From the journal Environmental Science: Atmospheres Peer review history

22-May-2022

Dear Dr Kutsuna:

Manuscript ID: EA-ART-04-2022-000039
TITLE: Laboratory Study on Uptake of Gaseous Molecular Iodine by Clay Minerals at Different Relative Humidity

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Reviewer 1

This paper reports experimental findings that the ground surface resistance for I₂ could differ greatly from that applied to atmospheric models. The findings have important implications for estimating the public health risk of the accidental release of radioiodine from nuclear power plants. The results are sufficiently novel and interesting. The paper is well written and Environmental Science: Atmospheres is an appropriate venue. Before recommending it for publication, I would like to see the following points addressed.
Line 11, Page 6: should it be 1 M standard aqueous sulfuric acid?
Table 1: Can the authors present the purities of clay minerals if this data is available?
Lines 12-13, page 9: What is the uncertainty for determining the I₂ mixing ratio?
Equation 4: Would the different wall loss rates of I₂ in the sampling line at different RH result in different P_in even at the same P_in-o?
Lines 16-18, page 24: It would be helpful to provide the details of how the uptake coefficients of I2 were estimated. For instance, what is the accommodation coefficient of I₂ adopted?
Line 22, page 24: What is the rationale that the uptake of I₂ on aqueous ascorbic acid is more representative than water and sulfuric acid for the gas-film layer above clay samples?
Figure 7: Why did the loss ratios for MONT, HALL, and AGRO increase with between RH 50%-80%? Is this related to the surface properties of different clay minerals?

Reviewer 2

Ground surface resistance of iodine is a crucial parameter in dry deposition for estimating the public health risk associated with accidental release of radioiodine from nuclear power plants. Reported deposition velocities of elemental iodine or I2 onto various surfaces vary from 0.02 to 26 cm s-1. However, in almost all models of the release of radioiodine from nuclear power plants, the species reported as I2 include not only molecular iodine but also other potential iodine species produced by atmospheric reactions. This work investigated the I2 uptake by various reactors, consisting of six different types of clay mineral particles and three different aqueous surfaces, to provide data for estimation Rg for I2 above soils. Many experiments and many results were conducted. The article can be considered to publish after major revision.
1. Environmental significance statement: Most important of this part is to give the environmental significant of this work. The authors have obtained so many results, they should give more clearly and detail discussions on “radioiodine dry deposition processes needed to be estimated separately before and after sunrise when I2 was released at night”. In other word, the authors should give the main conclusion of the experiments, then highlight the main significant of this work.
2. Through the whole article, I find so many methods are used to calculate different parameters, then the authors just give the data to let the audience to know the value of these data are high or low. However, the most important, “discussion” haven’t been given. For the better understanding of the work, the authors should give a part of “Discussion” or add some discussion at each part, to let the readers to know why this phenomenon happen, and give some mechanism of this phenomenon. The part of “3. RESULTS AND DISSCUSSION” only give the results without deeply and scientific discussion.
3. There are so many methods in the article, some less important part can be putted into the supporting information. Only the important part which can help the readers to better comprehend the work can be still in the main manuscript.
4. If these methods are important and innovation, the author should give some description and discussion in the part of Introduction.
5. In the part of “3.3.3. Simulation of I2 uptake by clay samples”, The authors mentioned “The simulation was conducted on the basis of a two-stage model in which the uptake of I2 by clay samples occurred via an initial stage of rapid sorption onto surface sites followed by a stage of slow sorption onto interior sites such as the mesopores and the micropores of clay samples.” Besides, ΔQloss decreased and ΔQads increased with successive runs; the sum of ΔQloss andΔQads, which represented the amount of I2 accommodated by clay samples, remained almost constant or decreased more slowly than ΔQloss with successive runs. Why? What’s the relationship between these two parts?

Response to the comments by Referee 1 and Referee 2.
The response to Referee 1 and Referee 2 follows sequence: (1) comments from Referees, (2) authors’ response, (3) authors’ changes in manuscript. The authors’ changes are marked in blue. Last, other revisions are represented.

1. To the comments by Referee 1:
Thank you very much for the positive comments.
I will replay to each comment as follows.

R1-1 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 1
Line 11, Page 6: should it be 1 M standard aqueous sulfuric acid?

(2) authors’ response
In the original manuscript, unit of N (normal) was used for standard aqueous sulfuric acid. In the revised manuscript, unit of M is used for all standard aqueous solutions.

