illustrated in the graphs, gas and particle number concentrations are also dependent on engine thrust level, although this is not the case for PM2.5 levels. The observation that PM2.5 concentration is highest during landing mode is likely to be due to the detection of larger particles produced during landing when aircraft wheels touch the runway producing a visible tire wear emission plume which mixes with the exhaust plume. The major sources of variance are likely to have been differences in dispersion due to wind direction and velocity, as well as possible engine type variations within the airframe 442

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type, and aircraft engine combustion processes including differences in the precise thrust level used for a given mode. 3.2. Aircraft Engine Particle Size Distribution. Particle size distributions obtained during measurements were also analyzed and classified base on aircraft type, and the mode of LTO cycle. The results obtained at the distance of 80 m from the aircraft are shown in Figures 2 and 3. Size distributions are presented as an average at each mode of LTO cycle, except for A320 during idle, due to insufficient available samples for this event. Particle size distributions (number and mass) were analyzed to identify the size distribution mode locations (nucleation, and accumulation modes). Measurement of B737 emitted particle size distributions were conducted using two sets of SMPS covering a wider size range from 4 to 710 nm size range. Comparing particle size distributions measured from B737 plumes to the background showed that particles larger than 118 nm originated from background aerosol rather than from the aircraft plume. Therefore, subsequent particle size distribution measurements were able to be conducted using a single SMPS covering the 4 to 157 nm particle size range. More information on particle size distributions is provided in the Supporting Information. In order to determine the particle concentrations associated with each size distribution mode, multimodal log-normal curves were fitted to the measured particle number size distributions using Microcal Origin, Version 6.0. The fitted curves were then used to calculate concentrations within the various modes. Particle mass size distributions were evaluated for each of the fitted modes by calculating particle volumes and assuming that the particles have a spherical shape and a density (F) of 1000 kg · m-3. The actual shape factor will depend strongly on the mechanism of particle formation which is beyond the scope of the current paper. The relationship between fractional contributions of particle number and mass from different modes in particle size distributions to measured total particle number and PM2.5 concentrations was also investigated. In general, size distributions exhibit similar modality during all phases of the LTO cycle. Figure 2 illustrate typical particle number size distributions for a representative background sample taken at the airport as well as for emissions from B737, B767, A320, A330 aircraft during different phases of LTO cycle, but not for B777 for which size distribution data was not available. The majority of the particles were found to be in the range of 4-100 nm in diameter in all cases. As two SMPSs were available for B737 aircraft plume measurements, full particles size distributions from 4 - 710 nm at each mode of LTO cycle are presented in this paper. These full size distributions were obtained by combining two separate SMPS scans taken for the aircraft plume sample. Results show that lower engine power settings result in lower concentrations, as can be seen from the size distributions for idle when compared to that of takeoff mode. According to ICAO (1), the thrust levels for taxi and idle are considered to be the same (7% of total thrust). However,

FIGURE 3. Typical B737, B767, A320, and A330 particle mass size distributions calculated for at different phases of LTO cycle (N ) nucleation mode, A ) Accumulation mode). The average background size distribution is also shown (sampling distance ) 80 m).

FIGURE 2. Typical B737, B767, A320, and A330 particle number size distributions measured at a distance of 80 m from the aircraft and at different phases of LTO cycle (N ) nucleation mode, A ) accumulation mode). The average background size distribution is also shown. particle size distributions obtained at idle show lower particle number and mass concentrations and are distinctly different from those for taxiing. This suggests that aircraft are operated at higher thrust levels when taxiing than is the case when idling. A very distinct nucleation mode was observed in all particle size distributions, except for A320 aircraft during taxiing and landing. This difference must be attributed to A320 engine properties and combustion, as measurement conditions were similar for all types of aircraft. Variation in particle size distribution due to different aircraft engine technologies was also mentioned by Herndon et al. (13) for takeoff and idle plumes. The nucleation mode consists of particles with diameters smaller than 30 nm and has the highest number concentration of all modes in all cases, excluding A320 aircraft during landing. This finding is consistent with the Rogers et al. (14), and Herndon et al. (15) results reporting a mode at 7 nm, Photometric Estimate of PM2.57 using DusTrak with Standard Calibration) particle number % concentration

a

PM2.5 % concentration

airframe type

LTO cycle mode

N

A

N

B737

idle taxi landing takeoff

99.8 99.4 99.9 99.6

0.2 0.6 0.1 0.4

9.6 9.2 1.2 20.6

0.9 1.3 0.8 11.8

B767

taxi landing takeoff

98.6 99.5 99.3

1.4 0.5 0.7

2.7 0.7 5.7

11.4 4.0 14.9

A320

idle taxi landing takeoff

96.5 71.8 93.6 97.3

3.5 28.2 6.4 2.7

0.6 NDa NDa 1.7

7.0 77.5 14.0 34.1

A330

takeoff BGb

99.9 99.3

0.1 0.7

6.2 7.6

4.9 7.9

ND ) no data was available.

b

A

BG refers to a representative background sample at airport.

