the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Nov 2023
Negligible Temperature Dependence of the Ozone-Iodide Reaction and Implications for Oceanic Emissions of Iodine
Abstract. The reaction between ozone and iodide is one of the main drivers of tropospheric ozone deposition to the ocean, due to the ubiquitous presence of iodide in the ocean surface and its rapid reaction with ozone. Despite the importance of this sea surface reaction for tropospheric ozone deposition, and also as the major source of atmospheric iodine, there is uncertainty in its rate and dependence on aqueous phase temperature. In this work, the kinetics of the heterogeneous second order reaction between ozone and iodide were investigated using conditions applicable to coupled ocean-atmosphere systems (1 × 10−7 – 1 × 10−5 M [iodide], 40 ppb ozone, 288 – 303 K, 15.0 psi). The Arrhenius parameters determined of A = 1.0 ± 4.6 ×1011 M−1 s−1 and Ea = 8.5 ± 10.9 kJ mol−1 show that the reaction has a negligible positive temperature dependence, which could be weakly negative within errors. This is in contrast to a previous study that found a strong positive activation energy and a pre-exponential factor many orders of magnitude greater than determined here. The re-measured kinetics of ozone and iodide were used to constrain a state-of-the-art sea surface microlayer (SML) model. The model replicated results from a previous laboratory study of the temperature dependence of hypoiodous acid (HOI) and molecular iodine (I2) emissions from an ozoneoxidised iodide solution. This work has significance for global modelling of dry deposition of ozone to the ocean and the subsequent emissions of iodine-containing species, thus improving understanding of the feedbacks between natural halogens, air quality, and climate change.
-
Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
-
Preprint
(762 KB)
-
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(762 KB) - Metadata XML
- BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2660', Anonymous Referee #1, 26 Nov 2023
This paper addresses the temperature dependence of the kinetics of the reaction between ozone and iodide dissolved in water, i.e., the reaction that dominates the dry deposition velocity of ozone to the ocean surface in the absence of high concentrations of dissolved reactive organics. This reaction also leads to the release of iodine to the atmosphere. The overall reaction rate is sufficiently slow, because of low dissolved iodide concentrations, that the aerodynamic resistance for ozone uptake is relatively minor, and so the rate constant of the reaction itself is particularly relevant for the atmosphere.
There is one reported temperature dependence for the rate constant of this reaction, by Magi et al., with a very high activation energy and pre-exponential factor. The present study finds minimal (potentially nil) temperature dependence to the rate constant, which has direct impact on the modeling of ozone dry deposition and iodine release to the atmosphere. The Magi et al. study was performed with high iodide concentrations (up to molar levels), roughly a million or more times higher than those in the ocean. Thus, the Magi et al. kinetics are almost certainly dominated by interfacial processes whereas the oceanic reaction occurs in the bulk, away from the interface, as delineated in a couple of papers by Moreno and co-workers. So, the kinetic parameters of Magi et al. are not relevant (nor are they likely correct, i.e., a rate constant that is so close to the diffusion limit cannot have such a large activation energy). Rather, the present study was conducted with atmospherically relevant concentrations of ozone and iodide. The finding of a minimal temperature dependence is not surprising given the size of the rate constant (i.e., approaching the diffusion limit) but nevertheless important to confirm.
I recommend publication after the following comments are addressed:
- Reaction 1 likely has a reversible (i.e., double arrowed) reaction of ozone and iodide to form IOOO-; after formation, this complex can either decompose back to reactants or go on to form IO- and O2. In this context, how does the minor temperature dependence in R1 arise? Is the rate determining step the formation of IOOO-, with a low barrier in the entrance channel to the reaction? Or, if the complex lives for a reasonably long time, how would the balance between the forward and backward reactions of IOOO- affect the temperature dependence for the overall reaction? The potential atmospheric significance is if something else in solution may affect the fate of IOOO-, such as dissolved organics.
- The care taken to avoid and assess iodide depletion is commendable. Indeed, the overall experimental setup is nicely configured and explained.
- Although the primary media was ozonized to remove potential contaminants, was there any evidence for the tubing or pump that feed the iodide solution to the flow tube to be a source of reactive contaminants?
