From the journal Environmental Science: Atmospheres Peer review history

08-Oct-2020

Dear Dr Hennigan:

Manuscript ID: EA-ART-09-2020-000005
TITLE: Investigating the evolution of water-soluble organic carbon in evaporating cloud water

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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Reviewer 1

This manuscript describes a study at Whiteface Mountain (near Wilmington, NY), where five cloudwater samples were collected, returned to lab, and analyzed by re-aspirating into a PILS system while comparing dried and undried aspiration flow paths. Oxalic acid, sulfate ions, and the sum of formic and acetic acid were also measured offline from the PILS outflow. The goal of the study was to quantify WSOC evaporation and brown carbon formation during drying of real cloudwater samples. It appears that this has not been done before, so the results will be of general interest to atmospheric scientists. This work should be published after revision to address the following comments.

General comments

In the opinion of this reviewer, a major result (summarized in Fig. 5) is that, compared with GECKO-A simulations, the evaporation of VOCs from cloud water in this study is much less than expected, perhaps indicating widespread aqueous-phase processing (oxidation + oligomerization) that GECKO-A’s gas-phase chemical processes cannot simulate in 2 days of aging of common precursor species. However, the conclusion and abstract emphasize a seemingly opposite conclusion that “a significant fraction of the organics in cloud water are reversibly partitioned to the aqueous phase.” Since the “significant fraction” appears to be only 11% of WSOC in this study (or perhaps more in a larger but less quantitative dataset of summit vs lodge samples at Whiteface Mtn.), this way of summarizing the manuscript’s conclusion is confusing at best, and possibly misleading.

By the same reasoning, the claim in the conclusion that “our observations are also consistent with detailed box model simulations” also seems confusing / misleading in light of Figure 5, especially when followed by “the modeling results underscore the broad aqSOA potential from the oxidation of many VOCs.” The source of this potential, if I understand the authors’ reasoning correctly, is precisely the mismatch between modeling and observations. So, I would like to suggest that the authors carefully reconsider how they summarize their results, especially in the abstract and conclusion.

Interestingly, less than 30% of the volatile species formic and acetic acid evaporated in this study – also much less than expected – and their evaporation fraction was positively correlated with the WSOC evaporation fraction. The authors suggest that this correlation indicates “that [formic and acetic acids] share similar sources and/or condensed-phase processes with this broader pool of WSOC compounds.” This inference seems reasonable in regard to condensed-phase processes, but not in regard to sources. Compound volatility and condensed-phase processing (which changes volatility and potentially viscosity) can control evaporation behavior, but I do not see how the emission source could do so.

The authors should justify the choice of 50% RH for the “dry” pathway measurements, as they did in their previous work. Is 50% RH an environmentally realistic drying level for droplets that have left a cloud, or would going to some other RH be better? In addition, 50% RH is not usually enough to remove all water from inorganic or organic aerosol droplets, so calling these conditions “dry” is somewhat misleading. (I should mention that the manuscript includes an excellent discussion on p. 14 of possible reasons browning was not observed upon drying, including possible effects of the limited extent of drying.)

Specific comments (using page numbers on the bottom right of each page)

p. 9: The manuscript compares evaporation in drying cloud water with previous studies on ambient aerosol. It is surprising that the measured WSOC evaporation fraction from ambient aerosol was larger than that measured in this study, given that “cloud droplets may undergo a greater extent of drying than aerosol …”. Before concluding the paragraph with a statement that future work is necessary, it would be helpful if the authors could address whether a similar level of “drying” was used in the previous studies of ambient aerosol. Did these other studies dry ambient aerosol to lower RH, or for longer times?

p. 11: Can the authors add a sentence summarizing what is known about the atmospheric sources of formic and acetic acid, to add context to the last sentence?

p. 15: High-NOx conditions in the simulation cause more volatile products, but the most polluted sample shows the least evaporation of WSOC. Can the authors comment on potential reasons for this?

Figure 2: Are the CW samples shown in numerical order in this figure? Labels or a caption statement about this is needed.

Figure 4: It would be helpful if the figure or caption would state the wavelength of the absorption measurement and also the meaning of “byp.” (This information is available elsewhere in the manuscript, of course.)

Last sentence of conclusion: “Detailed molecular measurements” could be more clearly expressed as “detailed but incomplete molecular measurements,” since the problem is not the detail but the existence of a large unidentified WSOC fraction.

Table S1: Why are some samples listed with collection time ranges, and other samples listed as collected at single times, rather than a collection time range?