TABLE 3. Measured Emission Factors during Different Phases of LTO Cycle Using PCAS. Uncertainty is Given as 2× Standard Error of the Mean EFNOx (g.kg-1) according to ICAO DataBank (18)

airframe type

aircraft ground operating mode

EFn (kg-1)

EFPM2.5 (g.kg-1)

EFNOx (g.kg-1)

B737

idle taxi landing takeoff

(1.28 ( 0.48) × 1016 (3.29 ( 0.20) × 1016 (4.26 ( 0.39) × 1016 (5.31 ( 0.58) × 1016

0.16 ( 0.02 0.20 ( 0.03 0.34 ( 0.08 0.27 ( 0.05

3.25 ( 0.44 4.13 ( 0.29 8.73 ( 0.54 16.54 ( 0.77

4.2 ( 0.1 4.2 ( 0.1 8.9 ( 0.3 20.8 ( 1.2

B767

idle taxi landing takeoff

(4.16 ( 2.27) × 1015 (3.78 ( 0.21) × 1016 (4.00 ( 1.17) × 1016 (4.86 ( 0.31) × 1016

NDa 0.03 ( 0.00 0.46 ( 0.20 0.18 ( 0.02

4.00 ( 1.32 6.77 ( 2.68 10.46 ( 1.35 16.18 ( 0.94

4.6 ( 0.1 4.6 ( 0.1 11.9 ( 0.3 26.9 ( 0.6

B777

idle taxi landing takeoff

NDa NDa (2.92 ( 0.11) × 1016 NDa

NDa NDa 0.72 ( 0.24 NDa

NDa NDa 9.96 ( 1.37 NDa

4.6 ( 0.5 4.6 ( 0.5 12.4 ( 0.4 47.2 ( 2.7

A320

idle taxi landing takeoff

NDa (6.57 ( 1.45) × 1015 (7.74 ( 1.47) × 1015 (2.09 ( 0.40) × 1016

NDa 0.14 ( 0.03 0.44 ( 0.17 0.23 ( 0.03

NDa 3.55 ( 0.69 8.10 ( 1.57 17.22 ( 2.97

4.1 ( 0.1 4.1 ( 0.1 9.0 ( 0.2 25.7 ( 1.2

A330

idle taxi landing takeoff

NDa NDa NDa (5.42 ( 1.40) × 1016

NDa NDa NDa 0.24 ( 0.06

NDa NDa NDa 37.94 ( 3.74

4.6 ( 0.0 4.6 ( 0.0 10.0 ( 0.1 41.5 ( 1.8

a

ND ) no data was available.

Calculation of the percentage of total particle number in the accumulation mode (40-100 nm) was performed using the fitted curves and the total particle number concentration recorded by the CPC. For each event, total particle number under the fitted accumulation mode was divided by the particle number concentration measured by CPC 3022. The CPC detects all particles larger than 7 nm. Therefore the concentration recorded by the CPC is assumed to consist of the nucleation mode which may extend below the range of the SMPS as well as the larger diameter modes. In order to find the true nucleation mode concentration, a reasonable assumption is adopted that the decline of number concentrations with increasing diameter, which is always seen in the size distribution measurements, will continue toward larger diameters beyond the SMPS range up to and beyond 444

