the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Mar 2023
Photoaging of Phenolic Secondary Organic Aerosol in the Aqueous Phase: Evolution of Chemical and Optical Properties and Effects of Oxidants
Abstract. While gas-phase reactions are well established to have significant impacts on the mass concentration, chemical composition, and optical properties of secondary organic aerosol (SOA), the aqueous-phase aging of SOA remains poorly understood. In this study, we performed a series of long-duration photochemical aging experiments to investigate the evolution of the composition and light absorption of the aqueous SOA (aqSOA) from guaiacyl acetone (GA), a semivolatile phenolic carbonyl that is common in biomass burning smoke. The aqSOA was produced from reactions of GA with hydroxyl radical (•OH-aqSOA) or a triplet excited state of organic carbon (3C*-aqSOA) and was then photoaged in water under conditions that simulate sunlight exposure in northern California for up to 48 hours. The effects of increasing aqueous-phase •OH or 3C* concentration on the photoaging of the aqSOA were also studied. High resolution aerosol mass spectrometry (HR-AMS) and UV-vis spectroscopy were utilized to characterize the composition and the light absorptivity of the aqSOA and to track their changes during aging.
Compared to •OH-aqSOA, the 3C*-aqSOA is produced more rapidly and shows less oxidation, a greater abundance of oligomers, and higher light absorption. Prolonged photoaging promotes fragmentation and the formation of more volatile and less light-absorbing products. More than half of the initial aqSOA mass is lost and substantial photobleaching occurs after 10.5 hours of prolonged aging under simulated sunlight illumination for 3C*-aqSOA and 48 hours for •OH-aqSOA. By performing positive matrix factorization (PMF) analysis of the combined HR-AMS and UV-vis spectral data, we resolved three generations of aqSOA with distinctly different chemical and optical properties. The first-generation aqSOA shows significant oligomer formation and enhanced light absorption at 340–400 nm. The second-generation aqSOA is enriched in functionalized GA species, while the third-generation aqSOA contains more fragmented products and is the least light-absorbing. Although photoaging generally increases the oxidation of aqSOA, a slightly decreased O / C of the •OH-aqSOA is observed after 48 hours of prolonged photoaging with additional •OH exposure. This is likely due to greater fragmentation and evaporation of highly oxidized compounds. Increased oxidant concentration accelerates the transformation of aqSOA and promotes the decay of brown carbon (BrC) chromophores, leading to faster mass reduction and photobleaching. In addition, compared with •OH, photoaging by 3C* produces more low-volatility functionalized products, which counterbalances part of the aqSOA mass loss due to fragmentation and evaporation.
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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-2023-443', Anonymous Referee #1, 04 Apr 2023
This study examines the evolution of chemical and optical properties of phenolic aqSOA generated via •OH- or 3C* oxidation during photoaging, as well as the effects of increased concentration of oxidants. The changes in the chemical composition and light absorption of aqSOA were tracked using HR-AMS and UV-Vis spectroscopy. The findings for the 3C*-aqSOA have been reported in a previous study by the same group (Jiang et al., 2021). Although this paper attempted to explain the differences between 3C*-aqSOA thoroughly and •OH-aqSOA, several statements appear unclear or not clearly supported by the results. The paper is well organized, but several points need clarification.
- Could the authors give examples of products likely resistant to fragmentation?
- Were the 4 aliquots prepared for further aging also continuously stirred?
- Section 3.2: why would •OH production from 3C* be more important during prolonged photoaging? is there any evidence for this?
- Section 3.2: Does higher light absorptivity always correlate with faster direct photodegradation? It seems not as, for example, Smith et al. (2016) attributed the essentially equal reactivity of syringaldehyde and acetosyringone against direct photodegradation to the greater light absorption by syringaldehyde and higher quantum efficiency for loss for acetosyringone. Also, how does this reconcile with the statement in section 3.1 about •OH-aqSOA being more vulnerable to photodegradation than 3C*-aqSOA?
- Section 3.2: Could the authors explain why acid formation is more pronounced in the aging of 3C*-aqSOA? In section 3.1, it was mentioned that •OH-aqSOA has a greater tendency to form volatile and semi-volatile compounds that evaporate from the condensed phase
- Section 3.2: What are the differences between the highly oxidized products from 3C* and •OH oxidation? Those from 3C* are stated to be resistant to degradation, while those from •OH are mentioned to degrade over long aging times. Also, why are some of the highly oxidized products less reactive with 3C* than with •OH, and that they can persist in the aqueous phase?
