A Study of the Low-Ozone Episode over Scandinavia and Northwestern Russia in March 2025

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. The topic is important, but the paper would benefit from a clearer statement of novelty. Please elaborate on how this event differs from previous low-ozone episodes and what new understanding is being offered.

2. The methodology section lacks detail about data resolution, error margins, and validation methods. Please provide more clarity on the analysis.

3. The manuscript especially linking to the polar vortex weakening or splitting would strengthen the interpretation.

4. Some figures consider improving their readability and ensuring all maps are properly labeled with time and location references.

5. A more analytical approach, comparing this event to past anomalies, could enhance context and scientific depth.

6. Adding more detailed attribution or discussion of contributing factors would be useful for the readers.

7. The objective is mentioned but not defined clearly. It would help to clearly state the research question and hypotheses at the end of the introduction section. 

8. A clearer justification for dataset selection and an assessment of accuracy (such as biases and uncertainties) would improve the methodology of the study.

Minor Comments:

1. Clarify the use of significant drop, quantify where possible.

2. Abstract: Consider stating the key findings more quantitatively.

3. Grammar: Minor grammatical errors noted throughout; recommend a thorough proofread.

4. References: Add a few more recent citations, especially from 2020 to 2025.

5. The manuscript needs minor editing for fluency. For example, passive voice is overused in some sections. A language polish would improve readability.

6. Terminology like ozone hole, ozone dip, and low-ozone episode are used interchangeably but may have different connotations. Please clarify and correct it accordingly.

Author Response

Authors are very grateful to all four anonymous reviewers for a careful reading and the all suggestions and recommendations. Our answers and changes in the text are marked by red color.

Below a new version of our manuscript is attached after Atmosphere’s language service.

Since some of the reviewers' questions relate to the same sections, sentences, and figures a common file with our answers is posted below.

 

 

Reviewer 1 Comments and Suggestions for Authors

1. The topic is important, but the paper would benefit from a clearer statement of novelty. Please elaborate on how this event differs from previous low-ozone episodes and what new understanding is being offered.

The following three sentences were included in the Introduction before the last paragraph:

The main advantage of this work is the investigation of the role of dynamic and chemical processes in the formation of a significant ozone anomaly in early March 2025, characterized by the second lowest TCO value for Moscow since 1986 and the first one for St. Petersburg since the early 1980s. These very low TCO values were only slightly above the generally excepted a threshold of the ozone hole in 220 DU periodically observed in the Antarctic stratosphere and much rare in the Arctic stratosphere. Also, associated with the such strong ozone layer anomaly an estimation of the UV radiation increase is presented firstly as far as we know. 

2. The methodology section lacks detail about data resolution, error margins, and validation methods. Please provide more clarity on the analysis.

Data resolution: the information about ERA5 reanalysis data resolution was added in the beginning of section 2.1

Validation: different TCO data were used in the present study to analyze the ozone mini hole: satellite observations, ground-based measurements as well as ERA5 reanalysis. 

The following sentence was added in the beginning of the section 2.2:

The TCO anomaly was analyzed using satellite observations, ground-based measurements, and ERA5 reanalysis data. 

3. The manuscript especially linking to the polar vortex weakening or splitting would strengthen the interpretation.

We suppose that this remark is related to the “shifting” of “splitting” type of SSW event. Therefore, at the end of the second paragraph after the Figure 2 the following sentence was included:

 This spatial structure of stratospheric polar vortex displays the shifting type of the major SSW event that was observed in the early March 2025.

4. Some figures consider improving their readability and ensuring all maps are properly labeled with time and location references.

Figures 1b, 2a,b, 3f were substituted to ones with higher resolution. Also figure 1a,c,b,d, 2d, 3f were modified (time axis particularly).

5. A more analytical approach, comparing this event to past anomalies, could enhance context and scientific depth.

The following sentences on peculiarities of the investigated ozone anomaly were included in the Introduction:

The main advantage of this work is the investigation of the role of dynamic and chemical processes in the formation of a significant ozone anomaly in early March 2025, characterized by the second lowest TCO value for Moscow since 1986 and the first one for St. Petersburg since the early 1980s. Also, associated with such the strong ozone layer anomaly an estimation of the UV radiation increase is presented firstly as far as we know.

6. Adding more detailed attribution or discussion of contributing factors would be useful for the readers.

The following sentence “Decrease of TCO values due to tropopause elevation and transport of low ozone air masses along the anticyclone western periphery as well as chemical ozone loss in the polar lower stratosphere were the main responsible processes for the ozone mini-hole formation.”

was added in the Discussion section.

7. The objective is mentioned but not defined clearly. It would help to clearly state the research question and hypotheses at the end of the introduction section. 

We suppose that including of additional two sentences before the last paragraph of Introduction (as our response to the 1^st Comment) clearly shows the objective as well as novelty of our study.

8. A clearer justification for dataset selection and an assessment of accuracy (such as biases and uncertainties) would improve the methodology of the study.

The following sentence was added in the sub-section 2.2:

Global daily TCO data measured by the TOMS, GOME, and OMI satellite instruments are the basic TCO data sets since the late 1970s and up to the present, validated in the comparison with the references ground based measurements, updated due to algorithms improvements and were successfully used in numerous studies of ozone layer variability and recovery over the recent decades [e.g., WMO, 2022].

Minor Comments:

1. Clarify the use of significant drop, quantify where possible.
2. Abstract: Consider stating the key findings more quantitatively.

The following two sentences of Abstract were rewritten: 

The temperature of the lower polar stratosphere increased that led to significant decrease of polar stratospheric clouds (PSC) type I (NAT) volume from ~80 million km³ to zero values.

