From the journal Environmental Science: Atmospheres Peer review history

25-Nov-2023

Dear Dr Zhao:

Manuscript ID: EA-ART-10-2023-000149
TITLE: Relative humidity affects molecular composition of secondary organic aerosol from α-pinene ozonolysis

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************

Reviewer 1

This study presents a comprehensive study with detailed insights into the molecular composition of secondary organic aerosols formed from a-pinene ozonolysis and its dependence on relative humidity. The findings are important for modeling SOA formation in the atmosphere, and they contribute significantly to the field of atmospheric chemistry.
I just have some minor comments:
L 57-60: The sentence is too long, and it could benefit from rephrasing for better clarity.
L 76-78: Recommend consistently using either °C or K for temperature.
L 88: Please clarify what “AS” refers to.
L 122-124: It is not clear how you deal with the background noise.
L 146-147: Could you provide details on the stability of the NaINa+ across all experiments?
L 149: Do the dimers decrease with increasing RH from 3% to 28%?
L 158: More details on the experiments conducted by Li et al. and Caudillo et al. would be beneficial for context.
L 162-164: This section might be an appropriate place to discuss the findings of Surdu et al.
L 178-180: Consider moving this sentence to a later section, possibly before L 204.
L183-184: Can you highlight any distinct changes in patterns? It’s currently not very clear.
L 213-214: Suggest removing “The changes in the …. their absolute abundance.” for brevity.
L 216: Since the figure is in the Supplementary Information, a more detailed description of the 7-fold increase across specific relative humidity ranges would be helpful.
L 222: As you discussed the decomposition here, could you also comment on whether C10H16O2 (e.g. in Fig. 4) are merely fragments?
L 243: I think RH also enhances the partitioning of less volatile products. The observed increase in more volatile products in the particle phase could be attributed to their higher gas-phase abundance.
L 274: Providing separate O/C ratios for monomers and dimers would be informative.
L 403-405: Consider the possibility that the dimers partly arise from particle-phase dimerization. Increases in monomers in the particle phase could lead to more dimers, even without acid-catalyzed reaction enhancements.
Line 430-433: The sentence is too long and complex. A rephrasing would improve readability.

Reviewer 2

Please see attached

We thank the reviewer for the helpful comments on our manuscript. The comments are greatly appreciated. We have addressed all the comments and believe that the revisions based on the comments help improve the quality of our manuscript. Below please find our responses to the comments one by one and the corresponding revisions made to the manuscript. The original comments are in italics. The revised parts of the manuscript are in blue.

Reviewer#1

Comments to the Author
This study presents a comprehensive study with detailed insights into the molecular composition of secondary organic aerosols formed from a-pinene ozonolysis and its dependence on relative humidity. The findings are important for modeling SOA formation in the atmosphere, and they contribute significantly to the field of atmospheric chemistry.
I just have some minor comments:
L 57-60: The sentence is too long, and it could benefit from rephrasing for better clarity.
Response:
We thank the reviewer for the advice. We have revised this sentence in line 60 as follows:
“Especially the role of highly oxygenated organic molecules (HOMs) with low or extremely low volatile organic compounds (LVOCs or ELVOCs) in the formation of SOA has been recognized,20-22 which firstly observed in α-pinene ozonolysis system and later in various oxidation systems7-9, 23, 24.”

L 76-78: Recommend consistently using either °C or K for temperature.
Response:
Accepted. We have revised -50 ℃ to -223 K in line 79.

L 88: Please clarify what “AS” refers to.
Response:
Accepted. We have added the clarification in line 91 as follows:
“…and absence of ammonium sulfate (AS) seed aerosol…”

L 122-124: It is not clear how you deal with the background noise.
Response:
We apologize for the unclear expression. We have clarified the description in line 136 of the revised manuscript as follows:
“An auto-valve was used to enable automatic alternates between direct sampling and sampling through a HEPA filter (AQ, Parker). The duration of direct sampling and filter sampling are 8 min and 7 min, respectively. The difference of spectra between direct sampling and sampling through the filter was used to derive the particle signal devoid of any instrument background signal (including residue gases passing through denuder and impurities from spraying solution).”

L 146-147: Could you provide details on the stability of the NaINa+ across all experiments?
Response:
The average signal intensity of NaINa+ is (1.6-1.9)106 ions/s with an average standard deviation of ~4.6% in the experiments with AS seed aerosol and (1.7-1.8)106 ions/s with an average standard deviation of ~2.3% in the experiments without seed aerosol. The detailed average signal intensity and relative standard deviations are shown below.
Table R1. The average signal intensity (Avg) and relative standard deviations (Std) in all experiments. Each condition is repeated three times. (AS seed: #1-#3; No seed: #4-#6.)
Conditions AS seed No seed
NO. #1 #2 #3 #4 #5 #6
Avg
(106 Ions/s) 1.9 1.7 1.6 1.8 1.8 1.7
Std
(%) 7.2 2.0 4.6 2.3 2.5 2.0


L 149: Do the dimers decrease with increasing RH from 3% to 28%?
Response:
Yes, the absolute abundance of total dimers increased with increasing RH in general despite a slight decrease from 3% to 28% RH. We have revised this sentence in line 168 of revised manuscript as follows:
“Despite the general invariant fractions of monomers and dimers with increasing RH, the absolute abundance of total monomers increased with increasing RH. The absolute abundance of total dimers also increased with increasing RH in general despite a slight decrease from 3% to 28% RH (Figure 2b).”

L 158: More details on the experiments conducted by Li et al. and Caudillo et al. would be beneficial for context.
Response:
Accepted. We have added more detail about Li et al. and Caudillo et al. in line 184 as follows:
“Li et al.(2019) 23 reported the concentrations of all main HOM (monomers and dimers) did not change in the RH ranged from 3% to 92% at 293 K. And also, Caudillo et al.(2021)35 reported that the RH change from 20% to 60% does not have a significant influence on the distribution of HOM products at 223 K.”

L 162-164: This section might be an appropriate place to discuss the findings of Surdu et al.
Response:
Accepted. We have added a sentence to make it clear in line 192 as follows:
“…Surdu et al (2021)33, who found a similar RH dependence of the absolute abundance of C10H16O2-8 in particle phase at 243 K33.”
More detailed findings of Surdu et al are discussed in a later section.

L 178-180: Consider moving this sentence to a later section, possibly before L 204.
Response:
As suggested, we have moved this sentence in line 231 in the revised manuscript.

