Review of the “emission dynamics of reactive oxygen species and oxidative potential in particles from a gasoline car and wood stove” by Utinger et al.

The study investigates the effects of chemical aging on the formation of reactive oxygen species (ROS) and oxidative potential (OP) of aerosol particles emitted from residential wood burning and car exhaust using semi real-time instruments. Overall, the study is designed well and the results provide some insights into the dynamics of OP and ROS evolution during chemical aging compared to primary emissions. However, the manuscript text and the communication of study design and results require improvements before the manuscript can be accepted for publication. I have provided some general and specific comments below:

General comments:

The objectives of the study have not been defined well. The authors state the work they carried out without saying why and how this study is different from their previous works using the same instruments. What this study aims to add to the existing literature in the field? The objectives need to be itemized at the end of the introduction section and correspond to sub-sections in the results section.

The discussions appear rather qualitative often without mentioning the measured quantities and how they compare in numerical terms. Moreover, statistical analysis is completely missing when describing the results. Considering that the study compares various experimental conditions and how they affect ROS and OP measurements, statistical analysis should be included and differences in results should be stated with statistical significance. For instance, considering the large variabilities in Figure 4, is ROSm “significantly” different in primary vs. aged emissions during 100 km/h cycle, or with primary emissions during 50 vs. 80 km/h cycle? This comment applies to other (similar) cases throughout the text.   

The method section does not discuss the use of aerosol measurement instruments that are shown in Figure 1, in particular AMS and PTR-MS. What were these used for? No data has been presented either. If they were part of this study, they should be clearly mentioned in the method section (with additional details provided in the supplement) and how the related data was used.

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More specific comments:

Page 2, Line 55-58: a more inclusive language should be used here. Exogenous reactive species exists in the gas phase too and this aspect has been neglected in this study.

Page 3, Line 87-91: the paragraph needs to be revised; the association of PM chemical composition, OP, and health effects is not highlighted well. This aspect has been studies in the past and association was found. The authors should clearly describe such links and provide appropriate citation. 

Page 3, Line 92-100: This paragraph needs to be revised: the authors should (a) clearly indicate the gap in knowledge this study is addressing, and (b) define the specific objective/s it aims to achieve. 

Page 4, Section 2.2.: the text should be re-arranged to follow the order of flowchart/Figure 1 (without skipping any step, as is the case now). Importantly, as mentioned in my general comments, the authors should provide more details about the aerosol measurement instrumentation used in this study (specially AMS, SMPS, and PTR-MS measurements, which appear to be positioned before OOPAAI and OPROSI). Details should be included in the supplement and referenced in the main text. Referencing previous publications is not sufficient.

Page 5, Line 116: provide more information (numerical values) about “the large differences in PM and gas phase concentrations” to make it clear why such different dilution ratios were chosen. The use of qualitative terms should be avoided; there are many such instances in this manuscript.     

Page 5, Line 141-142: please provide the reasons for different start times for OPROSI and OOPAAI. 

Page 5, 142-143: please provide a brief explanation of what the “filters” are used for and include a reference where more details can be found. Also, filters were changed for RWC experiments and only with OPROSI. While the reasons are provided in the results section, it is useful to make a reference here for clarity (e.g. ‘see section x for details’).

Page 5, Line 144-145: please provide a brief explanation of how the calibration was performed, and the rationale for calibrating the instruments once a week and not more often. For offline analysis, calibration is performed per sample batch, which happens daily, if not more often.

Page 6, Line 153: what is the rationale for choosing pH 6.8? The typical pH in the lower respiratory tract under normal conditions is just above 7.

Page 6, Line 172-174: There was no mention of ELPI (and other aerosol instruments used in this study) until this point (see my earlier comments). Also, which data was “averaged to 10s”? the AA and ROS data (they have 10-min resolution)!

Page 7, Line 179: regarding particle densities, Paul et al. mentions that 1.6 g/cm3 was “assumed”. What was the rationale for choosing this value in this study? This is important, as the value is a basis for calculating EFs.

Page 7, Line 182-184: regardless of the reference provided, a description of the method used for calculating EFs should be provided in the supplement and referenced in the text (specially considering that this aspect forms a section in this manuscript).

Page 7, Line 195: please include the detection limits of the online instruments.

Page 7, Line 202-204: a note should be added describing which load conditions generated the highest and lowest OP and ROS as well as PM mass (despite being evident in the figure).

Page 8, Figure 2: The figure’s key should follow the color bars in the figure, starting with idling, followed by 50, 100, and 80 km/h. Also, it would be useful to include the data for when PEAR was offline.

Page 8, Line 214-217: statistical significance should be included when comparing the results.

Page 9, Line 243-245: comparison of short vs. medium ageing should be performed with statistical significance (considering the variabilities with the observations). Also, authors should include an explanation for the higher variability seen between short and medium ageing during each load cycle.

Page 9, Line 249-251: the sentences need rephrasing. It is not entirely clear what the authors are implying here.

Page 10, Line 284-286: discussing changes should include values, e.g. increased or decreased from x to y, or x-fold, etc. Qualitative discussion is not appropriate. Also, please mention the actual batch-to-batch variability seen with RWC experiments (in numerical terms).

Page 11, Line 287-288: “In contrast to the car emissions, both instruments responded to the primary RWC emissions due to the significantly higher primary PM emissions”: Differences here should be stated in numerical terms (e.g. significantly higher?).         

Page 11, Line 290: Figure S6 does not exist in the supplement. Is this referring to Figure S5??

Page 11, Line 297: “Figure S 2 shows a clear decrease of the ROS signal…” The figures in the supplement should be rearranged to follow the order of appearance in the main text. It is not the case in this manuscript. This needs to be addressed as well as the following issues:

- Provide the full caption for Figure S2

- There are two Figures labeled S1 in the supplement (second one appearing after Figure S3)!   