Reviewer 2

This paper presents experimental and modeling results on the WSOC changes in cloud water upon drying, and found WSOC loss but no BrC formation. It is overall a good work and reveals important WSOC behaviors in real cloud cycling. The reviewer overall recommends its publication with a few minor comments below:
(1) On average, 11% WSOC loss was observed, and the authors argue that this was beyond the measurement uncertainty, then what is the exact measurement uncertainty, and how this influence your results? And in details, what is the measurement uncertainty for formate, acetate and oxalate?
(2) Did the authors conduct a detailed characterization of the cloud water WSOC? Such results can be linked with quantified loss and BrC formation. What are the F+A fractions in the cloud water samples before drying? As the evaporated losses of F+A are very different in different samples? What other species can be in the evaporated WSOC? (Although no measurement was conducted, any reasonable guess?)
(3) There are quite a number of studies showing that aqueous-phase processing of biomass burning precursors can be a source of BrC (can cite: Sci Total Environ 2019,685:976-985; Environmental Science & Technology 2018;52:9215-9224). Nor BrC formation was observed here, besides the reasons in the manuscript, is it likely because the drying time of 7s is too short for significant aqueous oxidation to happen? Can the authors conduct the experiments in dark for comparison? In addition, how is the Abs365 value ? Is this value higher or lower than typical ambient level? Indeed, it is possible that the BB precursors are already reacted in cloud water before your experiment, in this case, the Abs365 value of your starting solution may be already high, some discussions might be useful.

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

Thank you for the detailed and constructive reviews. We have responded to each of the reviewers' comments and have updated the manuscript accordingly.

We would like to thank reviewers for their constructive and useful comments. Below are the reviewers’ comments (in black), our point-by-point response (in blue), and updates to the manuscript (in red). Attached to the end of this we present the updated manuscript with all changes highlighted.

