the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Dec 2023
The impact of gaseous degradation on the equilibrium state of gas/particle partitioning of semi-volatile organic compounds
Abstract. The partitioning of semi-volatile organic compounds (SVOCs) between gas and particle phases plays a crucial role in their long-range transport and health risk assessment. However, the accurate predicting of the gas/particle (G/P) partitioning quotient (KP') remains a challenge, especially for the light molecular weight (LMW) SVOCs due to their upward deviation from the equilibrium state. Based on the diurnal study of concentrations and KP' values for methylated polycyclic aromatic hydrocarbons (Me-PAHs), it was found that the diurnal variations of methylated naphthalenes (Me-Naps, one type of LMW SVOCs) were different from other Me-PAHs, that KP' values during daytime were higher than that during nighttime, and the regression lines of log KP' versus log KOA (octanol-air partitioning coefficient) for daytime and nighttime were non-overlap. It was found that the higher gaseous degradation of Me-Naps during daytime than that during nighttime should be responsible for their special diurnal variation of KP', which provided a new explanation for the non-equilibrium behavior of KP' of LMW SVOCs. Moreover, the influence of gaseous degradation on the deviation of KP' from the equilibrium state was deeply studied based on a theoretical model considering particulate proportion in emission (ϕ0). It was found that the deviation occurred when ϕ0FGR (FGR, degradation flux of gas phase) cannot be ignored when compared with FGP (flux from gas phase to particle phase). It can be concluded that the deviation was not only related to the gaseous degradation rate (kdeg), but also related to ϕ0. Furthermore, an amplification of KP' ranging from 1 to 8.4 times under different ϕ0 (0 to 1) in the temperature range of −50 to 50 °C was estimated based on the individual degradation rates of Me-Naps and three LMW PAHs. In summary, it can be concluded that the influence of gaseous degradation should also be considered for the G/P partitioning models of SVOCs, especially for the LMW SVOCs, which provided new insights into the related fields.
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CC1: 'Comment on egusphere-2023-2376', Yanqing Chen, 10 Dec 2023
In this study, the authors measured the gas and particle-phase methylated PAHs collected by coupling glass fiber filters and polyurethane foam (PUF) plugs. It was found that the gas/particle quotient (Kp’) of low molecular weight (LMW) PAHs (methylated naphthalenes) was higher during the day than at night, which is different from other Me-PAHs. The authors attributed the special diurnal variation of methylated naphthalenes (methyl NAP) to gaseous degradation. Due to the lack of direct evidence and possible sampling issues, the conclusions of this work may not be valid.
(1) Previous studies have shown that high volume samplers equipped with PUF are unsuitable for measuring LMW PAHs (for example, 2-, 3-ring PAHs) due to high breakthrough (Hart et al., 1992; Peters et al., 2000). The observed breakthrough values of 2-methyl NAP and 1-methyl NAP are around 50% (Peters et al., 2000), indicating excessive breakthrough. Therefore, the measurement results of this work may not reasonably show the gas/particle partitioning of methylated NAPs, and the diurnal variations of their Kp’ are most likely ascribed to the variations of breakthrough due to temperature changes.
In section 2.3, did the author evaluate the breakthrough of LMW PAHs when sampling gasses?
(2) Section 3.3. The gaseous degradation of LMW PAHs was not directly observed, but inferred from existing theories and empirical calculations.
References:
Hart, K.M., Isabelle, L.M., Pankow, J.F., 1992. High-volume air sampler for particle and gas sampling. 1. Design and gas sampling performance. Environmental Science & Technology 26, 1048-1052.
Peters, A.J., Lane, D.A., Gundel, L.A., Northcott, G.L., Jones, K.C., 2000. A comparison of high volume and diffusion denuder samplers for measuring semivolatile organic compounds in the atmosphere. Environmental Science & Technology 34, 5001-5006.
Citation: https://doi.org/10.5194/egusphere-2023-2376-CC1 -
AC1: 'Reply on CC1', Wan-Li Ma, 29 Mar 2024
We thank the reviewer for the time and effort engaging with our manuscript and providing us with valuable feedback. The manuscript was revised based on the your comments and suggestions, which looks much better than the original one. The detailed response and revisions can be found in the supplement.
