Nighttime atmospheric chemistry of iodine

Saiz-Lopez, Alfonso; Plane, John M. C.; Cuevas, Carlos A.; Mahajan, Anoop S.; Lamarque, Jean-François; Kinnison, Douglas E.

doi:https://doi.org/10.5194/acp-16-15593-2016

Articles | Volume 16, issue 24

https://doi.org/10.5194/acp-16-15593-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-16-15593-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 16, issue 24

Research article

|

19 Dec 2016

Research article |

| 19 Dec 2016

Nighttime atmospheric chemistry of iodine

Alfonso Saiz-Lopez, John M. C. Plane, Carlos A. Cuevas, Anoop S. Mahajan, Jean-François Lamarque, and Douglas E. Kinnison

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Final revised paper (published on 19 Dec 2016)
Supplement to the final revised paper
Preprint (discussion started on 15 Jun 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC1: 'review of manuscript', Florent LOUIS, 04 Jul 2016
- AC1: 'Response to Florent Louis comments', Carlos Alberto Cuevas Rodríguez, 02 Sep 2016
RC1: 'Review of “Iodine Chemistry After Dark” by Saiz-Lopez et al., submitted to ACPD 2016.', Howard K. Roscoe, 05 Jul 2016
- AC2: 'Rosponse to Howard Roscoe comments', Carlos Alberto Cuevas Rodríguez, 02 Sep 2016
RC2: 'modelling', Anonymous Referee #3, 13 Jul 2016
- AC3: 'Response to Anonymous Referre #3', Carlos Alberto Cuevas Rodríguez, 02 Sep 2016
RC3: 'review', Anonymous Referee #1, 16 Jul 2016
- AC4: 'Response to Anonymous Referee #1 comments', Carlos Alberto Cuevas Rodríguez, 02 Sep 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Carlos Alberto Cuevas Rodríguez on behalf of the Authors (19 Sep 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (22 Sep 2016) by Jens-Uwe Grooß

RR by Florent LOUIS (22 Sep 2016)

Suggestions for revision or reasons for rejection

I posted an interactive comment on "Iodine chemistry after dark" by Saiz-Lopez et al. on July 4th 2016. This paper represents a very important contribution dealing with atmospheric iodine chemistry. I made several comments. There is still one comment for which the authors do not take into account my recommendation.

It concerns the spin-orbit correction (SOC) for iodine-containing species involved in the studied reactions. I previously reported "The authors stated on page 6 line 12 that "spin-orbit splittings of -17 and -5 kJ mol-1 were applied to energies of I and IO". These values do not correspond to the well known value for I atom (-30.3 kJ mol-1 from C.E. Moore, Atomic Energy Levels, USGPO, Vols. II and III. NSRDS-NBS 35, Washington, DC, 1971). Over the last years, my group performed theoretical calculations to get the SOC values for numerous iodine-containing species using the CASPT2/RASSI methodology. The corresponding values for I, IO, and HOI are -30.0, -14.4, and -5.9 kJ/mol (Meciarova et al., CPL, 2011, 517, 149; Khanniche et al., JPCA, 2016, 120, 1737; Sulkova et al., JPCA, 2013, 117, 771). These calculations were also validated by comparison to few available data. I recommend the authors to update their energetics according to the correct SOC values".

There are large differences between SOC values for I and IO (-17 and -5 kJ mol-1) in the manuscript and the literature data (-30 and -14 kJ mol-1). I would expect the authors will revise their energetics according to the most reliable data. Kinetic parameters should be also re-evaluated.

For the second comment dealing with the treatment of low frequency modes in the intermediate species, I would suggest the authors to mention only in the manuscript : "The rigid-rotor harmonic oscillator approximation has been used for all species". Hindered rotor treatment for low frequency modes could influence the entropy contribution. Of course, it will depend on the rotational barrier height, its periodicity and the model used. It is often used without any available experimental data.

Other comments have been properly answered by the authors.