Referee 1
Comments to the Author
This manuscript describes a study at Whiteface Mountain (near Wilmington, NY), where five cloudwater samples were collected, returned to lab, and analyzed by re-aspirating into a PILS system while comparing dried and undried aspiration flow paths. Oxalic acid, sulfate ions, and the sum of formic and acetic acid were also measured offline from the PILS outflow. The goal of the study was to quantify WSOC evaporation and brown carbon formation during drying of real cloudwater samples. It appears that this has not been done before, so the results will be of general interest to atmospheric scientists. This work should be published after revision to address the following comments.
General comments
In the opinion of this reviewer, a major result (summarized in Fig. 5) is that, compared with GECKO-A simulations, the evaporation of VOCs from cloud water in this study is much less than expected, perhaps indicating widespread aqueous-phase processing (oxidation + oligomerization) that GECKO-A’s gas-phase chemical processes cannot simulate in 2 days of aging of common precursor species. However, the conclusion and abstract emphasize a seemingly opposite conclusion that “a significant fraction of the organics in cloud water are reversibly partitioned to the aqueous phase.” Since the “significant fraction” appears to be only 11% of WSOC in this study (or perhaps more in a larger but less quantitative dataset of summit vs lodge samples at Whiteface Mtn.), this way of summarizing the manuscript’s conclusion is confusing at best, and possibly misleading.
Author response: We agree with the reviewer that the discrepancy between GECKO-A model simulations to the observation was likely due to aqueous-phase processing. We have revised the sentence and discuss aqueous-phase processing as a possible cause of this discrepancy. We also revised the abstract to reflect this change. The revised text (page 20-21) now reads as – “This observation confirms that organics in cloud water are reversibly partitioned to the aqueous phase, and likely represents a lower bound on the evaporation of organics during an actual cloud cycle (due to experimental measurement constraints). The relatively low evaporative loss (~11%) also suggests the presence of aqueous processing of organics in cloud water. Formic and acetic acid account for a small fraction, 19% on average, of this reversibly partitioned organic material. This may be an important process that affects the distribution and lifetime of reactive carbon in the atmosphere. The lifetime of semi-volatile organics is lower in the gas phase than in the condensed phase, so reversible partitioning to cloud water may reduce deposition losses and Page 3 of 87 Environmental Science: Atmospheres enhance the transport of these compounds.1 Although our laboratory analysis included a relatively small sample size (n = 5), the results from long-term observations at WFM show systematically lower organic aerosol concentrations downwind of the summit than in cloud water. This supports our laboratory measurements and suggests the nearly constant presence of volatile WSOC reversibly partitioned to cloud water. GECKO box model simulations predict abundant formation of compounds that reversibly partition to cloud water from the photooxidation of ten different VOCs under high- and low-NOx conditions. However, the observations were clustered together and show smaller WSOC evaporation that GECKO model predictions for most of the VOCs. This difference in observations and GECKO model predictions highlight the likely presence of aqueous processing in analyzed cloud water samples, that is not accounted for in GECKO simulations. No evidence of drying-induced BrC formation was observed in the cloud water samples. The most likely reason appears to be the airmass reaching WFM may have been depleted of BrC precursors.2,3 Many studies have reported the loss of absorbance of BrC due to photooxidative aging, which could also contribute to the depletion of BrC and BrC precursors during transport.4 Studies have confirmed the need to account for photobleaching of BrC in order to accurately constrain BrC and global radiative balance, so this may have had an impact on the observations.5 The work highlights the potential importance of the large unidentified fraction of WSOC in cloud and fog water, as well, suggesting that detailed molecular measurements of cloud water will likely be incomplete and may not capture broader trends in the behavior of organics.”
By the same reasoning, the claim in the conclusion that “our observations are also consistent with detailed box model simulations” also seems confusing / misleading in light of Figure 5, especially when followed by “the modeling results underscore the broad aqSOA potential from the oxidation of many VOCs.” The source of this potential, if I understand the authors’ reasoning correctly, is precisely the mismatch between modeling and observations. So, I would like to suggest that the authors carefully reconsider how they summarize their results, especially in the abstract and conclusion.
Author response: We thank the reviewer for pointing out this apparent discrepancy. We agree with the reviewer’s assessment that a more nuanced discussion of the modeling and experimental results is needed. We have updated the text so that it now reads: “This observation confirms that organics in cloud water are reversibly partitioned to the aqueous phase, and likely represents a lower bound on the evaporation of organics during an actual cloud cycle (due to experimental measurement constraints). The relatively low evaporative loss (~11%) also suggests the presence of aqueous processing of organics in cloud water. Formic and acetic acid account for a small fraction, 19% on average, of this reversibly partitioned organic material. This may be an important process that affects the distribution and lifetime of reactive carbon in the atmosphere. The lifetime of semi-volatile organics is lower in the gas phase than in Environmental Science: Atmospheres Page 4 of 87 the condensed phase, so reversible partitioning to cloud water may reduce deposition losses and enhance the transport of these compounds.1 Although our laboratory analysis included a relatively small sample size (n = 5), the results from long-term observations at WFM show systematically lower organic aerosol concentrations downwind of the summit than in cloud water. This supports our laboratory measurements and suggests the nearly constant presence of volatile WSOC reversibly partitioned to cloud water. GECKO box model simulations predict abundant formation of compounds that reversibly partition to cloud water from the photooxidation of ten different VOCs under high- and low-NOx conditions. However, the observations were clustered together and show smaller WSOC evaporation than the GECKO model predictions for most of the VOCs. This difference in observations and GECKO model predictions highlight the likely presence of aqueous processing in analyzed cloud water samples that produces low volatility organics, a process not accounted for in the GECKO simulations. No evidence of drying-induced BrC formation was observed in the cloud water samples. The most likely reason appears to be the airmass reaching WFM may have been depleted of BrC precursors.2,3 Many studies have reported the loss of absorbance of BrC due to photooxidative aging, which could also contribute to the depletion of BrC and BrC precursors during transport.4 Studies have confirmed the need to account for photobleaching of BrC in order to accurately constrain BrC and global radiative balance, so this may have had an impact on the observations.5 The work highlights the potential importance of the large unidentified fraction of WSOC in cloud and fog water, as well, suggesting that detailed molecular measurements of cloud water will likely be incomplete and may not capture broader trends in the behavior of organics.”
Interestingly, less than 30% of the volatile species formic and acetic acid evaporated in this study – also much less than expected – and their evaporation fraction was positively correlated with the WSOC evaporation fraction. The authors suggest that this correlation indicates “that [formic and acetic acids] share similar sources and/or condensed-phase processes with this broader pool of WSOC compounds.” This inference seems reasonable in regard to condensed-phase processes, but not in regard to sources. Compound volatility and condensed-phase processing (which changes volatility and potentially viscosity) can control evaporation behavior, but I do not see how the emission source could do so. Many recent studies have suggested that formic and acetic acid are secondary, and produced from biogenic sources.6–9 For example, Millet et al., (2015) showed in SOAS (Southern Oxidant Aerosol Study) campaign a large concentration of formic acid in south-eastern United States, which is dominated by biogenic emissions in summertime.7 Similarly, Paulot et al., (2011) reported biogenic emissions as a major source of global formic and acetic acid concentrations.8 In our case, as we mentioned in the manuscript (page 4), we expect the air mass at WFM to be more influenced by biogenic emissions.10 In addition, studies have also established WSOC in particles and the gas phase is produced from biogenic sources, at least in summertime in the eastern United States, when cloud water samples were collected. 11–13 El-Sayed et al., (2018) Page 5 of 87 Environmental Science: Atmospheres showed that water soluble organic gases and WSOC in particle were produced from isoprene oxidation in summertime.11 Therefore, it is likely that F+A and broader pool of WSOC share similar sources as secondary compounds produced from biogenic precursors, though the specific biogenic precursor and the chemical pathways cannot be confirmed at this time. We have revised the text in page 11, and added references and a discussion to address the concern. The revised text reads as – “While the majority of evaporated WSOC was not identified, this suggests that F+A share similar sources and/or condensed-phase processes with this broader pool of WSOC compounds.6– 9,14 For example, Millet et al., (2015) showed in SOAS (Southern Oxidant Aerosol Study) campaign a large concentration of formic acid in south-eastern United States, which is dominated by biogenic emissions in summertime.7 Similarly, Paulot et al., (2011) reported biogenic emissions as a major source of global formic and acetic acid concentrations.8 As mentioned earlier, airmasses at WFM are expected to be predominantly influenced by biogenic emissions.10 In addition, a number of studies have also established WSOC in particles and the gas phase is produced from biogenic sources, at least in summertime in the eastern United States, when cloud water samples were collected.11–13”

