the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Aug 2023
Influences of downward transport and photochemistry on surface ozone over East Antarctica during austral summer: in situ observations and model simulations
Abstract. Studies of atmospheric trace gases in remote, pristine environments are critical for assessing the accuracy of climate models and advancing our understanding of natural processes and global changes. We investigated the surface ozone (O3) variability over East Antarctica during the austral summer of 2015–2017 by combining surface and balloon-borne measurements at the Indian station Bharati (69.4° S, 76.2° E, ~35 m above mean sea level) with EMAC atmospheric chemistry-climate model simulations. The model reproduced the observed surface O3 level (18.8 ± 2.3 nmol mol-1) with negligible bias and captured much of the variability (R=0.5). Model simulated tropospheric O3 profiles were in reasonable agreement with balloon-borne measurements (mean bias: 3–11 nmol mol-1). Our analysis of a stratospheric tracer in the model showed that about 40–50 % of surface O3 over the entire Antarctic region was of stratospheric origin. Events of enhanced O3 (~4–10 nmol mol-1) were investigated by combining O3 vertical profiles and air mass back trajectories, which revealed the rapid descent of O3-rich air towards the surface. The photochemical loss of O3 through its photolysis (followed by H2O+O(1D)) and reaction with hydroperoxyl radicals (O3+HO2) dominated over production from precursor gases (NO+HO2 and NO+CH3O2) resulting in overall net O3 loss during the austral summer. Interestingly, the east coastal region, including the Bharati station, tends to act as a stronger chemical sink of O3 (~190 pmol mol-1 d-1) than adjacent land and ocean regions (by ~100 pmol mol-1 d-1). This is attributed to reverse latitudinal gradients between H2O and O(1D), whereby O3 loss through photolysis (H2O+O(1D)) reaches a maximum over the east coast. Further, the net photochemical loss at the surface is counterbalanced by downward O3 fluxes, maintaining the observed O3 levels. The O3 diurnal variability of ~1.5 nmol mol-1 was a manifestation of combined effects of mesoscale wind changes and up- and downdrafts, in addition to the net photochemical loss. The study provides valuable insights into the intertwined dynamical and chemical processes governing the O3 levels and variability over East Antarctica.
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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.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2221 KB) - Metadata XML
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Supplement
(1787 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1524', Anonymous Referee #1, 13 Sep 2023
Overall Comment. The paper presents new observations of ozone over the East Antarctic Bharati station and interprets them in terms of stratospheric-tropospheric transport using a chemistry-climate model and trajectories back from soundings. It will be of interest to readers who expect to see the data online. Main recommendation: post the ozonesonde data- have you sent this to woudc.org and registered the site with them? Your institutional link should be a primary location but the ozone profile user community will look for the data at woudc.org. Thank you.
Minor Comments:
- Although this reference is not new, it includes examples of chemical processes at the snow-ice interface that could be relevant to your study: Biogeochemical Cycles and Ice Cores, NATO ASI Series I-30, ed. R. J. Delmas. ISBN 3642511740; ISBN-13 978-3642511745. Did you consider gases interacting with ozone that may come from snow-pack during the summer?
- Lines 280-286 – Vague. How would blizzards, etc, affect the observations? You imply model improvements need to be made to give a more accurate simulation. Be more specific.
- Lines 505-510. The R for Syowa disagreement suggests that the model is not doing well in winter at al. What does this mean? Reasons?
Citation: https://doi.org/10.5194/egusphere-2023-1524-RC1 - AC1: 'Reply on RC1', Imran A. Girach, 05 Dec 2023
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RC2: 'Comment on egusphere-2023-1524', Anonymous Referee #2, 26 Sep 2023
Girach et al. analyze surface ozone measurements at the Indian East Antarctic Bharati station in comparison with simulations from the chemistry climate model EMAC. The model is used to discriminate the role of subsidence from the stratosphere versus photochemical production in the troposphere or at surface level.
My main criticism is that the model description is not clear enough if or how chemical processes at the snow surface are included, in particular NOx emissions from the snow pack halogen/bromine chemistry, and the dry deposition of ozone. Even if it is argued that these processes may not be relevant for this particular study, at least it should be made clearer in Secton 2.2 which chemical processes are considered by the present EMAC simulations. Relevant EMAC sub-models seem to be available for polar bromine chemistry (Falk and Sinnhuber, 2018, https://doi.org/10.5194/gmd-11-1115-2018) and this latter study also investigated the uncertainty in ozone dry deposition velocities (as did other studies).
Overall the manuscript is well written and will be an interesting contribution to the literature, investigating the processes that contribute to ozone variability and trends in a data sparse region. I recommend publication in Atmos. Chem. Phys. after the following comments are taken into account.
Specific comments:
l.217: The statement on the tropopause fold occurrence frequency is somewhat disconnected and it is not clear how the conclusion can be made that the stratosphere to troposphere ozone flux is dominated by tropopause folds in contrast to slow subsidence through the tropopause.
l.250: O3s and O3t are correlated “due to mixing during the transport from the tropopause”. This statement confuses me. So does that mean O3s and O3t do not really represent the stratospheric and tropospheric contributions any more, but rather a mixture of the two? How useful are they then as a diagnostic??
l.252: “Strong local O3 production (e.g., through NOx from snow)”: again, is O3 production through NOx from snow included in the EMAC simulations? If not, is this some kind of circular reasoning? If it is included, would be good to give a few more details.
l.285: “to further improve the model in future studies”: how? Can you give some hints what may need to be improved?
Fig. 6b/c: The modelled net chemical tendencies (up to around 15 pmol/mol/h) are 1 order of magnitude smaller than the mean observed O3 tendencies (on average around 0.2 nmol/mol/h during morning and noon). Are the observed O3 tendencies in 6c not statistically significant? Or are there removal processes missing?
l.423: can you give us some idea what the “other” ozone production includes?
