From the journal Environmental Science: Atmospheres Peer review history
Wavelength-resolved quantum yields for vanillin photochemistry: self-reaction and ionic-strength implications for wildfire brown carbon lifetime
Round 1
Manuscript submitted on 05 ene. 2024![](https://www.rsc-cdn.org/pubs-core/2022.0.148/content/NewImages/icon-letter.png)
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05-Feb-2024
Dear Dr Drozd:
Manuscript ID: EA-ART-01-2024-000002
TITLE: Wavelength-resolved quantum yields for Vanillin Photochemistry: Self-Reaction and Ionic-Strength Implications for Atmospheric Brown Carbon Lifetime
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.
After careful evaluation of your manuscript and the reviewers’ reports, I will be pleased to accept your manuscript for publication after revisions.
Please revise your manuscript to fully address the reviewers’ comments. When you submit your revised manuscript please include a point by point response to the reviewers’ comments and highlight the changes you have made. Full details of the files you need to submit are listed at the end of this email.
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Yours sincerely,
Dr Lin Wang
Associate Editor, Environmental Science: Atmospheres
Environmental Science: Atmospheres is accompanied by companion 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
This manuscript presents results of a study of photochemistry of vanillin in aqueous solutions as a function of the wavelength of the irradiation source and ionic strength. The experiments are supplemented by the calculation of different excited state processes. The measured quantum yield for the photochemical loss of vanillin has a strong wavelength dependence, mirroring that of the rate of intersystem crossing from the singlet to triplet manifold, which is a very nice result. Another important result is quantification of the ionic strength effect on the photochemical loss rate – such measurements are not common but needed to understand chemistry in deliquesced aerosol particles. I would be in favor of publishing this work.
COMMENTS
1). The authors go into a discussion of how not taking wavelength dependence of the quantum yield in the explicit consideration and simply scaling the photolysis rate with available UV flux would lead to mistakes. This should not be presented as a new result, which the tone of the discussion appears to suggest, it is a well-known fact. This is precisely why absorption cross sections have been tabulated as a function of wavelength for important atmospheric species, for example in the NASA JPL data evaluation (https://jpldataeval.jpl.nasa.gov/). So this discussion can be shortened significantly so that the authors can focus on their measurement results, which are interesting and important.
2). The lifetime of vanillin is calculated assuming that vanillin in the triplet excited stated would react with itself. It certainly does this under the measurement conditions, as the authors convincingly demonstrate (isosbestic point, direct observation of the dimer product, observed kinetics). But it would be worth pointing out that in the real atmosphere there will be many other reaction partners capable of reacting with Van*. This will affect the lifetime discussion related to Figure 6. I do not suggest this for this paper but for the following experiments for the next paper in this series, the authors may consider adding a mix of organics, perhaps an extract from a biomass burning sample, to see how the presence of other reactants affect the effective lifetime.
EDITORIAL
3). The “Environmental Significance Statement” is too technical and needs to be simplified so that it can be better understood by broader audience.
4). P5, “All three of these functions are wavelength dependent”. I think the authors meant that I0, quantum yield, and epsilon are wavelength dependent but the way this sentence is structured makes it seem as if they are talking about the equation for the fraction of light absorbed. Rewording is needed here.
5). Equation 1: It is not clear how the units work out in this equation. The authors use the normal units of the flux, photon/(nm*s*cm^2). This equation is normally written using absorption cross section (sigma, measured in cm^2/molecule) instead of molar absorptivity. If using the absorption cross section, pathlength “l” would not be part of this equation. It would help to explicitly state the units used in this equation to avoid ambiguity. Same applies to Eq. 4 below.
6). P8, top line: not clear what “100” stands for
7). Figure 1. This figure needs several corrections:
a). y-axis caption: the text on page 8 says it is base-10 molar absorptivity, however, the symbol used in the caption is that for base-e absorption cross-section. Please verify and correct as needed.
b). The way this figure is plotted it is not clear what scale the LED fluxes should be used. Why not have a separate panel for them with an actual Y-axis scale and no shift from zero?
c). It is not clear whether the left axis starts at 0 or not. This is critical to know one cannot tell whether there is any absorbance at longer wavelengths.
