the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Nov 2023
Formation and Loss of Light Absorbance by Phenolic Aqueous SOA by OH and an Organic Triplet Excited State
Abstract. Brown carbon (BrC) is an important component of biomass burning (BB) emissions that impacts Earth’s radiation budget. BB directly emits primary BrC as well as gaseous phenolic compounds (ArOH), which react in the gas and aqueous phases with oxidants – such as hydroxyl radical (●OH) and organic triplet excited states (3C*) – to form light-absorbing secondary organic aerosol (SOA). These reactions in atmospheric aqueous phases, such as cloud/fog drops and aerosol liquid water (ALW), form aqueous SOA (aqSOA), i.e., low-volatility, high molecular weight products. While these are important routes of aqSOA formation, the light absorption and lifetimes of the BrC formed are poorly characterized. To study these aspects, we monitored the formation and loss of light absorption by aqSOA produced by reactions of six highly substituted phenols with ●OH and 3C*. While the parent phenols absorb very little tropospheric sunlight, they are oxidized to aqSOA that can absorb significant amounts of sunlight. The extent of light absorption by the aqSOA depends on both the ArOH precursor and oxidant: more light-absorbing aqSOA is formed from more highly substituted phenols and from triplet reactions rather than ●OH. Under laboratory conditions, continued reactions with ●OH diminish sunlight absorption by aqSOA on timescales of hours, while continued reactions with 3C* reduce light absorption much more slowly. Estimated lifetimes of light-absorbing phenolic aqSOA range from 3 to 13 hours in cloud/fog drops, where ●OH is the major sink, and from 0.5 to 8 hours in ALW, where triplet excited states are the major sink.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-2719', Anonymous Referee #1, 09 Dec 2023
The authors presented experimental results of aqueous secondary organic aerosol (aqSOA) formation from six phenolic compounds by OH radical or triplet state (^3C*) oxidation. The formation and loss of light absorption of aqSOA are the foci of this work. By rigorously treating the reaction kinetics (both precursor decay and absorption change), the atmospheric lifetimes (scaled to Davis winter solstice) on the basis of light absorption were estimated for these commonly found particulate phenols. Such estimations were done for both cloud/fog waters (CLW) and aerosol liquid waters (ALW), providing parameterizable results useful for model simulations. The main conclusions are that 1) OH oxidation quickly diminishes sunlight absorption while ^3C* reduces sunlight absorption much more slowly from laboratory results, and 2) OH is the major sink in CLW while ^3C* is the major sink for most phenols in ALW.
The experiments were well designed, and the interpretation of the results was scientifically sound. The findings of this study provide new insights into the dynamics of light-absorbing chromophores from aqueous-phase reactions in both CLW and ALW. The manuscript is also well written with clear logics that is easy to follow. I therefore recommend Minor Revision, with a few comments as below.
Main:
- It seems that the yields of aqSOA (mass yield? Molar yield?) was used for the calculation of MAC (eq. 1). My question is, how was the mass of aqSOA obtained? By measuring total organic carbon then minus the remaining precursor phenol? Of just using the original phenols to minus the remaining phenol? If the former, it might involve some assumptions of how much oxygen atoms (on average) are incorporated into the aqSOA products. If the latter, it might underestimate the aqSOA mass, thereby affecting MAC estimation. Please clarify the potential uncertainty in this estimation.
- 5: Are the R_abs notations in the upper and lower panels the same thing? If so, the natural log of the upper might not give the numerical values of the lower. Or is there a difference in the unit (min vs. s)? This is also why that putting units in the y axis titles might be helpful.
- By saying reaction with ^3C* (as opposed to reaction with OH radicals), do the authors mean only direct reactions with ^3C*, or it includes secondary reactions with potentially formed oxidants other than ^3C*?
Technical:
- 6/7: are the lifetimes in these two figures referring to the same thing? If so, please use consistent notation. Besides, “Lifetime of R_abs” reads a bit wired. R_abs is the rate of photon absorption, which does not have a lifetime. It is the process of light absorption that has the lifetime, right?
- P5/L125: is the Ox in the subscript denoting OH radicals and ^3C*? It might be confused with odd oxygen (Ox = NO_2 + O_3). What about [O] or another notation, and specifying it?