The authors should justify the choice of 50% RH for the “dry” pathway measurements, as they did in their previous work. Is 50% RH an environmentally realistic drying level for droplets that have left a cloud, or would going to some other RH be better? In addition, 50% RH is not usually enough to remove all water from inorganic or organic aerosol droplets, so calling these conditions “dry” is somewhat misleading. (I should mention that the manuscript includes an excellent discussion on p. 14 of possible reasons browning was not observed upon drying, including possible effects of the limited extent of drying.)

Author response: We believe a 35% RH reduction (from 85% to 50%) is a reasonable drying for cloud water droplets. As per the box-plot presented in figure R1 (shown as figure S1 in the revised manuscript), the median RH at the summit was 98%, consistent with the frequent occurrence of clouds, while the median RH at the downwind lodge site was 80%. It should be noted here that the airmasses may experience both an increase in RH or further drying downwind of the lodge, so a range of dry channel RH levels should be investigated. However, limited sample volumes constrained the extent of drying RH values in this study. A discussion is added in the manuscript. The revised paragraph (page 7-8) reads as - Environmental Science: Atmospheres Page 6 of 87 Figure R1: Box-plots of relative humidity (RH) measured at the WFM summit and lodge in the month of August 2017. Give the number of observations represented. Circular symbols are respective means. “A 35% RH reduction (from 85% to 50%) appears to be a reasonable drying based on RH measurements at WFM summit and downwind lodge site. As per figure S1, the median RH at the summit was 98%, consistent with the frequent occurrence of clouds, while the median RH at the downwind lodge site was 80%. It should be noted that the airmasses may experience both an increase in RH or further drying downwind of the lodge, so a range of dry channel RH levels should be investigated. However, limited sample volumes constrained the extent of drying RH values in this study.” The purpose of using the word “dry” is to indicate the particles were dried to some extent in comparison to the particle produced. To avoid confusion, we noted it early on in the manuscript (section 2.2, page 6).

Specific comments (using page numbers on the bottom right of each page) p. 9: The manuscript compares evaporation in drying cloud water with previous studies on ambient aerosol. It is surprising that the measured WSOC evaporation fraction from ambient aerosol was larger than that measured in this study, given that “cloud droplets may undergo a greater extent of drying than aerosol …”. Before concluding the paragraph with a statement that future work is necessary, it would be helpful if the authors could address whether a similar level of “drying” was used in the previous studies of ambient aerosol. Did these other studies dry ambient aerosol to lower RH, or for longer times?

Author response: Thank you for the comment. We have revised the text to include the drying RH and drying time information of previous studies, and added additional discussion. The revised paragraph (page 9- 10) now reads as – 110 100 90 80 70 60 50 40 RH (%) Summit Lodge Page 7 of 87 Environmental Science: Atmospheres “To our knowledge, this represents the first direct measurements of organic evaporation from cloud water undergoing drying. In contrast, previous studies conducted on ambient aerosols in the eastern United States during summer have shown 10-30% evaporation of particulate WSOC when dried.15,16 The drying RH in both previous studies was lower (41% and 35% respectively) than the one in the current study (~50%), but underwent the same residence time in the drying system of 7s.15,16 .17 Future work should investigate variable drying configurations and temperatures to characterize a wider range of atmospheric conditions.”