Page 11, Line 297-299: “…clear decrease of the ROS signal with higher concentrations of charcoal.” This is a clear limitation for OPROSI and should be mentioned in the manuscript.

Page 12, Line 302: “Photochemical ageing led to a significant increase…” How significant? Statistically significant or else? This should be expressed in numerical terms and include a statistical assessment.

Page 12, Line 307: “…resulted in low and stable OPM values in the same range as the car emissions…” Please mention the actual values/ranges.

Page 13, Line 325: “…it was observed that more ageing does not induce a higher OP.” This is not entirely true. It depends on various factors including the assay type. Indeed, some studies found an increase in OP (with thiol antioxidants) with OH oxidation or organic matter up to 8 days. This aspect should be commented on.

Page 13, Figure 7: If longer aging of RWC emissions results in reduced OP (Figure 6) but increased ROS (Figure 7), should one conclude that peroxides have negligible effect on AA oxidation? The authors should provide a short discussion on this.

Page 14, Line 383-385: “EFOP of both aging conditions (blue and blue stripes for the car and short and medium for RWC) are very similar for both sources.” The sentence needs a better phrasing (mentioning the actual ageing conditions, providing numbers, and comparing values with statistical significance).

Page 15, Line 384-385: “For EFROS and EFOP gasoline car emissions are up to 8 times higher than RWC values, which is the opposite compared to mass normalized OPM and ROSM concentrations.” What is the reason for this counter intuitive observation. This reiterates the need to describe in the text how EFs were calculated; this would make it easier for the reader to understand the observations.

Page 16, Line 416-417: I do not follow the logic behind the closing sentence. The study certainly sheds light on dynamic processes behind OP observations but how does it provide weight of evidence that OP should be considered as a valid metric to evaluate health effect of PM. I do not think this study touches that area. Please revise and provide an appropriate closing statement.  

Minor/Technical comments:

Page 2, Line 60: change “while simultaneously deplete” to ‘and to deplete’ or ‘simultaneously depleting’

Page 2, Line 61: change “linked to causing negative health effects” to ‘linked to negative health effects’

Page 3, Line 98: change “potentially allowing to assess health risks” to ‘enabling the assessment of health risks’  

Page 4, Line 105: add the abbreviation for “o-phenylenediamine”

Page 5, Line 120-121: change “…to measure both primary and secondary emissions.” to ‘…to measure OP and ROS from both primary and secondary emissions.’  

Page 6, Line 157: For clarity, please indicate that the text is now discussing the ROS measurement. The same should be done for the AA assay.

Page 7, Line 196: change “…OPV and ROSV concentrations…” to “…OPV and ROSV values…”. 

Page 11, Line 289: I suppose the authors are referring to Figure 6a when saying “…during the aged experiments (6)…”. Please revise.

Page 12, Line 310: “The average OPM values…” Please mention the ageing condition this refers to.

Page 12, Line 312: change “The large increase in OPM…” to ‘For medium ageing condition, the large increase in OPm…’

Page 12, Line 314: “…similar to short aging conditions (Figure 6C).” I think this is referring to Figure 6B. Please revise.

Page 12, Figure 6: for clarity, labels should be added to the individual plots to indicate various ageing conditions.

Page 13, Figure 7: The plots are labeled as A and B while the caption refers to B and C. Please revise.

Figure S1 in the supplement (Mass concentrations measured by the SMPS and ELPI during a stove (A) and car (B) exhaust measurement): It is nearly impossible to read the color coding for PM0.3, 0.9, 2.2, and SMPS. This needs improvement. Also, please number this figure correctly (in the order of appearance in the text).

Dear Editor,

In this paper titled “Emission dynamics of reactive oxygen species and oxidative potential in particles from a gasoline car and wood stove”, authors report studies related to the OP and ROS activity of particles of  gasoline passenger car emission and of residential wood combustion (RWC), both generated as fresh and photochemically aged (to simulate the atmospheric ageing), by using two different online instruments. The reported tests to measure OP and ROS being AA and DCFH assays established. Based on these analysis Authors stated that no primary particle were observed in fresh gasoline car emission, probably thanks to the presence of a gasoline particles filter (GPF) to reduce primary exhaust emissions in 6d gasoline car. Otherwise an increment in OP and ROS particles were observed after photochemically aging. For RWC  the increase of OP and ROS contribution is also observed between fresh and aged RWC. The increase of OP and ROS was associated  in both samples to the formation of secondary particles due to the photochemical ageing. In my opinion the experiments are of interest to give information about the chemical effects of the  photochemical aging and the experimental set-up is well designed.

However, I will point out that there are some drawbacks to be improved before accepting for publication. In particular, the experimental set-up has been reported and discuss from the authors almost in two previous  papers. In the present paper the authors just applies the same set-up to obtain two different anthropogenic sources of particles generated to studies their OP and ROS properties at different aging level. To  strengthen the results they need to correlate the OP and ROS outcomes to the chemical composition of both particles typologies by experimentally measuring the chemical components of particles also at different level of aging (i.e. carbon and metal contents. In my opinion there is a lack of chemical information’s to be filled to better define and discusses these indicators.

Finally, for completeness I suggest to cite in the introduction an appropriate recent published paper titled: “Characterisation of the correlations between oxidative potential and in vitro biological effects of PM10 at three sites in the central Mediterranean” from Guascito et al., 2024 (Journal of Hazardous Materials Volume 448, 15 April 2023, 130872), were was investigate the correlations among acellular and intracellular toxicity indicators, the variability among the sites, and how these correlations were influenced by the main sources by using PMF receptor model coupled with MLR.