(3) authors’ changes in manuscript
Section 2.1, 1st block, lines 2–5:
Sodium iodide (99.5%), 1/240 M standard aqueous potassium iodate (NaIO3) solution, 0.5 M standard aqueous sulfuric acid (H2SO4) solution, (L+) ascorbic acid (99.6 %), and 1 M standard aqueous sodium hydroxide (NaOH) solution were purchased from FUJIFILM Wako Chemical.

R1-2 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 1
Table 1: Can the authors present the purities of clay minerals if this data is available?

(2) authors’ response
The data about the purities of clay minerals is not available. Illite, allophane, montmorillonite, kaolinite, and halloysite are pure clay samples provided by Iwamoto Minerals. AgroMAT AG-1 sample contains organic matter by 3.7% in weight, based on loss-on-ignition method. Section 2.2 is revised as shown in (3).

(3) authors’ changes in manuscript
Section 2.2, 2nd block, lines 1–2:
Pure clay minerals of kaolinite, halloysite, montmorillonite, allophane, and illite were purchased from Iwamoto Mineral.

R1-3 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 1
Lines 12-13, page 9: What is the uncertainty for determining the I2 mixing ratio?

(2) authors’ response
In this study, as shown in eq 4, the I2 mixing ratio was determined from both the partial pressure of I2 in I2–air(o), Pin-o, and the ratio of Fs / (Fs+F3+F4+F5). The uncertainty of the latter was much smaller than that of the former. The former was experimentally determined from four sets of ion chromatograph analysis (Table S2). Standard deviation of Pin-o for measurements in Table S2 was 0.09 Pa, that is, 1% of Pin-o. In the ion chromatography analysis, molar ratios of I− to IO3− were within 3% of the ratio expected from reaction R2, as described in section S1.2 (1st block, lines 1–2 from the bottom). The uncertainty for determining the I2 mixing ratio is thus about 3%. Section 2.3 in the main manuscript and section S1.2 in the Supporting Information are revised as shown in (3).

(3) authors’ changes in manuscript
Section 2.3, 1st block, lines 1–5:
As shown in the Supporting Information, Pin-o was experimentally determined at 7.1 ± 0.2 Pa on the basis of ion chromatograph analysis of I− and IO3− in 20 mM NaOH solutions that had captured I2 from I2–air(o). Letting P0 be the value of Pin in our experiments, when 12.5 mL of I2–air(o) was injected in 300 s at a flow rate Fs of 4.17 × 10−5 dm3 s−1, P0 was 41 ± 1 mPa, yielding a concentration of 0.40 ± 0.01 ppmv.

<Section S1.2, 2nd block>
The value of Pin-o was calculated at 7.08 ± 0.09 Pa from the IO3− concentration, and the volume of I2–air(o) introduced into the solution as the average of four measurements. Errors represent a standard deviation. Uncertainty of Pin-o was estimated at about 3% (± 0.21 Pa) because molar ratios of I− to IO3− were within 3% of the ratio expected from reaction R2. From this value, the partial pressure of I2 in the synthetic air passing through the PFA bottle (Figure 1, a) was calculated at 15.3 ± 0.05 Pa, or 0.88 times the saturated vapor pressure (17.14 Pa) of I2 reported at the preset temperature (288.2 K),46 and the partial pressure of I2 in I2–air(s), with P0 as Pin with a syringe pump speed (Fs) of 4.17 × 10−5 dm3 s−1, was calculated at 40.5 ± 1.2 mPa, for a 0.40 ± 0.01 ppmv concentration.

R1-4 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 1
Equation 4: Would the different wall loss rates of I2 in the sampling line at different RH result in different Pin even at the same Pin-o?

(2) authors’ response
“No” is the answer to the query. As shown in Equation 4 and in Figure 2, Pin is a constant value for the same Pin-o. The different wall loss rates of I2 in the sampling line at different RH result in different time-profiles of Pin f(t).
Pin is P0 or 0.5P0 and f(t)=f_"m" (t)+τ_"m" /2 (df_"m" (t))/dt (Eq. 16). Since dfm(t)/dt is approximately zero when t = 300 s, f(300) = fm(300) = b1 + b4 + b6 + b8 (Eq. 15). In the revised Supporting Information, Table S3 lists values of f(300) on the bottom row. As seen in the revised Table S3, Pin f(t) is a constant value within 1% and 3%, respectively, for Pin = P0 and 0.5P0 at 20–96%RH.
The manuscript and Table S3 are revised as shown in (3).