- Some of the figures/tables in SI are actually quite important, and I suggest the authors to put a few that come with extensive discussion back to the main text. For example, Fig. S10 is quite informative.
Citation: https://doi.org/10.5194/egusphere-2023-2719-RC1 -
RC2: 'Comment on egusphere-2023-2719', Anonymous Referee #2, 13 Dec 2023
General comments:
In this work, the formation and loss of light-absorbing SOA from phenol reaction with OH and organic triplet excited states were investigated. The mass absorption coefficients, rates of sunlight absorption and lifetimes of the formed light-absorbing SOA were discussed in the manuscript. My comments are listed below that I kindly ask the authors to address.
Major:
- As SA can undergo direct photodegradation, the blank experiments should be performed for SA in the absence of aqueous oxidant, and this data should be provided in the left panel of Figure 1.
- Throughout the manuscript, the authors compared the optical properties of aqSOA produced from phenols reaction with OH and 3C*, respectively. However, the reaction of phenols with 3C* can lead to the formation of H2O2 (Anastasio Cort et al. Sci. Technol. 1997, 31, 218−232), which is a source of OH in phenol + 3C* reaction. So in the phenol + 3C* reaction solution, OH also reacts with phenols forming light-absorbing SOA. Did the author exclude the contribution of OH to the formation and loss of light-absorbing SOA when refer to the ‘phenol + 3C* reaction’? Please clarify this.
Minor:
- Line 60: Atmospheric aqueous oxidants also contain reactive nitrogen species. In addition, replace ‘triplet excited states of brown carbon (3C*)’ with ‘organic triplet excited states (3C*)’.
- Please add error bar to the left panel of Figure 1, Figure 5 and Figure 6.
- Line 301: Revise the sentence to ‘Triplets-mediated reaction efficiently forms oligomers, while OH-mediated reaction tends to form hydroxylated products that eventually fragment.’
Citation: https://doi.org/10.5194/egusphere-2023-2719-RC2 -
RC3: 'Comment on egusphere-2023-2719', Anonymous Referee #3, 24 Dec 2023
This manuscript focuses on the evolution of light absorption by aqSOA formed from reactions of highly substituted phenols with ●OH and 3C* during continued illumination. The mass absorption coefficient (MAC), rate of sunlight absorption by aqSOA throughout the reactions, and lifetimes of absorbance were calculated. Continued illumination of ●OH-derived phenolic aqSOA led to faster photobleaching than 3C*-derived phenolic aqSOA. The discussion is logically structured, but the manuscript lacks background information on related references that this study seems to build upon. For example, the introduction mentioned the scarcity of information on how phenol-derived brown carbon is photobleached with continued reaction, but this has been studied by Jiang et al. (2023), along with experiments involving additional ●OH and 3C*, and corresponding kinetic and chemical analyses. In addition, a more detailed literature search should be performed as more recently published studies discuss aqSOA formation by 3C* chemistry and their impact on aqSOA light absorption.
- How do the results here compare with those in Jiang et al. (2023)? They reported faster decay of 3C*-aqSOA compared to •OH-aqSOA, which they attributed to the higher light absorptivity of the former contributing to faster direct photodegradation.
- Line 23: This statement is a bit confusing as it may be interpreted as aging with additional ●OH and 3C*.
- How representative are the highly substituted phenols examined? This should be stated in the text, and showing their structures in one figure would be helpful.
- Why were different concentrations used for ArOH, H2O2, and 3C* precursor? Does this affect the major findings of this study?
- Tables S3 and S4: Previous studies have reported that the reaction of phenols with 3C* is faster than with •OH due to higher oxidant concentration in 3C*-mediated reaction (Smith et al., 2014; Yu et al., 2016; Jiang et al., 2023). Could the authors explain why this work observed the opposite, considering the higher steady-state concentration of 3C* than •OH?
- Line 200: But doesn’t the aqSOA absorbance generally decrease regardless if •OH or 3C drives the reaction*, except for TYR?
- Line 259: Why would the weak absorption of parent SyrAcid lead to a greater increase in Rabs than for GA? For example, parent SyrAcid has a significant absorbance above 300 nm (line 178), whereas GA does not.
