General comments

The authors reported the volatile organic compound (VOC) concentrations measured at a temperate forest site. The research highlights the impact of high temperatures and wildfires on VOC concentrations from various sources. This topic is important due to the increasing frequency of heatwaves and fire events, and it fits within the scope of ACP. The results from this study are interesting and could contribute to understanding the impact of these events on air quality and climate. The paper is generally well-written, and the analysis conducted is reasonable and solid. However, the manuscript still requires additional adjustment in the format and clarification before it is ready for publication.

1. According to the requirements of ACP (https://www.atmospheric-chemistry-and-physics.net/submission.html#figurestables), figure panel labels should be included. This is currently not the case throughout the manuscript, including the figures in the supplementary materials.
2. The method section (Section 2.3) is not clear enough. The content in Lines 173-177 should be expanded with more detailed explanations. For example, the authors mentioned, “The NNMF was applied for a 10-factor series with 30 replicates, 1000 iterations, and a multiplicative update algorithm,” which is too vague for readers.
3. Some content from the first paragraph of Section 3.4 should be placed in the methods section, as it should not be mixed with the results.

Specific comments:

Line 231: When monoterpene shows a peak during the day, it implies that the emission of monoterpenes from plants is also light-dependent, similar to isoprene (e.g., Kuhn et al., 2004). This is not necessarily related to the isomers of monoterpenes.

Line 274: Align the paragraph.

Line 289: It is well-known that BVOC emissions increase exponentially with temperature. Why did you choose linear regression here? Could you try using the traditional exponential equation to fit the data? One interesting point I noticed in Figure 3 is that monoterpenes respond differently across different temperature ranges. This might be related to the stress response of monoterpenes. For example, monoterpene emissions may increase dramatically after surpassing a certain temperature threshold (Nagalingam et al., 2023). Therefore, I wonder whether the varying responses observed here are indicators of a stress response in plants or simply an artificial effect caused by the choice of fitting equation.

Line 364-368: From the perspective of ozone formation, the interactions between fire plumes and BVOCs could be more significant. Since rural regions are usually VOC-limited due to the lack of NOx, the transportation of fire plumes could bring NOx or PANs to the site and promote ozone formation (Xu et al., 2021). In this case, the increase in benzene is not a key factor for ozone formation at this site, which is abundant in isoprene. Additionally, even though the benzene concentration increased significantly, I wonder if the OFP of benzene could be as high as that of isoprene. I suggest either removing this part or providing a more comprehensive discussion.

Reference

Kuhn, U., Rottenberger, S., Biesenthal, T., Wolf, A., Schebeske, G., Ciccioli, P., Brancaleoni, E., Frattoni, M., Tavares, T.M. and Kesselmeier, J. (2004), Seasonal differences in isoprene and light-dependent monoterpene emission by Amazonian tree species. Global Change Biology, 10: 663-682. https://doi.org/10.1111/j.1529-8817.2003.00771.x

Nagalingam, S., Seco, R., Kim, S., & Guenther, A. (2023). Heat stress strongly induces monoterpene emissions in some plants with specialized terpenoid storage structures. Agricultural and Forest Meteorology, 333, 109400.

Xu, L., Crounse, J. D., Vasquez, K. T., Allen, H., Wennberg, P. O., Bourgeois, I., ... & Yokelson, R. J. (2021). Ozone chemistry in western US wildfire plumes. Science Advances, 7(50), eabl3648.

General Summary:

The work presented here reports the change in mixing ratio and distribution of several biogenic and anthropogenic VOCs (measured by CIMS) in a forest as a response to increased temperature and transported biomass burning plumes. The authors underscore the variability of VOCs as a result of heat and wildfire and claim to present a comprehensive analysis of the whole mass spectra performed in this study. While the subject matter of how the VOC distribution changes in response to extreme wildfire and temperature events in the context of future climate scenarios is highly relevant, the surface level results presented here unfortunately do not do well to support the conclusions and claims made by the authors. In its current form, the (sometimes incorrectly) drawn conclusions do little to further current knowledge. On this basis, and described in more detail in specific comments below, I recommend rejection.

