Atmospheric Measurements of the Physical Evolution of Aircraft

Jan 28, 2013 - chemical evolution of aircraft exhaust plumes on the time scale of 5 s to ... As the plume ages, the particle number emission index ini...
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Atmospheric Measurements of the Physical Evolution of Aircraft Exhaust Plumes M. T. Timko, E. Fortner, J. Franklin, Z. Yu, H. -W. Wong, T. B. Onasch, R. C. Miake-Lye, and S. C. Herndon* Aerodyne Research, Inc., 45 Manning Road, Billerica Massachusetts 01821, United States S Supporting Information *

ABSTRACT: Drawing from a series of field measurement activities including the Alternative Aviation Fuels Experiments (AAFEX1 and AAFEX2), we present experimental measurements of particle number, size, and composition-resolved mass that describe the physical and chemical evolution of aircraft exhaust plumes on the time scale of 5 s to 2−3 min. As the plume ages, the particle number emission index initially increases by a factor of 10−50, due to gas-to-particle formation of a nucleation/growth mode, and then begins to fall with increased aging. Increasing the fuel sulfur content causes the initial increase to occur more rapidly. The contribution of the nucleation/growth mode to the overall particle number density is most pronounced at idle power and decreases with increasing engine power. Increasing fuel sulfur content, but not fuel aromatic content causes the nucleation/growth mode to dominate the particle number emissions at higher powers than for a fuel with “normal” sulfur and aromatic content. Particle size measurements indicate that the observed particle number emissions trends are due to continuing gas-to-particle conversion and coagulation growth of the nucleation/growth mode particles, processes which simultaneously increase particle mass and reduce particle number density. Measurements of nucleation/growth mode mass are consistent with the interpretation of particle number and size data and suggest that engine exit plane measurements may underestimate the total particle mass by as much as a factor of between 5 and 10.

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INTRODUCTION Aircraft gas turbine combustion represents a small ( “moderately diluted” high sulfur JP-8 plumes (gray markers in Figure 1, no. 3 in the list) > “heavily diluted” low sulfur JP-8 plumes (the blue line in Figure 1, and no. 4 in the list) > “slightly diluted” JP-8 plumes captured at the engine test stand (the black line in Figure 1, and no. 1 in the list). Measurements made at 30 m and in the plume (shown in the SI as Figure S-4 clearly indicate that EIm-soot (and by implication EIn-soot) is invariant with sampling location and fuel sulfur content, as is expected to be. Therefore, differences in EIn-total between nos. 2, 3, and 4 must be due to formation of new particles in the plume. Because the smallest particles that we can detect are >5.6 nm (using the EEPS), we

Figure 2. Particle size data measurements made using the EEPS for (a) “moderately diluted” JP-8 combustion plumes (category no. 2 JP-8 plumes) and (b) “heavily diluted” JP-8 combustion plumes (category no. 4 JP-8 plumes). Fuel sulfur content was 121 ppmw in both cases. Data are normalized by ΔCO2 to remove artifacts due to different dilution levels.

content, and EIn-total. Figure 2 plots total particle size data in volume mode, after normalizing the data by the plume ΔCO2. Figure 2a provides data for “moderately diluted” plumes (red line in Figure 1, category no. 2) and Figure 2b shows data for “heavily diluted” plumes (blue line in Figure 1, category no. 4). Figure 2a shows distinct size modes for soot (ranging from roughly 50 to 200 nm), nucleation/growth particles (200 nm). In Figure 2a, the relative ratio of the soot mode and nucleation/growth modes varies with power; at low power, the two modes have roughly equal weighting and the soot mode 3516