- Section 3.3: Why would the higher oxidation degree of •OH-aqSOA lead to the destruction of chromophores?
- Section 3.4: Why would •OH be more reactive with non-phenolic organic compounds? For example, a study on the oxidation of green leaf volatiles (Richards-Henderson et al., 2015) comprising both non-phenolic and phenolic compounds did not show a general trend in the reactivity with 3C* and •OH.
- Section 3.4: Could the authors give some examples of these unique low-volatility, light-absorbing products that cannot be generated via •OH oxidation?
- Section 3.4: What do the authors mean by •OH only accounts for a small fraction of the total oxidant amount in the 3C*-initiated reaction system?
Minor comments and questions:
- Intro: Is hydroxylation an example of functionalization?
- Please provide more information on the HPLC method used to determine the concentration of GA and DMB.
- Please correct what [Org]t and [GA]t refer to
- There are other recently published articles regarding the formation of aqSOA by 3C* chemistry and the corresponding light absorption by reaction products (e.g., Li et al., 2022, https://doi.org/10.5194/acp-22-7793-2022; Mabato et al., 2023, https://doi.org/10.5194/acp-23-2859-2023)
References:
Jiang, W., Misovich, M. V., Hettiyadura, A. P. S., Laskin, A., McFall, A. S., Anastasio, C., and Zhang, Q.: Photosensitized reactions of a phenolic carbonyl from wood combustion in the aqueous phase—chemical evolution and light absorption properties of aqSOA, Environ. Sci. Technol., 55, 5199−5211, https://doi.org/10.1021/acs.est.0c07581, 2021.
Richards-Henderson, N. K., Pham, A. T., Kirk, B. B., and Anastasio, C.: Secondary organic aerosol from aqueous reactions of green leaf volatiles with organic triplet excited states and singlet molecular oxygen, Environ. Sci. Technol., 49, 268–276, https://doi.org/10.1021/es503656m, 2015.
Smith, J. D., Kinney, H., and Anastasio, C.: Phenolic carbonyls undergo rapid aqueous photodegradation to form low-volatility, light-absorbing products, Atmos. Environ., 126, 36−44, https://doi.org/10.1016/j.atmosenv.2015.11.035, 2016.
Citation: https://doi.org/10.5194/egusphere-2023-443-RC1 - AC1: 'Reply on RC1', Qi Zhang, 13 May 2023
- AC3: 'Reply on RC1', Qi Zhang, 13 May 2023
-
RC2: 'Comment on egusphere-2023-443', Anonymous Referee #2, 09 Apr 2023
Review comments on the manuscript "Photoaging of Phenolic Secondary Organic Aerosol in the Aqueous Phase: Evolution of Chemical and Optical Properties and Effects of Oxidants" in EGUsphere. The manuscript addresses the long-term aqueous aging of aqSOA formed from the photooxidation of guaiacyl acetone (GA) by OH radicals or the photosensitizer model compound 3,4-dimethoxy-benzaldehyde (DMB) using Pyrex tube experiments and performing positive matrix factorization (PMF) analysis of combined HR-AMS and UV-vis spectral data.
Questions and remarks:
Could the authors elaborate more the description of the experimental and analytical method, as it is a bit vague at the moment. It is not clear to the reader if the experiment is performed in a closed system or if it is, open to the atmosphere, if oxygen is present or not? Could the authors describe the potential contribution of singlet oxygen to the conversion within the system when the photosensitizer dimethoxybenzaldehyde and oxygen are present? Could the guaiacylacetone itself act as a photosensitizer? Would the authors expect a similar oligomer formation yield and conversion rate if 4-propylguaiacol had been used? What would be the general reaction mechanism in the photosensitizer system in the presence of GA and DMB after the first step of H-atom abstraction or electron transfer? Is addition of the photosensitizer possible? What would be the difference in the first oxidation products formed by the photosensitizers compared to OH radicals? What would be the result of the involvement of oxygen? Would the resulting peroxyl radicals lead to the formation of oligomers, what is the authors' opinion? If not, could this explain the lower yield of oligomers in OH radical-induced oxidation? How likely is alkyl/phenoxy-like radical recombination in the presence of oxygen at the steady-state concentrations used? In the authors' opinion, what are the main oxidation products of the OH radical reaction with GA? On page 7, line 166, it is mentioned that the products formed can evaporate more easily. Later (line 177), the formation of carboxylic acids and compounds formed by the degradation of aromatic rings is mentioned. How likely is it that these compounds will evaporate? Could the authors indicate how long the H2O2 (100 uM) as well as the DMB is present in the solution before it decays by the photochemistry? How many times is DMB involved in a reaction as a photosensitizer before it is degraded? Is it possible that GA acts as a photosensitizer in the GA + H2O2 system used and reacts with H2O2, which is subsequently more important than the production of OH radicals by H2O2 photolysis, since the absorption of GA is somewhat greater compared to H2O2 in the specific wavelength range? Did the authors do any experiments with GA in the absence of an oxidant? How justified here is the statement (on page 10 line 215) that the steady-state concentration of OH is the same or similar?