L183-184: Can you highlight any distinct changes in patterns? It’s currently not very clear.
Response:
Specifically, the fractions of C20 compounds decrease and the fractions of C16-19 compounds with low oxygen number increase in general with increasing RH. In the revised manuscript, we have added the following sentence in line 212 to clarify this point as follows:
“Such a speculation is substantiated by the observation that the distribution of dimer species exhibited distinct patterns between dry condition (3% RH) and other three RH conditions (Figure 3, Table S3). Specifically, the fractions of C20 decrease and the fractions of C16-19 with low oxygen number increase in general, indicating a change in dimer composition despite largely invariant total fractions of dimers and monomers.”
The T-test shows that dimer distribution at 3% RH are significantly different from those at other three RH (28%, 54%, and 84% RH). We have added the results of statistical test in the Table S3 of Supplement as follow:
Table S3 T-test results of the difference between dimer fractions at different RH in the presence and absence of AS seed
AS seed (3±1)%/ (28±2)% (3±1)%/ (54±2)% (3±1)%/ (84±3)% (28±2)%/ (54±2)% (28±2)%/ (84±3)% (54±2)%/ (84±3)%
t 4.62 3.11 2.14 -0.45 -0.48 -0.45
P <0.01 <0.01 0.04 0.66 0.64 0.66
No seed (<1)%/ (28±1)% (<1)%/ (58±2)% (<1)%/ (91±2)% (28±1)%/ (58±2)% (28±1)%/ (91±2)% (58±2)%/ (91±2)%
t 7.42 8.03 8.73 3.31 3.94 2.56
P <0.01 <0.01 <0.01 <0.01 <0.01 0.01


L 213-214: Suggest removing “The changes in the …. their absolute abundance.” for brevity.
Response:
Accepted. We have deleted this sentence.

L 216: Since the figure is in the Supplementary Information, a more detailed description of the 7-fold increase across specific relative humidity ranges would be helpful.
Response:
Accepted. We have added more detailed description in line 243 as follows:
“As RH increased from 3% to 84%, the increase in the total C9 species monomers (7 times) is much higher than the increase in the total products (0.9 times),…”

L 222: As you discussed the decomposition here, could you also comment on whether C10H16O2 (e.g. in Fig. 4) are merely fragments?
Response:
Considering the high vapor pressure of C10H16O2, it is generally expected to reside in the gas-phase. In order to investigate whether C10H16O2 is indeed present in the particle-phase or a fragment ion from other (likely more oxygenated) compounds, we examine the correlation between C10H16O2 and C10H16Ox=3-9 and C20H32Ox dimers. Such as method has been used by Bell et al., (2022) to judge potential fragmentation in ionization, who found strong correlation between fragment ions and parent ions. We found that in this study, the Pearson correlation (R2) between C10H16O2 and other C10H16Ox=3-9 was low (0.2-0.4), and the R2 between C10H16O2 and dimers was also low (R2<0.16). Therefore, we believe that the C10H16O2 measured is indeed present in particles and not a fragment product of dimer. In a recent study, C10H16O2 was also observed in the particle-phase from ozonolysis of α-pinene, which was also measured by EESI-CIMS, and the time series of C10H16O2 was very close to that of C10H16Ox=3,4 (Surdu et al., 2023).

L 243: I think RH also enhances the partitioning of less volatile products. The observed increase in more volatile products in the particle phase could be attributed to their higher gas-phase abundance.
Response:
We agree that RH also enhances the partitioning of less volatile products. However, compared to the more volatile products, the enhancement is expected to less strong. The larger enhancement as a function of RH for more volatile product is attributed to their volatility and unlikely attributed to the higher gas-phase abundance. The enhancement factor of a certain compound at two different RH (RH1 and RH2) can be expressed as follows.
f=(C_(p,i) (RH2))/(C_(p,i) (RH1)) (Eq.1)
Cp,i(RH2) and Cp,i(RH1) are the particle-phase concentrations of i at RH2 and RH1, respectively.
Assuming that i is in equilibrium between gas-phase and particle-phase, and organics and water form an idea solution. One can get
C_(g,i)=γ C_(p,i)/(C_(p,T)+C_w ) C^0, (Eq.2)
where Cg,i, γ, Cp,T, Cw, C0 denote the gas-phase concentrations of i, activity coefficient, particle-phase concentration of total organics, particle-phase concentration of water, and saturated gas-phase phase concentration of i.
The total concentration of i in the gas-phase and particle-phase can be defined as CT,i, i.e.,
CT,i= Cg,i+ Cp,i. (Eq.3)
From Eq. 2 and Eq. 3, one get
C_(p,i)=(C_(p,T)+C_w)/(C_(p,T)+C_w+γC^0 ) C_(T,i). (Eq.4)
Substituting Eq.4 into Eq. 1, one can get
f=(〖(C〗_(p,T)+C_(w,2))(C_(p,T)+C_(w,1)+γC^0))/(〖(C〗_(p,T)+C_(w,1))(C_(p,T)+C_(w,2)+γC^0)), (Eq.5)
where Cw,1 and Cw,2 are the particle-phase concentration water at RH1 and RH2, respectively.
According Eq.5, f is independent of total concentration of a compound but depends on its saturated concentration, i.e. volatility.

L 274: Providing separate O/C ratios for monomers and dimers would be informative.
Response:
Accepted. We have added the O/C ratio of monomers and dimers in line 306 of the revised manuscript as follows:
“The stable O/C ratio of the total SOA (0.60±0.03() for monomers and 0.15±0.02() for dimers), …”

L 403-405: Consider the possibility that the dimers partly arise from particle-phase dimerization. Increases in monomers in the particle phase could lead to more dimers, even without acid-catalyzed reaction enhancements.
Response:
We agree that enhanced monomers can contribute to enhanced dimers via particle-phase dimerization. We attribute the increase of dimer to acid-catalyzed reactions rather than only to the enhanced monomers because the distribution of dimer under humid condition (28%, 54%, and 84%) changed significantly compared to the dry condition (3%) in the presence of AS seed.

Line 430-433: The sentence is too long and complex. A rephrasing would improve readability.
Response:
Accepted. We have revised this sentence in line 448 as follows:
“Previous studies reported that HOM from α-pinene is independent of RH23,35(Li, Chee et al. 2019),(Caudillo, Rörup et al. 2021). Yet, in the ozonolysis of other monoterpenes e.g. limonene compared with Δ3-carene, RH-dependent product distribution in the gas-phase has also been reported to play a vital role in the composition and yield of SOA54 (Zhang, Du et al. 2023).”