- Line 270: For simpler phenols, could the increase of absorbance at shorter wavelengths with reaction time be related to their slower reactions?
- Line 278: Why would continued illumination enhance the absorption for VAL-aqSOA from triplet reaction between 300 and 425 nm then induce photobleaching at longer wavelengths, whereas only photobleaching was observed for VAL-aqSOA from •OH reaction?
- Line 335: What does this mean, and why would physical quenching be important for FA but not for the other phenols here?
- Line 345: How was the overall lifetime of absorbance by phenolic BrC for both •OH and 3C* for cloud and ALW conditions calculated?
- Line 350: How does the identity of the precursor phenol play a role in the BrC lifetime? Why would the lifetimes of BrC from TYR and FA, which are classified differently in this work (one is less substituted phenol, and the other is more substituted phenol), be longer than those from other phenols here?
Minor:
- Line 60: Are these second-order rate constants for highly substituted phenols? Please specify.
- Line 65: Which functionalization reactions are unique to the aqueous phase?
References:
Jiang et al., 2023, https://doi.org/10.1021/acsearthspacechem.3c00022.
Smith et al., 2014, https://doi.org/10.1021/es4045715.
Yu et al., 2016, https://doi.org/10.5194/acp-16-4511-2016.
Citation: https://doi.org/10.5194/egusphere-2023-2719-RC3 -
RC4: 'Comment on egusphere-2023-2719', Anonymous Referee #4, 24 Dec 2023
This manuscript describes studies of brown carbon formation by 6 substituted phenol species under bulk aqueous photooxidation by OH radicals or triplet carbon species. The data shows that triplet carbon photooxidation produces more brown carbon from each precursor, and that triplet carbon photooxidation destroys brown carbon more slowly than OH radicals. The authors extend their results with useful parameters such as the estimated atmospheric lifetimes of the precursor species under photooxidation by either oxidant as a function of liquid water content (clouds to aerosol particles), and by both oxidants together, at relevant concentrations. The authors conclude that the lifetime of brown carbon are controlled by OH photooxidation in clouds, but by triplet carbon in aerosol. The in-depth analysis and useful parameters make it very likely that this work will be of interest to climate modelers in addition to atmospheric chemists. The work can be published after minor revision to address the following point:
Table 1 lists the total R values for the first data point of each study, but the Figures show that this first data point were collected at different times, ranging widely from 20 to 150 minutes. How the times of the first data points collected were selected is not clear. It would seem that each reaction system would go through a maximum R value at some early point in the reaction, so the R values measured for precursors where the first data point was collected later might be expected to be biased low.
Citation: https://doi.org/10.5194/egusphere-2023-2719-RC4 - AC1: 'Comment on egusphere-2023-2719', Cort Anastasio, 28 Feb 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2719', Anonymous Referee #1, 09 Dec 2023
The authors presented experimental results of aqueous secondary organic aerosol (aqSOA) formation from six phenolic compounds by OH radical or triplet state (^3C*) oxidation. The formation and loss of light absorption of aqSOA are the foci of this work. By rigorously treating the reaction kinetics (both precursor decay and absorption change), the atmospheric lifetimes (scaled to Davis winter solstice) on the basis of light absorption were estimated for these commonly found particulate phenols. Such estimations were done for both cloud/fog waters (CLW) and aerosol liquid waters (ALW), providing parameterizable results useful for model simulations. The main conclusions are that 1) OH oxidation quickly diminishes sunlight absorption while ^3C* reduces sunlight absorption much more slowly from laboratory results, and 2) OH is the major sink in CLW while ^3C* is the major sink for most phenols in ALW.
The experiments were well designed, and the interpretation of the results was scientifically sound. The findings of this study provide new insights into the dynamics of light-absorbing chromophores from aqueous-phase reactions in both CLW and ALW. The manuscript is also well written with clear logics that is easy to follow. I therefore recommend Minor Revision, with a few comments as below.