The end of section 3.4 is difficult to follow and is full of speculation. Could the authors sharpen the end of the section for clarity and with concrete numbers, e.g., how likely evaporation is? This brings me back to the experimental description, where it is not clear whether evaporation may or may not play a role in this study, so the experimental design is not well described within the manuscript.
In summary, this manuscript, which certainly has its merits and is quite interesting in its present form, might be improved more in terms of the clarity and coherence of its scientific basis to enable a recommendation for acceptance.
Citation: https://doi.org/10.5194/egusphere-2023-443-RC2 - AC2: 'Reply on RC2', Qi Zhang, 13 May 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-443', Anonymous Referee #1, 04 Apr 2023
This study examines the evolution of chemical and optical properties of phenolic aqSOA generated via •OH- or 3C* oxidation during photoaging, as well as the effects of increased concentration of oxidants. The changes in the chemical composition and light absorption of aqSOA were tracked using HR-AMS and UV-Vis spectroscopy. The findings for the 3C*-aqSOA have been reported in a previous study by the same group (Jiang et al., 2021). Although this paper attempted to explain the differences between 3C*-aqSOA thoroughly and •OH-aqSOA, several statements appear unclear or not clearly supported by the results. The paper is well organized, but several points need clarification.
- Could the authors give examples of products likely resistant to fragmentation?
- Were the 4 aliquots prepared for further aging also continuously stirred?
- Section 3.2: why would •OH production from 3C* be more important during prolonged photoaging? is there any evidence for this?
- Section 3.2: Does higher light absorptivity always correlate with faster direct photodegradation? It seems not as, for example, Smith et al. (2016) attributed the essentially equal reactivity of syringaldehyde and acetosyringone against direct photodegradation to the greater light absorption by syringaldehyde and higher quantum efficiency for loss for acetosyringone. Also, how does this reconcile with the statement in section 3.1 about •OH-aqSOA being more vulnerable to photodegradation than 3C*-aqSOA?
- Section 3.2: Could the authors explain why acid formation is more pronounced in the aging of 3C*-aqSOA? In section 3.1, it was mentioned that •OH-aqSOA has a greater tendency to form volatile and semi-volatile compounds that evaporate from the condensed phase
- Section 3.2: What are the differences between the highly oxidized products from 3C* and •OH oxidation? Those from 3C* are stated to be resistant to degradation, while those from •OH are mentioned to degrade over long aging times. Also, why are some of the highly oxidized products less reactive with 3C* than with •OH, and that they can persist in the aqueous phase?
- Section 3.3: Why would the higher oxidation degree of •OH-aqSOA lead to the destruction of chromophores?
- Section 3.4: Why would •OH be more reactive with non-phenolic organic compounds? For example, a study on the oxidation of green leaf volatiles (Richards-Henderson et al., 2015) comprising both non-phenolic and phenolic compounds did not show a general trend in the reactivity with 3C* and •OH.
- Section 3.4: Could the authors give some examples of these unique low-volatility, light-absorbing products that cannot be generated via •OH oxidation?
- Section 3.4: What do the authors mean by •OH only accounts for a small fraction of the total oxidant amount in the 3C*-initiated reaction system?
Minor comments and questions:
- Intro: Is hydroxylation an example of functionalization?
- Please provide more information on the HPLC method used to determine the concentration of GA and DMB.
- Please correct what [Org]t and [GA]t refer to
- There are other recently published articles regarding the formation of aqSOA by 3C* chemistry and the corresponding light absorption by reaction products (e.g., Li et al., 2022, https://doi.org/10.5194/acp-22-7793-2022; Mabato et al., 2023, https://doi.org/10.5194/acp-23-2859-2023)
References:
Jiang, W., Misovich, M. V., Hettiyadura, A. P. S., Laskin, A., McFall, A. S., Anastasio, C., and Zhang, Q.: Photosensitized reactions of a phenolic carbonyl from wood combustion in the aqueous phase—chemical evolution and light absorption properties of aqSOA, Environ. Sci. Technol., 55, 5199−5211, https://doi.org/10.1021/acs.est.0c07581, 2021.