Despite the above, the manuscript has potential to be highly novel and impactful should the authors take steps to further the analysis and provide additional clarification in the methods. A refinement of the results would also allow for comparison to regional and global assessments, which would make the work far more reaching and useful. A review of recent literature would be beneficial in relating the relevance (and significance) of the work presented here to work already published.

Broadly speaking, the difference in quantitative metrics (for example estimated OH reactivity, estimated reactive organic carbon, change in species abundance, etc) between typical conditions and high temperature and smoke impacted conditions are lacking and could be better analyzed and presented. Several of the figures are missing estimates of error and need to be further refined. Results presented and implied conclusions are missing context or relevance. The comparison to previous studies and literature is lacking and needs to be elaborated on. For instance, it would be useful to know how many of the species measured here by the CIMS are not included in climate predictions (e.g models). Presumably, it’s quite a few given that global prediction models lack the complexity of emissions measured here. For models that discuss the importance in certain VOCs, it would be useful to know what those VOCs are and if they were also measured here. These are just a few examples of how the manuscript can be improved and resubmitted. Additional comments are provided below.

Specific Comments:

Prior to resubmission I highly encourage and recommend the author review formatting and composition guidelines presented by ACP.

The background is fairly general and could benefit from more specificity when referencing literature. Specifically, elaboration on how the reviewed literature fits into the context of this study could help strengthen the results presented.

Coordinates need a degree sign and space when naming the direction. There are a few spots in the methods where the authors only added the lat, long in parentheses. This needs to be corrected.

For methods, It would be useful to know how tall the tower is and where the instrument was situated.

L44: Not sure what is meant by “components”, consider rewording for clarity.

L53: Current wording of this sentence is awkward. Consider rephrasing to, “One potential effect of overall atmospheric warming is the change in global wildfire frequency”

L55: Cite Juang et al., 2022 as a reference to enhancements in wildfire and soil moisture/aridity: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021GL097131

L63/64: Do you have a reference that provides evidence of this? Please cite.

L66: Change to say, “the abundance of volatile organic compounds are expected…”

L74-75: Can you specify what “future” refers to here, or provide a time frame? The cited study is nearly 20 years old now, so it would be useful in context.

L109-110: Not sure what the significance is of including averages from January and July? Consider removing if not relevant.

L111: Specify if this is an annual average, and change units to cm.

L139: Specify what “calibrated regularly” means. How often and for how long? (e.g every 5 days for 20 minutes, daily for 20 minutes, etc).

L141: Are the mixing ratios for the entire mixture or for each compound?

L141-142: It’s unclear what is meant by the sentence, “The same compounds were used to calculate the mixing ratio of other compounds using the transmission efficiency and first-order kinetic reaction.” Please elaborate or reference to a manuscript that explains the method further.

L150-152: How tall was the tower? How long was the measurement sample line or inlet? Was a filter applied for particles?

Figure 2: Please specify the time resolution of the averaged data. Is this hourly, daily, etc? It would be useful to also have some estimate of error or variability on the graphs of diurnal cycles.

L190: How many is several? Define.

L206: What is BB? Don’t think the abbreviation has been defined yet.

L236: It’s unclear if the particle diameter is supposed to be greater than or less than 50 nm.

L264-267: While acetonitrile was more enhanced at this site during the BB event compared to other studies, it is not necessarily fair to say that the increase in acetonitrile alone ‘highlights the severe impact of BB on atmospheric VOC distribution and reactivity.’ It would instead be more appropriate to change the language to “implying the severe impact of BB…” or even better, providing a metric to confirm this. One idea is to compare the estimate average OH reactivity (OHr) of the measured VOC species during ambient times and compare it to the OHr during BB impacted times when acetonitrile is elevated. This would also help to strengthen the message of the manuscript.