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concentrations of particle precursors. As power is increased, however, EIn-total for the FTJF plumes exceeds that measured for both the low and high sulfur JP-8 plumes. This result is also understood as the result of differences in the interactions between nucleation mode and soot mode produced by combustion of the different fuels. Combustion of zero-aromatic FTJF reduces soot emissions (EIm-soot) by 10 (at idle) and 3 (at 100% takeoff).18 Therefore, the soot surface area available for coagulation of nucleation mode particles in the FTJF plumes is greatly reduced relative to the JP-8 combustion plumes, minimizing the soot-nucleation mode interaction, leading to a nucleation/growth mode that dominates at higher power, and overall increasing EIn-total compared to JP-8 combustion. AMS particle mass measurements serve to corroborate the conclusions reached from analysis of Figures 1 and 2 and provide some additional understanding. Figure 3a shows EImorganic and Figure 3b shows EIm-sulfate for 4 and 7% idle combustion plumes for various fuel types, plotted as functions of ΔCO2. We focus Figure 3 on idle data because Figure 2 shows more pronounced microphysical evolution of the nucleation mode at idle conditions than at higher powers.

becomes increasingly dominant as power increases. This behavior is to be expected based on the competition between soot mode accumulation and formation of new particles that has been explained previously here and elsewhere.6,35 Figure 2b shows that for “heavily diluted” plumes the clear distinction between nucleation mode and soot mode has vanished, an apparent result of nucleation particle coagulation-mediated growth. The increased size of the nucleation/growth mode particles shown in Figure 2 supports the conclusion that the EIn-total behavior observed for categorie nos. 2 and 4 is due to the nucleation/growth mode particles in the early dilution process (gray markers, category no. 2), followed by a decrease in EIn-total due to coagulation of the nucleation mode particles (blue line, category no. 4). The effect of increasing fuel sulfur content is to accelerate these processes so that the initial maximum in EIn-total occurs at earlier plume ages (and higher ΔCO2) than we can access safely from the mobile laboratory. Turning our attention now to data presented in Figure 1 for combustion of nonpetroleum fuels, we consider first the data points associated with HRJF combustion plumes. The HRJF data set is restricted to 4 and 7% data because these were the only combustion plumes that could be cleanly attributed to HRJF; all other HRJF plumes suffered some degree of mixing with JP-8 combustion plumes due to the fact that only one engine burned HRJF at a time (as discussed in the SI). The limitation in data notwithstanding, the available HRJF EIn-total data show several interesting features. First, as expected, the HRJF EIn-total data are lower than those measured for the JP-8 combustion plumes, irrespective of their sulfur content or age. This is expected since sulfur species generated by combustion of fuel sulfur compounds play an important role in particle formation events. Second, and more surprisingly, the HRJF plume EIn-total data are greater than measured for JP-8 combustion gases at the 30 m test stand. This observation shows that reduction of fuel sulfur below 10 ppmw does not eliminate particle formation and nucleation in aircraft exhaust plumes for this CFM56 engine, as had previously been reported for a PW308 engine.7 Relative to the previous work performed with a PW308 engine, the CFM56 engine studied here produces between 5 (at takeoff) and 10 (at idle) less soot. Therefore, the soot-nucleation mode interaction is much more important in the PW308 exhaust plume than the CFM56 exhaust plume, even for combustion of low aromatic fuels, like FTJF and HRJF. Finally, and in direct contrast to the JP-8 combustion plumes, EIn-total increases slightly with aging for the HRJF combustion plumes. The increasing EIn-total observed with aging of HRJF combustion plumes can likely be attributed to an increasing number of particles that grow to sizes large enough to be detected by the CPC (>7 nm) over time. In contrast, even at the earliest plume age measurements that we can capture on the mobile laboratory, the JP-8 combustion plume nucleation particles have already grown to sizes large enough for detection. The microphysical evolution that we can sample for JP-8 combustion plumes is dominated by coagulation and growth to larger sizes than the CPC cutoff diameter. Figure 1 presents data obtained for sulfur-spiked FTJF plumes that adds a final element to the picture. Interestingly, at idle powers the FTJF plumes behave similarly to the high sulfur JP-8 plumes. On consideration, this result is easily reconciled by understanding that, at idle, interaction between nucleation mode materials and soot surfaces is not important due to the fact that soot emissions are relatively low compared to the

Figure 3. Plume measurements of (a) EIm-organic and (b) EIm-sulfate. Data are restricted to plumes generated at 4 and 7% idle power. ΔCO2 is used as a measure of plume dilution and age; decreasing ΔCO2 indicates “older” exhaust. Measurements made using the AMS. 3517

