Overall remarks:

Comparing average April to September total column ozone data from four years (1980, 1988, 1997, 2020), the author defines two ozone recovery indices ORI1 and ORI2. ORI1 compares 1988 and 2020 total column ozone. ORI2 compares 2020, 1997, and 1980 total column ozone. The author calculates ORI1 and ORI2 for 16 northern mid-latitude stations, on the basis of ground-based observations, space-based observations, and two assimilated data sets. In addition to using four data sets, the author also uses two different smoothing methods, as well "raw" time series and time series with variability removed by standard multiple linear regression (MLR). In each case, uncertainty for the ORIs is estimated using boot-strapped residuals. 

All together, this provides a pretty comprehensive picture of the ORIs at the stations and their uncertainties. All four data sets, and the different smoothing and variability removal techniques give very similar ORIs. Exceptions are ground-based data at Uccle and Arosa, and a larger than usual effect of the MLR variability removal at the Bismark station.

The ORI values are then compared to what might be expected for chemical ozone depletion on the basis of Equivalent Effective Stratospheric Chlorine (EESC) loading for specific years (1980, 1988, 1997, 2020). For EESC, the author uses the Ozone Depleting Gas Index of Montzka et al. (2022). According to the (mid-latitude) ODGI, EESC loadings in 1988 and 2020 should be the same. Accordingly the total ozone difference between these two years (ORI1) should be 0. 

The other ozone recovery indicator ORI2(T), compares ozone recovery from 1997 to T with the expected maximum ozone depletion, from 1980 to 1997. ORI2(T) would be 100% for recovery to 1980s ozone levele, and would be 0% in 1997. This seems to be inconsistent with the EESC / ODGI plot in Fig. 1, where ODGI is at 100% in 1997, at 51.7% in 2020, and at 0% in 1980. I therefore believe that the expected value for ORI2(2020) should be 48.3% (=100%-51.7%), not 51.7%. Both values are close to 50%, though, and this difference / inconsistency is minor. It should however be checked carefully and (if necessary) fixed throughout the paper.

A second and larger problem, to me, is not mentioning the considerable uncertainty associated with using one EESC / ODGI time series only. Depending on the assumed age of air distribution, bromine contribution, data sources, ... there is a considerable spread between various EESC time series, see e.g. Newman et al. (2007), Engel et al. (2018), or the various WMO/UNEP Ozone Assessments. Thus, I think it is absolutely essential to discuss the substantial uncertainties associated with the ORI references values of 0% and 51.7% (or 48.3% ?). I think, Fig. 1 should show some uncertainty (e.g. by at least adding the Antarctic EESC /ODGI curve, see also my atttached Fig. 1). In Figs. 3 and 4 of the manuscript, the reference lines should really be reference ranges, indictae e.g. by much thicker lines, to reflect their uncertainty. Uncertainty of the EESCs and reference values, in my opinion, needs to be dicussed in nearly all the sections of the paper, because it is important for expected total ozone columns / reference values for the ORIs.

Overall, however, this is a good manuscript which presents useful new metrics for ozone layer recovery. It is well suited for ACP and should be published, after addressing my questions above, and the following minor suggestions.

Minor suggestions: 

Lines 19 to 21: It would be good to add the respective years here: 1980, 1988, 1997, 2020. 

Line 23 and many other places: 51.7% or 48.3%? as mentioned above. Please check, throughout the paper.

Line 61: Should be NOAA, not NASA. 

Line 61 to 70: This is one of the places where a dicussion of different EESC realizations and the accompanying uncertainties is necessary.

Fig. 1, caption: Data source needs to be mentioned (https://gml.noaa.gov/odgi/ ?). Additional EESCs? Uncertainty?

Lines 105 to 110: Also mention that dynamical variability is much smaller in these months, so total ozone variability is smaller and chemical changes are potentially more clear.

Figs. 3 and 4: As mentioned, show some uncertainty for the ORIs, e.g. make ORI reference lines wider, to account for EESC uncertainty.

The striking difference between the European stations and the other stations in Figs 3 and 4 should be discussed a bit more. It is mentioned very briefly in context with Coldewey-Egbers (2022) in the conclusions (around line 265), but it should be mentioned already when discussing the Figures. The paragraph in the conclusions should be expanded a bit as well.

The recent 2022 WMO/UNEP Ozone Assessment (https://csl.noaa.gov/assessments/ozone/) mentions tropospheric ozone changes as a potentially important factor in total column ozone changes in recent years. Improved air quality in Europe, for example, might have contributed to the lack of increases in total column ozone. I think it would be good to add some thoughts on this, e.g. an additional paragraph in the conclusions.

Reviewer Figure

Figure 1: Different estimates of Effective Equivalent Stratospheric Chlorine (EESC), mostly from the GSFC automailer (Newman et al. 2007), for different stratospheric mean ages of air (_Xyr_), distribution widths (_Ywd_) and Bromine contribution factors (_ZBr). Values are all normalized to 1 at the maximum and 0 at zero equivalent chlorine. This is different from the ODGI of Montzka et al. (2022), which is normalized to 1 at maximum und 0 at 1980s level. Note that the two Montzka et al. (2022) ODGIs are plotted as well, but with normalization modified to be consistent with the other plotted time series.