(3) authors’ changes in manuscript
Section 2.4.2, 1st block, lines 9–11:
Table S3 lists the values of ai and bi. As seen on the bottom row in Table S3, the peak height of the rectangular input pulse (Pin) is a constant value (P0 or 0.5P0) within 1% and 3%, respectively, for Pin = P0 and 0.5P0 at 20–96%RH. Figure 2 shows the time series of fm(t) curves fitted to the experimental data obtained at each RH value (see Figure S3 for plots of the underlying data).

<First and last columns in Table S3>
Input, P0 fm(t) Input, 0.5P0 fm(t)
20%RH 50%RH 80%RH 96%RH 20%RH 50%RH 80%RH 96%RH

f (300) =
b1 + b4 + b6 + b8 1.0065 1.0066 1.0093 1.0092 0.4918 0.4908 0.4887 0.4855


R1-5 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 1
Lines 16-18, page 24: It would be helpful to provide the details of how the uptake coefficients of I2 were estimated. For instance, what is the accommodation coefficient of I2 adopted?

(2) authors’ response
The details of how the uptake coefficients of I2 were estimated were described in section S6.4 in the Supporting Information. The accommodation coefficient of I2 adopted was 10−2 on the basis of the reported value (Reference 36). In the revised manuscript, referring to section S6.4 in the Supporting Information and the accommodation coefficient of I2 are clearly described in the manuscript as shown in (3).

(3) authors’ changes in manuscript
Section 3.2, 2nd block, lines 1–4 from the bottom:
Referring to the reported physicochemical properties of I2,34, 36-38 we estimate uptake coefficients (γ) of I2 at 2.6 × 10−3 for 10 mM ascorbic acid and 1.0 × 10−3 for 1 mM ascorbic acid (section S6.4. in the Supporting Information). In this estimate, 10−2 was adopted as the accommodation coefficient of I2.36 In the absence of the gas-film transfer limitation, the Rg values for I2 above ascorbic acid would be 10 s m−1 and 26 s m−1, respectively (the Supporting Information).

R1-6 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 1
Line 22, page 24: What is the rationale that the uptake of I2 on aqueous ascorbic acid is more representative than water and sulfuric acid for the gas-film layer above clay samples?

(2) authors’ response
As described in the first block in section 3.2, the rate-limiting step for the uptake of I2 on aqueous ascorbic acid was mass transfer in the gas-film layer. The uptake of I2 on water or sulfuric acid proceeded much more slowly than that on aqueous ascorbic acid, and its rate-limiting step was not mass transfer in the gas-film layer. The uptake of I2 on aqueous ascorbic acid is hence more representative for the gas-film layer above clay samples. However, as shown in section 3.3.3, mass-transfer resistance of I2 in the gas-film layer above aqueous ascorbic acid was greater than that above clay samples because the surface roughness of clay samples decreased resistance for mass transfer in the gas-film layer.

(3) authors’ changes in manuscript
No change in manuscript.

R1-7 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 1
Figure 7: Why did the loss ratios for MONT, HALL, and AGRO increase with between RH 50%-80%? Is this related to the surface properties of different clay minerals?

(2) authors’ response
The reason that the loss ratios for MONT, HALL, and AGRO increased with RH between 50% and 80% is partially the increase of the loss ratios in the blank. But, as seen in Figure 11, the loss ratios for MONT, HALL, and AGRO increased with RH between 50% and 80% regardless of the increase of the loss ratios in the blank. The reason is discussed on the basis of eq 40 as follows.
Letting the first term and the second term in the right hand of eq 40 be r1 and r2, respectively, values of Bpn/q∞/Kads, r1, and r2 are plotted against RH for each clay sample in Figures S18 (for small basins) and S19 (for large basins). Since eq 40 represents surface resistance for I2 above clay samples, the loss ratios increase as values of Bpn/q∞/Kads, r1, and r2 decrease. Values of Bpn/q∞/Kads decreased with RH between 50% and 80% for all clay samples (Figures S18 and S19). It meant that adsorption of I2 on clay samples increased with increasing RH. Hence if kf, Kpads, and kloss-p were constant, values of r1 and r2 would decrease with increasing RH and the loss ratios would increase for all clay samples. However, r1 or r2 or both increased with increasing RH for illite, allophane, and kaolinite samples (Figures S18 and S19) because Kpads decreased with increasing RH. In contrast, values of Kpads increased or remained almost constant with increasing RH between 50% and 80% for montmorillonite, halloysite, and AgroMAT AG-1. This is the reason that the loss ratios for MONT, HALL, and AGRO increased with RH between 50% and 80%. It suggests that relative humidity may destabilize I2 in the interior as much as in the surface for the latter three clays (MONT, HALL, and AGRO) and more than in the surface for the former three clays (ILLI, ALLO, and KAOL). This difference might come from differences in pH values of water films on the interiors and buffering capacity against absorption of CO2 among the clay samples.
In the revised manuscript, this point is discussed in section 3.3.3 as shown in (3).