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RR by Anonymous Referee #1 (23 Sep 2016)

Suggestions for revision or reasons for rejection

In my opinion, the scientific content of the revised version of the
manuscript is sufficient to justify publication in ACP. Unfortunately,
however, the authors ignored several suggestions from the reviewers to
improve the presentation quality which is also important for a
high-quality article. In particular, I would like to reiterate the
following points:

- Thank you for showing the iodine reaction mechanism in the supplement.
You mention that the scheme is referenced in page 22 line 12 in the
main text but I cannot find it there. Although the supplement now
shows all iodine reactions, the non-iodine reactions in the model are
still not listed. For example, IO + BrO is listed but BrO + BrO is not
listed, even though this reaction will also have an indirect effect on
reactive iodine concentrations.

- I know that LT refers to local time. However, this may not be clear to
all readers. I think that all acronyms should be explained in the text
when they are used for the first time.

- It may be customary to quote vibrational frequencies as wave numbers
but it is incorrect. It is like saying "the frequency of green light
is 500 nm".

- Provided that enough significant digits are shown, it is fine to show
the numbers in the plots of Figs. 4 and 5 instead of using a color
scale. Nevertheless, I still find it confusing if two plots in the
same row use slightly different color schemes. It makes a comparison
unnecessarily complicated.

- The information that "nighttime" refers to the average between
midnight and 01:00 local time is important and should be shown in the
figure caption. If you want to avoid repetition, it could be removed
from the text.

- The effect of the new chemistry on DMS is quite indirect. You mention
that DMS changes because of NO3. In addition, the new iodine chemistry
will most probably also affect bromine chemistry, and different BrO
will also affect DMS concentrations. I think that either this should
also be discussed, or figure 9 should be removed, as another reviewer
had suggested.

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ED: Reconsider after major revisions (23 Sep 2016) by Jens-Uwe Grooß

Dear authors,

In my impression, you should have considered more of the reviewers comments.
So please consider the remaining points expressed by the two reviewers as well as the following
technical points:

Title: "after dark" means to my knowledge something like "after it did get dark".
It could be mis-understood as "after the dark period", which would be at morning.
You also mention the peak at sunrise, so I also would prefer to be the title as
suggested by reviewer #1 ("Nighttime Iodine chemistry" or so.)

"buildup of HOI"
In your figure 4 (HOI scenario 1) HOI is clearly decreasing during night and
it is not building up, although the shown HOI emissions are increasing.
You could likely visualize the nighttime values better, if the used colour label
differences would not be linear, but e.g. logarithmic. But in the text should
be clarified, that part of the emitted HOI has a faster or slower sink depending
on scenario.

"which is converted into I2/IBr/ICl through heterogeneous recycling" (2 times)
What do you mean by that? According to your reaction list in the attachment there
the HOI would react to IBr/ICl on the aerosols but not to I2.

"Figures 4 and 5" please use "local time" a x-coordinate as suggested by the reviewer
and mention the two-day spin-up elsewhere.

"Figure 4 middle right" please do not truncate the colour labels of the contour plot

"Figure 5": please use also IONO2 instead of INO3 in the bottom panels to be consistent with the text.

"Figure 6": please explain in more detail which NO3 value is plotted (certain local time,
average nighttime or what)

"figure 7-9, colour bar label" use (greek) Delta HOI and Delta I2 as you show a difference.

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AR by Carlos Alberto Cuevas Rodríguez on behalf of the Authors (02 Dec 2016) Author's response Manuscript

ED: Publish subject to technical corrections (05 Dec 2016) by Jens-Uwe Grooß

AR by Carlos Alberto Cuevas Rodríguez on behalf of the Authors (05 Dec 2016) Author's response Manuscript

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Short summary

Electronic structure calculations are used to survey possible reactions that HOI and I₂ could undergo at night in the lower troposphere, and hence reconcile measurements and models. The reactions NO₃ + HOI and I₂ + NO₃ are included in two models to explore a new nocturnal iodine radical activation mechanism, leading to a reduction of nighttime HOI and I₂. This chemistry can have a large impact on NO₃ levels in the MBL, and hence upon the nocturnal oxidizing capacity of the marine atmosphere.