8). End of Page 9: the variation of sample volume from experiment to experiment makes sense, but could be problematic for divergent light sources [check SI for details]
9). Page 10: clarify how the pH was adjusted. Presumably with sulfuric acid?
10). Figure 2: In my opinion, the figure caption and panel labels are unnecessarily complicated. Consider placing labels like S2 (pi*) right next to the corresponding image instead of labelling them with letters.
11). Table 1: Is yield Fi-H the same as Fi-1 used in Equation 5?
Reviewer 2
This study used a combination of experiments and theoretical computations to measure the quantum yield of vanillin photolysis. Several parameters were considered, including the light wavelength, vanillin concentration, and ionic strength of solution. A significant dependence of the yield on the wavelength has been observed. This indicates that previous measurements performed with broadband solar simulators using reference compounds that absorb in a different spectral range may produce significant inaccuracies. Since vanillin is a good representative of phenolic compounds present in BrC aerosols, this has important atmospheric implications, e.g., related to photobleaching lifetime and climate impact. The manuscript is well written and I believe it can be published after a minor revision as outlined below.
Why only a single water molecule was used in a complex with vanillin in calculations? Did this or other study explored the dependence of the obtained values on the number of complexed water molecules?
Define uncommon abbreviations, such as “DAD”
P3L5: Remove “Quantum yields in”, i.e., the sentence will begin with “Higher energy,”
P3, line 2 from the bottom: Add that the quantum yield decreases from 1 to 0
P5L2: “Upon photon absorption in this range AND INTERCONVERSION,”
P5 last line: what does “large derivatives” refer to? Please clarify.
P6, eq. 1: Use an apostrophe to distinguish log10 and ln based molar extinction coefficients
P6, middle: The meaning of “relatively consistent” is unclear; consider rewording
P6, fourth sentence from the bottom: Define “SZA” here in main text
P7, second line from the bottom: remove spaces before semicolons
P8, L1: What does “100” mean?
Figure 1 caption is confusing. Molar absorptivity is not mentioned. Please rewrite.
Eq.3: Add space between Phi_ISC and the ratio
P13 middle: Define “PCM” here, not on P.14
P14, middle: “All DFT CALCULATIONS were...”. What/where is PUHTI supercomputer? Define.
P14, two lines down: did you mean “time dependent (TD)”?
P14, line 5 from the bottom: use the previously defined abbreviation (TD) only
P16, middle: it would read better if you spell out “PhC”
Scheme 1 caption repeats the main text
P24: Clean up the last sentence on this page
P25, middle: In the figure a different wavelength is mentioned (320 nm).
Fig 5: Add a description for the dashed line in part (b) in the caption. What is “several wavelengths of solar radiation”? Do you refer to intensity? Log-linear is confusing. Simply state that one of the y-axes shows logarithm of the ratio.
The graphic TOC has too many fine details. Some of the gray lines are barely visible.
Fig S7: It appears that S1 and S2 states got swapped
In Eqs. X1 and X2 the Planck’s constant got lost in the PDF version
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[This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.]
REVIEWER REPORT(S):
Referee: 1
Comments to the Author
This manuscript presents results of a study of photochemistry of vanillin in aqueous solutions as a function of the wavelength of the irradiation source and ionic strength. The experiments are supplemented by the calculation of different excited state processes. The measured quantum yield for the photochemical loss of vanillin has a strong wavelength dependence, mirroring that of the rate of intersystem crossing from the singlet to triplet manifold, which is a very nice result. Another important result is quantification of the ionic strength effect on the photochemical loss rate – such measurements are not common but needed to understand chemistry in deliquesced aerosol particles. I would be in favor of publishing this work.
We thank the reviewer for these thorough and thoughtful comments, as well as the suggestion for future investigations. We have replied to each comment and have adjusted the manuscript accordingly.
COMMENTS
1). The authors go into a discussion of how not taking wavelength dependence of the quantum yield in the explicit consideration and simply scaling the photolysis rate with available UV flux would lead to mistakes. This should not be presented as a new result, which the tone of the discussion appears to suggest, it is a well-known fact. This is precisely why absorption cross sections have been tabulated as a function of wavelength for important atmospheric species, for example in the NASA JPL data evaluation (https://jpldataeval.jpl.nasa.gov/). So this discussion can be shortened significantly so that the authors can focus on their measurement results, which are interesting and important.