p. 11: Can the authors add a sentence summarizing what is known about the atmospheric sources of formic and acetic acid, to add context to the last sentence?
Author response: We have added a few references discussing the sources of formic and acetic acid. The revised text reads as – “While the majority of evaporated WSOC was not identified, this suggests that F+A share similar sources and/or condensed-phase processes with this broader pool of WSOC compounds.6– 9,14 For example, Millet et al., (2015) showed in the SOAS (Southern Oxidant Aerosol Study) campaign a large concentration of formic acid in the southeastern United States, which is dominated by biogenic emissions in summertime.7 Similarly, Paulot et al., (2011) reported biogenic emissions as a major source of global formic and acetic acid concentrations.8 As mentioned earlier, airmasses at WFM are expected to be predominantly influenced by biogenic emissions.10 In addition, a number of studies have also established WSOC is produced in high abundance from biogenic sources, at least in summertime in the eastern United States, when cloud water samples were collected.11–13”

p. 15: High-NOx conditions in the simulation cause more volatile products, but the most polluted sample shows the least evaporation of WSOC. Can the authors comment on potential reasons for this?

Author response: We thank the reviewer for the observation. In polluted samples, it is true that VOCs likely experienced higher NOx during transport when the anthropogenic influence was high. However, it is also likely the atmospheric samples undergone some aqueous processing upwind of WFM, which can convert WSOC compounds that reversibly partition to cloud water into lower volatility material. Ervens et al., (2008) showed using a parcel model that high-NOx condition lead to higher aqSOA yield from isoprene.18 GECKO simulations do not account for any condensed-phase chemistry, including in clouds, which could produce differences between the simulations and the observations. This is reflected in the amount of organic carbon evaporated (~11%) from drying in cloud water samples. We are cognizant of the model-measurement limitations in our study and do not want to misrepresent the results or extrapolate to unreasonable conditions. There are a number of reasons that could underlie the differences shown in Fig. 5, including aqueous processing during transport to WFM (a process not represented in the GECKO simulations), one of which is that the CW samples were collected within one week. We have added the following text to acknowledge this point: Environmental Science: Atmospheres Page 8 of 87 “The modeling and long-term WFM results in Fig. 5 suggest that a much broader range of WSOCdry/WSOCcw values would be observed if the laboratory measurements were expanded to include cloud water samples collected throughout the summer or across multiple summers.”

Figure 2: Are the CW samples shown in numerical order in this figure? Labels or a caption statement about this is needed.

Author response: Thank you pointing this out. We revised the caption to include the information about CW symbols in the figure 2.
Figure 4: It would be helpful if the figure or caption would state the wavelength of the absorption measurement and also the meaning of “byp.” (This information is available elsewhere in the manuscript, of course.)
Author response: We have added the absorption wavelength and the meaning of “byp” in the caption of the figure 4.
Last sentence of conclusion: “Detailed molecular measurements” could be more clearly expressed as “detailed but incomplete molecular measurements,” since the problem is not the detail but the existence of a large unidentified WSOC fraction.
Author response: We have rephrased the sentence to address the reviewer’s comment. The revised sentence read as – “The work highlights the potential importance of the large unidentified fraction of WSOC in cloud and fog water, as well, suggesting that current detailed molecular measurements of cloud water will likely be incomplete and may not capture broader trends in the behavior of organics.”

Table S1: Why are some samples listed with collection time ranges, and other samples listed as collected at single times, rather than a collection time range?
Author response: Thank you for pointing this. We have corrected the time/time range in the table ***********************************************************************