(3) authors’ changes in manuscript
Section 3.3.3, 4th block:
The corresponding equilibrium constant, Kpads, decreased with increasing RH between 50% and 80% for illite, allophane, and kaolinite samples while it increased or remained almost constant for montmorillonite, halloysite, and AgroMAT AG-1 samples. It suggested that the water film might destabilize I2 on the surface and in the interior to almost the same extent for the latter three clay samples since Kpads was the equilibrium constant of I2 between the surface and the interior of clay samples. This equal destabilization might be the reason that the loss ratios for the latter three clay samples increased with RH between 50% and 80% (Figures 7 and 11) as discussed below (see eq 40).

<Section 3.3.3, 6th block>
Figures S18 and S19 plot Bnq/q∞/Kads, r1, and r2 against RH for each clay sample in small basins (Figure S18) and large basins (Figure S19) where r1 and r2 are the first term and the second term, respectively, in the right hand of eq 40. Figures S18 and S19 show that values of Bpn/q∞/Kads decreased with RH between 50% and 80% for all clay samples. It meant that adsorption of I2 on the surfaces of clay samples increased with increasing RH. Hence if kf, Kpads, and kloss-p were constant, values of r1 and r2 would decrease with increasing RH and the loss ratios would increase for all clay samples. However, r1 or r2 or both increased with increasing RH for illite, allophane, and kaolinite samples because Kpads decreased with increasing RH. In contrast, values of Kpads increased or remained almost constant and the loss ratios increased with increasing RH between 50% and 80% for montmorillonite, halloysite, and AgroMAT AG-1. It suggested that relative humidity might destabilize I2 in the interior more than on the surface for the former three clay samples and to almost the same extent as on the surface for the latter three clay samples. This difference might come from the difference in pH values of the water film in the interiors and buffering capacity against absorption of CO2 among the clay samples.

Figures S17 and S18 in the Supporting Information:

Figure S18. Bpn/q∞/Kads (panels 1), r1, and r2 (panels 2) versus RH for I2–air(s) mixtures flowing over small basins of (a) illite, (b) allophane, (c) montmorillonite, (d) kaolinite, (e) halloysite, and (f) AgroMAT AG-1 samples. Bpn/q∞/Kads, r1, and r2 are calculated from the parameters obtained in the simulation (Table S14).


Figure S19. Same as Figure S18, but for I2–air(s) mixtures flowing over large basins.
 
2. To the comments by Referee 2:
Thank you very much for the constructive comments.
I will replay to each comment as follows.

R2-1 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 2
Environmental significance statement: Most important of this part is to give the environmental significant of this work. The authors have obtained so many results, they should give more clearly and detail discussions on “radioiodine dry deposition processes needed to be estimated separately before and after sunrise when I2 was released at night”. In other word, the authors should give the main conclusion of the experiments, then highlight the main significant of this work.

(2) authors’ response
The conclusion of the experiment is summarized as follows.
Uptake of I2 by clay samples proceeded but its loss rates decreased to finite values with repeated experiments on the same clay samples. The ground surface resistances for I2 were estimated from the finite loss rates to be greater than the resistances adopted in atmospheric model calculations which considered no difference between I2 and iodine species produced by photolysis of I2 for dry deposition. Dependence of the former on relative humidity for some clay samples was different from that of the latter.
The manuscript is revised as shown in (3).

(3) authors’ changes in manuscript
Environmental Significance Statements:
Ground surface resistance of iodine is a crucial parameter in dry deposition for estimating the public health risk associated with accidental release of radioiodine from nuclear power plants. We conducted rectangular pulse experiments on uptake of I2 by clay samples and confirmed I2 deposition on the clay samples. The deposition rates decreased with successive runs and remained finite through each set of experimental runs. The resultant ground surface resistances for I2 were greater than the resistances adopted in atmospheric model calculations which considered no difference between I2 and its photodegradation products. The relative-humidity dependence was different between these resistances. Radioiodine dry deposition processes therefore need to be estimated separately before and after sunrise when I2 is released at night.

