| 02 Jun 2026
The influence of hydrogen addition on carbonaceous aerosols produced by an ethylene flame
Abstract. Combusting hydrogen alongside carbon-based fuels has been proposed to reduce CO2 emissions and combat climate change. However, combustion-generated aerosol particles can also cause a significant radiative forcing on climate. Since addition of novel fuels alters the combustion process, it also influences particle formation inside the fame and consequently the properties of the emitted aerosols. To investigate this, combustion-generated particles from various ethylene/hydrogen mixtures are sampled in the post-flame regime. The size distribution and light absorption properties of the particles are measured using a scanning mobility particle sizer (SMPS) and a multi-wavelength aethalometer. In addition, the particles are sampled on quartz-fiber filters and the mass concentrations of organic, elemental and total carbon (OC, EC, and TC) are measured using a thermo-optical OC-EC analyzer. The geometric mean diameter of the emitted particles decreased from 300 nm down to 150 nm upon increasing the hydrogen mole fraction in the fuel from 0 % to 50 %, while the EC/TC fraction decreased from 70 % to 35 %. The light absorption of methanol-dissolved OC were measured using UV-vis analysis, showing no dependence on flame parameters or fuel composition, and no significant light absorption at wavelengths larger than 500 nm. For combustion-generated particles, the mass absorption cross section σ of the total carbonaceous aerosol (i.e. the absorption coefficient normalized to TC mass concentration) is reported as a function of EC/TC ratio at wavelengths of 370, 590 and 880 nm. At a wavelength of 880 nm, σ is slightly higher than expected of an external mixture of OC and EC, indicating some absorption enhancement due to OC coating. At wavelengths of 590 and 370 nm, σ is much higher than that expected for a mixture of colorless OC and EC and this enhancement is attributed to light absorbing non-refractory species, also called brown carbon (BrC). The absorption Ångström exponent (370–660 nm) increased from 1.3 up to 3.8 with increasing hydrogen mole fraction in the fuel, especially at lower flame temperatures, indicating an increasing contribution of BrC to the light absorption of the emitted particles. It is concluded that BrC is a precursor to EC during particle formation, in line with the existing literature, and that it matures less efficiently into EC in the hydrogen containing flame.
Review of ar-2026-21
General Comments
The manuscript presents results from the investigation of the influence of hydrogen addition on the optical properties of generated combustion articles. The study investigates the effect of flame temperature and fuels mixtures of fossil fuel (ethylene) and hydrogen (H2) on the light absorbing properties of the generated combustion particles. Both Black and Brown Carbon are considered in the experimental approach.
The goal of the study is to examine potential impacts of combustion aerosol on the climate if fuel use switches from purely fossil to mixtures of fossil fuel and hydrogen. Since this mitigation strategy is one potential short-term option for reducing anthropogenic impacts on climate, the study makes a substantial contribution to an important research area.
Overall, the manuscript is well structured and fits very well into the scope of Aerosol Research. However, before being acceptable for publication it requires modifications of the presentation in general and of the explanation and discussion of results in particular. Suggested revisions are discussed in the following paragraphs. A separate language check is also recommended.
SPECIFIC COMMENTS
1| The terminology used here should be adjusted to the largely accepted terminology as described by Petzold et al. (2013). This can be introduced, e.g., in line 43 where the authors state that “BC is sometimes used as synonymous to EC”. Since this situation has been overcome by the introduction of the agreed terminology, this statement should be adjusted accordingly.
2| The Discussion section contains a part which might belong to the results, particularly the paragraph describing Fig. 8. Please consider rearranging the presentation.
3| The Conclusion section is very descriptive and summarises the findings. What is missing is a real conclusion by connecting the various findings into one “picture” of what has been gained in the study. In the current version, it is difficult to get the real take-home massages. Re-working the Conclusions section is strongly recommended.
4| The effect of the uncertainty of the Aethalometer C-factor is discussed e.g. on line 140 and following, and on line 352 and following but there is no reference to the review of Aethalometer inversion algorithms by Collaud Coen et al. (2010). It is highly recommended to analyse the potential effect of this landmark study on the presented results.
