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Identification of Lubrication Oil in the Particulate Matter Emissions from Engine Exhaust of In-Service Commercial Aircraft Zhenhong Yu,*,† Scott C. Herndon,† Luke D. Ziemba,‡ Michael T. Timko,† David S. Liscinsky,§ Bruce E. Anderson,‡ and Richard C. Miake-Lye† †

Aerodyne Research, Inc., Billerica, Massachusetts 01821, United States NASA Langley Research Center, Hampton, Virginia 23681, United States § United Technologies Research Center, East Hartford, Connecticut 06108, United States ‡

S Supporting Information *

ABSTRACT: Lubrication oil was identified in the organic particulate matter (PM) emissions of engine exhaust plumes from in-service commercial aircraft at Chicago Midway Airport (MDW) and O’Hare International Airport (ORD). This is the first field study focused on aircraft lubrication oil emissions, and all of the observed plumes described in this work were due to near-idle engine operations. The identification was carried out with an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF AMS) via a collaborative laboratory and field investigation. A characteristic mass marker of lubrication oil, I(85)/I(71), the ratio of ion fragment intensity between m/z = 85 and 71, was used to distinguish lubrication oil from jet engine combustion products. This AMS marker was based on ion fragmentation patterns measured using electron impact ionization for two brands of widely used lubrication oil in a laboratory study. The AMS measurements of exhaust plumes from commercial aircraft in this airport field study reveal that lubrication oil is commonly present in organic PM emissions that are associated with emitted soot particles, unlike the purely oil droplets observed at the lubrication system vent. The characteristic oil marker, I(85)/I(71), was applied to quantitatively determine the contribution from lubrication oil in measured aircraft plumes, which ranges from 5% to 100%.

1. INTRODUCTION The potential impact of aircraft gas turbine engine emissions upon the atmosphere has drawn extensive attention in recent years.1−10 A large number of particulate and gaseous pollutant detection and sampling techniques have been developed to monitor and evaluate the total emissions from modern aircraft engine exhausts.10,11 Particulate matter (PM) emissions from gas turbine engines are typically viewed to be products of the combustion process,12 such as black carbon soot particles. The sophisticated machinery of modern airplane engines requires lubrication systems, which are not typically considered as sources of PM emissions. Due to the low volatility of lubrication oil,13 any emitted oil vapor or drops will add to the condensed mass and contribute to the organic PM in the wake of the aircraft. The highly efficient fuel combustion of modern aircraft engines significantly reduces PM emissions, therefore emissions from the lubrication system can be a significant contributor to the overall emission signature. In some engine designs, the release from the engine lubrication system is vented into the engine core flow exhaust, while in other engines it is vented through the nacelle, which is the housing for the engine mounted on an aircraft. In the reported chemical analyses of diesel engine exhaust, lubrication oil was found in both the nucleation and soot modes.14,15 Although lubrication oils are thermally and © 2012 American Chemical Society

chemically stable liquids with low vapor pressures, the potential environmental impacts associated with the use of synthetic lubrication oils still need careful investigation. Human health impacts of aircraft lubrication oils including those from the volatile pyrolytic degradation products and the performance additives such as tricresyl phosphate (TCP) have been examined extensively and reported by a number of research groups.16−19 The main goal of this investigation was to study the contribution of lubrication oil to aircraft engine PM emissions. Characteristic ion fragmentation patterns of lubrication oil were investigated with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF AMS) in a dedicated engine test that measured the lubrication system vent.20 The current study extends the measurement to track the fate of lubrication oil for in-service aircraft where the vented oil vapor interacts with the aircraft combustion exhaust. In this study, we were able to identify lubrication oil in organic PM in commercial aircraft engine exhaust. Received: Revised: Accepted: Published: 9630