This section was shortened significantly. It now reads:
“The rate constant for photochemical loss, j, for an optically thin system and a simple reaction (e.g. isomerization) is
j=∫▒〖Φ(λ)*I_0 (λ)*ε(λ)*l dλ〗 (1)
where Φ is the quantum yield, I_0 the volume-averaged incident photon flux (photons/(〖cm〗^3 sec)), and ε the base-e molar absorptivity (M-1 cm-1), and l pathlength (cm). While ε(λ) and I_0 (λ) are readily determined for any species and light source, the wavelength dependence of the quantum yield for loss of PhC species due to photochemical loss, Φ_loss (λ), is generally not known.15 Calculation of the photolysis rate constant using the explicit wavelength dependence avoids errors in extrapolating from measurements made at a single SZA. There is significant variation (20 - 100%) in the relative intensity of I_(0,sun) (λ) between 0 – 60° SZA in the range 300 – 340nm (Figure S1). The variation in the solar spectrum in this region may cause inaccuracies if only changes in total photon flux are used to predict reaction rate as a function of solar zenith angle (SZA).”
2). The lifetime of vanillin is calculated assuming that vanillin in the triplet excited stated would react with itself. It certainly does this under the measurement conditions, as the authors convincingly demonstrate (isosbestic point, direct observation of the dimer product, observed kinetics). But it would be worth pointing out that in the real atmosphere there will be many other reaction partners capable of reacting with Van*. This will affect the lifetime discussion related to Figure 6. I do not suggest this for this paper but for the following experiments for the next paper in this series, the authors may consider adding a mix of organics, perhaps an extract from a biomass burning sample, to see how the presence of other reactants affect the effective lifetime.
Thank you for this thoughtful suggestion. We have in fact already begun such experiments and will present these in forthcoming work. The presence of other H-atom donors slows the loss of phenolic carbonyls and shifts the reaction order to nearly first order in vanillin.
EDITORIAL
3). The “Environmental Significance Statement” is too technical and needs to be simplified so that it can be better understood by broader audience.
We have revised the statement to read:
“Solar radiation transforms organic material in atmospheric aerosol, affecting how strongly they absorb light and how quickly they oxidize. We studied the photochemistry of vanillin, an oxidized aromatic in wildfire emissions, using UV LEDs. These measurements can be directly generalized to the solar spectrum under any conditions (time/day/location). This photochemistry speeds up at salt concentrations relevant to atmospheric aerosol. Generally, previous measurements require extrapolation to varying solar conditions and are limited to low salt concentrations. Our measurements reveal the wavelength dependence of photochemistry efficiency (quantum yield), and calculations indicate their origin is changing rates of electron processes after excitation by UV light (intersystem crossing). We demonstrate the importance of wavelength resolved measurements at conditions relevant to atmospheric aerosol.”
4). P5, “All three of these functions are wavelength dependent”. I think the authors meant that I0, quantum yield, and epsilon are wavelength dependent but the way this sentence is structured makes it seem as if they are talking about the equation for the fraction of light absorbed. Rewording is needed here.
This sentence was removed in focusing the scope of the introduction. See response to Comment 1.
5). Equation 1: It is not clear how the units work out in this equation. The authors use the normal units of the flux, photon/(nm*s*cm^2). This equation is normally written using absorption cross section (sigma, measured in cm^2/molecule) instead of molar absorptivity. If using the absorption cross section, pathlength “l” would not be part of this equation. It would help to explicitly state the units used in this equation to avoid ambiguity. Same applies to Eq. 4 below.
There were inconsistencies in the photon flux units in the manuscript, and these have been reconciled. Because we are working in aqueous solution, we have chosen to use molar absorptivity to describe light absorption. In Figure 1. LED irradiance is displayed with units of photon/( nm 〖cm〗^2 sec) for comparison to the solar spectrum. All calculations were done using volume-averaged fluxes, as that is the direct result of our actinometry measurements.