Bell, D. M., Wu, C., Bertrand, A., Graham, E., Schoonbaert, J., Giannoukos, S., Baltensperger, U., Prevot, A. S. H., Riipinen, I., El Haddad, I., and Mohr, C.(2022): Particle-phase processing of α-pinene NO3 secondary organic aerosol in the dark, Atmos. Chem. Phys., 22, 13167–13182, https://doi.org/10.5194/acp-22-13167-2022, 2022
Caudillo, L., B. Rörup, M. Heinritzi, G. Marie, M. Simon, A. C. Wagner, T. Müller, M. Granzin, A. Amorim, F. Ataei, R. Baalbaki, B. Bertozzi, Z. Brasseur, R. Chiu, B. Chu, L. Dada, J. Duplissy, H. Finkenzeller, L. Gonzalez Carracedo, X.-C. He, V. Hofbauer, W. Kong, H. Lamkaddam, C. P. Lee, B. Lopez, N. G. A. Mahfouz, V. Makhmutov, H. E. Manninen, R. Marten, D. Massabò, R. L. Mauldin, B. Mentler, U. Molteni, A. Onnela, J. Pfeifer, M. Philippov, A. A. Piedehierro, M. Schervish, W. Scholz, B. Schulze, J. Shen, D. Stolzenburg, Y. Stozhkov, M. Surdu, C. Tauber, Y. J. Tham, P. Tian, A. Tomé, S. Vogt, M. Wang, D. S. Wang, S. K. Weber, A. Welti, W. Yonghong, W. Yusheng, M. Zauner-Wieczorek, U. Baltensperger, I. El Haddad, R. C. Flagan, A. Hansel, K. Höhler, J. Kirkby, M. Kulmala, K. Lehtipalo, O. Möhler, H. Saathoff, R. Volkamer, P. M. Winkler, N. M. Donahue, A. Kürten and J. Curtius (2021). "Chemical composition of nanoparticles from α-pinene nucleation and the influence of isoprene and relative humidity at low temperature." Atmospheric Chemistry and Physics 21(22): 17099-17114.
Li, X., S. Chee, J. Hao, J. P. D. Abbatt, J. Jiang and J. N. Smith (2019). "Relative humidity effect on the formation of highly oxidized molecules and new particles during monoterpene oxidation." Atmospheric Chemistry and Physics 19(3): 1555-1570.
Surdu, M., H. Lamkaddam, D. S. Wang, D. M. Bell, M. Xiao, C. P. Lee, D. Li, L. Caudillo, G. Marie, W. Scholz, M. Wang, B. Lopez, A. A. Piedehierro, F. Ataei, R. Baalbaki, B. Bertozzi, P. Bogert, Z. Brasseur, L. Dada, J. Duplissy, H. Finkenzeller, X. C. He, K. Hohler, K. Korhonen, J. E. Krechmer, K. Lehtipalo, N. G. A. Mahfouz, H. E. Manninen, R. Marten, D. Massabo, R. Mauldin, T. Petaja, J. Pfeifer, M. Philippov, B. Rorup, M. Simon, J. Shen, N. S. Umo, F. Vogel, S. K. Weber, M. Zauner-Wieczorek, R. Volkamer, H. Saathoff, O. Mohler, J. Kirkby, D. R. Worsnop, M. Kulmala, F. Stratmann, A. Hansel, J. Curtius, A. Welti, M. Riva, N. M. Donahue, U. Baltensperger and I. El Haddad (2023). "Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity." Environ Sci Technol.
Zhang, S., L. Du, Z. Yang, N. T. Tchinda, J. Li and K. Li (2023). "Contrasting impacts of humidity on the ozonolysis of monoterpenes: insights into the multi-generation chemical mechanism." Atmospheric Chemistry and Physics 23(18): 10809-10822.

We thank the reviewer for the helpful comments on our manuscript. The comments are greatly appreciated. We have addressed all the comments and believe that the revisions based on the comments help improve the quality of our manuscript. Below please find our responses to the comments one by one and the corresponding revisions made to the manuscript. The original comments are in italics. The revised parts of the manuscript are in blue.

Comments re: EA-ART-10-2023-000149, “Relative Humidity Affects Molecular Composition of Secondary Organic Aerosol from α-pinene ozonolysis,” by Luo et al.

General comments
The present study used online mass spectrometry to investigate the effect of relative humidity (RH) on the molecular composition of SOA formed from the dark ozonolysis of α-pinene. The authors also investigated the influence of seed aerosol on the composition of SOA formed under high RH.
The authors show that, in the presence of ammonium sulfate (AS) seed aerosol, the ratio of dimers and monomers formed from α-pinene-derived SOA is mostly unchanged with an increase in RH from 3 – 84%, but there is a greater absolute yield in both dimers and monomers. They attribute the increase in monomer production to the likely enhanced partitioning of LVOC gas-phase products at high RH; increased dimer production is attributed to condensed-phase acid-catalyzed reactions from AS seed aerosol.
In the absence of seed aerosol, absolute yields of monomers increased with increasing RH, whereas dimer production decreased; this is attributed to the wall effect and enhanced condensation of organic vapors at high RH.
I think the subject matter is timely and worthy of publication. However, the manuscript is quite difficult to understand. Some important terms are vague or not defined (for example, “wall effect”), and there is critical discussion lacking. Though the figures provide evidence for general trends in α-pinene-derived SOA composition, some of the discussion is repetitive and vague. Figure captions are not complete, incorrect figures are referenced in some sections and important details (see below) are missing. Finally, extensive grammar and English editing is required.
Response:
We address the specific comments as below. Additionally, we have polished the grammar and English extensively, including omitting some repetitive discussion.

Specific comments
I suggest a change of the title to “Effect of Relative Humidity on molecular composition…”
Response:
Accepted. We have revised the title as suggested.

Lines 42-43 (Abstract): The authors’ explanation for monomer and dimer production in the absence of seed is confusing and vague.
Response:
We have revised this sentence in line 45 in the revised manuscript as follows to improve clarity:
“…These changes were attributed to a combination of different extents of condensation enhancement of monomer and dimer vapors by increasing RH and different vapor wall losses of monomers and dimers.”

Line 74: Unclear what the authors mean by “increase inversely with increasing RH.”
Response:
We have revised this sentence in line 75 in the revised manuscript as follows to improve clarity:
“Kidd et al.34 also reported that the viscosity of SOA decreases with increasing RH, and carboxylic acids increase inversely with increasing RH,…”

Line 94: What are the dimensions and volumetric flow rate of all gases through the reactor?
Response:
We have added the detailed description in line 96 as follows:
“The flow tube has an inner diameter of 20 cm and a volume of 62.8 L. The typical flow rates used are 2.85-3.00 LPM. The residence time in the flow tube for the experiments in this study was ~14.5 min, which can be adjusted by a movable sampling inlet.”

Line 99: While the interested reader can go to the reference cited for details, a brief description of the diffusion technique of delivering VOC should be provided.
Response:
Accepted. We have added the description in line 105 of revised manuscript as follows:
“In brief, the volatile organic compounds were placed in a cylindrical diffusion vial with a volume of 4 ml, covered with a headspace cap and penetrated with a PEEK tube. The diffusion vial is placed in a glass bottle, which is temperature-controlled with water bath. Zero air enters the diffusion bottle from the bottom through the air inlet, flows upward through the diffusion vial and exits at the top of the glass bottle, finally into the flow tube. Such as device has been used in previous studies37, 38 (Mentel et al., 2009; Gautrois et al., 1999).”

Line 102: The methodology section states RH was increased from 2 (± 2) – 91 (± 2) %, but in the Abstract (line 33) the RH range is reported as 3 to 84%.
Response:
We apologize for the unclear statement. We have modified this sentence in line 112 of revised manuscript as follows:
“The RH were gradually increased ranging from (3±1)% to (84±3)% in the presence of AS seed, and from <1% to (91±2)% in the absence of seed.”