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2. EXPERIMENTAL SECTION 2.1. Field Measurements. The field measurements at Midway International Airport (MDW) and O’Hare International Airport (ORD) in Chicago were performed on February 17−18, 2010 with the help of Southwest Airlines, United Airlines, and the City of Chicago. Emission plumes of the aircraft were investigated using a variety of gaseous and particulate measurement instruments located on board the Aerodyne mobile laboratory.21,22 Engine exhaust plumes were sampled through 1-in. outside-diameter stainless tubing in front of the laboratory and drawn into individual instruments. During the field measurement period, the Aerodyne mobile laboratory was located at downwind locations to monitor air advected from the active taxiways (30−150 m). The measured meteorological conditions (e.g., wind speed and direction) were used to choose the mobile laboratory location. A camera located in front of the mobile laboratory provided additional information for aircraft identification and attribution of the plume origin. The gaseous measurements of CO, NO, and NO2 were used to assess the engine state. NO, NO2, and CO were measured by tunable infrared differential absorption spectroscopy (TILDAS) with pulsed quantum cascade lasers.23,24 CO was monitored using the infrared absorption line at 2183.2 cm−1, NO2 using the absorption line at 1606.37 cm−1, while the NO absorption doublet at 1915 cm−1 was used. Emission index of CO (EI-CO) decreases with engine power, while EI-NOx increases, so the ratio of EI-CO/EI-NOx is a very reliable indicator of engine power.25 The particle measurement instruments employed during these tests included a multi-angle absorption photometer (MAAP) (Thermo Environmental), a condensation particle counter (CPC, model 3776, TSI) and an engine exhaust particle sizer (EEPS, model 3090, TSI). These instruments provided information about particle absorption, number density, and mobility-based size distribution. Carbon dioxide concentration measurements were provided by a Li-Cor 820 CO2 analyzer. A HR-ToF AMS was deployed to detect semivolatile organic PM from aircraft engine exhaust. At present, AMS is the only available instrument capable of providing quantitative size and chemical mass loading information simultaneously in real time for submicrometer aerosol particles (50−700 nm). It measures the nonrefractory PM composition of the particles through thermal ablation on a hot metal surface followed by mass spectrometric detection of the vaporized compounds. Technical details of the AMS have been described in previous publications.26−28 In this study, the HR-ToF AMS used a newly developed fastMS (FMS) mode of data acquisition.29 In the applied FMS mode, a given run contained only chopper-open or chopperclosed data, and data were acquired in cycles according to the following sequence: two chopper-closed runs, each of 1-s duration, were acquired followed by 20 chopper-open runs, also each of 1-s duration, and so on. The AMS-measured organic PM predominantly came from black carbon soot particles emitted from aircraft engines since the limitation on transmission efficiency from the applied aerodynamic lens system (50−700 nm) keeps us from measuring the smaller nucleation mode particles (10−20 nm). 2.2. Laboratory Investigations. To characterize lubrication oil with the AMS, we performed a dedicated laboratory

investigation of lubrication oils from ExxonMobil and BP. On the basis of industrial marketing materials, the market share of ExxonMobil and BP for aviation lubrication oil used in the world’s engines is more than 83%. The oil samples were provided by the commercial airlines that took part in the APEX 1-3 (aircraft particle emissions experiment) field missions.30 The base stocks of lubrication oils are essentially a mixture of C5−C10 fatty acid esters of pentaerythritol,31 whose structure is shown in Figure 1. Each of the four branches of fatty acid

Figure 1. Molecular structure of the fatty acid ester of pentaerythritol, the main base stock of aircraft lubrication oils.

moieties contain between 5 and 10 carbon atoms depending on the synthesis procedure. Although the presence of the molecular ion has been considered as the most important information of analytical mass spectrometry, Tou32 reported more than three decades ago that, under electron impact ionization (70 eV), polyfunctional pentaerythritol derivatives exhibit extensive ion fragmentations and no information on the molecular ion can be obtained. Therefore identification of lubrication oil from the observed mass spectrum must be primarily based on its specific ion fragmentation pattern. In the laboratory study, a continuous flow of lubrication oil aerosol was generated with a pneumatic nebulizer and then size-selected to a diameter of 250 nm by a differential mobility analyzer (DMA, TSI, model 3081). The nebulized monodisperse oil aerosol was then introduced into a HR-ToF AMS. The laboratory results provide a direct comparison to the field measurements.