Associated with the TCO anomaly, the difference relative to standard ozone level in the ultraviolet (UV) indices over Moscow, St. Petersburg, and Helsinki in early March 2025 reached up to 60%-100%.

3. Grammar: Minor grammatical errors noted throughout; recommend a thorough proofread.

We will send the manuscript to Atmosphere’s language service after the review.

4. References: Add a few more recent citations, especially from 2020 to 2025.

The following two references were added:

 

Crespo-Miguel, R.; Ordóñez, C.; García-Herrera, R.; Schnell, J.; Turnock, S. Large-scale ozone episodes in Europe: Decreasing sizes in the last decades but diverging changes in the future. Science of The Total Environment, 2024, 949, 175071, https://doi.org/10.1016/j.scitotenv.2024.175071.

 

Orsolini, Y.J. and Nikulin, G.  A low-ozone episode during the European heatwave of August 2003. Q.J.R. Meteorol. Soc. 2006, 132: 667-680. https://doi.org/10.1256/qj.05.30

5. The manuscript needs minor editing for fluency. For example, passive voice is overused in some sections. A language polish would improve readability.

We agree that a language polish is desirable to improve readability of the manuscript. Therefore, we will send the manuscript to Atmosphere’s language service after the review.

6. Terminology like ozone hole, ozone dip, and low-ozone episode are used interchangeably but may have different connotations. Please clarify and correct it accordingly.

The following sentence was added at the end of the sub-section 2.2:

As TCO anomaly investigated in the present study was characterized by the very low TCO values that were only slightly above the generally excepted a threshold of the ozone hole in 220 DU (periodically observed in the  Antarctic stratosphere and much rare in the Arctic stratosphere) is denoted further as low-ozone episode or ozone mini-hole.

Please see the attachment .

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The authors present a comprehensive study of ozone depletion over Moscow, St. Petersburg, and Helsinki during the 2024-2025 winter season. The authors analyzed multiple data sources (including satellite and ground-based sensors) to confirm that multiple measurements led to consistent conclusions about the ozone mini-hole that occurred over this region during the 2024-2025 winter season. The study was thorough and rigorous, and included simulation modeling as well as analyzing experimental measurements. The simulations in Figures 7 to 9 offer particularly compelling insights into the dominant role of dynamical processes on the ozone lowering event. The modeling indicated that dynamical processes were the largest factor leading to the formation of this ozone mini-hole.

The following minor corrections are needed:

(1) In the Discussion and Conclusions section, please clarify whether “The influence of processes on polar stratospheric clouds is less significant than the influence of gas-phase chemical reactions and heterogeneous processes on sulfate aerosol.” is intended to refer to dynamical processes or heterogeneous processes?

(2) In Figures 9b and 9b, the colors of the dotted and dashed lines do not match those of the corresponding solid curves. Please correct this and use the same color code scheme as was used in panel 9a for the dotted and dashed curves. Please make this same correction for Figures 10b and 10c, and for Figures 3A (b) and (c).

(3) Page 4, “Ocean Optics. Inc.” should be changed to “Ocean Optics, Inc.”

(4) Page 7, “mean over the whole winter season 20224-2025 till” should be “mean over the whole winter season 2024-2025 till”

(5) Please improve the image resolution of the figures to make them more clearly visible.

(6) In Figure 2a, the legend for OMI 2025 should indicate a blue line (not a black line).

(7) In Section 3.2, the two references to Figure 4 b, c, d, the reference to Figure 4e, the reference to Figure 4f, the reference to Figure 4g should be to Figure 3 b, c, d, to Figure 3e, to Figure 3f, and to Figure 3g, respectively.

Author Response

Authors are very grateful to all four anonymous reviewers for a careful reading and the all suggestions and recommendations. Our answers and changes in the text are marked by red color.

Below a new version of our manuscript is attached after Atmosphere’s language service.

Since some of the reviewers' questions relate to the same sections, sentences, and figures a common file with our answers is posted below.

Reviewer 2  Comments and Suggestions for Authors

The authors present a comprehensive study of ozone depletion over Moscow, St. Petersburg, and Helsinki during the 2024-2025 winter season. The authors analyzed multiple data sources (including satellite and ground-based sensors) to confirm that multiple measurements led to consistent conclusions about the ozone mini-hole that occurred over this region during the 2024-2025 winter season. The study was thorough and rigorous, and included simulation modeling as well as analyzing experimental measurements. The simulations in Figures 7 to 9 offer particularly compelling insights into the dominant role of dynamical processes on the ozone lowering event. The modeling indicated that dynamical processes were the largest factor leading to the formation of this ozone mini-hole.

The following minor corrections are needed:

(1) In the Discussion and Conclusions section, please clarify whether “The influence of processes on polar stratospheric clouds is less significant than the influence of gas-phase chemical reactions and heterogeneous processes on sulfate aerosol.” is intended to refer to dynamical processes or heterogeneous processes?

It is clarified that this statement is referred to heterogeneous chemical processes.

(2) In Figures 9b and 9b, the colors of the dotted and dashed lines do not match those of the corresponding solid curves. Please correct this and use the same color code scheme as was used in panel 9a for the dotted and dashed curves. Please make this same correction for Figures 10b and 10c, and for Figures 3A (b) and (c).

Figures 9, 10, and 3A were modified: the TCO, ClO, and NO values are presented as anomalies from January – March 2025 mean.   

(3) Page 4, “Ocean Optics. Inc.” should be changed to “Ocean Optics, Inc.”

Done

(4) Page 7, “mean over the whole winter season 20224-2025 till” should be “mean over the whole winter season 2024-2025 till”

Done

(5) Please improve the image resolution of the figures to make them more clearly visible.

Figures 1b, 2a,b, 3f were substituted to ones with higher resolution. Also figures 1a,c,b, 2d, 3f were modified.