-
AC1: 'Reply on CC1', Wan-Li Ma, 29 Mar 2024
-
RC1: 'Comment on egusphere-2023-2376', Anonymous Referee #2, 26 Dec 2023
Semi-volatile organic compounds (SVOCs) were typical pollutants in atmosphere. The gas and particle partitioning of SVOCs is important for their long-range atmospheric transport and health to human. Therefore, the study of gas and particle partitioning has attracted more attentions recently. However, the mechanism of the gas and particle partitioning for some types SVOCs was not well clarified. In this study, the impact of gaseous degradation of SVOCs on the equilibrium state of gas and particle partitioning was comprehensively discussed and studied. Some new findings were provided for this topic, which will improve our understanding of the mechanism of gas and particle partitioning.
I have some comments and suggestions to the study:
(1) In the title of the manuscript, semi-volatile organic compounds were used, however, in the main manuscript, only PAHs were studied and discussed. Therefore, semi-volatile organic compounds should be replaced by PAHs or Me-PAHs.
(2) Abstract, what kind of theoretical model? More details should be added.
(3) Introduction Section, the authors mentioned the scientific problem was the deviation between the prediction of models and monitoring for Kp with LMW SVOCs. The deviation or the problem needs to be quantified.
(4) Section 3.1. for the comparison with other studies, the numbers and names of Me-PAHs should be mentioned. If different Me-PAHs were compared, the conclusion was not reasonable.
(5) Fig.1, if different seasons were separated for discussion. I don’t think it is necessary for the figure of “All seasons”.
(6) Section 3.2, the equations of (3)-(5) were not easily for understanding.
(7) Section 3.3, for equation (6), more derivation process or steps are necessary for reading, or maybe in SI.
(8) Section 3.3, the last two sentences: “It can be found that, the impact caused by the gaseous degradation on KP' deviation was in the range of 1 to 8.4 times under different ϕ0 (0 to 1) in the temperature range of −50 to 50℃. However, due to the limited consideration of the gaseous degradation (only reaction with hydroxyl radicals) in this study, the actual impact of the gaseous degradation on KP' deviation was expected to be higher than the range.” I have two questions here: first, the uncertainty analysis of results is needed for the model; second, the two appearances with “the gaseous degradation” between the first sentences and the second sentence were confused for me, please modify the writing.
(9) Fig. 4, the title of Y-axis is not clear.
(10) This kind of writing was confused for reading: in the range of 0.429 to 0.887 (KP': 2.68 to 7.70 times increased). Pleased modify the writing for the similar problem through the manuscript.Citation: https://doi.org/10.5194/egusphere-2023-2376-RC1 -
AC2: 'Reply on RC1', Wan-Li Ma, 29 Mar 2024
We thank the reviewer for the time and effort engaging with our manuscript and providing us with valuable feedback. The manuscript was revised based on the following comments and suggestions, which looks much better than the original one. The detailed response and revisions can be found in the supplement.
-
AC2: 'Reply on RC1', Wan-Li Ma, 29 Mar 2024
-
RC2: 'Comment on egusphere-2023-2376', Anonymous Referee #1, 26 Feb 2024
In this manuscript, Zhu et al. reported their field observations, highlighting two interesting and important key findings: (1) significant diurnal variation in the gas-phase and particle-phase concentrations of methylated polycyclic aromatic hydrocarbons (Me-PAHs), and (2) remarkably higher gas-particle partitioning quotients (logKp') for lighter Me-PAHs during daytime compared to nighttime. To explain the latter observation, the authors propose that "the higher gaseous degradation of [Me-PAHs] during daytime than that during nighttime should be responsible for their special diurnal variation".
The authors arrived at this hypothesis as they investigated another hypothesis and found it insufficient for explaining the observed diurnal variation in logKp'. For another hypothesis, they assessed whether the logKp' observed for the same chemical at different temperatures correlates with the calculated logKOA at those temperatures, where logKOA at different temperatures were calculated using a simple regression (A/T + B). The authors found no significant correlation (Figure 3), which led them to conclude that the temperature-dependent variability in logKOA does not adequately explain the observed diurnal variation in logKp'. They then turned to an alternative hypothesis that the temperature-dependent variability in the gaseous degradation rate "should be responsible" for the observed diurnal variation in logKp', given that the temperature-dependent variability in the gaseous degradation rate can lead to a much more pronounced variation in logKp' (Figure 4).
Honestly, I do not believe the authors' reasoning is convincing. It is not logically sound to accept an alternative hypothesis as valid simply because another has been invalidated - unless the two hypotheses are mutually exclusive. So my first recommendation is that the authors reframe their argument to state that "the temperature-dependent variability in logKOA does not sufficiently explain the observed diurnal variation in logKp'", rather than asserting that "temperature-dependent variability in the gaseous degradation rate *should be responsible* for the observed diurnal variation in logKp'". This adjustment would present their conclusion as a more measured interpretation of the data, rather than a definitive explanation (actually, it is only a speculation).