3. RESULTS AND DISCUSSIONS 3.1. Characteristic Ion Fragmentation Pattern of Lubrication Oils. Monodisperse aerosols of two aircraft lubrication oils showed a characteristic ion fragmentation pattern from the laboratory AMS measurements, as shown in parts A and B of Figure 2. The mass spectra of lubrication oils are compared to the decane (C10H22) mass spectrum obtained from NIST Chemistry WebBook.33 Since kerosene-based aviation fuel is a mixture of hydrocarbons with a carbon number distribution between 8 and 16, we chose decane as a suitable reference. Similar to decane (Figure 2C), two homologous series of 14 atomic mass unit (amu) intervals starting at m/z = 27 and 29, which are the so-called Δ = 0 and 2 9631

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The two major brands of lubrication oils in the market yield different values for the ion intensity (peak area) ratio of m/z = 85 over m/z = 71, I(85)/I(71). The ratio of I(85)/I(71) for the BP sample is 3.7 ± 0.2 (Figure 2A) and 8.6 ± 0.5 for the sample from ExxonMobil (Figure 2B). In addition, lubrication oil from BP yields appreciable ion fragments at m/z = 113 (C6H13CO+ and C8H17+) and 127 (C7H15CO+ and C9H19+), while for the ExxonMobil oil these ion peaks are negligible. In general, the sum of ion intensity at m/z = 113 and 127 is more than 50% of ion peak at m/z = 85 for the BP oil, but much less than 50% for the ExxonMobil oil. The identification of the two oils from AMS mass spectrum was based on this comparison. According to the archived mass spectra by NIST,30 the ratio of I(85)/I(71) is insensitive to molecular weight of alkanes. For instance it is 0.66 for decane (C12H26), 0.64 for hexadecane (C16H34), and 0.69 for dodecane (C22H46). The value of 0.66 ± 0.03 assumed for oil-free organic PM emitted from aircraft engines will be applied to the following analysis. Although the AMS produces a mass spectrum that is very comparable to those from NIST, the difference in various instrumental details like the temperature of ionization may result in a slight variation in relative peak intensities. On the basis of a recently published AMS analysis on a large number of AMS data sets,38 the ratio of I(85)/I(71) was determined to be 0.55 ± 0.31 for hydrocarbon-like organic aerosols (HOA). This experimental AMS result is consistent with our assumption based on the NIST database. The significant difference in I(85)/I(71) between lubrication oil and n-alkanes allows the AMS to make rapid identification of organic PM emissions that contain a large portion of aircraft lubrication oil. In the following analysis, we assume that the combustion procedure yielded organic PM with an I(85)/I(71) ratio of 0.66 ± 0.03 as n-alkanes. The oil contribution was identified base on a threshold, such that any measured value of I(85)/I(71) larger than 0.66 indicates the presence of lubrication oil. We investigated the mass spectra of monodisperse oil droplets from 100 to 350 nm and did not observe any noticeable difference in the spectra across this range of particle size. However, a dependence of lubrication oil ion fragmentation on source temperature has been observed in the laboratory investigation. In this study, the lubrication oil droplets were generated via atomization of lubrication oil, which was placed in a heated oven. The source temperature inside the oven was carefully controlled. The AMS measurement (Figure 3S in Supporting Information) indicates that the ratio of I(85)/I(71) decreases slightly with the heating temperature due to thermal pyrolysis. Therefore, temperature to which the emitted oil is exposed is one of the limiting factors for the quantification of lubrication oil emissions. For example, the exhaust temperature of CFM-56 7B22 engine at idle condition is about 400 °C.30 3.2. Identification of Lubrication Oils from Aircraft Engine Exhaust Plumes at MWD and ORD. 3.2.1. Plumes Measurements at MDW. While at Chicago Midway Airport on February 17, 2010, engine exhaust plumes were identified using CO2, CO, and NOx time series measurements. The particle number concentration, black carbon concentration, and the particle size information were also recorded. The AMS quantified the nonrefractory mass loading and composition of semivolatile organic PM present in the exhaust plumes. All of the identified plumes described in this work were due to nearidle engine operation determined by the ratios of CO and NOx to CO2: the ratio of EI-CO/EI-NOx is significantly larger than