Main:
- It seems that the yields of aqSOA (mass yield? Molar yield?) was used for the calculation of MAC (eq. 1). My question is, how was the mass of aqSOA obtained? By measuring total organic carbon then minus the remaining precursor phenol? Of just using the original phenols to minus the remaining phenol? If the former, it might involve some assumptions of how much oxygen atoms (on average) are incorporated into the aqSOA products. If the latter, it might underestimate the aqSOA mass, thereby affecting MAC estimation. Please clarify the potential uncertainty in this estimation.
- 5: Are the R_abs notations in the upper and lower panels the same thing? If so, the natural log of the upper might not give the numerical values of the lower. Or is there a difference in the unit (min vs. s)? This is also why that putting units in the y axis titles might be helpful.
- By saying reaction with ^3C* (as opposed to reaction with OH radicals), do the authors mean only direct reactions with ^3C*, or it includes secondary reactions with potentially formed oxidants other than ^3C*?
Technical:
- 6/7: are the lifetimes in these two figures referring to the same thing? If so, please use consistent notation. Besides, “Lifetime of R_abs” reads a bit wired. R_abs is the rate of photon absorption, which does not have a lifetime. It is the process of light absorption that has the lifetime, right?
- P5/L125: is the Ox in the subscript denoting OH radicals and ^3C*? It might be confused with odd oxygen (Ox = NO_2 + O_3). What about [O] or another notation, and specifying it?
- Some of the figures/tables in SI are actually quite important, and I suggest the authors to put a few that come with extensive discussion back to the main text. For example, Fig. S10 is quite informative.
Citation: https://doi.org/10.5194/egusphere-2023-2719-RC1 -
RC2: 'Comment on egusphere-2023-2719', Anonymous Referee #2, 13 Dec 2023
General comments:
In this work, the formation and loss of light-absorbing SOA from phenol reaction with OH and organic triplet excited states were investigated. The mass absorption coefficients, rates of sunlight absorption and lifetimes of the formed light-absorbing SOA were discussed in the manuscript. My comments are listed below that I kindly ask the authors to address.
Major:
- As SA can undergo direct photodegradation, the blank experiments should be performed for SA in the absence of aqueous oxidant, and this data should be provided in the left panel of Figure 1.
- Throughout the manuscript, the authors compared the optical properties of aqSOA produced from phenols reaction with OH and 3C*, respectively. However, the reaction of phenols with 3C* can lead to the formation of H2O2 (Anastasio Cort et al. Sci. Technol. 1997, 31, 218−232), which is a source of OH in phenol + 3C* reaction. So in the phenol + 3C* reaction solution, OH also reacts with phenols forming light-absorbing SOA. Did the author exclude the contribution of OH to the formation and loss of light-absorbing SOA when refer to the ‘phenol + 3C* reaction’? Please clarify this.
Minor:
- Line 60: Atmospheric aqueous oxidants also contain reactive nitrogen species. In addition, replace ‘triplet excited states of brown carbon (3C*)’ with ‘organic triplet excited states (3C*)’.
- Please add error bar to the left panel of Figure 1, Figure 5 and Figure 6.
- Line 301: Revise the sentence to ‘Triplets-mediated reaction efficiently forms oligomers, while OH-mediated reaction tends to form hydroxylated products that eventually fragment.’
Citation: https://doi.org/10.5194/egusphere-2023-2719-RC2 -
RC3: 'Comment on egusphere-2023-2719', Anonymous Referee #3, 24 Dec 2023
This manuscript focuses on the evolution of light absorption by aqSOA formed from reactions of highly substituted phenols with ●OH and 3C* during continued illumination. The mass absorption coefficient (MAC), rate of sunlight absorption by aqSOA throughout the reactions, and lifetimes of absorbance were calculated. Continued illumination of ●OH-derived phenolic aqSOA led to faster photobleaching than 3C*-derived phenolic aqSOA. The discussion is logically structured, but the manuscript lacks background information on related references that this study seems to build upon. For example, the introduction mentioned the scarcity of information on how phenol-derived brown carbon is photobleached with continued reaction, but this has been studied by Jiang et al. (2023), along with experiments involving additional ●OH and 3C*, and corresponding kinetic and chemical analyses. In addition, a more detailed literature search should be performed as more recently published studies discuss aqSOA formation by 3C* chemistry and their impact on aqSOA light absorption.