Equation 1 was changed to read:
“The rate constant for photochemical loss, j, for an optically thin system and a simple reaction (e.g. isomerization) is
j=∫▒〖Φ(λ)*I_0 (λ)*ε(λ)*l dλ〗 (1)
where Φ is the quantum yield, I_0 the volume-averaged incident photon flux (photons/(〖cm〗^3 sec)), ε the base-e molar absorptivity (M-1 cm-1), and l the pathlength (cm)”
Equation 4 was adjusted slightly for clarity:
(d[Van])/dt= -[Van]*∫_λ^ ▒〖(Φ_ISC (k_(SR,λ) [Van] )/(k_1+k_(SR,λ) [Van] )+Φ_1 )*I_0 (λ)*ε(λ)*l dλ〗 (4)
We have updated the description of fluxes in Section 2.2 to read:
“Typical volume-averaged fluxes for LED varied between 3-9*10^15 photon/(〖cm〗^3 sec).”
The Fig 1. Caption now reads:
“LED peak irradiance varied by experiment in the range of 3-9*10^14 photon/(nm 〖cm〗^2 sec).”
6). P8, top line: not clear what “100” stands for
This was a typo and was deleted.
7). Figure 1. This figure needs several corrections:
Figure 1. was revised and split into two panels.
a). y-axis caption: the text on page 8 says it is base-10 molar absorptivity, however, the symbol used in the caption is that for base-e absorption cross-section. Please verify and correct as needed.
Indeed the plot displays base-e values, to emphasize their use in j-value calculations. We have corrected the caption to read “base-e”.
b). The way this figure is plotted it is not clear what scale the LED fluxes should be used. Why not have a separate panel for them with an actual Y-axis scale and no shift from zero?
The figure was split into two panels, with clear axes labels.
c). It is not clear whether the left axis starts at 0 or not. This is critical to know one cannot tell whether there is any absorbance at longer wavelengths.
The axis ticks were fixed to indicate the range appropriately.
8). End of Page 9: the variation of sample volume from experiment to experiment makes sense, but could be problematic for divergent light sources [check SI for details]
A control experiment was also performed for 2 uM vanillin, without aliquot removal, using a fresh 1mL solution for each time point. These results were consistent with the aliquot method.
9). Page 10: clarify how the pH was adjusted. Presumably with sulfuric acid?
pH was adjusted with HCl. Several tests were performed with sulfuric acid to verify there were no specific effects from chloride ions.
The following clarification was added:
“Several tests with sulfuric acid did not change results compared to hydrochloric acid, showing no specific acid or anion effects.”
10). Figure 2: In my opinion, the figure caption and panel labels are unnecessarily complicated. Consider placing labels like S2 (pi*) right next to the corresponding image instead of labelling them with letters.
The letter labels were replaced with orbital types, and the caption was updated to read:
“Figure 2. Natural transition orbitals (NTO) describing the π→π^*excitations for the S0 to S2 and S0 to S3 transitions.”
11). Table 1: Is yield Fi-H the same as Fi-1 used in Equation 5?
Yes, the table was updated to be consistent with Equation 5.
Referee: 2
Comments to the Author
This study used a combination of experiments and theoretical computations to measure the quantum yield of vanillin photolysis. Several parameters were considered, including the light wavelength, vanillin concentration, and ionic strength of solution. A significant dependence of the yield on the wavelength has been observed. This indicates that previous measurements performed with broadband solar simulators using reference compounds that absorb in a different spectral range may produce significant inaccuracies. Since vanillin is a good representative of phenolic compounds present in BrC aerosols, this has important atmospheric implications, e.g., related to photobleaching lifetime and climate impact. The manuscript is well written and I believe it can be published after a minor revision as outlined below.
We thank the reviewer for these thorough and thoughtful comments. We have replied to each of them and have adjusted the manuscript accordingly.
Why only a single water molecule was used in a complex with vanillin in calculations? Did this or other study explored the dependence of the obtained values on the number of complexed water molecules?