L268-272: Is it possible to estimate an age of the BB event using a back trajectory or the abundance of compounds? For instance, furan containing species are often associated with fresh combustion and can be used to estimate smoke age. This would help to contextualize the compounds reported here.

L274: Instead of saying “During some parts” it would be useful to have a quantitative measure (e.g number of days/hours). You could reference the supplemental histogram here.

Figure 3: Error bars should be added to the bar charts. It’s unclear why a linear regression was used to fit the correlation between isoprene and temperature when the relationship is known to be exponentially related. This needs to be corrected.  Equations should be added to the correlation analysis figures and errors. Units are needed on the y axis. For linear regression, consider orthogonal regression over linear regression, as orthogonal distance regression takes into account error in both the x and y axis.

L290-L291: It is inappropriate to use a linear regression for isoprene and temperature. The relationship is known to be exponential. There several papers in the literature that show this. (Guenther at al., 1993, 2006; Rasulov et al., 2010; Hu et al., 2015; Selimovic et al., 2022; etc).

L292: It’s simpler to just say  “was three time higher than conditions…”

L293-297: See my earlier comments about which fit to apply for monoterpenes. Also the authors state, “the tenfold increase … had several implications for the distribution and chemical reactivity in the forest,” but then provide no evidence or metric for reactivity to support this. An estimate of the change to reactive organic carbon (ROC) or OHr as a result of the increase would support this statement.

L300-301: Why? What is the significance of calculating this ratio? The wording could be changed.

L302-L303: This statement lacks specificity. What values needs to be exceeded in which aerosol formation is suppressed? What is the optimum temperature? Why is it interesting or relevant that this occurs?

L307-L311: What is the enhancement range in values for the temperature increase reported here? Would be useful to report so that a direct comparison can be made to previous literature.

L306-L314: Can you report the ratio of isoprene to MACR+MVK during “low” temperatures and elevated temperature? This would provide an assessment of the lifetime and a metric for how oxidation changes between the two events. It would also be useful to compare during BB and non-BB events. See Hu et al., 2015, and Selimovic et al., 2022 for a discussion of this metric.

L319: AVOCs has not been previously defined.

L319-320: It’s not clear why a negative correlation between colder nighttime temperatures and AVOC would exist, especially when in the previous sentence the authors seem to imply the opposite is true, and that AVOCs are enhanced when the boundary layer is reduced (presumably at colder temperatures)?

L321-322: I see no direct evidence to confirm this is due to higher temperature. A plot would be useful.

L327-328: This  needs to be reworked on the basis of extensive literature historically showing exponential relationships with temperature.

L328-L332. It would be useful to know what VOC compounds the literature refers to here and whether or not they were also measured in the results reported in this manuscript.

L335: Do you have a reference for this?

Some of the information in Section 3.3 would be better introduced prior to discussion of BB impacts in Section 3.1 and 3.2. Earlier introduction would help to provide context.

L343-L345: Specificity on how wildfire smoke pollution periods were determined and separated would be beneficial. Were there times when both temperature was high >32 and BB was present? If so, how did the authors handle these in their comparisons?

L345: What defines stronger enhancement between the two? Can you provide some metric (e.g PM2.5, acetonitrile mixing ratios, etc?)

L348-L351: How old is the air mass based on these trajectories? How long are the trajectories? What inventory was used? What is the resolution? What heights were the runs initialized at?

Figure 4: What is “smoke” in the figure? Is this a combination of VOCs? Is this PM2.5? There is no definition for what is included in the smoke measurement.

L360-363: How many non-BB days were compared? How were non-BB days defined? Did non-BB days include extreme temperature events? If so, how was this separated?

L363-L364: Not sure how the authors came to this conclusion?