R2-2 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 2
Through the whole article, I find so many methods are used to calculate different parameters, then the authors just give the data to let the audience to know the value of these data are high or low. However, the most important, “discussion” haven’t been given. For the better understanding of the work, the authors should give a part of “Discussion” or add some discussion at each part, to let the readers to know why this phenomenon happen, and give some mechanism of this phenomenon. The part of “3. RESULTS AND DISSCUSSION” only give the results without deeply and scientific discussion.

(2) authors’ response
In the original manuscript, mechanism of I2 loss was discussed in section 3.3.3 on the basis of the experimental results but this could not be clearly described. We revised the manuscript in the following three points as shown in (3). Part of section 3.3.2 is put into the Supporting Information as mentioned in R2-3. These revisions would let the readers know why this phenomenon happen, and give some mechanism of this phenomenon as pointed out by the referee.
a) We considered that the loss of I2 involved irreversible adsorption of I2 in micropores of clay minerals because of relatively large size of I2, as reported for I2 uptake by active carbons. In the original manuscript, this point is described only in section 4. In the revised manuscript, this point is described in both sections 3.3.1 and 4.
b) A two-stage model with two kinds of active sites on the surface and in the interior for I2 loss is discussed on the basis of experimental results such as behaviors of ΔQloss and ΔQads with successive runs in section 3.3.3. However, it was not clearly described in the original manuscript. In the revised manuscript, this is also described in section 3.3.1.
c) Dependence of loss rates of I2 on relative humidity is discussed on the basis of parameters obtained in the simulation in section 3.3.3. This is also pointed out by Referee 1 (R1-7). The manuscript is revised as shown in (3) in R1-7.

(3) authors’ changes in manuscript
Section 3.3.1, 1st block, lines 4–7:
These results showed that I2 was deposited on the clay samples and removed from the gas phase. Uptake of I2 by clay samples probably involved irreversible adsorption of I2 in micropores of clay minerals because of relatively large size of I2, as reported for I2 uptake by active carbons.40 This decrease of xm(t) varied among the clay samples, but the variation was small compared to that of the BET surface areas of the samples (Table 1).

Section 3.3.1, 2nd block, lines 1–4 from the bottom:
Loss ratios for montmorillonite, halloysite, and AgroMAT AG-1 decreased between 20%RH and 50%RH and increased between 50%RH and 80%RH. However, the latter increase was also evident in the blank experiments. This relative-humidity dependence is discussed in section 3.3.3 (see eq 40).

Section 3.3.1, 3rd block, lines 1–5 from the bottom:
ΔQloss decreased and ΔQads increased with successive runs; the sum of ΔQloss and ΔQads, which represented the amount of I2 accommodated by clay samples, remained almost constant or decreased more slowly than ΔQloss with successive runs. This behavior could be explained by a two-stage model with two kinds of active sites on the surface and in the interior for loss of I2. One kind was consumed on the surface, and the other remained its activity in the interior for uptake of I2, as shown in section 3.3.3. Similar behavior of ΔQloss/Qin and ΔQads/Qin over successive runs was observed at 20%RH and 50%RH (Figures S7 and S8) and for even-numbered runs (Figures S9–S11).

Section 3.3.3, 4th and 6th blocks, and Figures S18 and S19:
Same as (3) in R1-7.

R2-3 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 2
There are so many methods in the article, some less important part can be putted into the supporting information. Only the important part which can help the readers to better comprehend the work can be still in the main manuscript.

(2) authors’ response
The following parts are put into the Supporting Information to help the readers to better comprehend the work.
a) Simulation carried out for uptake by aqueous ascorbic acid (section 3.2, 4th block)
b) Part of the 1st block in section 3.3.2
The main manuscript and the Supporting Information are revised as shown in (3). The following equations in each manuscript are renumbered.

(3) authors’ changes in manuscript
Revision a)
Section 3.2, 4th block:
Simulations were carried out to distinguish the uptake of I2 by ascorbic acid and the contactor (section S6.3 in the Supporting Information). Letting km be the mass-transfer coefficient of I2 in the gas-film layer above ascorbic acid, values of km are estimated at 0.91 ± 0.01 cm s−1 and 0.78 ± 0.01 cm s−1 for the small and large basins, respectively (errors at 90% confidence level). These values are approximately equal to the estimated values of kg-a. Overall, Rg is estimated at 120 ± 5 s m−1 from 16 data of Rg-a for 1 mM and 10 mM ascorbic acid (Table S6). Errors represent a standard deviation.