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To place the EIm-sulfate measurements in context, we estimated the SO2-to-SO3 conversion efficiency implied by the AMS data. Initially, the measured conversion efficiency is approximately 0.1%, consistent with previous test stand measurements reported by Timko et al.6 With increasing dilution, the estimated conversion efficiency increases to 0.5− 1%, consistent with previous aircraft plume measurements.45 The data presented in Figures 1−3 provide a consistent picture of the physical characterization of aircraft plume evolution. Figure 4 is a simplified schematic describing

Interpretation of Figure 3 benefits from recalling that the instrument used to gather Figure 3 data (the AMS) is sensitive to particles >80 nm,43,44 whereas the instruments used to gather data shown in Figures 1 and 2 (the CPC and EEPS, respectively) are sensitive to particles as small as 7 nm. Figure 3 includes EIm data measured at the 30-m test stand during AAFEX-2; the 30-m test stand data are a composite average, representative of all power conditions and plotted arbitrarily at ΔCO2 of 250 ppm. The “low dilution” EIm data (i.e., ΔCO2 > 200 ppm) start at values comparable to those measured at the test stand. However, as the plume ages (and ΔCO2 goes to 0), the EIm-organic and EIm-sulfate tend to increase, though combustion of different fuels leads to qualitatively different trends. EIm data in Figure 3 can be grouped into three categories, based on fuel type: (1) zerosulfur, zero-aromatic fuel (HRJF); (2) “high sulfur”, low aromatic fuel (the sulfur spiked FTJF); (3) “normal aromatic”, “normal sulfur” fuels, a category that includes pure JP-8, blends, and different fuels for the different engines, provided that at least one engine is running on fuel with aromatic content and the other is running on fuel with sulfur content. Focusing on category no. 1, the zero-sulfur, zero-aromatic HRJF shows no evolution of EIm-organic or EIm-sulfate, behavior that is consistent with Figure 1 that showed that HRJF combustion produces much lower particle number densities than the other fuels. Category no. 3 fuels show that, provided that the fuel contains at least 50% of the aromatic and sulfur present in “normal” JP-8, EIm data monotonically increase from their measured values at the test stand (on the order of 2−8 μg m−3 for EIm-organic and 0−3 for EIm-sulfate) to about 40 ± 10 μg m−3 and 3.5 ± 0.5 μg m−3, respectively. We interpret the increasing values of EIm-organic and EImsulfate observed for increasing dilution levels to be associated with an increase in the overall particle size of organic/sulfate bearing particles into a size mode that is detected in higher efficiencies by the AMS. Therefore, the observed increases in EIm-organic and EIm-sulfate are consistent with the decreased EIn-total measured by the CPC and reported in Figure 1 and the increased particle size measured by the EEPS and reported in Figure 2. Removing the aromatic content but not the sulfur content of the fuel (as was done with the sulfur-spiked FTJF), causes EImorganic to increase more slowly at first; but, the values obtained at the longest aging times are similar for sulfur-spiked FTJF and EIm-organic fuels. This is consistent with FTJF combustion producing lower amounts of soot PM; the AMS requires that the FTJF nucleation mode grow in size to the point that they can be detected, whereas in the JP-8 combustion plumes nucleation/growth mode material grows into a size mode that is visible to the AMS by coagulating and coating the surfaces of soot particles. Like EIm-organic, EIm-sulfate evolution for the sulfur-spiked FTJF combustion plume may be slightly delayed relative to the JP-8 containing plumes. However, the high sulfur content of the sulfur-spiked FTJF causes EIm-sulfate to attain higher values after increased dilution than the lower-sulfur containing JP-8 plumes. Regardless, the ratio of organic to sulfate materials in plumes in categorie nos. 2 and 3 is about 10:1, though Figure 3a shows that EIm-organic may be approaching a maximum, whereas Figure 3b shows that EImsulfate has not yet reached a maximum. Therefore, Figure 3 shows that the ratio of EIm-sulfate to EIm-organic is slowly increasing with increasing dilution levels. The explanation of this effect is not clear.