Figure 2. Characteristic mass spectra of 250 nm monodisperse oil droplets obtained from C-ToF AMS measurements with (A) lubrication oil from BP; (B) lubrication oil from Mobil Oils; and (C) NIST standard mass spectrum of decane (C10H22). Due to the significant difference in ion fragmentation pattern between aircraft lubrication oil and long-chain alkanes, the ratio of I(85)/I(71) will be considered as the mass marker in the AMS measurements to identify lubrication oil.

series by McLafferty and Turecek,34,35 are apparent for both oil samples. Compared to the same series of decane, the stronger Δ = 0 series for lubrication oils indicates the presence of oxygenated alkyl chains as one of the major moieties of lubrication oils. However, unlike the mass spectra of n-alkanes such as decane, the mass spectra of lubricant oils give significantly different fragments patterns at the mass range above m/z = 57, specifically the ion signal at m/z = 85, but also m/z = 113, 127, and 155. Even though an ion signal at m/z = 85 is common for both alkanes and oils, the ratio of m/z = 85 over m/z = 71 allows their unambiguous differentiation. The less abundant ion signals at m/z = 113 and 127 vary appreciably between the two lubrication oils, probably a result of slightly different synthetic procedures and raw materials. According to the early mass spectrometric study on aliphatic and aromatic esters by Sharkey, McLafferty, and co-workers,36,37 C−O bond cleavage is one of the dominant pathways to generate fragment ions under electron impact ionization. From our AMS measurements on aircraft lubrication oils, the C−O bond breaking to form CnH2n+1CO+ is the primary ion fragmentation pathway. This mechanism was verified in this study by using a HR-ToF AMS. The HR-ToF AMS is capable of distinguishing C4H9CO+ from C6H13+ based upon their slightly different masses (85.070 vs 85.102 amu). We found that the pronounced m/z = 85 peaks in mass spectra of lubrication oils were primarily due to C4H9CO+, which came from the C− O cleavage of C5 fatty acid ester. The high-resolution MS measurement suggests that C5 fatty acid esters of pentaerythritol are the major components of the base fluids, in agreement with the specifications from the manufacturers. By comparison to m/z = 85, the signal at m/z = 71 is very weak, consistent with the lack of the C4 fatty acid ester moiety in lubrication oil. 9632

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Figure 3. A portion of the results from the field measurements on in-service commercial aircraft, which was performed at MWD on February 17, 2010.

unity at idle operation.25 Tail numbers were recorded and used to verify the visual identification of engine/airframe. A segment of the exhaust plume data set is presented in Figure 3, in which the plume event 2 was marked by a significant increase in the CO2, particle, and black carbon concentrations, as well as organic PM. For all the gaseous and PM measurements, statistical analysis was performed over the measurement periods that were without any identified plumes. Such analysis yields measurement uncertainties and background correction for each measurement instrument. Since the AMS measures particles from 50 to 700 nm in diameter, the detected organic PM emissions were primarily associated with black carbon soot emissions since volatile nucleation mode particles are mostly smaller than the AMS measurement range. The rest of the observed plume events shown in Figure 3 showed

increases in CO2 and particle concentrations, while the signals in the MAAP and AMS measurements were below the instrumental detection limits. In this study, the detection limit of the MAAP was about 3 μg m−3 for 15 s integration, while the detection limit was approximately 50 ng m−3 for the AMS. This observation indicates that the low AMS signal is related to a low soot concentration. Due to the limitation of particle transmission efficiency decreasing at small particle sizes in the AMS,39 we cannot determine the composition of nucleation mode particles, which contribute significantly to by CPC measurements. For event 2, the ratio of I(85)/I(71) was determined to be approximately 5.9 ± 0.8, indicating predominant contribution from lubrication oil to organic PM. To identify the type of lubrication oil in use, we obtained the full organic PM mass spectra of the individual plume by 9633

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measured by the AMS and black carbon (EI-BC) by the MAAP were calculated via the following equation:40

integrating the AMS measurements through each plume and subtracting the ambient background from the resulting spectra, and then we compared the obtained plume mass spectra with the standards of the two lubrication oils (Figure 2A,B). The obtained mass spectrum from event 2 is presented in Figure 4A,