Line 107: What is the ratio of isopropanol to VOC used and how was it selected? As it is known that the chemistry is influenced by the choice of OH scavenger, why did the authors choose to use isopropanol?
Response:
The ratio of isopropanol to VOC used in this study is ~120. The ratio was selected so that more than 90% (93% in this study) of OH reacted with OH scavenger. The ratio of OH scavenger to VOC used in previous studies range from ~5 to 800 (Jonsson et al., 2008; Ahmad et al., 2017). The ratio used in this study is within the range in the literature.
We agree that all OH scavengers can affect RO2 chemistry. In this study, OH scavenger was added to investigate the effect of RH on SOA from pure ozonolysis and to exclude potential influence of heterogeneous reaction of OH with aerosol. Isopropanols are chosen because alcohols are commonly used OH scavengers according to previous studies (Bell et al., 2023; Jonsson et al., 2008; Keywood et al., 2004).

Line 116: Give a brief description of the EESI setup and then the interested reader could refer to the citation provided for more details.
Response:
Accepted. We have added a brief description in line 129 of revised manuscript as follows:
“…In brief, particles and gases are sampled continuously through a charcoal denuder which can removes most volatile gas species efficiently. After denuder, the particle flow intersects a charged droplets spray and then collides with the electrosprayed droplets. Soluble components in particles are extracted by solvents from charged droplets and then the droplets evaporate through a heated stainless-steel capillary, finally yielding charged aerosol ions that are detected by time-of-flight mass spectrometer.”

Lines 121-122: Are the “8 min” and “7 min” referring to the duration of either on-line sampling to the MS or filter sampling, respectively? Please clarify.
Response:
We apologize for the unclear statement. We have added more descriptions in line 136 of the revised manuscript as follows:
“An auto-valve was used to enable automatic alternates between direct sampling and sampling through a HEPA filter (AQ, Parker). The duration of direct sampling particles and filter sampling are 8 min and 7 min, respectively. The difference of spectra between direct sampling and sampling through the filter was used to derive the particle signal devoid of any instrument background signal (including spray).”

Lines 122-124: This statement is confusing. To what “background noise” are the authors referring? If filter sampling had higher temporal resolution (7min vs 8 min), why is on-line sampling needed? How was this procedure carried out?
Response:
We apologize for the unclear statement.
We have changed the “background noise” to “background signals”, which refers to the signals during the sampling through the instrument inlet with a HEPA filter. The background signals include contributions from impurities in working solution and residue gases after passing the denuder. We have revised in line 139 of the revised manuscript as follows:
“The difference of spectra between direct sampling and sampling through the filter was used to derive the particle signal devoid of any instrument background signal (including residue gases passing through denuder and impurities from spraying solution).”

Line 125: If I have estimated correctly, each reported value is an average of about 100 mass spectra (5s/measure x 8 min). If so, I’m surprised that the error bars are so large (±30-40%).
Response:
We found that the values of the errors were improperly calculated and apologize for the problem. In the revised manuscript, we have updated the errors which were calculated according the 13-min stable signal during aerosol sampling. We have checked and updated the error bars in Figure 2(b) and Figure 5(b). The updated error bars are not large (~4.7 %).

Line 135: From Figure 2, I see an increase in the monomer fraction upon increasing RH from 3% to 28%. Did the authors mean a “decrease in the dimer fraction” or “an increase in the monomer fraction”?
Response:
We meant an increase in monomer fraction. We have modified this sentence in line 155 of revised manuscript as follows to improve clarity:
“…except for a slight increase in the monomer fraction when RH increased from 3% to 28% RH.”

Figure 2: What do the error bars represent? 1σ? 2σ? 3σ? Also, with such large error bars is the difference between experiments statistically significant? For example, dimers normalized peak intensity between 54% and 84% RH appear to be the same withing experimental error? Some discussion must be provided.
Response:
New errors were calculated according the 13-mins stable signal. The new error bar represents 1σ. Based on the new error, the difference of absolute intensity between different RH was statistically significant (p values between two adjacent RH conditions in T-test are all below 0.01). We have checked and updated the error bar in Figure 2(b). The updated error bars are not large.
We have added more description in line 171 of revised manuscript as follows:
“The difference of absolute intensity between different RH was statistically significant (p values between two adjacent RH conditions in T-test are all below 0.01).”
And added in the caption of Figure 2 as follows:
“The error bar represents 1σ.”

Figure 2: Please use different hash fills on bars to aid readers with visual difficulties.
Response:
Accepted. We have updated Figure 2 as follows:

Figure 2. (a) The fractions of monomers and dimers in SOA in the presence of AS seed aerosol different RH ((3±1)%, (28±2)%, (54±2)%, and (84±3)%). The fraction of each class is normalized to the total products. Misc_monomer denotes other monomers except C9 and C10 monomers. Misc_dimer denotes other dimers except C17-20 dimers. Misc denotes the other SOA components which are not identified. (b) The absolute intensity of total monomers (in red, left y axis) and total dimers (in blue, right y axis) in the presence of AS seed aerosol under RH ramp. The error bar represents 1σ. Note that the peak intensity in mass spectra was normalized to the peak intensity of NaINa+ (m/z 173), which is the most abundant peak from ion source.

Figure 2. Why were 3, 28, 54, and 84% RH selected as representative humidities for monomer/dimer contribution? If it is true that the ratio of monomer/dimer production is invariable with increased RH in the presence of seed aerosol, it would be more realistic to show averaged monomer contribution for various RH ranges (e.g. 2 – 10%, 11 – 20%, 21 – 30%, etc.).
Response:
We have conducted the experiments in the RH condition ranged from 2% to 91%. And the gradient of RH rise is designed to be at an interval of ~25% RH. In the experiments, RH was not controlled to be at exactly 25% RH interval. However, RH for each of our experiments was stable.

Figure 2b. Make it more clear on the figure that the left y-axis is for monomer and right y-axis is for dimer.
Response:
Accepted. We have updated Figure 2(b) as shown above.

Line 152: This statement assumes that the analytical sensitivity of the measurements for all compounds reported is identical, which is almost certainly not the case. So, while trends in monomer signal as a function of RH or dimer signal as a function of RH (independently) may be appropriate, it is not appropriate to compare absolute abundance signals of monomers to those of dimers.
Response:
While we agree that different compounds may have different sensitivity in EESI-CIMS, we did not intend to compare the absolute abundance signals of monomers to those of dimers. We apologize for the confusion caused by this sentence. In the revised manuscript, we have revised this sentence in line 174 as follows:
“For a change of RH from 3% to 84%, the absolute abundance of monomers increased by a factor of 2.9 (Figure S2(b, d)), and dimers increased with a comparable factor (3.3). These changes finally led…”
In addition, previous studies show a good correlation of total signals of EESI with total organic aerosol concentration measured by a scanning mobility particle sizer (SMPS) (Lopez-Hilfiker et al., 2019; Surdu et al., 2021) or an aerodyne mass spectrometer (AMS)(Brown et al., 2021; Qi et al., 2019), which suggests that for an ensemble of compounds (e.g. all monomers), it is plausible to assume a linearity between overall signals of EESI-CIMS with its total concentrations.