Section S6-3 in the Supporting Information:
Simulations were carried out to distinguish the uptake of I2 by ascorbic acid and the contactor. Because the mass transfer in the gas-film layer is the rate-limiting step in I2 uptake above aqueous ascorbic acid, on the basis of eq 26 we assumed
("d" x_"out" (t))/("d" t)=1/τ_"c" ×(f(t)-x_"out" (t))-S/(S_c h_c ) 〖k_m×x〗_out (t)-k_"bm" ×{x_"out" (t)-((y_"b" (t))/K_"bads" )^2 } (S11)
where km is the mass-transfer coefficient of I2 in the gas-film layer above ascorbic acid. Simulations using eqs 12, 27, 28, and 30 were conducted to reproduce time series of xm(t). Although the simulation with the small basin successfully reproduced the time series of xm(t) with km = 0.91 cm s−1, the simulation with the large basin never reproduced the data. We assumed additional Freundlich-type adsorption of I2 onto the contactor for the large-basin case and performed the simulation with
("d" x_"out" (t))/("d" t)=1/τ_"c" ×(f(t)-x_"out" (t))-S/(S_c h_c ) 〖k_m×x〗_out (t)-k_"bm" ×{x_"out" (t)-((y_"b" (t))/K_"bads" )^2 }
-k_bma×{x_"out" (t)-((y_"ba" (t))/K_"badsa" )^2 } (S12)
("d" y_"ba" (t))/("d" t)=A_"pn" k_bma×{x_"out" (t)-((y_"ba" (t))/K_"badsa" )^2 } (S13)
where kbma is the mass-transfer coefficient of I2 for additional adsorption between gas and the contactor (determined as 0.0481), Kbadsa is the corresponding parameter of Freundlich adsorption equilibrium (determined as 1.38), and yba(t) is the additional amount of I2 adsorbed onto the contactor. Figure 4 shows the obtained values of xm(t) as solid curves. Values of km are thus estimated at 0.91 ± 0.01 cm s−1 and 0.78 ± 0.01 cm s−1 for the small and large basins, respectively (errors at 90% confidence level).

Revision b)
Section 3.2.2, 1st and 2nd blocks:
We calculated I2 loss rates through surface uptake U(t), the rate constants k1-a(t), and the deposition rates kg-a(t) during the injection period (13–289 s) for each run of the clay experiments at the two basin sizes. Figure 10 shows the results of runs 1 and 5 with illite at 80%RH. We then used a procedure analogous to our procedure for the aqueous solutions (section 3.2) to obtain the apparent surface resistance, Rg-a, above the clay samples (Tables S8–S13) as described in section S7.3 in the Supporting Information.
If Rg-cont is the resistance for mass transfer of I2 in the gas-film layer above clay samples in the contactor, the surface resistance for I2 above clay samples, Rg-a-clay, is given by
R_"g-a-clay" =R_"g-a" -R_"g-cont" (30)
The Rg-cont-w value above ascorbic acid (120 ± 5 s m−1; section 3.2) was taken as the value of Rg-cont in eq 30. Figure 11 shows the Rg-a-clay values thus calculated from the average of Rg-a values for each clay sample with the Rg-cont-w value.

Section S7.3. in the Supporting Information:
We used a procedure analogous to our procedure for the aqueous solutions (section 3.2) to obtain the values of k1-a, kg-a, and Rg-a from 10 values of k1-a(t) at times of 253–289 s. Values of k1-a in blank experiments in which values of P0 or 0.5P0 were used as Pin at each value of RH (Table S7) were used to calculate k1-a values for the clay samples (Tables S8–S13).
Table S7. Values of k1-a in Blank Experiments
RH Pin xout(t) a 102 k1-a (s−1) a
20 P0 0.943 0.08 ± 0.02
0.5P0 0.450 0.11 ± 0.02
50 P0 0.904 0.14 ± 0.02
0.5P0 0.422 0.22 ± 0.02
80 P0 0.861 0.23 ± 0.02
0.5P0 0.396 0.30 ± 0.02
a Each value is the average of 10 data points from 253–289 s. Uncertainties (±) are standard deviations.
As shown in Figure 10, uptake of I2 decreased with time after about 50 s, although the gas-phase concentration of I2 increased. Values of k1-a(t) and kg-a(t) decreased by an order of magnitude or more. The similarity of the decrease we observed for all clay samples suggested that there was a non-linear relationship between the I2 uptake rates and gaseous concentrations of I2.
Rg-a(t) increased with time during the injection period, but if surface resistance were a constant value equal to Rgc, then ΔQloss/Qin would be given by (see section S7.4)
(ΔQ_"loss" )/Q_"in" =1/(1+T_0/T F_"g" /S R_"gc" ) (S21)
Figure 9 and S7–S11 show the values of ΔQloss/Qin calculated with eq S21 and assuming Rgc = Rg-a. The calculated values agreed with the observed values within an error range of 30%, except for two runs (run 5 and run 7 for halloysite with the small basins at 20%RH). Values of Rg-a thus represent the surface resistance for I2 above the clay samples.