Figure 4. Schematic representation of particle size distribution (for both soot and nucleation mode PM), EIn, and EIm. The lower panel shows how the nucleation/growth and soot modes evolve over time; the top panels show EIn and EIm evolution. The horizontal axis represents times ranging from 0.1 s to 3 min, in log units. The vertical axis in the lower panel represents particle diameter.

microphysical gas-to-particle conversion processing for idle conditions characterized by a high initial precursor to soot ratio. We generated Figure 4 using the microphysical algorithm described previously.35,36 The microphysical algorithm contains expressions for gas-to-particle conversion (nucleation and growth) and particle−particle coagulation. Although the algorithm does not capture the correct time scales quantitatively (and hence we omit the time scale from Figure 4), it does capture the qualitative features of the plume evolution process that we have observed. At the engine exit plane, soot is emitted as particles; a range of organic and sulfate precursors exist as vapors in the hot exhaust. Dilution and cooling processes that occur during exhaust plume aging promote new particle nucleation and growth through gas-to-particle conversion of these precursors. These materials form a nucleation/growth mode on the time scale of seconds, faster than our current measurement capability. On the scale of minutes, our measurements, summarized in Figures 1 and 2, show that the nucleation mode particles grow in size, and the schematic captures this growth as the result of condensation of additional gas-phase precursors and coagulation. As a result of gas-toparticle conversion processes, the EIn-total decreases, EIm-total increases, and particle size increases. AMS measurements of particle mass (which are limited more severely to larger particles than are the measurements of particle number density) are consistent with steady particle growth during the first several minutes of the plume aging process. After an estimated 3518

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Force, Arnold Engineering Design Center); Edwin Corporan (Air Force Research Laboratory); Matt DeWitt (University of Dayton, Research Institute); Dave Liscinsky (United Technologies); Anuj Bhargava (Pratt & Whitney); Phil Whitefield, Don Hagen, and Prem Lobo (Missouri University of Science and Technology).

2−3 min of aging, EIm-organic is 4-times greater than measured at the 30 m test stand, whereas EIm-sulfate has increased about 7-fold. Currently, our schematic representation captures the dominant features of plume evolution by considering only physical processes of gas-to-particle conversion. Preliminary data obtained at AAFEX-2 suggest that chemical aging may be taking place at time scales of tens of seconds or minutes. We include these preliminary data as Figure S-5 in the SI. Engine power and fuel composition plays an important role in the PM evolution process, at least for “major” departures from JP-8 combustion. In both cases, the driving force is the ratio of particle precursors (both organic and sulfate) to soot. Increasing power decreases the precursor to soot ratio, resulting in greater interaction of the precursors with the soot and reducing EIn. Removing the aromatic content of the fuel reduces the soot concentration in the exhaust plume and forces microphysical evolution to favor self-coagulation of nucleation mode particles, rather than interaction with the soot. Increasing the sulfur content accelerates the nucleation of new particles, which grow via condensation and coagulation processes during aging. Test stand extraction measurements provide an initial snapshot of the aircraft plume. Airport measurements, taken beyond the fenceline, provide a second snapshot. Here, we show field measurements that tie these two sets of measurements together in a framework of microphysical processes. Continuing evolutionon the time scales of seconds to minutesmeans that using data based on measurements made either at the test stand or at a fixed point beyond the airport fenceline will lead to incomplete projections of potential environmental impacts, especially for neighborhoods surrounding airports.

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ASSOCIATED CONTENT

S Supporting Information *

An aerial view of the AAFEX-2 test site, overlaid with the Mobile Laboratory positions logged by the GPS; a table describing the instruments used during the AAFEX campaigns; a discussion of size cut-offs and diameter measurements; a figure showing data from a representative plume encounter, the expressions used for EI calculation, a description of quality control protocol; description of plume age estimation methods; safety notes; a figure comparing EIm-soot measurements made in the plumes to those made at the engine test stand; and a figure providing preliminary data of chemical plume aging. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 978-932-0266, e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS NASA led the AAFEX-1 and AAFEX-2 missions, and DAOF graciously hosted the emissions measurements. We acknowledge support from FAA (PARTNER contracts through MS&T) and NASA (NC07CB57C). The experiments described in this manuscript were possible due to contributions from Bruce Anderson and Andreas Beyersdorf (NASA Langley); John Kinsey (U.S. EPA); Robert Howard (U.S. Air 3519

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