the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 May 2026
Measurement report: Significant ozone loss during winter 2020 measured from ground based microwave radiometer
Abstract. Ground-based microwave observations from MIRA2 situated in Kiruna, Sweden, were used to investigate Arctic stratospheric ozone during the winter 2019/2020. A comparison of O3 retrievals with coincident measurements of Aura MLS between 1 October 2019 and 30 April 2020 shows good agreement across the investigated pressure levels (74, 56, 46, and 10 hPa). Remaining differences are well within the retrieval uncertainty of MIRA2. This demonstrates the capability of MIRA2 to provide robust ozone measurements for studies of stratospheric variability.
A tracer-based approach was applied to derive cumulative chemical ozone loss on isentropic surfaces. At the 475 K level, ozone depletion increased from late winter into early spring, reaching a maximum loss of 2.19±0.90 ppmv in early April 2020. The magnitude and timing of the loss are consistent with the exceptional Arctic ozone depletion stated by model simulations and satellite-based estimates during the winter 2019/2020 and agree well. Despite limited temporal sampling, the tracer-based method enables a consistent estimate of seasonal ozone loss from ground-based observations. The results highlight the ability of ground-based microwave radiometers to quantify chemical ozone depletion and its temporal evolution.
This study also demonstrates the value of ground-based measurements as an independent and complementary component of the global observing system, providing continuity and reducing reliance on satellite data for monitoring stratospheric ozone.
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Status: open (until 19 Jun 2026)
- RC1: 'Comment on egusphere-2026-2216', Anonymous Referee #1, 17 May 2026 reply
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RC2: 'Comment on egusphere-2026-2216', Chris Boone, 28 May 2026
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This paper shows interesting results from ground-based microwave (MIRA2) measurements of O3, using atmospheric tracer information (in the form of N2O) from MLS to derive catalytic ozone losses within the polar vortex. Overall, the study is clearly described and is worthy of publication. I had a relatively minor issue with some of the discussion regarding things happening inside the polar vortex while MIRA2 was measuring outside the vortex, where the discussion seemed to imply that the things happening inside the vortex were shown in the figures, when they were not, as I will address later. I would also have liked a discussion of how the ozone loss analysis approach might work once MLS reaches end of life and its N2O measurements are no longer available to use as a tracer.
Perhaps there could be a few more words regarding details of the data set. What is the sampling time? Do measurements require extensive averaging? Are measurements taken continuously, day and night, or are only a few measurements collected per day? Continuous operation would improve the chances of a satellite mission finding a good coincidence, so that might be useful information. Observations in the paper are distilled down to focus solely on coincidences with MLS. I assume the data set is more extensive than that. I could browse through the data to see what it held, but it would be helpful to provide a sense of that in the paper describing the measurement set. If the data consisted of relatively short measurement times and a variety of elevation angles, one might consider analyzing multiple measurements simultaneously to potentially improve the altitude resolution. Conversely, if measurement times are long and you don’t have multiple observations over a relatively short period of time at a variety of elevation angles, that might not be a viable option. From the paper itself, without browsing the data, I can’t tell what the sampling or extent of the data set might be.
You might consider saying “Arctic winter” in the title rather than just “winter.”
>Line 77: In the above mentioned frequency range, observation of HNO3, N2O, ClO and O3 is possible. However, due to instrumental limitations MIRA2 has been operated continuously in ozone observation mode only.
As discussed several times in the article, MLS is not expected to continue producing data for much longer. The ozone loss analysis is completely reliant on an outside source of coincident measurements of O3 and at least one atmospheric tracer (like N2O). Without that information, it is not possible to separate chemical losses from transport effects. However, the above statement seems to imply that the microwave radiometer itself could also measure N2O, which would remove the need for an outside source of measurements from MLS. It wasn’t clear to me what the instrumental limitations were. Is it possible to have a self-contained ground-based instrument that could derive ozone loss using its own set of measurements (and have ClO on top of that, to infer chlorine processing)? I wondered why this capability was mentioned but never explored.