(6) In Figure 2a, the legend for OMI 2025 should indicate a blue line (not a black line).

Done

(7) In Section 3.2, the two references to Figure 4 b, c, d, the reference to Figure 4e, the reference to Figure 4f, the reference to Figure 4g should be to Figure 3 b, c, d, to Figure 3e, to Figure 3f, and to Figure 3g, respectively.

Done

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Please see the attachment.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

As stated above and in the attached file.

Author Response

Authors are very grateful to all four anonymous reviewers for a careful reading and the all suggestions and recommendations. Our answers and changes in the text are marked by red color.

Below a new version of our manuscript is attached after Atmosphere’s language service.

Since some of the reviewers' questions relate to the same sections, sentences, and figures a common file with our answers is posted below.

Reviewer 3 

 This is the first-round review of the manuscript titled “A Study of the Low-Ozone Episode over Scandinavia and the northwestern Russia in March 2025” by P. Vargin and co-authors. It is a case study of a low ozone (O3) concentration over the Arctic at the beginning of the current year. The behavior of O3 as well as other chemical species was analyzed over three cities: Helsinki, Moscow and Saint Petersburg. The case study included the modelling of different scenarios in order to evaluate the role played by dynamical and chemical processes in the evolution of these species. This investigation has a potential interest to readers as it attempts to analyze an event with low O3 concentrations in the Arctic that had precedents with similar conditions in the past. All this said, the manuscript is not well organized and the English used throughout is in need of improvement. More specifically, the following points should be addressed before this manuscript could be considered for publication:

 

1. a) Abstract: It does not provide an accurate summary of the entire manuscript.

Abstract was edited. Estimation of some key findings was included.

1. b) Introduction: In general, the section is not written in a way that helps linking the background of the study topic with your aims. Please rework.

The first part of Introduction was decreased to highline the ozone mini holes formation and its effects on the surface UV radiation levels.

1. c) A similar O3 depletion event that took place in 2020 is mentioned several times but no comparisons between this event and the presented case study were made.

The following four sentences were included in the text after the Figure 1.

The Arctic ozone anomaly of the winter season 2019- 2020 was the strongest over the entire period of observations as a result of a strong and persistent the stratospheric polar vortex observed till the late March 2020. Due to the weakening of stratospheric polar vortex in the late February and especially in the early March 2025 as a result of the major SSW the minimal polar cap temperature rapidly increased and reached climate mean values in the late March 2025 whereas it was on 15-20 degrees less during March 2020 (Figure 1a). As a result of that the monthly mean PSC type I NAT volume values were significantly less in March 2025 in a comparison with March 2020 (Figure 1b). Therefore, due to weaker ozone destruction the monthly polar cap TCO mean was higher in March 2025 in a comparison with March 2020: 414 DU and 323 DU respectively (not shown).

 

1. d) I generally see the Results section as a mere description of the Figures with no added value (e.g. possible causes that lead to the results)

 

We are agree that most part of the Results section as a mere description of the figures. It is due to these figures aimed to illustrated using different data sets (ground-based observations, satellite measurements, and reanalysis data) revealed in the present study a rather unique TCO anomaly over the highly populated region of Russia and Finland. The possible dynamical causes (anticyclone and low ozone air transport along its the western periphery) are also shown. Further figures of model simulations show the estimates of contributed factors (dynamical vs chemical processes) in the low TCO values formation.  

 

In line with the foregoing considerations, and also taking into account some of the points included further down, I will require that this manuscript undergo major revisions. I’d like to emphasize on the major character of the revisions required in this case. This is a manuscript that is on the border of rejection, but I think the authors must have the opportunity to sort it out. A list of specific comments can be found below.

 

1. 20‒22: Please revise this sentence.

The temperature of the lower polar stratosphere increased that led to significant decrease of polar stratospheric clouds (PSC) type I (NAT) volume. =>

Increase of the lower polar stratosphere temperature led to significant decrease of polar stratospheric clouds (PSC) type I (NAT) volume.

 

37. 36‒37. I’d split the list of keywords so that it includes “ground-based ozone observations” and “satellite ozone observations.”

Done

 

1. 40‒48: Please provide references to support these claims.

The following two references were added:

1. World Meteorological Organization (WMO), Scientific Assessment of Ozone Depletion: 2022, GAW Report 278, 509 pp., WMO, Geneva, 2022
2. Baldwin, M., Birner, T., Brasseur, G., Burrows, J., Butchart, N., Garcia, R., et al. 100 Years of Progress in Understanding the Stratosphere and Mesosphere. Monogr. 2019. 59, 27.1–27.61. https://doi.org/10.1175/AMSMONOGRAPHS-D-19-0003.1.

 

1. 53: “Over all the years of observations...” Please specify the observation period.

Over all the years of observations, the greatest depletion of the Arctic ozone layer was observed in the spring of 2020, e.g., …. =>

Since the beginning of the 1980s the greatest depletion of the Arctic ozone layer was observed …..

 

1. 61‒64: Please revise the grammar in this paragraph. Separately, please consider merging it into the previous paragraph to improve text flow. Overall, there are stray paragraphs across the Introduction. Please consider merging.

Done, two sentences were rewritten and included in the previous paragraph.

 Also, four other paragraphs in the Introduction with the first sentences “Recovery of the global ozone layer….” and “However, due to a decrease in the temperature of the stratosphere caused by continuing …”

and

“As a result of ongoing climate warming, the likelihood of longer blocking anticyclones increases…” and “Thus, given the vital importance of the ozone layer…..”  were merged.

 

1. 66: Are you attempting to compare 2020 with 2011? Please clarify in the text.

The following sentence was rewritten.