Line 161. Please expand upon this statement. Simply stating effect of RH on SOA composition is likely due to multi-phase processes is vague.
Response:
Accepted. We have modified and moved this sentence in line 187 of revised manuscript as follows:
“…Therefore, HOM in the gas phase is likely not affected by RH. As HOM contribute to most of SOA from α-pinene ozonolysis22, the influence of RH on SOA composition here is likely attributed to the influence on multi-phase processes such as particle-phase reactions and gas-particle partitioning of organics.”

Figure 3: Without a discussion of error and statistical significance, this figure does not support the authors’ conclusions that product distributions exhibited distinct patterns as a function of RH. What is the magnitude of the error on each pixel?
Response:
Accepted. The statistical significance test shows that dimer distribution at 3% RH is significantly different from the other three RH conditions (28%, 54%, and 84%RH). And the relative standard deviation of each pixel in the same RH condition ranged from 4.3% to 6.1%. We have added the results of statistical significance test in the Table S3 of Supplement as follows:
Table S3 T-test results of the difference between dimer fractions at different RH in the presence and absence of AS seed
AS seed (3±1)%/ (28±2)% (3±1)%/ (54±2)% (3±1)%/ (84±3)% (28±2)%/ (54±2)% (28±2)%/ (84±3)% (54±2)%/ (84±3)%
t 4.62 3.11 2.14 -0.45 -0.48 -0.45
P <0.01 <0.01 0.04 0.66 0.64 0.66
No seed (<1)%/ (28±1)% (<1)%/ (58±2)% (<1)%/ (91±2)% (28±1)%/ (58±2)% (28±1)%/ (91±2)% (58±2)%/ (91±2)%
t 7.42 8.03 8.73 3.31 3.94 2.56
P <0.01 <0.01 <0.01 <0.01 <0.01 0.01

Line 215: The authors refer to Figure S4b, but they haven’t referred to Figure S3 yet.
Response:
We apologize for the typo. Here the “Figure S4b” should be “Figure S3b”. We have checked numbering of figures throughout the manuscript.

Lines 251-257: How were the equilibration timescales estimated? References? Also in Figure S3, use different line styles to aid the visually impaired reader.
Response:
Accepted. We have updated the Figure S3 and added more description in line 281 of revised manuscript as follows:
“…vapor-wall equilibration (v,w) compared to that of vapor directly condensing on particles (vapor-particle, (v,p)) (Figure S5. These time scales were calculated according to the method used by Huang et al. (2016) and references therein and more details of the timescale estimates are provided in the Supplement).”.
And we have added more detailed description in the Supplement as follows.
“The timescale of vapor-particle partitioning (τv,p) refers to the partitioning of vapors directly condensing on particles, which can be quantified as follows [Huang et al., 2016; Seinfeld and Pandis, 2016]:
τv,p = (2πNpD ̅pg f (Kn, αp))-1;
where Np and D ̅p are the total number concentration and number mean particle diameter of suspended particle, respectively; g means the gas-phase diffusion coefficient of a specific organic vapor; Kn is Knudsen number, which is equal to 2λ/Dp; αp is vapor accommodation coefficient on particles, and for α-pinene SOA, αp ranges from 0.01-0.1 [Stanier et al., 2007; Saleh et al., 2013; Vaden et al., 2011]; f (Kn, αp) denotes the correction factor for non-continuum diffusion and imperfect accommodation.
The timescale of vapor-wall equilibrium partitioning (τv,w) represents the partitioning between the vapor deposited on the reactor wall and particle, which can be obtained as follows [Zhang et al 2015]:
τ_(v,w)=1/(A/V)(((α_w v)/4)/(1+(πα_w v ̅)/(8(D_g K_e )^(1/2) )))(1+C^*/C_w )
where A, V, and Ke are the surface area, volume, and the coefficient of eddy diffusion of the reactor, respectively; v ̅ represents the species mean thermal speed. C* means the mass saturation concentration of the vapor species, and Cw represents the mass concentration of absorbing organic material on the wall. αw means the vapor accommodation coefficient on the reactor wall, which is related to both the mass transfer resistances at the vapor-wall interface and in the wall layer itself. The parameter αw can be obtained according to the inverse dependence of αw on C*, which is similar to the treatment of vapor-particle partitioning.”

Lines 216-217: Several times in the manuscript, the authors compare monomer and dimer extent at the two extremes of RH (3% and 84%), however, this assumes a predictable trend in behavior as a function of RH, which is not what is observed. For example, it does not seem appropriate to report a change in the normalized peak intensity of dimers for <1% vs 91% (Figure 5), when intensities at intermediate points are greater than at both extremes.
Response:
Although the normalized dimer abundance is not completely monotonic as a function of RH, in general it shows an increasing trend with RH. Therefore, in order to quantitatively describe the extent of changes, we compare the abundance at the lowest and highest RH. In the revised manuscript, we have added a note to explain this point in line 177 as follows:
“We would like to note that while the normalized dimer abundance is not completely monotonic as a function of RH, in general it shows an increasing trend with RH. We compare the abundance at the lowest and highest RH in order to quantitatively describe the extent of changes of abundance as a function of RH.”

Line 219: Do C9 monomers not also form dimers?
Response:
Yes, we agree that C9 monomers could also form dimers. The increase fraction of C9 monomers suggests that the enhanced partitioning at higher RH outweighed the consumption by reactions, e.g. forming dimers. We have modified this sentence in line 247 of the revised manuscript as follows:
“…The increase of C9 species with increasing RH can be attributed to the enhanced partitioning at higher RH as discussed above. The less increase of absolute abundance of C10 than total products may be attributed to their larger reactive loss forming dimers than C9 as C20 dimers, which is likely formed from C10 compounds, increased more than C18 dimer, which is likely formed from C9 compounds.”

Line 251: I assume the authors are referring to wall vapor losses, but that is not clear, especially when the authors refer to “vapor-wall contribution to SOA.”
Response:
We apologized for the unclear expression. We have modified in line 279 of revised manuscript as follows:
“…influence of vapor wall losses, i.e. vapors deposited on the reactor walls to particles…”
Also we have checked and updated this throughout the manuscript.