>Line 261: Measurements within the polar vortex are indicated by red crosses, and white markers denote dates when MIRA2 sampled vortex air masses.
In Figure 5, there are red squares (not crosses) and no white markers that I can see.
>Line 276: During February and March, these periods also show enhanced ClO volume mixing ratios
In Figure 5, there are no enhancements in ClO seen in March, but judging from the red squares the measurements in March are outside the polar vortex. The discussion appears to strongly imply that Figure 5 shows evidence of chlorine activation in March, but it does not because it reports MLS ClO coincident with MIRA2, which is measuring outside the vortex, where ClO levels are close to zero.
>Line 283: The air masses sampled by MIRA2 have undergone substantial depletion during the period of peak ClO activation in mid-March 2020
My understanding is that one typically refers to chlorine activation, not ClO activation. Again, nothing presented in this paper substantiates that ClO inside the vortex peaked in March. It would be advisable to add a figure showing ClO measured by MLS within the polar vortex (not outside the polar vortex in March, like Figure 5) or provide a reference to a paper where that information has explicitly been shown.
The idea is that the Arctic polar vortex was (as usual) not stationary, as mentioned in Section 3.3. The ground-based site was sometimes inside and sometimes outside the vortex because the vortex was moving. During March 2020, the MIRA2 site was outside the vortex, at which time significant chlorine processing destroyed ozone inside the vortex, but that was “off-stage” to MIRA2 with its measurements being outside the region of ozone destruction. In April, the vortex moved such that the MIRA2 site was once again back inside, at which point the MIRA2 measurements saw the aftermath of the chlorine processing, reduced ozone levels. This is addressed to some extent in the text, but there is also discussion of things happening off-stage as though the figures are showing information on chlorine processing in March, which they are not. It would be good to have a clearer distinction in the text as to what relates to the figures and data provided in the paper, and what relates to “outside” knowledge (like ClO peaking in mid-March).
>Line 285: By early April, ClO has largely been deactivated
Again, my understanding is that it would be phrased chlorine deactivation, not ClO deactivation.
Minor (mostly wording) issues
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Should the Abstract perhaps be a single paragraph?
>Line 20: During the second half of the 20th century holes in the ozone layer began to form due to Ozone Depleting Substance (ODS)
The wording here makes me picture a Swiss-cheese type atmosphere with persistent holes. The ozone hole forms in the Antarctic during Southern Hemisphere winter/spring but goes away when the polar vortex breaks up. There was also overall thinning of the ozone layer globally. I do not get that picture from this statement.
>Line 22: The absolute majority of the ODS gasses does not have a natural origin
The word “absolute” seems unnecessary
>Line 23: This prompted the Montreal Protocol, banning industries and other actors to produce ODS
The Montreal Protocol did not ban production; it gradually phased it out.
>Line 58: denitrification through sedimentation of HNO3 reduces reactive nitrogen
The wording seemed odd to me, in that it is not gaseous HNO3 that is sedimenting. I would instead say something like sedimentation of HNO3-containing PSC particles. That is probably just personal preference.
>Line 98: observations at a given site may both sample air inside the vortex, within the vortex edge region, or outside the vortex altogether
I would remove the word “both.” It implies two options, but three are given.
Citation: https://doi.org/10.5194/egusphere-2026-2216-RC2
Data sets
MIRA2 measurement and retrieval data from 20191001-20200501 R. Johansson, U. Raffalski, and J. Groß https://zenodo.org/records/19608988
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The paper describes stratospheric ozone profile data obtained in winter 2019/2020 by a microwave radiometer at Kiruna, northern Sweden. The ground-based microwave data are compared with and supplemented by data from the satellite-based microwave limb sounder instrument. Ozone profiles from both instruments compare well over the pressure / altitude range from 70 hPa to 1 hpa. Using a tracer relationship between O3 and N2O from the MLS data, the authors then compare observed O3 with passive O3 from the tracer relation. This provides estimates of the large chemical ozone loss that occured over the Arctic in the unusually cold stratopsheric winter of 2019/2020. The derived ozone loss is consistent with previous studies.