R2-4 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 2
If these methods are important and innovation, the author should give some description and discussion in the part of Introduction.

(2) authors’ response
Features of these methods are described in the Introduction section as shown in (3).

(3) authors’ changes in manuscript
Introduction section, 4th block, lines 4–9:
We used a rectangular pulse method22 to distinguish loss of I2 from adsorption onto the reactor. This method is able to be conducted under lower-atmospheric conditions such as humidified air at 1 atm. The pulse method is useful for determining the loss rates for the reactors which tend to deactivate with exposure to I2 because it can control the exposure to I2 precisely at low concentration levels of I2. As a disadvantage, it needs estimate of mass-transfer resistances in gas-film layers above the solid or the liquid reactors for determination of the surface resistance above the reactors. The clay samples consisted of five pure clay minerals and a standard clay soil, and the aqueous surfaces were water, aqueous sulfuric acid, and aqueous ascorbic acid.

R2-5 ---------------------------------------------------------------------------------------------------------
(1) comments from Referee 1
In the part of “3.3.3. Simulation of I2 uptake by clay samples”, The authors mentioned “The simulation was conducted on the basis of a two-stage model in which the uptake of I2 by clay samples occurred via an initial stage of rapid sorption onto surface sites followed by a stage of slow sorption onto interior sites such as the mesopores and the micropores of clay samples.” Besides, ΔQloss decreased and ΔQads increased with successive runs; the sum of ΔQloss and ΔQads, which represented the amount of I2 accommodated by clay samples, remained almost constant or decreased more slowly than ΔQloss with successive runs. Why? What’s the relationship between these two parts?

(2) authors’ response
This behavior of ΔQloss and ΔQads can be explained by a two-stage model with two kinds of active sites on the surface and in the interior for loss of I2. One kind of active site was consumed on the surface, and the other remained its activity in the interior for uptake of I2, as described in section 3.3.3. This is described shortly in 3rd block in section 3.3.1 as shown in (3).

(3) authors’ changes in manuscript
Section 3.3.1, 3rd block, lines 1–5 from the bottom:
ΔQloss decreased and ΔQads increased with successive runs; the sum of ΔQloss and ΔQads, which represented the amount of I2 accommodated by clay samples, remained almost constant or decreased more slowly than ΔQloss with successive runs. This behavior can be explained by a two-stage model with two kinds of active sites on the surface and in the interior for loss of I2. One kind was consumed on the surface, and the other remained its activity in the interior for uptake of I2, as shown in section 3.3.3. Similar behavior of ΔQloss/Qin and ΔQads/Qin over successive runs was observed at 20%RH and 50%RH (Figures S7 and S8) and for even-numbered runs (Figures S9–S11).

3. Other revisions
3-1. Revision in Reference 15 ------------------------------------------------------------------------
In reference 15, manuscript number (L04112) is missing. The revisions are made as follows.

Before revision
15. A. Saiz-Lopez and J. M. C. Plane, Novel iodine chemistry in the marine boundary layer, Geophys. Res. Lett., 2004, 31.

After revision
15. A. Saiz-Lopez and J. M. C. Plane, Novel iodine chemistry in the marine boundary layer, Geophys. Res. Lett., 2004, 31, L04112.

3-2. Revisions in eq 37 in the revised manuscript and the related equations in the revised Supporting Information -----------------------------------------------------------------------
There are typos in eq 37 and in the related equations in the Supporting Information. The revisions are made in these equations as described in (a)–(d). The revisions are marked in blue. If these revisions are made, there are no change in the following equations (eqs 38−40) and in 3.4 section.