Referee: 2
Comments to the Author
This paper presents experimental and modeling results on the WSOC changes in cloud water upon drying, and found WSOC loss but no BrC formation. It is overall a good work and reveals important WSOC behaviors in real cloud cycling. The reviewer overall recommends its publication with a few minor comments below: Page 9 of 87 Environmental Science: Atmospheres
(1) On average, 11% WSOC loss was observed, and the authors argue that this was beyond the measurement uncertainty, then what is the exact measurement uncertainty, and how this influence your results? And in details, what is the measurement uncertainty for formate, acetate and oxalate?
Author response: The measurement uncertainty of the WSOCdry/WSOCbyp was ± 3% and individual measurement uncertainty was between 1-4 ppb for measurements ranging from 45 – 177 ppb. The uncertainty suggests that the evaporation of organics is indeed taking place in the dry channel. The absolute uncertainty in the formate and acetate concentrations is higher than the uncertainty of the ratio of formate + acetate in the two measurement channels. The uncertainty in oxalate measurement was between 27-34%. The uncertainty in (F+A+O/WSOC)byp was between 17- 29%, (O/WSOC)byp was between 27-34% (Figure 3(a)). The uncertainty in (F+A)dry/(F+A)byp was between 1-8% (Figure 3(b)). Figure 3 is updated with relevant uncertainties.
(2) Did the authors conduct a detailed characterization of the cloud water WSOC? Such results can be linked with quantified loss and BrC formation. What are the F+A fractions in the cloud water samples before drying? As the evaporated losses of F+A are very different in different samples? What other species can be in the evaporated WSOC? (Although no measurement was conducted, any reasonable guess?)
Author response: With the limited amount of cloud water samples (mentioned in the manuscript, page 7), our focus was on the measurement of drying-induced evaporative loss of organics and brown carbon formation. Therefore, we could not perform detailed characterization of bulk cloud water. In this study, the results between the “dry channel” and “bypass channel” were compared to quantify the effect of particle drying on the organic content and BrC formation. Figure 3(a) in the manuscript provides F+A fractions in bypass channel, and it can be noticed that fractions vary with WSOC. It is difficult for us to speculate the species that could have been evaporated. Evaporated species were likely be volatile organic gases that partitioned into cloud droplets due to some affinity to water (non-zero henry’s constant).
(3) There are quite a number of studies showing that aqueous-phase processing of biomass burning precursors can be a source of BrC (can cite: Sci Total Environ 2019,685:976-985; Environmental Science & Technology 2018;52:9215-9224). Nor BrC formation was observed here, besides the reasons in the manuscript, is it likely because the drying time of 7s is too short for significant aqueous oxidation to happen? Can the authors conduct the experiments in dark for comparison? In addition, how is the Abs365 value? Is this value higher or lower than typical ambient level? Indeed, it is possible that the BB precursors are already reacted in cloud water before your experiment, in this case, the Abs365 value of your starting solution may be already high, some discussions might be useful.
Author response: We believe the reviewer may be unclear on our experimental methods. We have not added any oxidants to the aerosols or bulk CW samples before they were nebulized. The 365 nm light Environmental Science: Atmospheres Page 10 of 87 source is used only for the UV-Vis absorbance measurement, for the purpose of quantifying BrC, not for driving any aqueous chemistry. We have updated the Methods description (section 2.2) to clarify this point. The text now reads as - “Note here, no oxidants were added in bulk cloud water samples or in the aerosol phase, and no UV light source was used for reaction. The drying induced BrC formation chemistry takes place due to already present reactants in cloud water.” Even though the drying time of 7s seems too short, previous studies have shown that the drying time of 7s was sufficient to produce drying induced BrC formation.15,19 We have updated section 2.2 (page 8) and section 3.1.2 (page 15) to include a discussion on drying time. “The residence time of ~7 s is sufficient to achieve equilibrium for water, however, organics may take longer time to equilibrate.20–23 Therefore, our method likely provides a lower limit on WSOC evaporation. In addition, further evaporation of WSOC is expected at longer drying times and lower RHs.” “Even though the drying time of ~7 s appears small, it was found to be sufficient to observe drying induced BrC formation in previous laboratory studies.15,19 The effect of drying time on BrC formation has not been systematically studied in past, and additional studies would be required to test these hypotheses.”
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Sciare, A. Nenes and R. J. Weber, Atmospheric Evolution of Molecular Weight Separated Brown Carbon from Biomass Burning, Atmos. Chem. Phys., 2019, 19, 7319–7334. Page 11 of 87 Environmental Science: Atmospheres 6 T. Stavrakou, J.-F. Müller, J. Peeters, A. Razavi, L. Clarisse, C. Clerbaux, P.-F. Coheur, D. Hurtmans, M. De Mazière, C. Vigouroux, N. M. Deutscher, D. W. T. Griffith, N. Jones and C. Paton-Walsh, Satellite evidence for a large source of formic acid from boreal and tropical forests, Nat. Geosci., 2012, 5, 26–30. 7 D. B. Millet, M. Baasandorj, D. K. Farmer, J. A. Thornton, K. Baumann, P. Brophy, S. Chaliyakunnel, J. A. De Gouw, M. Graus, L. Hu, A. Koss, B. H. Lee, F. D. LopezHilfiker, J. A. Neuman, F. Paulot, J. Peischl, I. B. Pollack, T. B. Ryerson, C. Warneke, B. J. Williams and J. Xu, A large and ubiquitous source of atmospheric formic acid, Atmos. Chem. Phys., 2015, 15, 6283–6304. 8 F. Paulot, D. Wunch, J. D. Crounse, G. C. Toon, D. B. Millet, P. F. Decarlo, C. 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Hennigan, No evidence for brown carbon formation in ambient particles undergoing atmospherically relevant drying, Environ. Sci. Process. Impacts, 2020, 22, 442–450. 16 M. M. H. El-Sayed, D. Amenumey and C. J. Hennigan, Drying-Induced Evaporation of Secondary Organic Aerosol during Summer, Environ. Sci. Technol., 2016, 50, 3626–3633. Environmental Science: Atmospheres Page 12 of 87 17 J. H. Seinfeld and S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2006. 18 B. Ervens, A. G. Carlton, B. J. Turpin, K. E. Altieri, S. M. Kreidenweis and G. Feingold, Secondary organic aerosol yields from cloud-processing of isoprene oxidation products, Geophys. Res. Lett., 2008, 35, 4–8. 19 A. K. Y. Lee, R. Zhao, R. Li, J. Liggio, S.-M. Li and J. P. D. Abbatt, Formation of Light Absorbing Organo-Nitrogen Species from Evaporation of Droplets Containing Glyoxal and Ammonium Sulfate, Environ. Sci. Technol., 2013, 47, 12819–12826. 20 W. C. 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15-Nov-2020