The study was restricted to one water molecule partly for computational reasons. Clusters of vanillin with multiple water molecules will inevitably have very many different conformers, and the expensive excited-state calculations would need to be performed on all of them, vastly increasing the cost (and introducing a new source of uncertainty as quantitative conformer equilibria are also difficult to compute). Furthermore, it is typically the case that when implicit solvation (using continuum models, in this study PCM) is insufficient, explicit solvation can (and should) be modelled by including only the solvent molecule or molecules directly interacting (in this case H-bonding) with the relevant functional groups (in this case the chromophore aldehyde). Adding further solvent molecules would amount to double counting, as the more indirectly interacting solvation shell is then modelled both explicitly, AND with the continuum model. For example, Kelly et al (https://pubs.acs.org/doi/full/10.1021/jp055336f) found that precisely one explicit solvent is sufficient fo accurate pKa calculations. While a systematic investigation of the effect of the number of water molecules on the predicted photophysical properties of vanillin might be interesting, we believe it is outside the scope of this study.
Define uncommon abbreviations, such as “DAD”
DAD was defined in section 2.4.
P3L5: Remove “Quantum yields in”, i.e., the sentence will begin with “Higher energy,”
This edit was made.
P3, line 2 from the bottom: Add that the quantum yield decreases from 1 to 0
As the quantum yield is dependent on the initial concentration, the wavelength dependence was normalized to the maximum value in Figure 4. We believe it would require too many qualifications to state the values of the quantum yield at this point in the manuscript.
P5L2: “Upon photon absorption in this range AND INTERCONVERSION,”
This sentence now reads:
“Upon photon absorption and intersystem crossing in this range, PhC readily form triplet excited states that drive oxidation processes through a complex mechanism.”
P5 last line: what does “large derivatives” refer to? Please clarify.
This sentence was deleted in editing. That paragraph now reads:
“The rate constant for photochemical loss, j, for an optically thin system and a simple reaction (e.g. isomerization) is
j=∫▒〖Φ(λ)*I_0 (λ)*ε(λ)*l dλ〗 (1)
where Φ is the quantum yield, I_0 the volume-averaged incident photon flux (photons/(〖cm〗^3 sec)), and ε the base-e molar absorptivity (M-1 cm-1), and l pathlength (cm). While ε(λ) and I_0 (λ) are readily determined for any species and light source, the wavelength dependence of the quantum yield for loss of PhC species due to photochemical loss, Φ_loss (λ), is generally not known.15 Calculation of the photolysis rate constant using the explicit wavelength dependence avoids errors in extrapolating from measurements made at a single SZA. There is significant variation (20 - 100%) in the relative intensity of I_(0,sun) (λ) between 0 – 60° SZA in the range 300 – 340nm (Figure S1). The variation in the solar spectrum in this region may cause inaccuracies if only changes in total photon flux are used to predict reaction rate as a function of solar zenith angle (SZA).”
P6, eq. 1: Use an apostrophe to distinguish log10 and ln based molar extinction coefficients
We have revised the manuscript to include only base-e values, and defined the first usage of “ε" to be base-e, as shown in our response to the previous comment (“P5 last line…”)
P6, middle: The meaning of “relatively consistent” is unclear; consider rewording
For clarity this sentence was removed, see response to the comment on “P5 last line”
P6, fourth sentence from the bottom: Define “SZA” here in main text
SZA was re-defined here.
P7, second line from the bottom: remove spaces before semicolons
This edit was made.
P8, L1: What does “100” mean?
This was a typo and was removed.
Figure 1 caption is confusing. Molar absorptivity is not mentioned. Please rewrite.
This figure was re-made and the caption re-written.
Figure 1.a) Base-e molar absorptivity, ε, of vanillin (magenta), and solar irradiance at 0^° (blue) and 60^° (yellow) SZA. Solar spectrum data was obtained from the NCAR TUV Model (ozone: 300 DU, albedo: 0.1). 16 b) LED irradiance source profiles for nominal wavelengths of 295nm, 310nm, 325nm, 340nm, 365 nm, 375 nm, 385 nm. LED peak irradiance varied by experiment in the range of 3-9*10^14 photon/(nm 〖cm〗^2 sec).
Eq.3: Add space between Phi_ISC and the ratio
This space was added.
