the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 May 2025
In-flight emission measurements with an autonomous payload behind a turboprop aircraft
Abstract. This paper reports on successfully testing a new, autonomously operating measurement system on a Grob G 520 Egrett aircraft for in-flight aerosol and trace gas measurements of engine exhaust. A suite of in-house-built and commercially available instruments was selected, modified, and adapted to the unpressurized compartment of the Egrett to operate over a wide range of ambient temperatures and pressure levels. We performed first in-flight emission measurements at cruise altitudes behind a twin-turboprop aircraft, the Piper Cheyenne, powered by Garrett/Honeywell TPE 331-14 engines, over Texas in April 2022.The instrumentation and inlets on the Egrett were designed to measure non-volatile particulate matter (nvPM), total particulate matter (tPM), nitrogen oxides (NO and NO2), water vapor (H2O), carbon dioxide (CO2), and contrail ice particles. All instruments were operated in relevant plume conditions at cruise altitudes between 7.6 and 10.4 km (FL250 and FL340) at distances ranging from 100 to 1200 m between the two aircraft. The instruments proved to have high reliability, a large dynamic range, and sufficient accuracy, which is adequate for measuring the emissions of the turboprop engine.
We derived the emission indices (EI) for tPM, nvPM, and NOx at cruise. The particulate emission indices range from 9.6 to 16.2 ×1014 kg−1 (particles per kg fuel burned) for EItPM and from 8.1 to 12.4 ×1014 kg−1 for EInvPM (medians). For NOx we find rather low EINOx between 7.3 and 7.7 g kg−1 for EINOx (medians). Furthermore, aerosol size distributions have been measured in the exhaust plume. The analysis of the size-resolved emission index indicates a log-normal distribution with geometric mean and standard deviation at Dg = 34.7 ± 1.9 nm. This geometric diameter value is in the range of jet engine soot emissions previously measured in flight. The measurements help to constrain the climate impact of current turboprop engines and provide a benchmark for future alternative H2 propulsion systems such as fuel cells and direct combustion engines.
Competing interests: CR and JC are employed by Airbus Operations. RV and AV are employed by AV Experts LLC. All the other authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: open (until 26 Jun 2025)
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RC1: 'Comment on egusphere-2025-2026', Anonymous Referee #1, 11 Jun 2025
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Summary: The impact is well-motivated, showing the importance of studying short range flights with turboprop aircraft. Significance of the impact, difficulty of the measurement, and lack of comparable datasets highly motivate the scientific significance. The method is sound and calibrations and subsequent analyses are thorough, including variance and error propagation. Some explanation as to why the aerosol instruments need to be pressurized would be preferable, since I was confused by the explanation of sampling stability. I am suspicious of the particle sizing and concentration accuracy at the small sizes ~10nm, due to the cut size of the particle counters (exacerbated by diffusion losses) leading to large correction factors, thus the mode size may be slightly overestimated; however, results compare well to previous airborne measurements with similar instrumentation from Moore et al., 2017. The author did a very good job characterizing instrument response to sample and environmental factors, and uncertainty. There are a couple of typos.
L15-17: Are the size distributions presented from the total or nonvolatile aerosol? If total, suggest removing “soot” and generalizing as jet engine emissions, since the soot implies non-volatile particulates.
L25-26: Incomplete sentence. Suggest combining with previous sentence.
L97: L228 is the first time bringing up isokinetic, but it would be helpful to mention at the introduction of the aerosol inlet.
L122-123: Can you expand on what “ensuring stable sampling conditions” means? Why would the instruments need to be in a pressurized vessel? It introduces a higher deltaP and increases the potential for dilution/leak into the sample.
L206-208 & F5b: The curve fit deviates from the measured counting efficiency right around your critical operating environment. The operating environment is barely captured in your data points. Higher resolution in the region that you measure (more points between 375-250 hPa) would result in more precise correction.
L223: Is the 90-degree tube bend sufficiently large enough radius to be negligible for inertial impaction of large particles? Valuable to mention if negligible here when describing other loss mechanisms.
L259: Typo. soot soot.
L267: Why the range in sheath flow? Is the range from intentional changes, e.g., compensating for pressure to achieve the same size range in a scan, or fluctuation due to environment? What is the corresponding sample flow?
L270: Typo on mSEMS. “Is able to operate at 5 s scan time”, does that mean you did operate a 5 s scan? What was the lag time from the sample out of the DMA to the aMCPC? For a 5-second scan, the smearing may be significant. You don’t mention operation scan times until L509, and it’s worth mentioning how it was operated in section 2.1.8. 17s scan while in a highly variable plume seems too slow for samples shown in F7 without a large lag chamber. Were scans averaged to suppress the noise, and if so, how many scans are used for averaging?
L289-290: Is it supposed to read “sizing” instead of “size”? Why are instrument sizing and flow calibration major sources of uncertainty? Are you talking about the physical size due to unknown refractive indices?
F9, F5c, F5d: The combined effects of the aMCPC size cut, counting efficiency with pressure, and diffusion losses, hurts the confidence in the size distributions below 20 nm. I expect the entire left-side falling edge of the curve in Figure 9 in plume would have increasing error bars associated with it, which may provide context/caution in interpreting the mode size from the fit in F9b. The aMCPC may not be the best choice for engine emission characterization since its cut size is near to the exhaust particle size range. There may be a significant number of sub-10 nm particles missing from the tPM when calculating the EI. Perhaps the EI should be specified as the EI_tPM>10nm at the top of the document. This may be less of a concern for this generation of engine, but consider ultra fine CPCs when testing future generation engines that combust more efficiently that they may have a smaller mode size where the aMCPC will completely misinform/bias the peak.
Citation: https://doi.org/10.5194/egusphere-2025-2026-RC1
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