the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Measurements of particle emissions of an A350-941 burning 100 % sustainable aviation fuels in cruise
Abstract. In order to reduce aviation’s CO2 emissions and comply with current climate targets, the European Union plans a mandatory quota of 2 % sustainable aviation fuel (SAF) by 2025, rising up to ≥ 70 % SAF by 2050. In addition to a reduction of life cycle CO2-emissions the use of SAF can also have a positive impact on particle emissions and contrail properties. In this study we present observations from the ECLIF3 (Emission and CLimate Impact of alternative Fuels) aircraft campaign, which investigated exhaust and contrail characteristics of an Airbus A350-941 equipped with Rolls-Royce Trent XWB-84 engines. For the first time, non-volatile and total particle emissions of 100 % HEFA-SPK (Hydro-processed Esters and Fatty Acids – Synthetic Paraffinic Kerosene) SAF, a blended fuel and a reference Jet A-1 were measured in flight. A maximum reduction in non-volatile particle number emissions of ∼41 % compared to the reference Jet A-1 fuel was measured at low cruise engine power settings when using 100 % HEFA-SPK. The reduction decreases to ∼29 % for typical cruise engine settings and to ∼22 % at high cruise engine power settings. The size of non-volatile particles was slightly smaller for HEFA-SPK compared to Jet A-1. We show a comprehensive analysis of the hydrogen content of globally available fuels. Our results demonstrate the impact of the fuel composition in terms of its aromatic, hydrogen and sulfur content as well as of the effect of engine power settings on particle emissions. We demonstrate that the use of HEFA-SPK can significantly reduce particle emissions and thus contrail ice particles, and therefore can provide an aviation climate benefit.
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RC1: 'Comment on egusphere-2024-1224', Anonymous Referee #1, 08 May 2024
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This manuscript is generally well written and clear, and on an important topic. The findings are appropriate for publication in ACP, since sustainable aviation fuel (SAF) emissions are needed to understand the atmospheric effects of future aircraft fleets. This manuscript reports unique results from in-flight testing of different fuel types using a modern high-efficiency engine; results show a lower production of nonvolatile (soot) particles using the SAF compared with Jet-A. This is consistent with the higher hydrogen and lower aromatic content of the SAF.
The manuscript is well organized. Only relatively minor revision is needed to make some aspects of the presentation clearer, as follows:
Line 40: What is meant by a "positive effect" on contrail properties? Is it "positive" in the climate sense that it reduces contrail occurrence and radiative effect, or "positive" in the sense of a positive feedback in abundance?
Line 42: A "seed droplet" is not the right description for a solid soot particle serving as an ice-forming nucleus.
Fig. 1: The black and blue lines are difficult to tell apart; could you please use grey or a lighter blue color for one of these lines?
Line 102: How well could the fuel tanks and lines be emptied? It would be worthwhile to state what the level of contamination from residual fuel might be (<1%?).
Fig. 2: The figure and caption are unclear. Please draw horizontal lines between the data for conventional, blended, and HEFA fuel types to clearly show which data belong to which category. In the caption, explain that the hexagonal symbols are the measured fuel hydrogen content, and the box-and-whisker symbols are the world average fuel data. Which is how I interpret the figure; is this correct? What do the different symbol colors indicate?
Line 122: What is meant by "low" sulfur levels--do you mean "measurable"? How much is present in the HEFA-SPK fuel after the aircraft has been fueled?
Line 181: What is a "simplified mission" in the context of the MEEM methodology?
Line 189: Define "T30" and "P30" here.
Line 197: Define "AFR".
Line 204: Define ICAO and provide a reference for the Aircraft Engine Emissions Databank.
Fig. 4 and other similar figures: The symbol sizes and color choices make the figures hard to read, especially if lighting is not perfect or screen color is not good. The green and blue colors are fairly similar and hard to distinguish. The symbols are pretty small, and only solid (filled) symbols are used. Please work to improve the readability of this figure and other similar ones throughout the manuscript by adjusting the color choices, symbol sizes and types, and fill types. I understand you are trying to maintain consistency across different figures, but they need to all be more easily readable.
Fig. 7: This figure is also hard to read because the black hatched bars are on top of fairly dark backgrounds. Please use much lighter shading for the colored bars, which are just indicating the fuel type used. I also suggest flipping the axes so that flight level is the left axis, which makes intuitive sense. I also don't think you have defined flight level anywhere; this term might be unfamiliar to modelers who might make use of these data.
Line 273: I don't understand the sentence beginning "Assuming the size distribution of nvPM measured on the ground. . . ." What ground-based measurements are you referring to?
Figure 9. There need to be units on the axes. What is the dashed line?
Lines 332-333: I'm quite confused by the reference to "filled symbols" in Fig. 9. Only the left panel has any symbols that are not "filled". Are you referring to the "x" symbols in this panel? These aren't "filled". If you label the panels "a" and "b" and refer to these in the text the meaning would be clearer. It's also not directly evident that "the model prediction skill is clearly strongest for average cruise flight conditions." There are a lot of scattered data on these plots.
Lines 345-348: The Pearson correlation coefficient is not the only relevant metric; the slope is also important to assess how well the model reproduces the measurements. Could you please provide slopes as well as r^2 values?
Line 363: Were samples "captured" or measured continuously?
Data availability: Is the model and analysis software code available for download? Where can the (proprietary) MEEM model be found?
Line 398: I don't understand this "correction function" equation at all. It looks like it would have units of #/cm^3, since the "reference concentration" is on the right side of the equation. So this doesn't appear to be a correction factor. Please provide values for the coefficients m, alpha, and b. And shouldn't the counting efficiency be a function of diameter as well as pressure?
Fig. A1: Is this counting efficiency curve for 35 nm particles only? I don't find this very informative. I would much rather see efficiency curves as a function of size at different pressures as in the right panel of Fig. A2 for all the CPCs used. You should also show the calibration data rather than just the fitted curves, so the reader can get a sense of the variability and uncertainty in the measurements. This might require more panels in the figure; one for each CPC.
Line 428: What is the "correction factor" that is applied? Is this the F(P) from the equation?
Line 443: Isn't contamination from shattered ice crystals on the inlet the primary concern for using measurements made within contrails or cirrus?
References: Some reference titles are capitalized; some are not. Please check all references for consistency with the Copernicus editorial guidelines.
Citation: https://doi.org/10.5194/egusphere-2024-1224-RC1
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