Dear Dr Hennigan:

Manuscript ID: EA-ART-09-2020-000005.R1
TITLE: Investigating the evolution of water-soluble organic carbon in evaporating cloud water

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to one of the previous reviewers and I have now received his report which is copied below.

After careful evaluation of your manuscript and the reviewers’ reports, I will be pleased to accept your manuscript for publication after revisions.

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Dr Lin Wang
Associate Editor, Environmental Science: Atmospheres

Environmental Science: Atmospheres is accompanied by sister journals Environmental Science: Nano, Environmental Science: Processes and Impacts, and Environmental Science: Water Research; publishing high-impact work across all aspects of environmental science and engineering. Find out more at: http://rsc.li/envsci

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Reviewer 1

The authors have admirably addressed the issues brought up by the reviewers, but there are two exceptions that still need to be addressed. Then this article will be suitable for publication.

The reviewer 1 comment beginning “Interestingly, less than 30% of the volatile species formic and acetic acid evaporated…” stated that similarity in sources cannot be inferred using a correlation between the evaporation fraction of these acids and that of WSOC. The authors argue that recent studies show that both these acids and WSOC in this study are dominated by biogenic emissions, and this argument convinces this reviewer that the sources of both classes of compounds are indeed similar. However, this knowledge of similar sources comes from the recent studies that are now cited in the manuscript, not from the correlation observed in this work. The text still claims (in the last line of p. 11) that the correlation suggests similar sources and/or condensed phase processes, but only the latter of the two suggestions is valid. The first should be removed.

The reviewer 2 comment (3) about drying times and how the Abs365 value compares to ambient levels are critical issues for putting this research in context but which are not yet fully addressed by the authors. In the referenced work by Lee et al. 2013, dried aerosol particles were collected on a filter for a minimum of 2 hours before extraction and analysis, so this experiment does not provide usable evidence that 7 s of drying time is adequate. Can the authors compare their Abs365 values to ambient levels, as requested by reviewer 2, perhaps by converting them to mass absorption coefficients? In a similar vein, can a detection limit be provided as an upper limit of the browning not observed in the drying channel in their study (bottom of p. 14) – and can this detection limit be compared to ambient absorption levels? This would help readers place their results in context.

Technical corrections:

Last line of p. 5 and first line of p. 6: no hyphen required in “dry-channel” or “bypass-channel”

Figure 2 caption: glyoxal is a volatile species, until it encounters water.

It would be nice to know in Figure 4a which of the points on the graph are influenced by biomass burning.

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

We have addressed all reviewer comments. These are detailed in the updated (v2) response to reviewer file, which includes the updated manuscript with all changes highlighted.