P13 middle: Define “PCM” here, not on P.14
This definition was moved.
P14, middle: “All DFT CALCULATIONS were...”. What/where is PUHTI supercomputer? Define.
PUHTI is one of the supercomputers at CSC IT Center for Science in Espoo, Finland.
The specific computer node is not necessary, so for clarity we have removed the reference to PUHTI.
P14, two lines down: did you mean “time dependent (TD)”?
The method used was time-independent; this abbreviation was wrong, so it was removed. The sentence now reads:
“The rate constants for fluorescence (kr), intersystem crossing (kISC) and internal conversion (kIC) between the considered electronic states were calculated using time independent theory and the code described in the details of recent publications.”
P14, line 5 from the bottom: use the previously defined abbreviation (TD) only
This edit was made.
P16, middle: it would read better if you spell out “PhC”
This edit was made.
Scheme 1 caption repeats the main text
The text was edited to include fluorescence, and the caption was shortened to read:
“Mechanism for photochemical loss of Vanillin. Radical propagation through reaction of excited triplet state vanillin with ground state vanillin as well as direct loss of a hydrogen atom lead to a dimer product.”
P24: Clean up the last sentence on this page
To avoid any confusion, we have removed this sentence.
P25, middle: In the figure a different wavelength is mentioned (320 nm).
The sentence was changed to read: “The photochemical loss of vanillin and 320 nm solar radiation have a similar dependence on SZA.”
Fig 5: Add a description for the dashed line in part (b) in the caption. What is “several wavelengths of solar radiation”? Do you refer to intensity? Log-linear is confusing. Simply state that one of the y-axes shows logarithm of the ratio.
To avoid confusion, the dashed line fit was described and the caption shortened to read :
“Figure 5. a) Solar zenith angle (SZA) dependence for: j_(1 μM) normalized to j(1 μM,0°) (blue), the normalized intensity of several wavelengths of solar radiation (black, dashed), and three common gas-phase photochemical reactions (red, dashed). Solar spectra and photolysis rate data were obtained from the NCAR TUV Model.16 b) Effect of ionic strength on the photochemical loss of 10 μM vanillin. The dashed line shows a linear fit to 〖Log〗_10 (Φ_(salt,pH=2)⁄Φ_(pH=2) ) derive the kinetic salting coefficient (b = 0.15).”
The graphic TOC has too many fine details. Some of the gray lines are barely visible.
We have enlarged fonts removed some objects and thickened all fine lines to improve clarity.
Fig S7: It appears that S1 and S2 states got swapped
The figure is correct, as noted in the text the energy ordering of the S1 and S2 states (notation defined by their ordering at the ground state S0 geometry) is inverted at the S2 geometry: “After vibrational relaxation, S2 becomes the lowest energy excited state at its equilibrium geometry.”
The caption was updated to read:
Figure S7. The Jablonski diagram for vanillin in water, at the S2 equilibrium geometry. Note that after vibrational relaxation, S2 becomes the lowest energy excited state at its equilibrium geometry. The kr are radiative rate constants, kISC are intersystem crossing rate constants and kIC are internal conversion rate constants.
In Eqs. X1 and X2 the Planck’s constant got lost in the PDF version
Equations with Planck’s constant were re-rendered, and supplemental equations were relabeled as S1, etc.
Round 2
Revised manuscript submitted on 25 feb. 2024![](https://www.rsc-cdn.org/pubs-core/2022.0.148/content/NewImages/icon-letter.png)
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28-Feb-2024
Dear Dr Drozd:
Manuscript ID: EA-ART-01-2024-000002.R1
TITLE: Wavelength-resolved quantum yields for Vanillin Photochemistry: Self-Reaction and Ionic-Strength Implications for Wildfire Brown Carbon Lifetime
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Transparent peer review
To support increased transparency, we offer authors the option to publish the peer review history alongside their article. Reviewers are anonymous unless they choose to sign their report.
We are currently unable to show comments or responses that were provided as attachments. If the peer review history indicates that attachments are available, or if you find there is review content missing, you can request the full review record from our Publishing customer services team at RSC1@rsc.org.