⎛ mg ⎞ ΔM T EI⎜ × 4478 ⎟= ΔC(CO2 ) P ⎝ kg fuel ⎠

where ΔM is the measured mass in μg m−3, ΔC(CO2) is the difference between the sample and ambient CO2 concentrations (in ppm), T is the sampling temperature in kelvin, and P is the pressure (in torr). On the basis of the characteristic EICO2 of 3160 g/kg fuel for conventional aviation fuel and neglecting engine combustion inefficiency,41 we calculated the emission indices of organic PM and black carbon for the observed plume events and listed the results in Table 1. The listed uncertainties in Table 1 were calculated from the obtained measurement uncertainties. During the field measurements at MWD, the signal of organic PM and black carbon emissions for a large number of the plume events were below the instrumental sensitivity, except for event 2, in which we measured more than 1 order of magnitude higher EI-org compared to the previously published results.40 The analysis above demonstrates that the predominant contribution to organic PM in event 2 came from lubrication oil. 3.2.2. Plume Measurements at ORD. On February 18, 2010, analogous measurements were conducted at Chicago O’Hare International Airport (ORD). Figure 5 presents exhaust emission data for seven engine events (labeled as events 6− 12), all of which were taxiing based on the CO and NOx emission indices. The near constant delay (∼60 s) between the appearance of the taxiing aircraft in the center of the video camera frame and wind speed-based estimates of the advection distance corroborate that these events were from taxiing aircraft with engines operating near-idle. The sources of the plume events described in Figure 5 are listed in Table 1. The obtained organic PM and black carbon emissions are comparable with other published data. For instance, EIm-org and EIm-soot for the PW 4158 engine were 8 ± 1 and 39 ± 6 mg kg−1 from this study, compared to 2 ± 1 and 80 ± 20 mg kg−1 for the similar PW4077 from the APEX2/3 study.40 We observed more than 100 plume events during the two campaigns, but a large number of them were contaminated with other ground activities. The presented plume events are periods with little interference. The emission

Figure 4. The averaged mass spectra of organic PM from three observed runway plume events: (A) event 2 (MWD); (B) event 6 (ORD); and (C) event 12 (ORD).

along with those from other events. In comparison with the mass spectra in Figure 2A,B, especially the sum of ion peaks at m/z = 113 and 127 to that at m/z = 85, we identified the type of lubricant oil applied for the aircraft, and the results were listed in Table 1. Since we identified from the AMS measurement that the applied lubrication oil for event 2 was from ExxonMobil, which gives I(85)/I(71) = 8.60, the lubrication oil contribution to organic PM is determined to be approximately 66 ± 10% by assuming that the organic PM from engine combustion yields I(85)/I(71) of 0.66 ± 0.03. The emission characteristics of a combustion source is usually evaluated in terms of emission index (EI), which is mass of the emitted pollutant per mass fuel consumed, in the unit of mg kg−1 for PM mass. In this study, EI of organic PM (EI-org)

Table 1. Emission Indices of Organic PM (EIm-org) and Black Carbon (EIm-BC) Due to the Observed Aircraft Engine Plume Events As Well As the Determined Oil Contribution to Organic PMa,b eventc

engine

EIm-org (mg kg−1)

EIm-BC (mg kg−1)

1 2 3 4 5 6 7 8 9 and 10d 11 12

CFM56-7B24 BR715C1-30 CF34-3B1 CFM56-7B24 PW150A PW4077 V2522-A5 PW2037 PW2037 and CF34-3B1 AE3007A1P CF4-8E5G01

386 ± 49 8±1 10 ± 2 13 ± 2 12 ± 2 46 ± 2

200 ± 47 95 ± 54 39 ± 6 206 ± 10 214 ± 9 141 ± 8 117 ± 31 -

I(85)/I(71) 5.9 1.1 3.7 3.5 4.1 2.4

± 0.8

± ± ± ±

0.5 0.7 1.0 1.1

± 0.4

oil type ExxonMobil ExxonMobil ExxonMobil ExxonMobil BP BP

oil contribution 0.66 0.05 0.38 0.36 1.00 0.55

± 0.10

± ± ± ±

0.06 0.09 0.13 0.35

± 0.13

a

Hyphens (-) in the table indicate that the detected signals are less than the instrumental sensitivity because of the restricted distance of the Aerodyne mobile laboratory to the observed aircraft. bThe uncertainty corresponds to 1σ. cEvents 1−5 are obtained from MDW while 6−12 are from ORD. dEvents 9 and 10 overlapped, so the oil contribution applies to only one of those engines, but we cannot determine which one. 9634

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Figure 5. A portion of the results from the field measurements at ORD.