Before 2020, the greatest Arctic ozone depletion was observed in the spring of 2011 [9 Manney et al., 2011]. Importantly, under conditions of comparable strong Arctic ozone depletion to the spring of 2011, increased UV radiation levels may persist in the mid - high latitudes of the Northern Hemisphere until the summer months inclusive [10 Karpechko et al., 2013]. =>

Importantly, under conditions of strong Arctic ozone depletion to the spring of 2011 [9 Manney et al., 2011] (the largest one observed before 2020), increased UV radiation levels may persist in the mid - high latitudes of the Northern Hemisphere until the summer months inclusive [10 Karpechko et al., 2013].

 

1. 96: What are the units these values are expressed in?

For this OMH, it was shown that ozone depletion did not have a significant impact on surface UV radiation levels (primarily due to low solar zenith angles) [25 Chubarova et al., 2019].

For this OMH, it was shown that ozone depletion did not have a significant impact on surface UV radiation levels (estimated as UV index) primarily due to low solar zenith angles [25 Chubarova et al., 2019].

 

1. 134‒138: This sentence is too long. Consider splitting.

Done.

The present study is aimed to analyze the features of the Arctic stratosphere circulation in the winter season of 2024-2025 and to study the dynamic and chemical processes responsible for the formation of a significant ozone anomaly formed in early March over northwestern Russia and Scandinavia based on the analysis of satellite and ground-based observations, as well as numerical modeling.  =>

The present study is aimed to analyze the features of the Arctic stratosphere circulation in the winter season of 2024-2025 and to study the dynamic and chemical processes responsible for the formation of a significant ozone anomaly formed in early March over northwestern Russia and Scandinavia.

 

1. 141‒146: These lines are too short for describing the two reanalysis datasets. Please expand accordingly.

The following two sentences were included in the sub-section 2.1:

The horizontal resolution of NCEP reanalysis is 2.5⁰ ×2.5⁰, the upper boundary is at the pressure level 10 hPa (~30 km). The horizontal resolution of ERA5 reanalysis is 0.25⁰ × 0.25⁰, the upper boundary is at the pressure level 1 hPa (~ km).

 

1. 143‒145: Unintelligible.

 

The following two sentences were rewritten:

 

Large-scale dynamical processes were analyzed using daily data of NCEP [37 Kalnay et al., 1996] and ERA5 reanalysis [38 Hersbach et al., 2020]. =>

Dynamical processes in the extratropical boreal troposphere and stratosphere that can impact the ozone layer were analyzed using daily data of NCEP [37 Kalnay et al., 1996] and ERA5 reanalysis [38 Hersbach et al., 2020].

 

Know that such processes including main aspects of SSW events are represented comparable in modern reanalysis [37 Ayarzagüena et al., 2019]. =>

An overall good agreement across modern reanalyses was found in the representation of the most abrupt phenomena of the boreal winter time stratospheric circulation variability and with a strong impact on ozone layer - SSW events [37 Ayarzagüena et al., 2019].

 

The next sentence “A good agreement among reanalyses was also found for triggering mechanisms, tropospheric precursors, and surface response of SSWs” was deleted.

 

4. 193: The first figure to be called in the text is Figure 1, not Figure 4. Figures must be called in numerical order, from the lowest to the highest. Please correct this across the manuscript.

The results of DOAS measurements of total ozone content in the St. Petersburg region in January-March 2025 are shown in Figure 4 =>

The results of DOAS measurements of total ozone content in the St. Petersburg region in January-March 2025 are analyzed.

 

1. 203: Please remove the word “Obviously.” What is obvious to you cannot be that obvious to others.

Done

 

1. 216‒217: Where do these values come from? Please add references to them.

The following two references were added:

Dorokhov, V.M., Ivlev, G.A., Privalov, V.I. et al. Technical equipment of ground-based stations for total ozone measurements in Russia and prospects of modernization. Atmos. Ocean Opt. 2014, 27, 566–572. https://doi.org/10.1134/S1024856014060050

Guide to Instruments and Methods of Observation. Volume I – Measurement of Meteorological Variables, WMO-No.8, 2018. https://community.wmo.int/en/activity-areas/imop/wmo-no_8

1. 222‒293: In general, there is a much better description in this part of the methodology than in the previous lines. The entire section needs to be as precise as possible so that your results could be reproduced.

 

The information on spatial resolution of employed reanalysis data sets was included in the section 2.2. We suppose that our results of data analysis could be easily reproduced using the description of data sets and employed methods of analysis.

The section 2.3 (Chemistry-Transport Model Experiments) and section 2.4 (Surface UV radiation) also contain the description of performed model simulations.  The section 2.4 also contains a link to the open on-line model employed to estimate UV radiation changes caused by low ozone.

 

1. 226: What reanalysis data? You said you employed both the ERA5 and the NCEP/NCAR datasets. Please clarify.

 

Separately, did you follow [43] to do this? If so, you should state this explicitly. The way it is written, it seems the meteorological fields used in your study came from [43].

 

The corresponding sentence in the beginning of the sub-section 2.3 was modified as follows:

 

The temporal evolution of the Arctic stratospheric gases was simulated using meteorological fields from Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalysis data [49 Gelaro et al., 2017] as in [47 Smyshlyaev et al., 2020].

 

1. 299‒302: Please add references to support these assertions.

Done

The Arctic is characterized by PSCs type I (NAT), formed at temperatures below about 195 K (-78°C), e.g., [Zhang et al., 2025].

Over the past decades, the most severe ozone depletion in the Arctic occurred in the following winter seasons: 1996-97, 2010-11, 2015-16 and the record-breaking 2019-2020 e.g., [Manney et al., 2011; Manney et al., 2020; WMO 2022].

 

1. 302: The northern winter season must be defined somewhere.

The sentence “The winter circulation in the Arctic stratosphere persists usually till the late March, therefore this period is often called as “winter season.” was included in the Introduction.