- How do the results here compare with those in Jiang et al. (2023)? They reported faster decay of 3C*-aqSOA compared to •OH-aqSOA, which they attributed to the higher light absorptivity of the former contributing to faster direct photodegradation.
- Line 23: This statement is a bit confusing as it may be interpreted as aging with additional ●OH and 3C*.
- How representative are the highly substituted phenols examined? This should be stated in the text, and showing their structures in one figure would be helpful.
- Why were different concentrations used for ArOH, H2O2, and 3C* precursor? Does this affect the major findings of this study?
- Tables S3 and S4: Previous studies have reported that the reaction of phenols with 3C* is faster than with •OH due to higher oxidant concentration in 3C*-mediated reaction (Smith et al., 2014; Yu et al., 2016; Jiang et al., 2023). Could the authors explain why this work observed the opposite, considering the higher steady-state concentration of 3C* than •OH?
- Line 200: But doesn’t the aqSOA absorbance generally decrease regardless if •OH or 3C drives the reaction*, except for TYR?
- Line 259: Why would the weak absorption of parent SyrAcid lead to a greater increase in Rabs than for GA? For example, parent SyrAcid has a significant absorbance above 300 nm (line 178), whereas GA does not.
- Line 270: For simpler phenols, could the increase of absorbance at shorter wavelengths with reaction time be related to their slower reactions?
- Line 278: Why would continued illumination enhance the absorption for VAL-aqSOA from triplet reaction between 300 and 425 nm then induce photobleaching at longer wavelengths, whereas only photobleaching was observed for VAL-aqSOA from •OH reaction?
- Line 335: What does this mean, and why would physical quenching be important for FA but not for the other phenols here?
- Line 345: How was the overall lifetime of absorbance by phenolic BrC for both •OH and 3C* for cloud and ALW conditions calculated?
- Line 350: How does the identity of the precursor phenol play a role in the BrC lifetime? Why would the lifetimes of BrC from TYR and FA, which are classified differently in this work (one is less substituted phenol, and the other is more substituted phenol), be longer than those from other phenols here?
Minor:
- Line 60: Are these second-order rate constants for highly substituted phenols? Please specify.
- Line 65: Which functionalization reactions are unique to the aqueous phase?
References:
Jiang et al., 2023, https://doi.org/10.1021/acsearthspacechem.3c00022.
Smith et al., 2014, https://doi.org/10.1021/es4045715.
Yu et al., 2016, https://doi.org/10.5194/acp-16-4511-2016.
Citation: https://doi.org/10.5194/egusphere-2023-2719-RC3 -
RC4: 'Comment on egusphere-2023-2719', Anonymous Referee #4, 24 Dec 2023
This manuscript describes studies of brown carbon formation by 6 substituted phenol species under bulk aqueous photooxidation by OH radicals or triplet carbon species. The data shows that triplet carbon photooxidation produces more brown carbon from each precursor, and that triplet carbon photooxidation destroys brown carbon more slowly than OH radicals. The authors extend their results with useful parameters such as the estimated atmospheric lifetimes of the precursor species under photooxidation by either oxidant as a function of liquid water content (clouds to aerosol particles), and by both oxidants together, at relevant concentrations. The authors conclude that the lifetime of brown carbon are controlled by OH photooxidation in clouds, but by triplet carbon in aerosol. The in-depth analysis and useful parameters make it very likely that this work will be of interest to climate modelers in addition to atmospheric chemists. The work can be published after minor revision to address the following point:
Table 1 lists the total R values for the first data point of each study, but the Figures show that this first data point were collected at different times, ranging widely from 20 to 150 minutes. How the times of the first data points collected were selected is not clear. It would seem that each reaction system would go through a maximum R value at some early point in the reaction, so the R values measured for precursors where the first data point was collected later might be expected to be biased low.
Citation: https://doi.org/10.5194/egusphere-2023-2719-RC4 - AC1: 'Comment on egusphere-2023-2719', Cort Anastasio, 28 Feb 2024
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Stephanie Arciva
Lan Ma
Camille Mavis
Chrystal Guzman
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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- Final revised paper