While there are no major new findings this is still a well written paper that fits into the scope of ACP. I recommend publication after a few generally minor revisions.
My most major comment is about lack of information on how the passive ozone time series was derived. I think it is necessary to provide a plot (scatter plot?) of the underlying ozone vs. N2O data from MLS, before or after Fig. 5. More statements about the time period for the used relation, and how this would change over time is also needed. In Fig. 5 it would be good to also see the passive ozone plotted.
Also, in various places in the manuscript, the authors claim that MIRA data can independently quantify chemical ozone depletion and its temporal evolution (e.g. lines 11, 37, 298/299, 316/317). I question this claim, because the approach relies heavily on the N2O to ozone relation as well as on the N2O profiles provided by MLS. Without MLS or comparable data, I have a hard time seeing how the authors could derive ozone loss from the MIRA data alone. This claim either needs information about available ground-based data that support the approach, or the claim needs to me modified.
In Fig. 6, I am missing additional lines / more information for the ozone depletion over time found by Manney et al. 2020, and Wohltmann et al. 2021.
Lines 6, 256: maybe add 60 to 50 hPa somewhere after 475 K, to give additional information about the altitude range.
Line 26, 31: I suggest to add the 2022 WMO/UNEP Scientific Assessment of Ozone Depletion as a reference. In line 31, it is not clear whether lower stratospheric ozone is actually increasing, although this is expected. Maybe better to say: Although the decline of stratospheric ozone has been stopped, ...
Line 41-42: While I strongly support ground-based measurements, I don't think your work has done a lot to "highlight the need to ...". I suggest to formulate differently, and stick with what you have actually shown.
Line 58: active chlorine is Cl, not ClO, and the loss cycles start with Cl
Line 59: I would use the active forms: limits and prolongs
Line 72: I don't understand these temperatures. You cool the mixer diode to 70 K? With what? Liquid nitrogen that has 77 K? What is the meaning of 1250 K? I would assume that with 1250 K noise you would not be able to measure anything in the stratosphere, which has about 250 K. Please explain.
Line 76: Instead of "achieved", I would say "retrieved" If you had narrower bandwidth, you would probably not be able to derive much more atmospheric information? Also, this sentence needs a reference.
Line 84: Again, I think a reference is necessary here.
Line 117: instead of "and their associated uncertainties" I would say "on the basis of their associated uncertainties"
Fig. 1: Please indicate which time period is considered here. One day? One month?
Line 154: loss missing after O3
Line 201: I think you need to add "vertical" before transport. Horizontal transport on isentropic surface can and will change ozone. To a large degree you exclude that by only using high PV / vortex data, but it could happen.
Line 204: Which meteorological dataset is used for the MLS data? Is it MERRA? Please state and give a reference.
4.6: as stated above, it think it is necessary to show a plot of this correlation, and maybe explain a bit more, e.g. on what timescales the same correlation can be used. E.g. could you also use it 5 years later (probably not because of N2O trends).
Figure 5: would be good to also plot the passive O3 derived from the tracer relation to N2O, at least for the in-vortex times.
Line 279, 280: I don't see in this plot that ozone variability is linked to chemical processing. E.g. could be transport too. I suggest to drop this sentence.
Figure 6: It would be good to also plot ozone depletion lines from Manney et al. 2020, and Wohltmann et al. 2021 for a better comparison.
Line 289: I question that persistent cold vortex conditions are the main driver here. To me the main driver is activated chlorine and sunlight. The cold vortex conditions are necessary to activate the chlorine, but after that chlorine and sunlight do the job.
Overall, a neat paper using microwave radiometer data for ozone. I support publication after the comments above have been addressed.