 

1. 306: Please describe the content of Figure 1 (as you did in L. 470‒472 with Figure 6) before attempting to call to any of its panels. The same applies to the rest of the figures in the manuscript.

 

1. The following sentence “The Figure 1 displays Arctic minimum lower temperature variability in the winter season of 2024-2025 in comparison with the other winters with the strong ozone layer depletion and values of Vpsc type I (NAT) and Vpsc type II (ICE)” was included before a discussion of Figure 1 panels.

 

Additionally, we change the order of Figure 1a and 1b

 

2. The sentence “Characteristics of the ozone layer anomaly over St. Petersburg, Moscow, and Helsinki in early March 2025 are discussed further using the satellite and ERA5 reanalysis TCO data (Figure 2).” was included in the beginning of sub-section 3.2 Ozone layer
3. The sentence “A comparison of ground-based ozone measurements and satellite ozone measurements is presented on Figure 5” was included in the beginning of sub-section Filter ozonometer M-124 observations
4. A short information on Figure 6 was slightly modified

The Comparison of the results of the basic PSC experiment with calculations under noCHEM and noPSC scenarios over St. Petersburg is shown on Figure 6.

 

1. 309: 2024-2025

Done

 

1. 311: The vertical scale in Figure 1b) is in K, whereas the text describes ‒83°C. Please homogenise the units across the manuscript.

Done. The following sentence was included in the 1^st paragraph of Introduction:

“The temperature threshold values for the PSC type I and type II formation are about 195 K (-78 ⁰C) and 188 K (-83 ⁰C) respectively”.

Also, two following sentences were modified in the subsection 3.1:

“The Arctic is characterized by PSCs type I (NAT), formed at temperatures below about 195 K (-78°C).

The lowest minimum temperature was observed in early February 2025 when it dropped below 188 K (-83⁰ C) on about a week.”

 

1. 320‒325: The horizontal axes in panels (c) and (d) are unlabelled. Please check the rest of the figures for this very issue. Separately, what does the “MAX” curve represent and how was it calculated? Please explain.

The following two sentences was included in the Figure 1 caption:

Absсisa axis here and hereafter corresponds to dates in the format day/mon.

“MAX” curve corresponds to maximal values of Vpsc NAT and Vpsc Ice values over the period from 1979 to 2024.

 

1. 326‒L.468: The reasons for presenting many alternative descriptions of the O3 evolution of ozone is not clear.

The evolution of ozone associated with the investigated ozone mini-hole is shown using an available ground-based measurements (M124 & DAOS), satellite observations (OMI) and ERA5 reanalysis data to avoid a possible question on persistence of the observed low ozone values as well as its comparability.

 

2. 327‒331: Data description does not belong in this section. Where O3 measurements were obtained from is a matter for Section 2.

1^st sentence of the sub-section 3.2 was rewritten:

Analysis of TCO measurement data from the TOMS instrument (8th version, overpass data product) installed on the Nimbus-7 and Earth Probe satellites for the period from 1979 to 1993, GOME instrument on ERS-2 and MetOp satellites (1995-2010) and OMI instrument on Aura satellite (2002-2025) showed that the lowest spring TCO values of about 220 DU were observed in St. Petersburg in early March 2025 (Figure 2a).

=>

Analysis of TCO measurement data from the TOMS instrument for the period from 1979 to 1993, GOME instrument (1995-2010) and OMI instrument (2002-2025) showed that the lowest spring TCO values of about 220 DU were observed in St. Petersburg in early March 2025 (Figure 2a).

 

Also, the information on total ozone satellite data were slightly extended in the sub-section 2.2.

 

1. 337‒339: You do not show standard deviations. Did you calculate this? If so, please state it. If not, please reference it.

The standard deviation was estimated for TCO anomaly in the OMI satellite measurement data for the period of the lowest ozone values and described in the subsection 3.2

Lines 337-339 (in our initial submission)

The TCO anomaly for St. Petersburg and Moscow according to OMI data on March 6, 2025 was ~40% or ~3 standard deviation units relative to the average values ​​for the period 1978-1988.

 

Additionally, the following sentence in the Discussion was modified as follows:

This study analyzes the ozone anomaly formed over Scandinavia and the northwestern Russia in March 2025 with minimum TCO values ​​were near ~40% or ~3 standard deviation units relative to the average values ​​for the period 1978-1988 and close to the accepted conventional threshold of the ozone hole in Antarctica at 220 DU.

 

1. 370: Figure 3a shows the entire atmosphere from surface to 1 hPa, not just the “troposphere-stratosphere.” Regarding the latter term I guess you meant to say the upper troposphere-lower stratosphere (UTLS), which is by far more appropriate to your study. Following this, the limits of the UTLS should be defined a priori, via the position of the tropopause or other related metric.

 

Certainly, this range of heights also covers a lower mesosphere, but we don’t sure that it is necessary to mention here. 

 

The relevant sentence was amended as follows:

Figure 3a shows a deviation of geopotential height from zonal mean from surface to the pressure level of 1 hPa (~50 km) that includes the heights of troposphere – stratosphere over St. Petersburg in January – March 2025.

 

We agree that upper troposphere-lower stratosphere (UTLS) is well-known term and might be also appropriate for our study. However, our two altitude-time cross-sections (Figure 3a and 3f) show the changes of geopotential height deviation from zonal mean and ozone mixing ratio not only in the UTLS but at the higher levels up to the upper stratosphere. 

 

1. 380‒408: Inexistent panels of Figure 4 are called in these paragraphs, please correct this.

Done

 

1. 526: “Figure 8a” should be moved to the end of the sentence. Results for Helsinki are shown in Figure 8, and you are describing only one panel of it.

Done.