- Would the kinetics have been different with seawater concentrations of salts, in particular chloride? I know that ozone does not rapidly react with chloride but could the ionic strength of seawater affect the kinetics and its temperature dependence?
- Minor superscript and subscripts issues, e.g., lines 147, 179 and a number of other places where the 3 is O3 is not a subscript.
- The authors subtract the loss observed in blank experiments (without iodide present) from the results for iodide experiments. This assumes that the loss observed in the blank also occurs when iodide is present. I don’t know if this is necessarily true, i.e., without iodide present, ozone will diffuse deep into the solution until it finds something to react with. When iodide is present, ozone is constrained to the reacto-diffusive depth of the iodide solution (a few microns) and is less likely to react with those contaminants. So, it is possible that the blank should not be subtracted from the observed kinetics with iodide present. How much would the results of the paper be changed if the blank is not subtracted?
- The paper claims a connection to the lower stratosphere, where iodide oxidation may be occurring. I can see the potential connection but the temperature range investigated in this work is small when thinking about stratospheric conditions. Perhaps add a minor caveat?
- The open and closed symbols in Figure 7 are a bit confusing to me. For example, the data from Magi et al. have closed circles but the authors claim that that study is not environmentally relevant (and closed circles are meant to indicate environmental relevance) whereas the data from Garland et al. are open circles but their work is environmentally relevant ...
- Which rate constant is the “k” in the legend of Figure 10? First or second order? Also, the "ka" for HOI should be "Ka".
- Concerning the broader question of the temperature dependence of iodine release driven by iodide ozonolysis, I am guessing that one of the strongest uncertainties will be the uncertainties in the temperature dependencies of the Henry’s Law constants for HOI (and I2). Is that true? I didn’t see this point made in the paper, but I may have missed it. Figure 10 illustrates the strong dependence on the HOI Henry’s Law constant but what are the uncertainties in that temperature dependence?
Citation: https://doi.org/10.5194/egusphere-2023-2660-RC1 -
RC2: 'Comment on egusphere-2023-2660', Anonymous Referee #2, 07 Dec 2023
The manuscript entitled, "Negligible Temperature Dependence of the Ozone-Iodide Reaction. . ." submitted by Brown et al., details a very rigorous study of the aqueous phase reaction rate coefficient of I + O3. The authors have endeavored to determine this rate coefficient under atmospherically relevant concentrations (O3 and Iodide) and temperatures. This fills a gap in previous measurements of this reaction. The authors then used the updated rate coefficients and Arrhenius parameters to constrain a sea surface microlayer model. The importance of this reaction and rate coefficient, as the authors have argued, cannot be understated for understanding marine chemistry.
The experiments are carefully done and described in detail to allow other to replicate their approach. The authors have done a excellent job with experimental checks to minimize iodide depletion in their setup. The authors have done a good job analyzing previous measurements and have done an excellent job explaining where discrepancies exist and why. I recommend publication after the authors have addressed the following:
1. The data points in Figure 3 are hard to read due to the thick lines and the use of yellow. Please revise so a ready can clearly distinguish the data from the fit. Also in the text the residence time is cited as 24-66 seconds whereas in Fig. 3 the first data point is at 20 seconds?
2. Line 199 For clarity since there has been historical confusion please be explicit about what H = 3.63 means? i.e. Gas/Aqueous
3. Line 211 There is some discussion about if the mechanism proceeded via Langmuir-Hinshelwood an exponential increase in uptake with decreasing ozone would be observed. I don't understand this statement and as written it is not clear to me if this statement is in fact correct. The authors should take a few sentence to explain why this would be expected for a LH mechanism.
4. Table 1. The rate coefficients in column 1 seem to be missing and 10^9 factor?
5. Figure 7 The open closed symbols and colors are confusing to denote environmental applicability. Please revise figure for clarity.
6. Line 359 "For Eqs. X to X. . ." is confusing.
7. The authors should attempt to discuss the negligible (or slightly negative) temperature dependence in light of R1 in particular the formation of intermediate IOOO-. There have been previous theoretical calculations (see for example Ó. Gálvez, M. Teresa Baeza-Romero, M. Sanz and L. F. Pacios, A theoretical study on the reaction of ozone with aqueous iodide, Physical Chemistry Chemical Physics, 2016, 18, 7651-7660 and others) of this reaction that are useful context for a reader to understand if the temperature dependence observed in the experiment is consistent with theory and the proposed intermediate IOOO-. In other words, do these new measurements suggest previous theory is or is not correct?