Figure 4: Please use different hash fills for the bars.
Response:
Accepted. We have updated Figure 4 as follows:

Figure 4. The fractions of individual C10H16Ox(x=2~9) in the family under different RH in the presence of AS seed aerosol. The fraction of individual C10H16Ox is normalized to the total intensity of C10H16Ox family. The gradual color gradient ramps from light to deep represent RH range from (3±1)%, (28±2)%, (54±2)%, and (84±3)%.
In addition, in order to avoid ambiguity and to keep consistent with the text, we now plot fractions of C10H16Ox(x=2~9) in total C10H16Ox family rather than normalized to C10H16O6 the in the revised manuscript.
Lines 277-283: The authors explain the relatively unchanged O/C ratio as being due to some of the families increasing O/C as a function of RH. However, as shown in Figure S6, that is clearly not the case for all families (some experience significant decreases in O-content with increasing humidity).
Response:
We agree with the reviewer that some families showed decreases in O/C while others showed increases in O/C with increasing humidity. This point has been stated in the original manuscript (lines 274-277). The increases and decreases offset each other and thus, the overall changes in O/C of all products led to a generally invariant O/C of SOA.

Line 324: Should refer to Figure S3 not S8.
Response:
We apologize for the typo. We have corrected it in the revised manuscript.

Lines 330 – 331: How comparable is this metric of enhancement? The distribution of dimers formed vs. RH varies greatly between the experiments with and without seed aerosol. In addition, it seems misleading to report this as the ‘enhancement of dimer formation’, since the fraction of dimers formed dramatically decreases (⁓30% to ⁓5%) with an increase in RH in the absence of seed aerosol, even though the total abundance of dimers generally increases.
Response:
Here we compare the abundance of total dimers not the specific species. As clarified in the response to Question #17, previous studies show a good correlation of total signals of EESI with total organic aerosol concentration measured by a scanning mobility particle sizer (SMPS) (Lopez-Hilfiker et al., 2019; Surdu et al., 2021) or an aerodyne mass spectrometer (AMS)(Brown et al., 2021; Qi et al., 2019), which suggests that for an ensemble of compounds (e.g. all monomers), it is plausible to assume a linearity between overall signals of EESI-CIMS with its total concentrations.
We decide not to compare the “enhancement” and have deleted this sentence in the revised manuscript.

Line 334: The authors claim that Figure S7 shows that the “distribution of dimers did not change appreciably.” However, earlier in the manuscript they claim that Figure 3 “shows distinct patterns” as a function of RH. I would argue that Figure S7 shows much greater differences as a function of RH. What metric(s) are the authors using to quantify whether or not a distinction exists?
Response:
Accepted. The statistical significance test shows that the distribution of dimer at <1% RH are also different from those at the other three RH (28%, 58%, and 91%). And the relative standard deviation of each pixel in the same RH condition ranged from 2.3% to 4.7%. Therefore, in the revised manuscript we have removed the original corresponding discussion for the condition without seed and modified the corresponding discussion with AS seed in line 211 as follows:
“Such a speculation is substantiated by the observation that the distribution of dimer species exhibited distinct patterns between dry condition (3% RH) and other three RH conditions (Figure 3, Table S3). Specifically, the fractions of C20 decrease and the fractions of C16-19 compounds with low oxygen number increase in general, indicating a change in dimer composition despite largely invariant total fractions of dimers and monomers.”
We have added a description in the line 145 of revised manuscript as follows:
“Paired sample T-test was used to examine whether the dimer distribution at two different RH is significantly different.”
And also added the results of statistical significance test in the Table S3 of Supplement as follows:

Table S3 T-test results of the difference between dimer fractions at different RH in the presence and absence of AS seed
AS seed (3±1)%/ (28±2)% (3±1)%/ (54±2)% (3±1)%/ (84±3)% (28±2)%/ (54±2)% (28±2)%/ (84±3)% (54±2)%/ (84±3)%
t 4.62 3.11 2.14 -0.45 -0.48 -0.45
P <0.01 <0.01 0.04 0.66 0.64 0.66
No seed (<1)%/ (28±1)% (<1)%/ (58±2)% (<1)%/ (91±2)% (28±1)%/ (58±2)% (28±1)%/ (91±2)% (58±2)%/ (91±2)%
t 7.42 8.03 8.73 3.31 3.94 2.56
P <0.01 <0.01 <0.01 <0.01 <0.01 0.01

Line 336: Were the measurements corrected for wall-losses?
Response:
We did not correct the wall-losses.

Line 351: While it is true that very modest increases in the “low O-content” families increases with RH between 28% and 91%, all values are lower than the dry condition (<1%). Please discuss.
Response:
As shown in the updated Fig. 4, we now plot the fraction of each C10H16Ox in total C10H16Ox. We found that in the absence of seed, the trend does not exactly resemble the findings in the presence of seed. In the absence of seed, C10H16O3 decreased with the increasing RH, and the others were similar to those in AS seed condition (the fractions of O<6 increased with increasing RH, while the fractions of O>6 decreased with increasing RH. The difference may be attributed the wall-loss of vapors due to the varying concentration of aerosol surface area concentration. In order to avoid ambiguity, we decide to delete the relevant discussion in the revised manuscript.

Line 352: Unclear. The discussion throughout the manuscript about relative changes vs absolute abundance intensities is very confusing.
Response:
The dependence of fraction of a compound on RH is the result of the dependence of its absolute abundance and of total products on RH. As shown in the manuscript, the absolute abundance of monomers and dimers increased by different extent, which leads to a different response in fractions of monomers and dimers to the change of RH. If only focusing on the absolute abundance, one can only see the magnitude of the increase. However, the fraction can better inform the changes in the chemical composition of SOA, which determines its physiochemical properties such as cloud condensation nuclei activity as we discussed in the manuscript.
We have rephrased discussion related to the relative and absolute abundance throughout the manuscript to make the discussion more clearer.

Lines 357 – 362: Some of the concepts/explanations in this manuscript are repetitive. Attributing observations to the same justification is understandable, but a large portion of the results section repeats the same concept. Condensing some of the text would help to strengthen the manuscript.
Response:
Accepted. We have condensed these text in line 377 of revised manuscript as follows:
“The increase of C10H16Ox(x<6) with increasing RH can be attributed to the enhanced condensation by RH and/or wall effect as discussed above (Section 3.1.2 and Section 3.2.1).”

Lines 383 – 386: This statement is contradictory. The authors attribute the dependence of the wall-effect on RH to both the increase and decrease in abundance of C19H28O7-11. Please fix this statement.
Response:
We apologized for this typo. We have revised in line 400 of revised manuscript as follows:
“If the dependence on RH is resulted from the enhanced condensation by RH,…”

Summary
While potentially interesting, the manuscript suffers from many critical flaws, as discussed above. It may be publishable, but only after extensive revision.