The agreement between the calculations for the PSC baseline scenario and the OMI measurement data and MERRA2 reanalysis for Helsinki is slightly better than for St. Petersburg, which is close in distance (~300 km), while the same features of underestimation of the variation amplitudes are also observed (Figure 8a).

 

10. 538‒568: The text describes what is seen in the different panels of Figures 9 and 10. To what potential causes are the differences ascribed?

 

1. 540: Missing units

Done.

The main fluctuations in ozone are observed in the lower stratosphere in the altitude range of 10-20 km (green line).

 

1. 561: Figures 9 and 10 would be better presented if only the differences were included.

Figures 9, 10, and 3A were modified: the TCO, ClO, and NO column values are presented as anomalies from January – March 2025 mean.   

1. 626: Were the potential values shown here compared with the actual values on this date? If not, this is a useless experiment.

 

The potential UV indices estimation on April 6 under conditions of a low TCO values caused by a similar potential ozone mini hole is aimed to analyze a possible UV increase on one month later than on March 6, 2025.

It is assumed that such an event in the future cannot be excluded. Also obtained estimates of UV radiation increase are interesting and important as their values exceed a moderate category and require the adoption of restrictive and protective measures to avoid the negative impact of increased levels of UV radiation on human health.

 

A comparison of potential UV increase with the observed values on April 6, 2025 is beyond the framework of our study.

1. 685‒713: Some of the points in these lines repeat the information included in the foregoing lines of the same section. Please remove the duplicated material. On the other hand, I insist on the lack of potential causes for your results, which is the main aim of the Discussion section. Please expand this accordingly.

1) The following sentence was removed from the beginning of the Discussion section: “According to satellite observations since 1979 the lowest TCO values of 221 DU were detected over St. Petersburg on March 6, 2025. The very low values of 204 DU and 242 DU were revealed in OMI satellite observations over Helsinki and Moscow respectively.”   Several other sentences before conclusions were shortened.

2) The following sentences “Moreover, the Arctic stratosphere has shown a cooling trend due to the increase of carbon dioxide in recent years, e.g., [Smith, Polvani, 2014] that may enhance ozone depletion”

and

“Decrease of TCO values due to tropopause elevation and transport of low ozone air masses along the anticyclone western periphery as well as chemical ozone loss in the polar lower stratosphere were the main responsible processes for the ozone mini-hole formation.”

were added respectively after the second paragraph and before the conclusion of model simulations in the Discussion section.

1. 693: Please add references to support this claim.

Done, the sentence “Usually, this type of PSCs is observed only in Antarctica” was changed to “This type of PSCs is the rarest in Arctica in comparison to the PSC NAT, e.g., [Zhang et al., 2025]. “

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

Dear Editor,

Thank you for the opportunity to review the manuscript titled “A Study of the Low-Ozone Episode over Scandinavia and the northwestern Russia in March 2025” (Manuscript ID: atmosphere-3702833). This study analyzed the relationship among the polar vortex, UV, and ozone. The idea is innovative in integrating remote sensing data, ground-level monitoring, and modeling to estimate and understand important factors that impact total column ozone in three important cities: St. Petersburg, Moscow, and Helsinki. However, several substantial concerns must be addressed before the manuscript can be considered for publication.

General Comments:

1. Language and Sentence Structure

The manuscript requires substantial editing to improve clarity, flow, and grammar. Many sentences are unclear or incomplete, which makes it difficult to follow the authors' ideas. For example, Line 574–576:

“So, we analyzed UV changes, due to reduced TCO over northwestern Russia and Scandinavia during the January–March period, 2025.”

This is not a complete sentence and should be restructured for clarity. Similar issues can be found in other sections of the manuscript.

Additionally, some expressions are informal or uncommon in peer-reviewed scientific journals. For instance, Line 585:

“One can see a significant drop...” would be better stated as “A significant drop was observed...”

The stance can be improved to be clearer and have a more scientific tone. Similar issues can be found in other places this manuscript.

2. Quality of figures needs better resolution.

Almost all figures are blurred and do not have a clear legend. I suggest that the authors provide better resolution for the figures.  

I recommend a major revision focused on improving the language for the whole article.

 

Author Response

Authors are very grateful to all four anonymous reviewers for a careful reading and the all suggestions and recommendations. Our answers and changes in the text are marked by red color.

Below a new version of our manuscript is attached after Atmosphere’s language service..

Since some of the reviewers' questions relate to the same sections, sentences, and figures a common file with our answers is posted below.

Reviewer 4 

Dear Editor,

Thank you for the opportunity to review the manuscript titled “A Study of the Low-Ozone Episode over Scandinavia and the northwestern Russia in March 2025” (Manuscript ID: atmosphere-3702833). This study analyzed the relationship among the polar vortex, UV, and ozone. The idea is innovative in integrating remote sensing data, ground-level monitoring, and modeling to estimate and understand important factors that impact total column ozone in three important cities: St. Petersburg, Moscow, and Helsinki. However, several substantial concerns must be addressed before the manuscript can be considered for publication.

General Comments:

1. Language and Sentence Structure

The manuscript requires substantial editing to improve clarity, flow, and grammar. Many sentences are unclear or incomplete, which makes it difficult to follow the authors' ideas. For example, Line 574–576:

“So, we analyzed UV changes, due to reduced TCO over northwestern Russia and Scandinavia during the January–March period, 2025.” =>  Further UV changes associated with reduced TCO over northwestern Russia and Scandinavia in the late winter season 2024-2025 are studied.

This is not a complete sentence and should be restructured for clarity. Similar issues can be found in other sections of the manuscript.

Additionally, some expressions are informal or uncommon in peer-reviewed scientific journals. For instance, Line 585:   “One can see a significant drop...” would be better stated as “A significant drop was observed...”