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the range of the CPC. Therefore the difference between the particle number concentrations measured by the SMPS and the CPC is attributed entirely to undetected nucleation mode particles, too small to be included in the size distribution measurement. Particle mass fractions were estimated by calculating the particle mass under the fitted log-normal curves for each mode and dividing it by the measured PM2.5 concentration. As presented in Table 2, nucleation is the dominant mode in terms of particle number concentrations. Conversely, the accumulation mode frequently yielded higher mass fractions than the nucleation mode in terms of PM2.5 although there were a number of exceptions to this. The partitioning of the PM2.5 mass between the two modes is most pronounced in the case of the A320 during taxiing where 77.5% of PM2.5 is

located in the accumulation mode. Examination of Figure 2 reveals that this results from the fact that no prominent nucleation mode occurred. The measured A320 particle size distribution during taxiing exhibits a unimodal curve peaked at around 28 nm diameter. In fact the nucleation mode was less prominent for this aircraft in all modes. The exceptions mentioned above include the B737 during the LTO cycle and the A330 during takeoff. As shown in Figure 2, the accumulation mode was more prominent in the B767 and A320 plume size distributions. The sum of fractional contributions in terms of particle number and mass are less than 100%, as the remainder is attributed to the larger particles (>100 nm) which are beyond the accumulation mode particles. The sum of the PM2.5 mass fractions for B737 during landing is found to be 2%. This is due to the fact that thrust and hence engine emissions are low at the point of landing while high concentrations of larger tire wear particles, are produced as the aircraft wheels touch the runway, and these are detectable by the DustTrak but beyond the SMPS detection range (>710 nm). The observed differences in size distribution modality and hence in the mass fractions associated with the nucleation and accumulation modes are presumably due to differences in either the make up and temperature of the exhaust at the exit plane or to differences in the dynamics of particle formation in the dispersing plume which may depend on the exit velocity and plume dimension. As production of accumulation mode particles is mostly influenced by engine parameters rather than fuel effects, this dominant mode for most takeoff size distributions might be attributable to the fact that the aircraft engine is operated at close to full power during takeoff resulting in higher particle concentrations and temperatures at the engine plane and therefore more intensive coagulation of the primary particles (7, 16). These findings are in agreement with previous aircraft emission size distribution studies (5, 7, 17). 3.3. Aircraft Plume Emission Factors. Average particle number, particle mass (PM2.5), and NOx emission factors obtained during different phases of LTO cycle, along with available NOx emission factors from ICAO DataBank are listed in Table 3 for all the tested airframes (18). Emission factors presented here are the average values over all the aircraft engine plume samples for the given airframe and phase of LTO cycle. EFPM2.5 during idle mode was not included for B767 airframes as large statistical errors were observed with those results which were attributed to instrument (TSI DustTrak) malfunction during those sampling events. In general, measured EFNOx compare well with the ICAO DataBank, although ICAO has reported higher EFNOx for takeoff events. This could be due to the fact that the engine power was considered to be at 100% in ICAO test measurements while that may not always be the case. The exact takeoff engine power could be less than 100%, as it depends on factors such as weight of the aircraft. Emission factor results from this study suggest that aircraft thrust level is considerably higher during taxiing than idling mode. Results show that all emission factors increase with engine thrust level, except for that of PM2.5. This is in agreement with the literature proposing that particle mass and NOx emissions and power settings have a nearly linear relationship (7, 8). Lobo et al. (2007) reported the highest EFn at the highest fuel flow rates (0.85 kg · s-1), lowest between 0.2-0.4 kg.s-1 and a higher value at the lower fuel flow rates (0.1 kg · s-1) from a CFM56-2C1 engine and at the distance of 1 and 10 m (19). Their result at 30 m location showed highest EFn at lowest fuel flow rates. The variations in the results for EFn as a function of engine thrust can depend on the differences in the FSC, and aircraft engine type, sampling distance, and therefore depend on the atmospheric conditions which affect the nucleation mode particles (19, 20). It

is clear that EFn, and EFNOx are highest during takeoff, when aircraft are operating at full power, and lowest during idle when engines are operating at low thrust level. Emission factors quantified in this study can be applied for further environmental assessment at airport and developing an emission inventory at airport. The resulted database can help to determine the realistic aircraft exhaust emissions in real world situations.

Acknowledgments This project was supported as a collaborative research agreement between Queensland University of Technology (QUT), and Brisbane Airport Corporation (BAC), Brisbane, Australia. We thank Steve Goodwin, General Manager of Operations; Helen Clarke, Acting Environmental Manager; Tammy Loewe-Baker, Environment Administration Assistant; John McCaffery, Safety and Standards Manager; Mike Goller, Airport Operations Coordinator; from BAC; Prof. Ashantha Goonetilleke; from QUT; and also all airport operation staff at security Gate1 for their valued assistance and cooperation. Thanks also to Jim Drysdale and Bob Organ from QUT for their efforts in making the PCD.

Supporting Information Available Instrumentation, aircraft emission measurements, and three figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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