Citation: https://doi.org/10.5194/egusphere-2023-2660-RC2 -
RC3: 'Comment on egusphere-2023-2660', Anonymous Referee #3, 08 Dec 2023
This article titled “Negligible Temperature Dependence of the Ozone-Iodide Reaction and Implications for Oceanic Emissions of Iodine” by Brown et al. presents a thorough measurement of the temperature dependence of the ozone-iodide reaction. This reaction is especially important to understand in the marine troposphere and stratosphere since it drives ozone dry deposition and is the dominant source of gaseous iodine (as HOI and I2) into the atmosphere.
In this study, the authors did an excellent job describing the specific challenges associated with these laboratory measurements. These considerations include the depletion of iodide in the measurement system, making sure to operate at atmospherically relevant concentrations, and the presence of contaminants in their system. Additionally, the authors did a great job in describing the extent and limitations of previous studies measuring similar systems. I recommend publication after the following points are addressed:
- More details could be provided on the blank measurements and the subsequent iodide experiments. How was the solution ozonized, specifically, was it by bubbling through the solution or from the gas-phase only? Was the iodide spiked directly into the experimental set-up as is suggested on L117 – 118?
- I am a bit confused about the discussion on ‘chemical availability’ mentioned on L324 which includes: “the combined chemical availability of HOI and I2 at higher temperatures limits the emissions [of I2]… ” Does this refer to lower concentrations of HOI and I2 in solution from reduced iodide which results in lower emissions (i.e. R5 and R6)? Or rather, on L321, the authors state “the formation of I2 is dependent on both HOI and I- availability”, so perhaps the statement on L324 contains a typo?
- In Figure 11 and Figure F5, the authors refer to the variable [HOI x iodide] and I’m not sure exactly what this is referring to. Is this the sum of both concentrations? Or is it referring to the reactant availability (from a rate equation) from R3?
- The authors use ‘enrichment’ to discuss the relative concentration of iodide in the SML relative to the bulk, which is confusing because they are specifically quantifying the depletion in the SML relative to the bulk. Perhaps using the term ‘enrichment factor’ is clearer since it doesn’t immediately suggest that the concentrations are elevated. This also aligns with the terminology used to describe the relative concentration of organics in the SML to the underlying water.
- What is the depth of the SML in the model? Perhaps it is useful to define this, since the SML depth is defined operationally.
Citation: https://doi.org/10.5194/egusphere-2023-2660-RC3 - AC1: 'Comment on egusphere-2023-2660', Lucy Brown, 23 Feb 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2660', Anonymous Referee #1, 26 Nov 2023
This paper addresses the temperature dependence of the kinetics of the reaction between ozone and iodide dissolved in water, i.e., the reaction that dominates the dry deposition velocity of ozone to the ocean surface in the absence of high concentrations of dissolved reactive organics. This reaction also leads to the release of iodine to the atmosphere. The overall reaction rate is sufficiently slow, because of low dissolved iodide concentrations, that the aerodynamic resistance for ozone uptake is relatively minor, and so the rate constant of the reaction itself is particularly relevant for the atmosphere.
There is one reported temperature dependence for the rate constant of this reaction, by Magi et al., with a very high activation energy and pre-exponential factor. The present study finds minimal (potentially nil) temperature dependence to the rate constant, which has direct impact on the modeling of ozone dry deposition and iodine release to the atmosphere. The Magi et al. study was performed with high iodide concentrations (up to molar levels), roughly a million or more times higher than those in the ocean. Thus, the Magi et al. kinetics are almost certainly dominated by interfacial processes whereas the oceanic reaction occurs in the bulk, away from the interface, as delineated in a couple of papers by Moreno and co-workers. So, the kinetic parameters of Magi et al. are not relevant (nor are they likely correct, i.e., a rate constant that is so close to the diffusion limit cannot have such a large activation energy). Rather, the present study was conducted with atmospherically relevant concentrations of ozone and iodide. The finding of a minimal temperature dependence is not surprising given the size of the rate constant (i.e., approaching the diffusion limit) but nevertheless important to confirm.