The following sentence was added: “A significant drop was observed in ozone level during the mini hole events during all three months however, the most pronounced reduction was observed in March”.

The stance can be improved to be clearer and have a more scientific tone. Similar issues can be found in other places this manuscript.

1. Quality of figures needs better resolution.

Almost all figures are blurred and do not have a clear legend. I suggest that the authors provide better resolution for the figures.  

Figures 1b, 2a,b, 3f were substituted to ones with higher resolution. Also figure 1a,c,b,d, 2d, 3f, 9,10, and A3 were modified.

I recommend a major revision focused on improving the language for the whole article.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

References: Add a few more recent citations, especially from 2020 to 2025. But still the author added the year 2006 reference as mentioned in the response file. See below. 

"Orsolini, Y.J. and Nikulin, G.  A low-ozone episode during the European heatwave of August 2003. Q.J.R. Meteorol. Soc. 2006, 132: 667-680. https://doi.org/10.1256/qj.05.30"

Author Response

The authors are very grateful to all Reviewers for their comments and suggestions. 

Reviewer 1

References: Add a few more recent citations, especially from 2020 to 2025. But still the author added the year 2006 reference as mentioned in the response file. See below. 

"Orsolini, Y.J. and Nikulin, G.  A low-ozone episode during the European heatwave of August 2003. Q.J.R. Meteorol. Soc. 2006, 132: 667-680. https://doi.org/10.1256/qj.05.30"

The following reference was added:

Sofiev, M., Kouznetsov, R., Hänninen, R., and Sofieva, V. F.: Technical note: Intermittent reduction of the stratospheric ozone over northern Europe caused by a storm in the Atlantic Ocean, Atmos. Chem. Phys., 2020, 20, 1839–1847, https://doi.org/10.5194/acp-20-1839-2020.

Our answers to all other Reviewers is attached below.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

I read the authors' replies to my comments and suggestions, and found that they addressed most of them satisfactorily. However, I saw no response to my comment Re: L. 538‒568: "The text describes what is seen in the different panels of Figures 9 and 10. To what potential causes are the differences ascribed?" This was related to my request for providing possible causes for the presented results. On the other hand, I expected that all the necessary clarifications be included in the manuscript and not just in the author's replies. I ask the authors to check that this was done for all of my concerns requiring a clarification.

Author Response

Reviewer 3

I read the authors' replies to my comments and suggestions, and found that they addressed most of them satisfactorily.

However, I saw no response to my comment Re: L. 538‒568: "The text describes what is seen in the different panels of Figures 9 and 10. To what potential causes are the differences ascribed?" This was related to my request for providing possible causes for the presented results. On the other hand, I expected that all the necessary clarifications be included in the manuscript and not just in the author's replies. I ask the authors to check that this was done for all of my concerns requiring a clarification.

The following three paragraphs were included before the Figure 9 (instead of previous 3 paragraphs):

Ozone values across all altitude ranges over St. Petersburg in January-February show minor difference for three different scenarios (Fig. 9a), indicating the predominant influence of atmospheric dynamics and a minor influence of chemistry. This is further supported by the fact that there is an increase in ClO at altitudes of 20-30 km in January in the PSC scenario compared to other scenarios (Fig. 9c). Ozone levels at these altitudes in the PSC scenario are even slightly higher than in scenarios without PSC and chemistry (Fig. 9a). At altitudes of 10-20 km, and also in February at altitudes of 20-30 km, the ClO values for the PSC scenario vary sometimes being higher and sometimes lower than average. However, for altitudes of 10-20 km in February the ClO values are consistently below average. In contrast, for scenarios without PSC, ClO values are close to average, but this has practically no effect on the difference in ozone between scenarios (Fig. 9a). This confirms the predominant influence of dynamics on ozone variability over St. Petersburg in January-February. Furthermore, the minima and maxima of ozone and ClO (in the PSC scenario) changes generally align well, indicating the dominant influence of atmospheric transport on their variability. In January-February nitrogen oxides (Fig. 9c), typically show values below average with low day-to-day variability.

In March, ClO values at altitudes of 10-20 km increase significantly above average values. In the PSC scenario, high values persist until the end of the month, while in the scenario without PSC, high ClO values are only registered in early March, coinciding with the time when the minimum total ozone values were recorded (Fig. 9a). Consequently, in mid-March processes involving polar stratospheric clouds play a significant role along with dynamics at altitudes of 10-20 km. At altitudes of 20-30 km, In the PSC scenario at altitudes of 20-30 km ClO steadily decreases in March and remains below average values. This indicates that at these altitudes, ozone variability is not related to chlorine catalytic destruction. Conversely, nitrogen oxide content at these altitudes is significantly above average in March for scenarios with chemistry, creating potential for ozone destruction in nitrogen catalytic cycles.

In Moscow (Fig. 10), the main features of ozone, chlorine, and nitrogen variability characteristic of St. Petersburg are maintained. Atmospheric dynamics play a dominant role in January and February, but March has its own unique characteristics. For both the PSC and NO PSC scenarios, maximum ClO values are observed in early March, However, in the middle and latter half of the month, ClO values drop sharply for both scenarios. This suggests that ozone depletion potential is created through chemical reactions involving PSCs and without PSCs. Unlike in St. Petersburg, ozone values at altitudes of 10-20 km in Moscow show no differences between these scenarios (Fig. 10a). This indicates that in Moscow, the dynamic processes have an even more significant influence than in St. Petersburg.