Given that I(85)/I(71) was approximately 1.1 for event 6 (Figure 5), the oil contribution to the organic PM emission was small for this long-range wide-body aircraft. On the contrary, events 9 and 10, which were both due to small regional jets, show a significant lubrication oil contribution to the organic PM. On the basis of the determined I(85)/I(71) ratio and lubrication oil type, we calculated the contribution of lubrication oil to organic PM and presented the results in Table 1. Our results indicate that the oil contribution to the total organic PM EI ranges from 5 to 100% depending on engine type and condition.

plume, event 6 in Figure 5, was generated by a PW4077 engine installed on a Boeing 777-222 (ETOPS) airframe. Events 9 and 10 arrived too close to one another and are indistinguishable from each other in the measurements. Mass spectra of the organic PM obtained for all the plume events were obtained and compared to the laboratory results to determine which type of lubrication was used. The determined oil type is also listed in Table 1, showing that both BP and Mobil lubrication oils were in common usage. Mass spectra from two plume events are presented in Figure 4B,C, to illustrate the variation in ion fragmentation patterns found for the two different oils. 9635

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3.2.3. Issues Related to Quantification of Lubrication Oil Contribution to Organic PM. A number of factors need to be considered when quantifying lubrication oil contribution to organic PM emissions. Since there are two common aircraft lubrication oils, the mass marker, I(85)/I(71), can only provide quantitative information after the applied lubrication oil has been determined. Therefore, the mass spectra of organic PM must be obtained to determine the oil type before the oil contribution can be calculated. However, in the case of small lubrication oil composition, the determination of oil type becomes difficult because of the experimental uncertainty in the AMS measurements. We could only provide an estimate in such cases. Since the oil-free organic PM emissions from aircraft engine was assumed to yield a value of 0.66 ± 0.03 for I(85)/I(71), the accuracy of the assumed value could also impact the accuracy of the quantification of lubrication oil contribution. For instance, if the value is 0.3 for the oil-free organic PM, the lubrication oil contribution to organic PM becomes (68 ± 10)%. Compared to the listed result of (66 ± 10)%, this variation of 2% appears much smaller than the experimental uncertainty due to the AMS measurement. In summary, this study reveals the unambiguous identification of lubrication oil constituents in organic PM emissions from taxiing commercial aircraft. Comparing the AMS field measurements of various commercial aircraft engine exhausts with those from the well-controlled monodisperse lubrication oil aerosols generated in the laboratory, we found that lubrication oil was clearly present in the organic PM emissions from the commercial aircraft engine exhausts, often associated with emitted soot particles, and could become the predominant composition in some of the cases. The intensity ratio of I(85)/ I(71) from the AMS measurements serves as an excellent mass marker of lubrication oils. Although the quantitative correlation between lubrication oil emissions and engine model, thrust, age, and ambient conditions is still under investigation, this study demonstrates that lubrication oil emission can be a significant component of organic PM in aircraft engine exhaust.

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Brooks, and Tim Onasch at ARI during instrument preparation activities and laboratory studies.

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ASSOCIATED CONTENT

S Supporting Information *

Particle size distributions from events 2 and 9/10; plot of the vapor pressure of pentaerythritol tetrapentanate, and the mass spectra of lubrication oil at various temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Airport Cooperative Research Program (ACRP) project 02-03a, the U.S. Department of Defense, through the Strategic Environmental Research and Development Program (SERDP), and NASA (NRA #NNC07CB57C). We thank Southwest Airlines, United Airlines, and the City of Chicago for providing us access to taxiways in the airports and the invaluable support to conduct measurements at MDW and ORD. We are also grateful for the assistance and logistical support offered by John Jayne, Bill 9636

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dx.doi.org/10.1021/es301692t | Environ. Sci. Technol. 2012, 46, 9630−9637