In addition, I don't even think the observed absence of correlation between the observed logKp' and logKOA at different temperatures can lead to any meaningful conclusions, as many sources of uncertainties may contribute to the deviation of the logKp'-logKOA relationship. I just name a few:
(1) One critical assumption underlying the logKp'-logKOA relationship is that lipid-like organics predominantly control the partitioning of chemicals into the particle phase. Although this assumption may hold for a variety of organochlorines and organobromines, it may not be universally applicable to Me-PAHs. This is because, for PAHs, carbonaceous components (such as black carbon) can sometimes exceed lipid-like organics as the principal sorbents (Cornelissen et al., Environ. Sci. Technol. 2005, 39, 18, 6881–6895). Of course, Cornelissen et al. focused on the significant role of carbonaceous materials in the sorption of PAHs onto sediments and soils, but it also gives the possibility of sorption of PAHs by carbonaceous components in aerosol. So, it should not be unexpected to see significant deviations from the logKp'-logKOA relationship for Me-PAHs.
(2) Another potential reason for deviation is the uncertainties associated with the parameters in Equation 2. The values of A and B in this equation are derived from pp-LFER solute descriptors. However, Me-PAHs were not among the training chemicals used to develop these pp-LFER solute descriptors, and it is not sure whether these chemicals can be adequately predicted by these relationships.
(3) It has also been recognised that advection of air masses may also lead to deviation from the expected temperature dependence of logKp'. See Wania et al. Environ. Sci. Technol. 1998, 32, 8, 1013–1021.Of course, we may also name another set of possibilities responsible for the absence of correlation between the observed logKp' and logKOA at different temperatures. Clearly, all of these indicate a need for cautious interpretation of the use of the logKp'-logKOA relationship, especially when applied to chemicals like Me-PAHs that may not align perfectly with the assumptions and parameters used in its derivation. As such, I do not think the manuscript has discussed and excluded these possible counterexamples to well justify the authors' interpretation of the deviation of the logKp'-logKOA relationship. So my second recommendation is that the authors just end the manuscript with the statement that "the temperature-dependent variability in logKOA does not sufficiently explain the observed diurnal variation in logKp'", stop overinterpreting its implications, and more importantly, acknowledge and discuss a wide array of possible reasons (including but not limited to those outlined above) that leads to such a deviation from the logKp'-logKOA relationship.
Overall, I believe the two key findings are very interesting and important and deserve publication. However, the authors' explanations and interpretations do not fully convince me. It may be beneficial for the authors to consider either dropping much of the speculative discussion in Section 3.2 and all of Section 3.3 or pivoting towards discussing the potential reasons behind these findings in a more thorough, comprehensive manner.
Citation: https://doi.org/10.5194/egusphere-2023-2376-RC2 -
AC3: 'Reply on RC2', Wan-Li Ma, 29 Mar 2024
We thank the reviewer for the time and effort engaging with our manuscript and providing us with valuable feedback. The manuscript was revised based on the following comments and suggestions, which looks much better than the original one. The detailed response and revisions can be found in the supplement.
-
AC3: 'Reply on RC2', Wan-Li Ma, 29 Mar 2024
Status: closed
-
CC1: 'Comment on egusphere-2023-2376', Yanqing Chen, 10 Dec 2023
In this study, the authors measured the gas and particle-phase methylated PAHs collected by coupling glass fiber filters and polyurethane foam (PUF) plugs. It was found that the gas/particle quotient (Kp’) of low molecular weight (LMW) PAHs (methylated naphthalenes) was higher during the day than at night, which is different from other Me-PAHs. The authors attributed the special diurnal variation of methylated naphthalenes (methyl NAP) to gaseous degradation. Due to the lack of direct evidence and possible sampling issues, the conclusions of this work may not be valid.
(1) Previous studies have shown that high volume samplers equipped with PUF are unsuitable for measuring LMW PAHs (for example, 2-, 3-ring PAHs) due to high breakthrough (Hart et al., 1992; Peters et al., 2000). The observed breakthrough values of 2-methyl NAP and 1-methyl NAP are around 50% (Peters et al., 2000), indicating excessive breakthrough. Therefore, the measurement results of this work may not reasonably show the gas/particle partitioning of methylated NAPs, and the diurnal variations of their Kp’ are most likely ascribed to the variations of breakthrough due to temperature changes.
In section 2.3, did the author evaluate the breakthrough of LMW PAHs when sampling gasses?