Our answers to all other Reviewers is attached below.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

• Citation style correction
  The citation formats used in the text are inconsistent and not in the appropriate style.
  For example:
• Line 698: “see, e.g., [1-2 Baldwin et al., 2019; WMO 2022]”
• Line 704: “see, e.g., [1, 59 Smith, Polvani, 2014; WMO 2022]”

If the authors are using APA style, the Line 698 example should be written as:
see, e.g., Baldwin et al. (2019) and WMO (2022)

Please select one citation style and apply it consistently throughout the manuscript for all in-text literature citations.

• Introduction flow
  While the introduction has improved, the flow is still difficult to follow in places. Please consider consulting a native English editor to further improve the clarity and logical progression of the introduction.
• Figure quality
  The resolution of several figures needs improvement. Some figures remain blurry, and the legends in Figures 2a and 2b are still unreadable. Please update Figures 2, 4, and 5 to ensure high resolution and legible text.

 

Author Response

The authors are very grateful to all Reviewers for their comments and suggestions.

Reviewer 4

• Citation style correction
  The citation formats used in the text are inconsistent and not in the appropriate style.
  For example:
• Line 698: “see, e.g., [1-2 Baldwin et al., 2019; WMO 2022]”
• Line 704: “see, e.g., [1, 59 Smith, Polvani, 2014; WMO 2022]”

If the authors are using APA style, the Line 698 example should be written as:
see, e.g., Baldwin et al. (2019) and WMO (2022)

Please select one citation style and apply it consistently throughout the manuscript for all in-text literature citations.

All citation formats in the text was modified according to Atmosphere recommendation

• Introduction flow
  While the introduction has improved, the flow is still difficult to follow in places. Please consider consulting a native English editor to further improve the clarity and logical progression of the introduction.

The following sentences were modified: 

 

1. If the low temperature of the polar lower stratosphere persists until the spring (when solar irradiance begins to penetrate the polar stratosphere after the polar night), chemical destruction of the ozone layer occurs, which can lead to a significant decrease in the total ozone content [1]. =>

If the low temperature of the polar lower stratosphere persists until the spring (when solar irradiance begins to penetrate the polar stratosphere), chemical destruction of the ozone occurs, which can lead to a significant decrease in the total ozone content [1].

 

2. Despite this, due to the low solar zenith angles, elevated levels of UVR remained at the average safe values. However, in spring after a long winter, the human skin becomes more sensitive to increased UVR and, therefore, even a relatively small increase in UVR during this season must be taken into account. =>

Since solar zenith angles were low, UVR levels stayed within average safe limits. However, after winter, skin is more sensitive to UVR, so even slight increases in spring should be considered.

 

3. It was shown that ozone depletion did not have a significant impact on surface UVR levels (estimated as UV index) in the case of this OMH, primarily due to low solar zenith angles [25]. =>

It was shown that ozone depletion did not have a significant impact on surface UVR levels in the case of this OMH, primarily due to low solar zenith angles [25].

 

4. However, dynamic conditions may be conducive to strong ozone destruction in the Arctic for many years, even as far as the late 21st century. This is due to a decrease in the temperature of the stratosphere caused by the continuous increase in greenhouse gas concentrations [12-14]. =>

However, dynamic conditions may be conducive to strong ozone destruction in the Arctic as far as the late 21st century due to a decrease in the temperature of the stratosphere caused by the continuous increase in greenhouse gas concentrations [12-14].

 

5. The aim of the present study was to analyze the features of the Arctic stratosphere circulation in the winter season of 2024-2025, and to study the dynamic and chemical processes responsible for the formation of a significant ozone anomaly formed in early March over northwestern Russia and Scandinavia. =>

The aim of the present study was to analyze the features of the Arctic stratosphere circulation in the winter season of 2024-2025, and to study the dynamic and chemical processes responsible for the formation of a significant ozone anomaly noted above.

6. The main advantage of this work is the investigation of the role of dynamic and chemical processes in the formation of a significant ozone anomaly in early March 2025, characterized by the second lowest TCO value for Moscow since 1986 and the first one for St. Petersburg since the early 1980s. =>

In this paper, we investigate for the first time the role of dynamic and chemical processes in the formation of a significant ozone anomaly in early March 2025, characterized by the second lowest TCO value for Moscow since 1986 and the first one for St. Petersburg since the early 1980s.

 

7. Furthermore, to the best of our knowledge, this is the first study to present the association between such strong ozone mini-hole and an estimated increase in UVR. =>  deleted

 

8. These very low TCO values were only slightly above the generally accepted conventional threshold of the ozone hole at 220 DU, which is periodically observed in the Antarctic stratosphere and much more rarely in the Arctic stratosphere. => deleted

 

9. The aim of the present study was to analyze the features of the Arctic stratosphere circulation in the winter season of 2024-2025, and to study the dynamic and chemical processes responsible for the formation of a significant ozone anomaly noted above. => deleted

 

10. In addition, the impact of this ozone layer anomaly on surface UVR levels over St. Petersburg, Moscow and Helsinki in comparison to climate mean values was investigated. => was moved down on several lines

 

11. The obtained results on parameters of the low-ozone episode over northwestern Russia and Scandinavia in March 2025 obtained from satellite observations and ground-based measurements revealed dynamic processes in the extratropical northern stratosphere in the 2024–2025 winter season, including formation of the minor and major SSW events in late February and in early March, respectively. Surface UVR changes are described in Section 3 =>

The obtained results on parameters of the low-ozone episode in March 2025, revealed dynamic processes in the Arctic stratosphere, including formation of the minor and major SSW events in February - March 2025, and surface UVR changes are described in Section 3.

 

• Figure quality
  The resolution of several figures needs improvement. Some figures remain blurry, and the legends in Figures 2a and 2b are still unreadable. Please update Figures 2, 4, and 5 to ensure high resolution and legible text.

Figures 2a,b, 4, 5a,b were modified (including the size of legends was increased)

Our answers to all other Reviewers is attached below.

Author Response File: Author Response.docx