L366-L368: This seems to be only for one day? Can you expand this analysis for the observation period to strengthen your results? This is also only benzene and ozone, so it’s not fair to say that this one measurement is evidence of change to the overall chemical reactivity in the forest. On this note, it would be beneficial to have a measure of the regional applicability based on landscape and emission sources. Further, transported smoke plumes can also reduce the amount of sunlight getting to vegetation, impacting photolysis and potentially altering emissions of BVOC (notably isoprene) due to light and temperature reduction. This is an important consideration in the context of changing BVOC profiles due to changes in environmental factors. Based on Figure 5 it looks like the peak of smoke occurred during an extreme temperature event. Given the known relationship between ozone and temperature, how were the authors able to separate increases in ozone due to temperature versus the increase due to enhancements in VOC and BVOC precursors (e.g isoprene?)  

An expansion of Section 3.4 to include references to previous literature would be beneficial in contextualizing the results. As it is currently written it’s unclear what the significance of the reported results is. Previous studies (Brito et al., 2014) have utilized the O:C and H:C ratios as a marker for aging and to characterize organic aerosol.

L396-397: It seems disadvantageous to exclude these compounds from the analysis, considering their global abundance and the importance that was placed on them in the earlier part of the manuscript?

L397-L398: Is that the average VOC mixing ratio excluding those compounds? What is the standard deviation in the average VOC mixing ratio?

L401-L402: This conclusion is fundamentally incorrect. VOCs become more oxygenated as they are aged away from the biomass burning source. The oxidation of VOCs is what produces ozone and secondary organic aerosol. Multiple studies show this. The more likely explanation for a decrease in the O:C ratio is the increase in reactive organic carbon as a result of enhancements in VOC abundance due to BB and BB aerosol, which is overwhelmingly organic in nature. Additionally, the higher temperatures noted during the smoke period likely induce gas-particle portioning of transported BB aerosol, particularly VOC and IVOC compounds (classified in the manuscript) further contributing to a decrease in the O:C ratio.

Figure 6: It would be more useful to plot the VOC types as a fraction of the total, to assess the distribution and how it changes, rather than the total abundance. Does the analysis presented in Figure 6 exclude the compounds previously mentioned?

L421-L423: The change in distribution in the extended list alone does not validate the substantial influence of temperature and  BB on the overall chemical reactivity. A more appropriate measure of reactivity would be to calculate how the distribution of total reactive organic carbon (ROC) and OH reactivity (OHr) as a result of temperature and biomass burning influence.  

L426-L428: Many of the compounds listed here are tracers for (typically fresh <1 day old) wildfire emissions. That it increased with temperature is likely a result of concurrent “smoke” enhancements as well (evident in Figure 5). Given this fact it cannot be stated that they increased 100% s a result of enhanced temperature conditions alone, especially given the concurrence of the two events.

L488: This is presumably an average mixing ratio? Earlier the manuscript stated that isoprene reached a maximum of 75 ppb.

L489-490: There are no measurements of light or photosynthetically active radiation to support this conclusion that temperature had a greater effect than UV.

L496-497: There is no metric (SOA formation potential, OHr, change to ROC) that supports this conclusion.

L497-498: It would be useful to know what VOCs these are and if there is overlap with the ones presented here.

L510: It’s unclear why the authors all of a sudden switched to units of Kelvin? And what is smoke? How is it defined?

L514-L515: There is no correlation analysis presented to support this conclusion.

L518-L519: Some of these increases are likely associated with enhancement of wildfire emissions rather than temperature.

L525-527: This is likely broadly true but there is no evidence presented in the manuscript to support this conclusion. It would be useful to compare how the reactivity changes for each oxidant based on available kinetics data for the species measured.

References:

Isoprene and monoterpene emission rate variability: Model evaluations and sensitivity analyses - Guenther - 1993 - Journal of Geophysical Research: Atmospheres - Wiley Online Library: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/93JD00527, last access: 18 October 2024.

Rapid Growth of Large Forest Fires Drives the Exponential Response of Annual Forest‐Fire Area to Aridity in the Western United States - Juang - 2022 - Geophysical Research Letters - Wiley Online Library: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021GL097131, last access: 18 October 2024.

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