We would like to thank the reviewer for helpful comments. Below is our point-by-point response to reviewer’s comments. At the end, we have the updated manuscript will all changes highlighted.
REVIEWER REPORT(S):
Referee: 1 Comments to the Author
The authors have admirably addressed the issues brought up by the reviewers, but there are two exceptions that still need to be addressed. Then this article will be suitable for publication. The reviewer 1 comment beginning “Interestingly, less than 30% of the volatile species formic and acetic acid evaporated…” stated that similarity in sources cannot be inferred using a correlation between the evaporation fraction of these acids and that of WSOC. The authors argue that recent studies show that both these acids and WSOC in this study are dominated by biogenic emissions, and this argument convinces this reviewer that the sources of both classes of compounds are indeed similar. However, this knowledge of similar sources comes from the recent studies that are now cited in the manuscript, not from the correlation observed in this work. The text still claims (in the last line of p. 11) that the correlation suggests similar sources and/or condensed phase processes, but only the latter of the two suggestions is valid. The first should be removed. We have rephrased the sentence to address reviewer’s concern. We now show that our observation is consistent with prior work. The sentence now reads as – While the majority of evaporated WSOC was not identified, our correlation is consistent with prior observations that F+A share similar sources and/or condensed-phase processes with this broader pool of WSOC compounds.1–5 The reviewer 2 comment (3) about drying times and how the Abs365 value compares to ambient levels are critical issues for putting this research in context but which are not yet fully addressed by the authors. In the referenced work by Lee et al. 2013, dried aerosol particles were collected on a filter for a minimum of 2 hours before extraction and analysis, so this experiment does not provide usable evidence that 7 s of drying time is adequate. Can the authors compare their Abs365 values to ambient levels, as requested by reviewer 2, perhaps by converting them to mass absorption coefficients? In a similar vein, can a detection limit be provided as an upper limit of the browning not observed in the drying channel in their study (bottom of p. 14) – and can this detection limit be compared to ambient absorption levels? This would help readers place their results in context. We agree with the reviewer that in Lee et al. (2013) work, particles were collected onto the filter for 2 hours which may bring biases to the observation.6 However, in our previous work on ambient particles, we performed laboratory experiments in a similar experimental setup as in the current study, and observed BrC formation in drying particles undergoing ∼7 s of drying.7 Therefore, we still think ∼7 s of drying time is adequate to observe drying induced BrC formation. The revised text (page 15) now reads as – A drying time of ~7 s was sufficient to produce BrC formation in aqueous particles undergoing drying in previous laboratory studies.6,7 Notably, in our previous study conducted with a nearly identical experimental setup, we observed BrC formation in aqueous glyoxal and methylglyoxal droplets with only ~7 s of drying. 7 However, the effect of variable drying times on BrC formation has not been systematically studied. Can the authors compare their Abs365 values to ambient levels, as requested by reviewer 2, perhaps by converting them to mass absorption coefficients? We have added additional text to compare Abs365 measurements with ambient particle measurements performed in our previous study. The values are in same order of magnitude. The text (page 13) reads as – Figure 4(a) shows Abs365 and WSOC measurements on the bypass channel. The cloud water Abs365 values are slightly higher than measurements of ambient particles (mean Abs365 = 10.1 × 10-3) in our previous study.14 In a similar vein, can a detection limit be provided as an upper limit of the browning not observed in the drying channel in their study (bottom of p. 14) – and can this detection limit be compared to ambient absorption levels? We have provided the method detection limit of our absorbance measurement. The method detection limit was 1 × 10-3 A.U., and median absorption was 10.1 × 10-3 A.U. in our previous work on ambient particles.7 We have provided this information in the manuscript. The text (page 13 and 15) now reads as – Figure 4(a) shows Abs365 and WSOC measurements on the bypass channel. Abs365 measurement values are in the same order of magnitude to Abs365 measurements performed on ambient particles (mean = 10.1 × 10-3) in our previous study.14 The reason for relatively low values of Abs365 measurements is discussed later in this section. Similar to our prior results for the eastern US aerosol, we did not observe evidence of BrC formation in the aerosolized cloud water samples as a result of drying within our method detection limit of 1 × 10-3 A.U.14 Technical corrections: Last line of p. 5 and first line of p. 6: no hyphen required in “dry-channel” or “bypass-channel” Corrected. Figure 2 caption: glyoxal is a volatile species, until it encounters water. We changed “semi-volatile” “volatile” in the figure 2 caption. It would be nice to know in Figure 4a which of the points on the graph are influenced by biomass burning. All points are now identified in the figure, and the caption was changed to identify samples likely influenced by biomass burning.

24-Nov-2020

Dear Dr Hennigan:

Manuscript ID: EA-ART-09-2020-000005.R2
TITLE: Investigating the evolution of water-soluble organic carbon in evaporating cloud water

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, I am pleased to accept your manuscript for publication in its current form.

You will shortly receive a separate email from us requesting you to submit a licence to publish for your article, so that we can proceed with publication of your manuscript.

You can highlight your article and the work of your group on the back cover of Environmental Science: Atmospheres, if you are interested in this opportunity please contact me for more information.

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Thank you for publishing with Environmental Science: Atmospheres, a journal published by the Royal Society of Chemistry – the world’s leading chemistry community, advancing excellence in the chemical sciences.

With best wishes,

Dr Lin Wang
Associate Editor, Environmental Science: Atmospheres

Environmental Science: Atmospheres is accompanied by sister journals Environmental Science: Nano, Environmental Science: Processes and Impacts, and Environmental Science: Water Research; publishing high-impact work across all aspects of environmental science and engineering. Find out more at: http://rsc.li/envsci
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