I recommend publication after the following comments are addressed:
- Reaction 1 likely has a reversible (i.e., double arrowed) reaction of ozone and iodide to form IOOO-; after formation, this complex can either decompose back to reactants or go on to form IO- and O2. In this context, how does the minor temperature dependence in R1 arise? Is the rate determining step the formation of IOOO-, with a low barrier in the entrance channel to the reaction? Or, if the complex lives for a reasonably long time, how would the balance between the forward and backward reactions of IOOO- affect the temperature dependence for the overall reaction? The potential atmospheric significance is if something else in solution may affect the fate of IOOO-, such as dissolved organics.
- The care taken to avoid and assess iodide depletion is commendable. Indeed, the overall experimental setup is nicely configured and explained.
- Although the primary media was ozonized to remove potential contaminants, was there any evidence for the tubing or pump that feed the iodide solution to the flow tube to be a source of reactive contaminants?
- Would the kinetics have been different with seawater concentrations of salts, in particular chloride? I know that ozone does not rapidly react with chloride but could the ionic strength of seawater affect the kinetics and its temperature dependence?
- Minor superscript and subscripts issues, e.g., lines 147, 179 and a number of other places where the 3 is O3 is not a subscript.
- The authors subtract the loss observed in blank experiments (without iodide present) from the results for iodide experiments. This assumes that the loss observed in the blank also occurs when iodide is present. I don’t know if this is necessarily true, i.e., without iodide present, ozone will diffuse deep into the solution until it finds something to react with. When iodide is present, ozone is constrained to the reacto-diffusive depth of the iodide solution (a few microns) and is less likely to react with those contaminants. So, it is possible that the blank should not be subtracted from the observed kinetics with iodide present. How much would the results of the paper be changed if the blank is not subtracted?
- The paper claims a connection to the lower stratosphere, where iodide oxidation may be occurring. I can see the potential connection but the temperature range investigated in this work is small when thinking about stratospheric conditions. Perhaps add a minor caveat?
- The open and closed symbols in Figure 7 are a bit confusing to me. For example, the data from Magi et al. have closed circles but the authors claim that that study is not environmentally relevant (and closed circles are meant to indicate environmental relevance) whereas the data from Garland et al. are open circles but their work is environmentally relevant ...
- Which rate constant is the “k” in the legend of Figure 10? First or second order? Also, the "ka" for HOI should be "Ka".
- Concerning the broader question of the temperature dependence of iodine release driven by iodide ozonolysis, I am guessing that one of the strongest uncertainties will be the uncertainties in the temperature dependencies of the Henry’s Law constants for HOI (and I2). Is that true? I didn’t see this point made in the paper, but I may have missed it. Figure 10 illustrates the strong dependence on the HOI Henry’s Law constant but what are the uncertainties in that temperature dependence?
Citation: https://doi.org/10.5194/egusphere-2023-2660-RC1 -
RC2: 'Comment on egusphere-2023-2660', Anonymous Referee #2, 07 Dec 2023
The manuscript entitled, "Negligible Temperature Dependence of the Ozone-Iodide Reaction. . ." submitted by Brown et al., details a very rigorous study of the aqueous phase reaction rate coefficient of I + O3. The authors have endeavored to determine this rate coefficient under atmospherically relevant concentrations (O3 and Iodide) and temperatures. This fills a gap in previous measurements of this reaction. The authors then used the updated rate coefficients and Arrhenius parameters to constrain a sea surface microlayer model. The importance of this reaction and rate coefficient, as the authors have argued, cannot be understated for understanding marine chemistry.
The experiments are carefully done and described in detail to allow other to replicate their approach. The authors have done a excellent job with experimental checks to minimize iodide depletion in their setup. The authors have done a good job analyzing previous measurements and have done an excellent job explaining where discrepancies exist and why. I recommend publication after the authors have addressed the following:
1. The data points in Figure 3 are hard to read due to the thick lines and the use of yellow. Please revise so a ready can clearly distinguish the data from the fit. Also in the text the residence time is cited as 24-66 seconds whereas in Fig. 3 the first data point is at 20 seconds?