(a) eq 37 in the revised manuscript
Before revision
k_g0=(q_∞/B_"pm" 〖 k〗_"ls" (t))/(1/K_"ads" +1/(k_"ma" /(K_"pads" k_"loss-p" )+k_"ma" /k_2"f" )+(k_"ls" (t))/k_"ma" )+(q_∞/B_"pm" )/(q_∞/B_"pm" 1/k_"ma" +1/(K_"ads" k_"f" )+1/(K_"ads" K_"pads" k_"loss-p" )+(k_"ls" (t))/(k_"ma" k_"f" ) (1/K_"pads" +1) )

After revision
k_g0=(q_∞/B_"pm" 〖 k〗_"loss-s" (t))/(1/K_"ads" +1/(k_"ma" /(K_"pads" k_"loss-p" )+k_"ma" /k_2"f" )+(k_"loss-s" (t))/k_"ma" )+(q_∞/B_"pm" )/(q_∞/B_"pm" 1/k_"ma" +1/(K_"ads" k_"f" )+1/(K_"ads" K_"pads" k_"loss-p" )+(k_"loss-s" (t))/(k_"ma" k_"f" ) (k_"f" /(K_"pads" k_"loss-p" )+1) )

(b) eq 38 in the revised manuscript
Before revision
R_(g"-" clay"-" 0)=(q_∞/B_pn K_"ads" k_"ls" (t)+(q_∞/B_pn )/(1/(K_ads k_f )+1/(K_ads K_pads k_(loss"-" p) )))^(-1)

After revision
R_(g"-" clay"-" 0)=(q_∞/B_pn K_"ads" k_"loss-s" (t)+(q_∞/B_pn )/(1/(K_ads k_f )+1/(K_ads K_pads k_(loss"-" p) )))^(-1)


(c) eq S29 in the revised Supporting Information
Before revision
U_M1=k_1"u" n_1=k_3d n_3

After revision
U_M1=k_1"u" n_1=〖k_"2d" n_2+k〗_3d n_3

(d) eq S34 in the revised Supporting Information
Before revision
U_M1={k_"2d" /(1/K_1 +(K_2 k_3d)/(k_1f (1+k_3d/k_2b ) )+k_"2d" /k_1f )+1/(1/(K_1 K_2 k_3"d" )+1/(K_1 k_2"f" )+1/k_1"f" +k_"2d" (1/(K_2 k_"1f" k_"2f" )+1/(k_"1f" k_"2f" )) )}×n_1
After revision
U_M1={k_"2d" /(1/K_1 +(K_2 k_3d)/(k_1f (1+k_3d/k_2b ) )+k_"2d" /k_1f )+1/(1/(K_1 K_2 k_3"d" )+1/(K_1 k_2"f" )+1/k_1"f" +k_"2d" (1/(K_2 k_"1f" k_"3d" )+1/(k_"1f" k_"2f" )) )}×n_1

(e) eq S35 in the revised Supporting Information
Before revision
U_M1={(q_∞/B_"pm" 〖 k〗_"ls" (t))/(1/K_"ads" +1/(k_"ma" /(K_"pads" k_"lp" )+k_"ma" /k_2"f" )+(k_"ls" (t))/k_"ma" )+(q_∞/B_"pm" )/(q_∞/B_"pm" 1/k_"ma" +1/(K_"ads" k_"f" )+1/(K_"ads" K_"pads" k_"lp" )+(k_"ls" (t))/(k_"ma" k_"f" ) (1/K_"pads" +1) )}×n_1
After revision
U_M1={(q_∞/B_"pm" 〖 k〗_"loss-s" (t))/(1/K_"ads" +1/(k_"ma" /(K_"pads" k_"lp" )+k_"ma" /k_2"f" )+(k_"loss-s" (t))/k_"ma" )+(q_∞/B_"pm" )/(q_∞/B_"pm" 1/k_"ma" +1/(K_"ads" k_"f" )+1/(K_"ads" K_"pads" k_"loss-p" )+(k_"loss-s" (t))/(k_"ma" k_"f" ) (k_"f" /(K_"pads" k_"loss-p" )+1) )}×n_1

13-Jun-2022

Dear Dr Kutsuna:

Manuscript ID: EA-ART-04-2022-000039.R1
TITLE: Laboratory Study on Uptake of Gaseous Molecular Iodine by Clay Minerals at Different Relative Humidity

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Reviewer 2

The author revised all my comments, the manuscript can be accepted to publish.