Minor comments:
l.65: “increasing trend (<0.2 nmol/mol/y)”: the number refers to a trend, not a trend increase. So either give some numbers how the trend is increasing or delete the word “increasing” in this context. Moreover, the “<” means this is an upper limit for the trend; better give a lower limit if available.
l.209: I suppose “Summit” is on Greenland? Why is this included here? If Summit should be included, please include lat/lon and/or geographic reference.
Fig.1 Caption: suggest to include explicitly the time period shown
Fig.2 Caption: suggest to mention that O3s and O3t are on a different scale than O3. (It took me a while before I realized that, couldn’t make sense of it before)
Citation: https://doi.org/10.5194/egusphere-2023-1524-RC2 - AC2: 'Reply on RC2', Imran A. Girach, 05 Dec 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-1524', Anonymous Referee #1, 13 Sep 2023
Overall Comment. The paper presents new observations of ozone over the East Antarctic Bharati station and interprets them in terms of stratospheric-tropospheric transport using a chemistry-climate model and trajectories back from soundings. It will be of interest to readers who expect to see the data online. Main recommendation: post the ozonesonde data- have you sent this to woudc.org and registered the site with them? Your institutional link should be a primary location but the ozone profile user community will look for the data at woudc.org. Thank you.
Minor Comments:
- Although this reference is not new, it includes examples of chemical processes at the snow-ice interface that could be relevant to your study: Biogeochemical Cycles and Ice Cores, NATO ASI Series I-30, ed. R. J. Delmas. ISBN 3642511740; ISBN-13 978-3642511745. Did you consider gases interacting with ozone that may come from snow-pack during the summer?
- Lines 280-286 – Vague. How would blizzards, etc, affect the observations? You imply model improvements need to be made to give a more accurate simulation. Be more specific.
- Lines 505-510. The R for Syowa disagreement suggests that the model is not doing well in winter at al. What does this mean? Reasons?
Citation: https://doi.org/10.5194/egusphere-2023-1524-RC1 - AC1: 'Reply on RC1', Imran A. Girach, 05 Dec 2023
-
RC2: 'Comment on egusphere-2023-1524', Anonymous Referee #2, 26 Sep 2023
Girach et al. analyze surface ozone measurements at the Indian East Antarctic Bharati station in comparison with simulations from the chemistry climate model EMAC. The model is used to discriminate the role of subsidence from the stratosphere versus photochemical production in the troposphere or at surface level.
My main criticism is that the model description is not clear enough if or how chemical processes at the snow surface are included, in particular NOx emissions from the snow pack halogen/bromine chemistry, and the dry deposition of ozone. Even if it is argued that these processes may not be relevant for this particular study, at least it should be made clearer in Secton 2.2 which chemical processes are considered by the present EMAC simulations. Relevant EMAC sub-models seem to be available for polar bromine chemistry (Falk and Sinnhuber, 2018, https://doi.org/10.5194/gmd-11-1115-2018) and this latter study also investigated the uncertainty in ozone dry deposition velocities (as did other studies).
Overall the manuscript is well written and will be an interesting contribution to the literature, investigating the processes that contribute to ozone variability and trends in a data sparse region. I recommend publication in Atmos. Chem. Phys. after the following comments are taken into account.
Specific comments:
l.217: The statement on the tropopause fold occurrence frequency is somewhat disconnected and it is not clear how the conclusion can be made that the stratosphere to troposphere ozone flux is dominated by tropopause folds in contrast to slow subsidence through the tropopause.
l.250: O3s and O3t are correlated “due to mixing during the transport from the tropopause”. This statement confuses me. So does that mean O3s and O3t do not really represent the stratospheric and tropospheric contributions any more, but rather a mixture of the two? How useful are they then as a diagnostic??
l.252: “Strong local O3 production (e.g., through NOx from snow)”: again, is O3 production through NOx from snow included in the EMAC simulations? If not, is this some kind of circular reasoning? If it is included, would be good to give a few more details.
l.285: “to further improve the model in future studies”: how? Can you give some hints what may need to be improved?
Fig. 6b/c: The modelled net chemical tendencies (up to around 15 pmol/mol/h) are 1 order of magnitude smaller than the mean observed O3 tendencies (on average around 0.2 nmol/mol/h during morning and noon). Are the observed O3 tendencies in 6c not statistically significant? Or are there removal processes missing?
l.423: can you give us some idea what the “other” ozone production includes?
Minor comments:
l.65: “increasing trend (<0.2 nmol/mol/y)”: the number refers to a trend, not a trend increase. So either give some numbers how the trend is increasing or delete the word “increasing” in this context. Moreover, the “<” means this is an upper limit for the trend; better give a lower limit if available.
l.209: I suppose “Summit” is on Greenland? Why is this included here? If Summit should be included, please include lat/lon and/or geographic reference.
Fig.1 Caption: suggest to include explicitly the time period shown
Fig.2 Caption: suggest to mention that O3s and O3t are on a different scale than O3. (It took me a while before I realized that, couldn’t make sense of it before)
Citation: https://doi.org/10.5194/egusphere-2023-1524-RC2 - AC2: 'Reply on RC2', Imran A. Girach, 05 Dec 2023
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Narendra Ojha
Prabha R. Nair
Kandula V. Subrahmanyam
Neelakantan Koushik
Mohammed M. Nazeer
Nadimpally Kiran Kumar
Surendran Nair Suresh Babu
Jos Lelieveld
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2221 KB) - Metadata XML
-
Supplement
(1787 KB) - BibTeX
- EndNote
- Final revised paper