2. Line 199 For clarity since there has been historical confusion please be explicit about what H = 3.63 means? i.e. Gas/Aqueous
3. Line 211 There is some discussion about if the mechanism proceeded via Langmuir-Hinshelwood an exponential increase in uptake with decreasing ozone would be observed. I don't understand this statement and as written it is not clear to me if this statement is in fact correct. The authors should take a few sentence to explain why this would be expected for a LH mechanism.
4. Table 1. The rate coefficients in column 1 seem to be missing and 10^9 factor?
5. Figure 7 The open closed symbols and colors are confusing to denote environmental applicability. Please revise figure for clarity.
6. Line 359 "For Eqs. X to X. . ." is confusing.
7. The authors should attempt to discuss the negligible (or slightly negative) temperature dependence in light of R1 in particular the formation of intermediate IOOO-. There have been previous theoretical calculations (see for example Ó. Gálvez, M. Teresa Baeza-Romero, M. Sanz and L. F. Pacios, A theoretical study on the reaction of ozone with aqueous iodide, Physical Chemistry Chemical Physics, 2016, 18, 7651-7660 and others) of this reaction that are useful context for a reader to understand if the temperature dependence observed in the experiment is consistent with theory and the proposed intermediate IOOO-. In other words, do these new measurements suggest previous theory is or is not correct?
Citation: https://doi.org/10.5194/egusphere-2023-2660-RC2 -
RC3: 'Comment on egusphere-2023-2660', Anonymous Referee #3, 08 Dec 2023
This article titled “Negligible Temperature Dependence of the Ozone-Iodide Reaction and Implications for Oceanic Emissions of Iodine” by Brown et al. presents a thorough measurement of the temperature dependence of the ozone-iodide reaction. This reaction is especially important to understand in the marine troposphere and stratosphere since it drives ozone dry deposition and is the dominant source of gaseous iodine (as HOI and I2) into the atmosphere.
In this study, the authors did an excellent job describing the specific challenges associated with these laboratory measurements. These considerations include the depletion of iodide in the measurement system, making sure to operate at atmospherically relevant concentrations, and the presence of contaminants in their system. Additionally, the authors did a great job in describing the extent and limitations of previous studies measuring similar systems. I recommend publication after the following points are addressed:
- More details could be provided on the blank measurements and the subsequent iodide experiments. How was the solution ozonized, specifically, was it by bubbling through the solution or from the gas-phase only? Was the iodide spiked directly into the experimental set-up as is suggested on L117 – 118?
- I am a bit confused about the discussion on ‘chemical availability’ mentioned on L324 which includes: “the combined chemical availability of HOI and I2 at higher temperatures limits the emissions [of I2]… ” Does this refer to lower concentrations of HOI and I2 in solution from reduced iodide which results in lower emissions (i.e. R5 and R6)? Or rather, on L321, the authors state “the formation of I2 is dependent on both HOI and I- availability”, so perhaps the statement on L324 contains a typo?
- In Figure 11 and Figure F5, the authors refer to the variable [HOI x iodide] and I’m not sure exactly what this is referring to. Is this the sum of both concentrations? Or is it referring to the reactant availability (from a rate equation) from R3?
- The authors use ‘enrichment’ to discuss the relative concentration of iodide in the SML relative to the bulk, which is confusing because they are specifically quantifying the depletion in the SML relative to the bulk. Perhaps using the term ‘enrichment factor’ is clearer since it doesn’t immediately suggest that the concentrations are elevated. This also aligns with the terminology used to describe the relative concentration of organics in the SML to the underlying water.
- What is the depth of the SML in the model? Perhaps it is useful to define this, since the SML depth is defined operationally.
Citation: https://doi.org/10.5194/egusphere-2023-2660-RC3 - AC1: 'Comment on egusphere-2023-2660', Lucy Brown, 23 Feb 2024
Peer review completion
Journal article(s) based on this preprint
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 162 | 57 | 19 | 238 | 11 | 13 |
- HTML: 162
- PDF: 57
- XML: 19
- Total: 238
- BibTeX: 11
- EndNote: 13
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Ryan J. Pound
Lyndsay S. Ives
Matthew R. Jones
Stephen J. Andrews
Lucy J. Carpenter
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(762 KB) - Metadata XML