(2) Section 3.3. The gaseous degradation of LMW PAHs was not directly observed, but inferred from existing theories and empirical calculations.
References:
Hart, K.M., Isabelle, L.M., Pankow, J.F., 1992. High-volume air sampler for particle and gas sampling. 1. Design and gas sampling performance. Environmental Science & Technology 26, 1048-1052.
Peters, A.J., Lane, D.A., Gundel, L.A., Northcott, G.L., Jones, K.C., 2000. A comparison of high volume and diffusion denuder samplers for measuring semivolatile organic compounds in the atmosphere. Environmental Science & Technology 34, 5001-5006.
Citation: https://doi.org/10.5194/egusphere-2023-2376-CC1 -
AC1: 'Reply on CC1', Wan-Li Ma, 29 Mar 2024
We thank the reviewer for the time and effort engaging with our manuscript and providing us with valuable feedback. The manuscript was revised based on the your comments and suggestions, which looks much better than the original one. The detailed response and revisions can be found in the supplement.
-
AC1: 'Reply on CC1', Wan-Li Ma, 29 Mar 2024
-
RC1: 'Comment on egusphere-2023-2376', Anonymous Referee #2, 26 Dec 2023
Semi-volatile organic compounds (SVOCs) were typical pollutants in atmosphere. The gas and particle partitioning of SVOCs is important for their long-range atmospheric transport and health to human. Therefore, the study of gas and particle partitioning has attracted more attentions recently. However, the mechanism of the gas and particle partitioning for some types SVOCs was not well clarified. In this study, the impact of gaseous degradation of SVOCs on the equilibrium state of gas and particle partitioning was comprehensively discussed and studied. Some new findings were provided for this topic, which will improve our understanding of the mechanism of gas and particle partitioning.
I have some comments and suggestions to the study:
(1) In the title of the manuscript, semi-volatile organic compounds were used, however, in the main manuscript, only PAHs were studied and discussed. Therefore, semi-volatile organic compounds should be replaced by PAHs or Me-PAHs.
(2) Abstract, what kind of theoretical model? More details should be added.
(3) Introduction Section, the authors mentioned the scientific problem was the deviation between the prediction of models and monitoring for Kp with LMW SVOCs. The deviation or the problem needs to be quantified.
(4) Section 3.1. for the comparison with other studies, the numbers and names of Me-PAHs should be mentioned. If different Me-PAHs were compared, the conclusion was not reasonable.
(5) Fig.1, if different seasons were separated for discussion. I don’t think it is necessary for the figure of “All seasons”.
(6) Section 3.2, the equations of (3)-(5) were not easily for understanding.
(7) Section 3.3, for equation (6), more derivation process or steps are necessary for reading, or maybe in SI.
(8) Section 3.3, the last two sentences: “It can be found that, the impact caused by the gaseous degradation on KP' deviation was in the range of 1 to 8.4 times under different ϕ0 (0 to 1) in the temperature range of −50 to 50℃. However, due to the limited consideration of the gaseous degradation (only reaction with hydroxyl radicals) in this study, the actual impact of the gaseous degradation on KP' deviation was expected to be higher than the range.” I have two questions here: first, the uncertainty analysis of results is needed for the model; second, the two appearances with “the gaseous degradation” between the first sentences and the second sentence were confused for me, please modify the writing.
(9) Fig. 4, the title of Y-axis is not clear.
(10) This kind of writing was confused for reading: in the range of 0.429 to 0.887 (KP': 2.68 to 7.70 times increased). Pleased modify the writing for the similar problem through the manuscript.Citation: https://doi.org/10.5194/egusphere-2023-2376-RC1 -
AC2: 'Reply on RC1', Wan-Li Ma, 29 Mar 2024
We thank the reviewer for the time and effort engaging with our manuscript and providing us with valuable feedback. The manuscript was revised based on the following comments and suggestions, which looks much better than the original one. The detailed response and revisions can be found in the supplement.
-
AC2: 'Reply on RC1', Wan-Li Ma, 29 Mar 2024
-
RC2: 'Comment on egusphere-2023-2376', Anonymous Referee #1, 26 Feb 2024
In this manuscript, Zhu et al. reported their field observations, highlighting two interesting and important key findings: (1) significant diurnal variation in the gas-phase and particle-phase concentrations of methylated polycyclic aromatic hydrocarbons (Me-PAHs), and (2) remarkably higher gas-particle partitioning quotients (logKp') for lighter Me-PAHs during daytime compared to nighttime. To explain the latter observation, the authors propose that "the higher gaseous degradation of [Me-PAHs] during daytime than that during nighttime should be responsible for their special diurnal variation".