Reference
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Surdu, M., Pospisilova, V., Xiao, M., Wang, M. Y., Mentler, B., Simon, M., Stolzenburg, D., Hoyle, C. R., Bell, D. M., Lee, C. P., Lamkaddam, H., Lopez-Hilfiker, F., Ahonen, L. R., Amorim, A., Baccarini, A., Chen, D. X., Dada, L., Duplissy, J., Finkenzeller, H., He, X. C., Hofbauer, V., Kim, C., Kürten, A., Kvashnin, A., Lehtipalo, K., Makhmutov, V., Molteni, U., Nie, W., Onnela, A., Petäjä, T., Quéléver, L. L. J., Tauber, C., Tomé, A., Wagner, R., Yan, C., Prevot, A. S. H., Dommen, J., Donahue, N. M., Hansel, A., Curtius, J., Winkler, P. M., Kulmala, M., Volkamer, R., Flagan, R. C., Kirkby, J., Worsnop, D. R., Slowik, J. G., Wang, D. Y. S., Baltensperger, U., and el Haddad, I.: Molecular characterization of ultrafine particles using extractive electrospray time-of-flight mass spectrometry, Environmental Science-Atmospheres, 1, 434-448, 10.1039/d1ea00050k, 2021.
Keywood, M. D., Kroll, J. H., Varutbangkul, V., Bahreini, R., Flagan, R. C., and Seinfeld.J. H.: Secondary Organic Aerosol Formation from Cyclohexene Ozonolysis:  Effect of OH Scavenger and the Role of Radical Chemistry, Environmental Science & Technology, 38 (12), 3343-3350, DOI: 10.1021/es049725j, 2004
Jonsson, Å. M., Hallquist, M., and Ljungström, E.: Influence of OH Scavenger on the Water Effect on Secondary Organic Aerosol Formation from Ozonolysis of Limonene, Δ3-Carene, and α-Pinene, Environmental Science & Technology, 42 (16), 5938-5944, DOI: 10.1021/es702508y, 2008
Bell DM, Pospisilova V, Lopez-Hilfiker F, et al. Effect of OH scavengers on the chemical composition of α-pinene secondary organic aerosol. Environmental Science: Atmospheres, 3(1):115-123. DOI: 10.1039/d2ea00105e, 2023.

26-Mar-2024

Dear Dr Zhao:

Manuscript ID: EA-ART-10-2023-000149.R1
TITLE: Effect of relative humidity on molecular composition of secondary organic aerosol from α-pinene ozonolysis

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 minor revisions. Reviewer 2 gave constructive suggestions about minor revisions that could make the manuscript more robust.

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************

Reviewer 1

Upon reviewing the revised manuscript and the authors' responses to the review comments, I am satisfied that all concerns have been thoroughly addressed. The revisions significantly enhance the manuscript, making it a valuable contribution to the field. Therefore, I recommend the manuscript be accepted for publication.

Reviewer 3

The authors have revised the manuscript adequately according to the comments of the two reviewers. I support the publication of this study after the authors can address my additional minor comments.
(1) For your reply to the comment for “L 243” raised by Referee 1, though I agree that the enhancement of dimers is mainly because of the acid catalyzed reactions, and the contribution of the gas-particle partitioning is minor, I hold my opinion about the way the authors reply to that comment. First, the “increase” does not necessarily mean a “ratio” relation as the authors wrote; instead, Cp,i(RH2)-Cp,i(RH1) is also a reasonable way denoting the “increase”, then the particle phase concentration will be dependent on the gas phase concentrations as the Referee 1 pointed. I suggest the authors revise the sentence on Line 209 from “not sensitive to partitioning” to “less sensitive to partitioning”.
(2) For the discussion of the competition between gas-particle partitioning and gas-wall partitioning, the sentences should be revised to convey what the authors intended to explain in a clearer way. For example, Line 356, you could consider to add “particularly for low RH conditions in the experiments with the absence of seed particles”.

(3) Some of the references were not cited properly or some important studies from other groups were ignored. For example, Line 456, I am wondering would a field study characterize the SOA formed from α-pinene ozonolysis? Line 465, I suggest add studies from chemical transport models, i.e., (Pye et al., 2017), (Jathar et al., 2016). The works from Sergey Nizkorodov group are missing in this manuscript though that group did extensive experiments on RH effects on SOA composition. E.g., at Line 219-220, the authors could consider cite Wong et al. (2022) who showed the effect of acid catalyzed ageing on α-pinene SOA.
Besides, Andi Zuend, Manabu Shiraiwa, Rahul Zaveri groups did simulations on particle phase state which is affected by RH and evaluated the effects on multiphase processes; their works were not cited neither. Line 190, add (Shiraiwa et al., 2013; Zaveri et al., 2020) at the end of the sentence that “the influence of RH on SOA composition here is likely attributed to the influence on multi-phase processes such as particle-phase reactions and gas-particle partitioning of organic”. Line 69-70, “Although in theory RH may affect gas-particle partitioning”, the gas-particle partitioning has been simulated under different RH by various models thus it is not proper to say “may affect partitioning”; the RH effect on partitioning has been simulated by previous studies (Li and Shiraiwa, 2019; Gorkowski et al., 2019).

(4) Line 122, “Particle concentrations achieve a steady-state after ~60 min once all conditions are set.” Does “steady-state” mean gas-particle partitioning reaches equilibrium? At Line 98, the residence time in the flow tube is 14.5 m. Compared the two time lengths, does it indicate that the gas-particle partitioning in the flow tube is kinetically limited instead of thermodynamically limited since 14.5 m is less than 60 m?

Gorkowski, K., Preston, T. C., and Zuend, A.: Relative-humidity-dependent organic aerosol thermodynamics via an efficient reduced-complexity model, Atmos. Chem. Phys., 19, 13383-13407, 10.5194/acp-19-13383-2019, 2019.
Jathar, S. H., Mahmud, A., Barsanti, K. C., Asher, W. E., Pankow, J. F., and Kleeman, M. J.: Water uptake by organic aerosol and its influence on gas/particle partitioning of secondary organic aerosol in the United States, Atmospheric Environment, 129, 142-154, http://dx.doi.org/10.1016/j.atmosenv.2016.01.001, 2016.
Li, Y. and Shiraiwa, M.: Timescales of secondary organic aerosols to reach equilibrium at various temperatures and relative humidities, Atmos. Chem. Phys., 19, 5959-5971, 10.5194/acp-19-5959-2019, 2019.
Pye, H. O. T., Murphy, B. N., Xu, L., Ng, N. L., Carlton, A. G., Guo, H., Weber, R., Vasilakos, P., Appel, K. W., Budisulistiorini, S. H., Surratt, J. D., Nenes, A., Hu, W., Jimenez, J. L., Isaacman-VanWertz, G., Misztal, P. K., and Goldstein, A. H.: On the implications of aerosol liquid water and phase separation for organic aerosol mass, Atmos. Chem. Phys., 17, 343-369, 10.5194/acp-17-343-2017, 2017.
Shiraiwa, M., Zuend, A., Bertram, A. K., and Seinfeld, J. H.: Gas-particle partitioning of atmospheric aerosols: interplay of physical state, non-ideal mixing and morphology, Phys. Chem. Chem. Phys., 15, 11441-11453, 10.1039/C3CP51595H, 2013.
Zaveri, R. A., Shilling, J. E., Zelenyuk, A., Zawadowicz, M. A., Suski, K., China, S., Bell, D. M., Veghte, D., and Laskin, A.: Particle-Phase Diffusion Modulates Partitioning of Semivolatile Organic Compounds to Aged Secondary Organic Aerosol, Environmental Science & Technology, 54, 2595-2605, 10.1021/acs.est.9b05514, 2020.
Wong, C., Liu, S., and Nizkorodov, S. A.: Highly Acidic Conditions Drastically Alter the Chemical Composition and Absorption Coefficient of α-Pinene Secondary Organic Aerosol, ACS Earth and Space Chemistry, 6, 2983-2994, 10.1021/acsearthspacechem.2c00249, 2022.