5| Equation 1 identifies the equivalence ratio by upper case Greek symbol Φ while in the text the same property is referred to by the lower-case Greek symbol φ. Please check for consistency. Furthermore, Equation 1 is not really presented as an equation. The terms should be explained, and a blank space should be added between “combustion” and “products”.
MINOR ISSUES:
1| In most of the figures the axis titles and colour bar descriptions contain the property and the unit in brackets. E.g., in Fig. 2 the y-axis title is “Tflame in [K]” and the description of the colour code of the lines is “vexit in [cm s-1]. Since this way of describing the axes is confusing one of the two option is suggested: (1) Tflame [K], or (2) ) Tflame in K. Personally, I would prefer type (1).
2| The last sentence of the abstract is too general. BrC can be a precursor of BC but is not always. Instead, it is another form of carbonaceous aerosol in the atmosphere. What is probably meant here is that BrC is a precursor to EC during combustion particle formation in high-temperature combustion processes. This is different to BrC formation, e.g., in smouldering processes during biomass burning events where lots of BrC is formed but only very minor BC because of the low flame temperatures and the different air to fuel mixture. I suggest a clarification.
3| In Figure 3, the inserted size distribution plot uses Dd for “diameter” while in all other figures and equations “dp” is used. This should be harmonized.
4| In Figure 5, the panel labels (a) and (b) should be more visible and may be moved to the top right corner of each panel.
5| In Figure 6, The symbols of the Mie- and Rayleigh-calculations should be enhanced. Actually, they are easily overlooked.
6| On line 420, it might help the reader to mention that higher values of vexit indicate higher flame temperatures which is the real process affecting particle maturation. On line 402, the authors say that Figure 5 indicates no significant light absorption at wavelengths short than 500 nm. However, shouldn’t it mean instead “longer than” 500 nm”?
TYPOS
Line11: Please correct to “flame”.
Line 67: It is suggested to write “In a combustion-generated aerosol, coated particles …” Now, there is a repetition of the term “particles”.
Line 82: It should read: “… as a function of the hydrogen content …”.
In Figure 1, the first sentence of the figure caption should read “Schematic representation of the experimental setup”.
Line 163: Please correct to “…, whereas the sampling times range from …”.
Line 173: Please correct to “… was negligible when operating …”.
Line 180: Please check the style of Equation 4.
Line 310: To increase readability, it is suggested to add “Values of” before a300-500 .
Line 376: The sentence starting with “Only under the conditions …” should be rephrased. A suggested modification is “Only under the conditions of γ = 0.5 and vexit = 6 cm s-1, σEC values were lower than …”.
Line 425: Please correct “The mass absorption cross section values of EC …”.
Line 435: The sentence is incomplete; it should be either “a multi-wavelength in-situ method …” or “multi-wavelength in-situ methods …”.
Line 477: It should read “thermo-optical”.
REFERENCES
Collaud Coen, M., Weingartner, E., Apituley, A., Ceburnis, D., Fierz-Schmidhauser, R., Flentje, H., Henzing, J. S., Jennings, S. G., Moerman, M., Petzold, A., Schmid, O., and Baltensperger, U.: Minimizing light absorption measurement artifacts of the Aethalometer: evaluation of five correction algorithms, Atmospheric Measurement Techniques, 3, 457-474, https://doi.org/10.5194/amt-3-457-2010, 2010.
Petzold, A., Ogren, J. A., Fiebig, M., Laj, P., Li, S.-M., Baltensperger, U., Holzer-Popp, T., Kinne, S., Pappalardo, G., Sugimoto, N., Wehrli, C., Wiedensohler, A., and Zhang, X.-Y.: Recommendations for reporting “black carbon” measurements, Atmospheric Chemistry and Physics, 13, 8365–8379, https://doi.org/10.5194/acp-13-8365-2013, 2013.