Summary

This manuscript presents an interesting study on the effect of photochemical aging on aviation emissions. Such research is important as aerosol emissions can undergo significant chemical and morphological changes due to aging. This in turn can significantly change the effect of the emissions on human health and the environment. The detailed analysis of the gaseous and particulate emissions with many different methods provides a wealth of information on this topic that makes it worth publishing. However, some improvements could be made to this article in particular by better explaining how the results obtained here compare to the existing literature on this topic and the implications of these results. The conclusion does explain the implications of the results reasonably well, but it would be useful to elaborate on this in the discussion section.

Specific comments:

Pg. 2 – 3, Introduction: The introduction overall does a good job of summarizing the issue of particulate emissions from aviation however, what seems to be missing is a review of the current literature on photochemical aging of exhaust. A summary of any articles which measure PM surrounding airports would be useful to understand if the results obtained here correlate to real-world observations. In addition, if there are no or few studies on the photochemical aging of aircraft exhaust, similar studies conducted on exhaust from other combustion sources such as vehicles could be useful to put the current work in context with the broader scientific literature.

Pg. 3, lines 83 – 84: It is not clear what is meant by "An original combustion chamber taken a small jet engine with a net thrust of 180 N was used (Hupfer et al. 2012)." The work cited by Hupfer et al. 2012 is not available online and therefore, the jet engine used should be described in greater detail here. In particular, please explain the ways in which this combustion chamber does or does not represent real world conditions. In the following paragraph, the emissions are compared to commercial aircraft which typically have rated thrusts 1 – 3 orders of magnitude higher than the 180 N produced here. How do you know that the particles produced here are similar enough to that produced by commercial engines?

Pg. 3, lines 95 – 96: Even for the same type of jet fuel, there can be differences from batch-to-batch that have measurable effects on the resulting aerosol. Here, two types of fuel are reportedly used and an average composition was given. What were the differences between the Jet A1 and JP-8 fuel? Was there any correlation between the fuel used and the resulting particles (i.e. changes in particle number, size, etc.)?

Pg. 4, lines 105 – 108: The text in Figure 1, particularly the numbers on the GC x GC-MS chromatographs, is too small to read. Please increase the size of the text so that it is legible. If text such as the numbers on the chromatographs is not important due to the qualitative nature of the data, then remove the text rather than leaving too-small-to-read characters on the graph.

Pg. 4, lines 110 – 120: How long are the different sections of the sampling lines? What corrections, if any, have been made to account of diffusion losses in the sampling lines?

Pg. 6, line 149: Could you elaborate on how "…careful conditioning of the sample…" was achieved? What exactly was done to condition the sample?

Pg. 12, lines 342 – 343: What is approximately the range of pressures that can be expected in newer vs. older engines? How does this compare to the pressure in your set-up?

Pg. 12, section 3.1.2: Can you comment on the implications of the changes in VOC EI after photochemical processing?

Pg. 14, lines 395 – 397: Why do you think the particles produced here are more spherical than those observed in studies from real aircraft engines? If the particles are substantially different from what is observed in real engines, what differences might we expect to see in the aging of real aircraft exhaust?

Pg. 14, lines 402 – 403: What was average aerodynamic size of the fresh particles for comparison? Based off the rest of your analysis, do you know how the particles are growing? In other words, which compounds caused the growth and what was the growth mechanism?

Pg. 15, lines 409 – 411: This sentence is hard to understand because of the complicated grammatical structure. Consider rephrasing.

Pg. 16, lines 435 – 440: You state that the LDSA was 370 cm²/kg_fuel^-1 for fresh emissions and 8900 cm²/kg_fuel^-1 for aged emissions. However, you also state that "…for the size range of the fresh exhaust particles, any particle growth would decrease their deposition efficiency…" If the particles grew in size with aging as shown in Fig. 5, why did the LDSA increase? Is the increased LDSA due to the increased number of particles? Perhaps this is a misunderstanding, if so, please consider rephrasing this text to make the meaning clearer.

Pg. 23, lines 600 – 602: If the new particle mass formation cannot be explained by the oxidation of volatile precursors, what other potential mechanisms exist to explain the new particle mass formation observed?

Pg. 24, lines 626 – 633: While PM can radiative forcing effects, it is estimated to be a very small proportion of aviation's net-radiative forcing (https://doi.org/10.1016/j.atmosenv.2020.117834). Given this, do the changes in RF observed here represent a significant enough change to influence local warming or cooling?

Pg. 24, lines 634 – 636: How do the results observed here compare to measurements of ambient air around airports?

Review of “Photochemical aging of aviation emissions: transformation of chemical and physical properties of exhaust emissions from a laboratory-scale jet engine combustion chamber” (egusphere-2024-3836)

Hartikainen et al. presented a comprehensive study on the photochemical aging of emissions from a laboratory-scale jet engine with a focus on the transformation of organic species and absorption properties of secondary organic aerosols formed. Useful dataset on aging of aviation exhausts are presented in this manuscript, which surely expand our understanding on an important source of atmospheric pollutants. On the other hand, this reviewer believe that the authors should further clarify their experimental settings and relevant uncertainties to increase the robustness of the conclusions.

1. It is well known that flow tube experiments are characterized with branching ratios of reaction channels different from those in the ambient, which subsequently impact the yields of secondary organic aerosols, the chemical identities, and potentially the absorption properties of aerosol particles. Although such a deviation from the ambient is unavoidable, the authors are suggested to give a discussion, e.g., initial organic/NOx ratios, branching ratios, and any enhanced products that may contribute to absorption.
2. I assume that the black line was heated to 350 degree whereas the green and yellow dashed lines are not. So “the fresh” later in the manuscript stands for emissions after cooling and dilution? Maybe, also through the PEAR to take into account any condensation/loss in the flow tube?
3. Also, as the authors stated (Line 216), there was a potential issue to install QFFs in front of the adsorber tubes. I would like to see a more quantitative analysis on the accuracy of quantification of organics using this setup, and consequently, the results derived from these measurements.
4. (Line 260), did you see any negative value from the subtraction?
5. The authors made a number of assumptions during the calculation, e.g., a multiple scattering correction factor of 3.4 (Line 273), identical core volume fractions for all particle sizes (Line 305). A further justification is suggested.
6. (Line 340-345), what does “pressure” refer to?
7. My calculation indicates that 1 eqv. d = 1.5 x 10^6 cm^(-3) x 24 hr by the authors, which is suggested to be stated in the manuscript. Many people would prefer 12 hr in this definition. This also leads to my concern whether it makes sense to have such a long OH exposure if what we really want to know is the physicochemical properties of aged emission.