Richards-Henderson, N. K., Pham, A. T., Kirk, B. B., and Anastasio, C.: Secondary organic aerosol from aqueous reactions of green leaf volatiles with organic triplet excited states and singlet molecular oxygen, Environ. Sci. Technol., 49, 268–276, https://doi.org/10.1021/es503656m, 2015.
Smith, J. D., Kinney, H., and Anastasio, C.: Phenolic carbonyls undergo rapid aqueous photodegradation to form low-volatility, light-absorbing products, Atmos. Environ., 126, 36−44, https://doi.org/10.1016/j.atmosenv.2015.11.035, 2016.
Citation: https://doi.org/10.5194/egusphere-2023-443-RC1 - AC1: 'Reply on RC1', Qi Zhang, 13 May 2023
- AC3: 'Reply on RC1', Qi Zhang, 13 May 2023
-
RC2: 'Comment on egusphere-2023-443', Anonymous Referee #2, 09 Apr 2023
Review comments on the manuscript "Photoaging of Phenolic Secondary Organic Aerosol in the Aqueous Phase: Evolution of Chemical and Optical Properties and Effects of Oxidants" in EGUsphere. The manuscript addresses the long-term aqueous aging of aqSOA formed from the photooxidation of guaiacyl acetone (GA) by OH radicals or the photosensitizer model compound 3,4-dimethoxy-benzaldehyde (DMB) using Pyrex tube experiments and performing positive matrix factorization (PMF) analysis of combined HR-AMS and UV-vis spectral data.
Questions and remarks:
Could the authors elaborate more the description of the experimental and analytical method, as it is a bit vague at the moment. It is not clear to the reader if the experiment is performed in a closed system or if it is, open to the atmosphere, if oxygen is present or not? Could the authors describe the potential contribution of singlet oxygen to the conversion within the system when the photosensitizer dimethoxybenzaldehyde and oxygen are present? Could the guaiacylacetone itself act as a photosensitizer? Would the authors expect a similar oligomer formation yield and conversion rate if 4-propylguaiacol had been used? What would be the general reaction mechanism in the photosensitizer system in the presence of GA and DMB after the first step of H-atom abstraction or electron transfer? Is addition of the photosensitizer possible? What would be the difference in the first oxidation products formed by the photosensitizers compared to OH radicals? What would be the result of the involvement of oxygen? Would the resulting peroxyl radicals lead to the formation of oligomers, what is the authors' opinion? If not, could this explain the lower yield of oligomers in OH radical-induced oxidation? How likely is alkyl/phenoxy-like radical recombination in the presence of oxygen at the steady-state concentrations used? In the authors' opinion, what are the main oxidation products of the OH radical reaction with GA? On page 7, line 166, it is mentioned that the products formed can evaporate more easily. Later (line 177), the formation of carboxylic acids and compounds formed by the degradation of aromatic rings is mentioned. How likely is it that these compounds will evaporate? Could the authors indicate how long the H2O2 (100 uM) as well as the DMB is present in the solution before it decays by the photochemistry? How many times is DMB involved in a reaction as a photosensitizer before it is degraded? Is it possible that GA acts as a photosensitizer in the GA + H2O2 system used and reacts with H2O2, which is subsequently more important than the production of OH radicals by H2O2 photolysis, since the absorption of GA is somewhat greater compared to H2O2 in the specific wavelength range? Did the authors do any experiments with GA in the absence of an oxidant? How justified here is the statement (on page 10 line 215) that the steady-state concentration of OH is the same or similar?
The end of section 3.4 is difficult to follow and is full of speculation. Could the authors sharpen the end of the section for clarity and with concrete numbers, e.g., how likely evaporation is? This brings me back to the experimental description, where it is not clear whether evaporation may or may not play a role in this study, so the experimental design is not well described within the manuscript.
In summary, this manuscript, which certainly has its merits and is quite interesting in its present form, might be improved more in terms of the clarity and coherence of its scientific basis to enable a recommendation for acceptance.
Citation: https://doi.org/10.5194/egusphere-2023-443-RC2 - AC2: 'Reply on RC2', Qi Zhang, 13 May 2023
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Wenqing Jiang
Christopher Niedek
Cort Anastasio
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1667 KB) - Metadata XML
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Supplement
(1331 KB) - BibTeX
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- Final revised paper