The authors arrived at this hypothesis as they investigated another hypothesis and found it insufficient for explaining the observed diurnal variation in logKp'. For another hypothesis, they assessed whether the logKp' observed for the same chemical at different temperatures correlates with the calculated logKOA at those temperatures, where logKOA at different temperatures were calculated using a simple regression (A/T + B). The authors found no significant correlation (Figure 3), which led them to conclude that the temperature-dependent variability in logKOA does not adequately explain the observed diurnal variation in logKp'. They then turned to an alternative hypothesis that the temperature-dependent variability in the gaseous degradation rate "should be responsible" for the observed diurnal variation in logKp', given that the temperature-dependent variability in the gaseous degradation rate can lead to a much more pronounced variation in logKp' (Figure 4).
Honestly, I do not believe the authors' reasoning is convincing. It is not logically sound to accept an alternative hypothesis as valid simply because another has been invalidated - unless the two hypotheses are mutually exclusive. So my first recommendation is that the authors reframe their argument to state that "the temperature-dependent variability in logKOA does not sufficiently explain the observed diurnal variation in logKp'", rather than asserting that "temperature-dependent variability in the gaseous degradation rate *should be responsible* for the observed diurnal variation in logKp'". This adjustment would present their conclusion as a more measured interpretation of the data, rather than a definitive explanation (actually, it is only a speculation).
In addition, I don't even think the observed absence of correlation between the observed logKp' and logKOA at different temperatures can lead to any meaningful conclusions, as many sources of uncertainties may contribute to the deviation of the logKp'-logKOA relationship. I just name a few:
(1) One critical assumption underlying the logKp'-logKOA relationship is that lipid-like organics predominantly control the partitioning of chemicals into the particle phase. Although this assumption may hold for a variety of organochlorines and organobromines, it may not be universally applicable to Me-PAHs. This is because, for PAHs, carbonaceous components (such as black carbon) can sometimes exceed lipid-like organics as the principal sorbents (Cornelissen et al., Environ. Sci. Technol. 2005, 39, 18, 6881–6895). Of course, Cornelissen et al. focused on the significant role of carbonaceous materials in the sorption of PAHs onto sediments and soils, but it also gives the possibility of sorption of PAHs by carbonaceous components in aerosol. So, it should not be unexpected to see significant deviations from the logKp'-logKOA relationship for Me-PAHs.
(2) Another potential reason for deviation is the uncertainties associated with the parameters in Equation 2. The values of A and B in this equation are derived from pp-LFER solute descriptors. However, Me-PAHs were not among the training chemicals used to develop these pp-LFER solute descriptors, and it is not sure whether these chemicals can be adequately predicted by these relationships.
(3) It has also been recognised that advection of air masses may also lead to deviation from the expected temperature dependence of logKp'. See Wania et al. Environ. Sci. Technol. 1998, 32, 8, 1013–1021.Of course, we may also name another set of possibilities responsible for the absence of correlation between the observed logKp' and logKOA at different temperatures. Clearly, all of these indicate a need for cautious interpretation of the use of the logKp'-logKOA relationship, especially when applied to chemicals like Me-PAHs that may not align perfectly with the assumptions and parameters used in its derivation. As such, I do not think the manuscript has discussed and excluded these possible counterexamples to well justify the authors' interpretation of the deviation of the logKp'-logKOA relationship. So my second recommendation is that the authors just end the manuscript with the statement that "the temperature-dependent variability in logKOA does not sufficiently explain the observed diurnal variation in logKp'", stop overinterpreting its implications, and more importantly, acknowledge and discuss a wide array of possible reasons (including but not limited to those outlined above) that leads to such a deviation from the logKp'-logKOA relationship.
Overall, I believe the two key findings are very interesting and important and deserve publication. However, the authors' explanations and interpretations do not fully convince me. It may be beneficial for the authors to consider either dropping much of the speculative discussion in Section 3.2 and all of Section 3.3 or pivoting towards discussing the potential reasons behind these findings in a more thorough, comprehensive manner.
Citation: https://doi.org/10.5194/egusphere-2023-2376-RC2 -
AC3: 'Reply on RC2', Wan-Li Ma, 29 Mar 2024
We thank the reviewer for the time and effort engaging with our manuscript and providing us with valuable feedback. The manuscript was revised based on the following comments and suggestions, which looks much better than the original one. The detailed response and revisions can be found in the supplement.
-
AC3: 'Reply on RC2', Wan-Li Ma, 29 Mar 2024
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