Comments on the manuscript entitled “Measurement Report: Atmospheric aging of combustion-derived particles: impact on stable free radical concentration and its ability to produce reactive oxygen species in aqueous media”.

EPFRs character change and ability to induce OH formation under influence of light and water were studied to simulate the environmental fate of EPFRs in this research. EPFRs were characterized fully by many instrumental or fitting methods. The results are valuable. In general, after minor revision, the manuscript can be considered for publication on EGUsphere.

1. Have you verified that materials you used could not interference the results during the experiment? Transition metals were reported to influence formation and stabilization of EPFRs. The disposable aluminum baking dish and aluminum foil used in this research might have influences. Please clarify that.
2. Section 2.2, you mentioned there was a storage period of the EPFR samples. Considering the inevitable decay and high reactivity of free electron (even though in EPFRs), please provide the storage time of each sample and try to give evidence for not change of EPFRs during the storage.
3. Line 134, though the g value in samples A-G showed no obvious change, it still could not conclude whether there was no radical type change. There was no hyperfine splitting information in the EPR spectra (Figure A1), making it difficult to deduce the structure of EPFRs. In addition, except for the influence of temperature, there were probably more than one kind of radical in each of the samples to give a single, broad peak. The sentence “Changes in g-factor indicate a change in radical type, which was not observed in this study.” was suggested to be modified.
4. Line 145, please add discussion about possible composition change of samples based on the different ΔBp-p data.
5. Could you give possible EPFR structures based on the characterization data? And if possible, try to give the formation mechanism.

Comments for egusphere-2023-4

This manuscript investigated the impacts of atmospheric aging of combustion-derived particles on the EPFRs concentration and its ability to produce ROS in aqueous media. This issue and findings are significant, the experimental design and measurements are also ingenious. Some comments of improvements are suggested for considering as follows:

General comments:

Revisions are needed to present results and support discussions appropriately. There are three main points worthy to notice.

1. The conclusion and significance of this study should be carefully summed up. For examples:

First, the author emphasized the importance of soot in PM2.5 (Line 41), but there was no particle size analysis for samples in this study.

Second, the soot generated from hexane in this study as model particle is not the main byproducts of wildfire (Line 245). It's more likely from the fuel combustion, as the hexane is widely used in the fuel and industry.

Third, the EPFRs in soot from real combustion process was verified to be EPFRs-metal conjugate, which might pose potentially more health effects.

2. The materials and methods section seems uninformative to readers. Since many techniques have been used in this study, detailed and clearly descriptions of these methods are strongly recommended to support the coming results. For examples:

Line 60: How was the “final soot concentration was 45 ppm (m/v) ” prepared? Any micro-balance used for soot weight quantification should be told?

Line 89: Why this concentration of SBA used in this study? How to determine the concentration of SBA used? Were there any preliminary experiments for supports?

Line 90: Why 24 h for soot aged but 16 h for photoreactions here?

3. The results and discussion of OH production and measurement are not solid and convincible. This part should be paid cautious consideration.

Line 181: How were the controls done? It seems that there was no OH produced by soot slurries exposed to light and photo-aged previously. The results could come from SBA photo degradation, not only from the reaction with OH which was produced by soot samples.

Line 183-184: “This is consistent with a recent study reporting a decrease in OH concentrations in wildfire smoke plumes downfield from the point source, vs the same plume measured nearer the source (Akherati et al., 2022)”. But soot with dark-aged and then exposed to light can produce higher OH.

Line 185-188: That study (Gehling, 2014) measured the OH from dark-aged EPFRs containing-PM suspension, which is different from current study.

Line 228: “only EPFR on the surface of the particles are available for OH formation”, but the samples were well mixed and stirred in this study.

Line 233-234: “The lack of changes to chemical composition of the soot upon irradiation, as seen in the GC-MS and UV-Vis analysis, support the hypothesis that only EPFR on the surface of the particle are available for OH formation”. Doubt. The OH formation in this study seems more affected by the light.