We thank the reviewer for the helpful comments on our manuscript. The comments are greatly appreciated. We have addressed all the comments and believe that the revisions based on the comments help improve the quality of our manuscript. Below please find our responses to the comments one by one and the corresponding revisions made to the manuscript. The original comments are in italics. The revised parts of the manuscript are in blue.

Comments to the Author
The authors have revised the manuscript adequately according to the comments of the two reviewers. I support the publication of this study after the authors can address my additional minor comments.

(1) For your reply to the comment for “L 243” raised by Referee 1, though I agree that the enhancement of dimers is mainly because of the acid catalyzed reactions, and the contribution of the gas-particle partitioning is minor, I hold my opinion about the way the authors reply to that comment. First, the “increase” does not necessarily mean a “ratio” relation as the authors wrote; instead, Cp,i(RH2)-Cp,i(RH1) is also a reasonable way denoting the “increase”, then the particle phase concentration will be dependent on the gas phase concentrations as the Referee 1 pointed. I suggest the authors revise the sentence on Line 209 from “not sensitive to partitioning” to “less sensitive to partitioning”.
Response:
Accepted. We have revised this sentence as the reviewer suggested as follows in line 208.
“…and thus to be less sensitive to partitioning”.

(2) For the discussion of the competition between gas-particle partitioning and gas-wall partitioning, the sentences should be revised to convey what the authors intended to explain in a clearer way. For example, Line 356, you could consider to add “particularly for low RH conditions in the experiments with the absence of seed particles”.
Response:
Accepted. We have revised these sentence as suggested in the revised manuscript as follows.
In line 355:
“Therefore, influence of the competition between the vapor-particle partitioning and vapor-wall partitioning on the monomer abundance in particles could not be neglected.”
In line 359:
“the reactor walls besides by enhanced condensation by RH, particularly for low RH conditions in the experiments with the absence of seed particles.”

(3) Some of the references were not cited properly or some important studies from other groups were ignored. For example, Line 456, I am wondering would a field study characterize the SOA formed from α-pinene ozonolysis? Line 465, I suggest add studies from chemical transport models, i.e., (Pye et al., 2017), (Jathar et al., 2016). The works from Sergey Nizkorodov group are missing in this manuscript though that group did extensive experiments on RH effects on SOA composition. E.g., at Line 219-220, the authors could consider cite Wong et al. (2022) who showed the effect of acid catalyzed ageing on α-pinene SOA.
Besides, Andi Zuend, Manabu Shiraiwa, Rahul Zaveri groups did simulations on particle phase state which is affected by RH and evaluated the effects on multiphase processes; their works were not cited neither. Line 190, add (Shiraiwa et al., 2013; Zaveri et al., 2020) at the end of the sentence that “the influence of RH on SOA composition here is likely attributed to the influence on multi-phase processes such as particle-phase reactions and gas-particle partitioning of organic”. Line 69-70, “Although in theory RH may affect gas-particle partitioning”, the gas-particle partitioning has been simulated under different RH by various models thus it is not proper to say “may affect partitioning”; the RH effect on partitioning has been simulated by previous studies (Li and Shiraiwa, 2019; Gorkowski et al., 2019).
Response:
We thank the reviewer for pointing our overlooked citation and providing detailed advice. We have added and modified the suggested references.
We apologized for the confusing sentence in line 456 and revised it as followed:
“For example, the viscosity39 and volatility63 of α-pinene ozonolysis SOA have been found to be influenced by RH.”
We have added suggested references Pye et al., Z (2017) and Jathar et al., (2016) in line 465:
“…on the SOA formation in atmospheric numerical models.69, 70”
We have added more references (Bateman et al., 2009; Hinks et al., 2018; Nguyen et al., 2011) from Sergey Nizkorodov group on RH effects on SOA composition in line 70:
“…particle-phase chemistry as shown in other reaction systems and modeled by a number of previous studies30-38”
We have added suggested reference Wong et al. (2022) in line 218:
“…particularly in the case of oligomers in SOA from α-pinene ozonolysis.51-53, 56”
We have added suggested references Shiraiwa et al., (2013) and Zaveri et al., (2020) in line 189:
“…the influence on multi-phase processes such as particle-phase reactions and gas-particle partitioning of organics46, 47”
And we revised the sentence in line 69 as followed:
“Although in theory RH may affects gas-particle partitioning and lead to different gas- and particle-phase chemistry as shown in other reaction systems and modeled by a number of previous studies30-38”


(4) Line 122, “Particle concentrations achieve a steady-state after ~60 min once all conditions are set.” Does “steady-state” mean gas-particle partitioning reaches equilibrium? At Line 98, the residence time in the flow tube is 14.5 m. Compared the two time lengths, does it indicate that the gas-particle partitioning in the flow tube is kinetically limited instead of thermodynamically limited since 14.5 m is less than 60 m?
Response:
The “steady-state” mean particle concentration and RH in our flow tube setup reach a stable value. It does not necessarily mean that the gas-particle partitioning reaches equilibrium, as the time scale of gas-particle partitioning for some compounds may be longer than the residence time. If one assumes that the flow in the flow tube is a perfect plug flow, RH would reach stable at the residence time (14.5 min) once all conditions are set. The longer time to reach stable for RH is likely attributed to imperfect flow field compared with plug flow and the absorption of water vapor on the walls of flow tube. For particle concentrations, the interaction between organic vapors and tube walls may also contribute to the longer time for particle concentrations to reach stable than the residence time. Here one cannot distinguish whether the gas-particle partitioning in the flow tube reaches equilibrium only based the difference between the residence time (14.5 min) and time for particle concentrations to stable value (~60 min). In addition, as we discussed in the manuscript, the time to reach gas-particle partitioning depends on volatility of compounds and their diffusivity in particles, thus on individual compounds.
To avoid misunderstanding, we revised this sentence as follow in line 121:
“Particle concentrations and RH in flow tube both reach stable after ~60 min once all conditions are set.”

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03-Apr-2024

Dear Dr Zhao:

Manuscript ID: EA-ART-10-2023-000149.R2
TITLE: Effect of relative humidity on molecular composition of secondary organic aerosol from α-pinene ozonolysis

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