References

Engel, A., Bönisch, H., Ostermöller, J., Chipperfield, M. P., Dhomse, S., and Jöckel, P.: A refined method for calculating equivalent effective stratospheric chlorine, Atmos. Chem. Phys., 18, 601–619, https://doi.org/10.5194/acp-18-601-2018, 2018.

Hossaini, R., Atlas, E., Dhomse, S. S., Chipperfield, M. P., Bernath, P. F., Fernando, A. M., et al.: Recent trends in stratospheric chlorine from very short-lived substances. J. Geophys. Res., 124, 2318– 2335. https://doi.org/10.1029/2018JD029400 2019.

Montzka S. et al.: The NOAA Ozone Depleting Gas Index: Guiding Recovery of the Ozone Layer, https://gml.noaa.gov/odgi/  2022.

Newman, P. A., Daniel, J. S., Waugh, D. W., and Nash, E. R.: A new formulation of equivalent effective stratospheric chlorine (EESC), Atmos. Chem. Phys., 7, 4537–4552, https://doi.org/10.5194/acp-7-4537-2007, 2007. For updated EESC data see  https://acd-ext.gsfc.nasa.gov/Data_services/automailer/index.html user: Ertel pwd: geostrophic 

Referee comment on „Indicators of the ozone recovery for selected sites in the Northern Hemisphere mid-latitudes derived from various total column ozone datasets (1980-2020)” by J. Krzyścin.

Overall remark

In this study the author proposes two new ozone recovery indices (ORI1 and ORI2) in order to evaluate the current status of ozone recovery related to changes in the amount of Ozone Depleting Substances (ODSs).

ORIs are computed for 16 stations located in the middle latitudes of the Northern Hemisphere. Four time series based on ground-based (Brewer/Dobson) and satellite (MOD V8.7) measurements and two reanalysis projects (MERRA-2 and MSR-2) are investigated. For each data set two different smoothing approaches are applied to both the original time series and the time series with natural variability removed (using a standard multiple linear regression). The uncertainties of the ORIs are estimated using bootstrapping.

Total ozone values from four key years (1980, 1988, 1997, and 2020) are used to calculate the indices, which are then compared to reference values obtained from the EESC loading.

From this comparison the following conclusions were drawn:

• The author found a strong signal of slower ozone recovery for five stations (as expected from the decrease in EESC loading).
• For some stations an ongoing ozone decline was found.
• In general the results are quite consistent among the data records and do not depend on the smoothing approach or using the raw/non-proxy time series. However, in some cases ORIs and their significance can differ.

In my view the new indices provide a useful addition to the standard ozone trend analyses in order to assess the current stage of recovery.

The topic fits well into the scope of ACP and I recommend publication after having addressed my remarks.

General comments

The author uses ozone values that were averaged over the warm sub-period of the year (Apr-Sep), which is justified with (i) higher solar elevation (leading to more accurate observations) and (ii) higher UV indices (having detrimental biological effects). However, regarding long-term ozone changes, some seasonal dependence might be expected (see, e.g., Szelag et al., 2020 or Coldewey-Egbers et al., 2022). I think it is necessary to investigate and discuss the uncertainty of the ORIs associated with ozone values averaged over different periods of the year, e.g., annual mean, cold sub-period (Oct-Mar), or seasonal means (DJF, MAM, JJA, SON). I suggest to add a figure indicating this uncertainty.

Sec. 3: What is the impact of the Mt. Pinatubo eruption in 1991 and the extremely low ozone values in the following years on your results? Did you include these years in your analysis? Do all data records provide data during this time or do they have gaps?

Sec. 4: The discussion of Figures 3 and 4 should be expanded a bit. I suggest to elaborate on the apparent difference between the regions (Europe and North America / Japan) and on the agreement/differences between the data records (WOUDC/MOD/MERRA-2/MSR2).

Figs. 3 and 4: It might be helpful for the readers to somehow highlight those stations in the figures, which indicate a significant difference from EESC values.

Technical corrections

P1, L14: „obtained“ → „obtain“

P1, L35: „Earth surface“ → „Earth’s surface“

P1, L37: „Earth environment“ → „Earth’s environment“

P2, L51: „longitude/longitude“ → „latitude/longitude“

P2, L61: „NASA“ → „NOAA“

P3, L94: Please add also “Frith et al., 2014”.

P5, Fig.2 caption: Which dataset is shown here? Ground-based? Pleas add the information in the caption.

P7, L207: „0%e“ → „0%“

P11, LL346-348: Paper now published in ACP: Maillard Barras, E., Haefele, A., Stübi, R., Jouberton, A., Schill, H., Petropavlovskikh, I., Miyagawa, K., Stanek, M., and Froidevaux, L.: Dynamical linear modeling estimates of long-term ozone trends from homogenized Dobson Umkehr profiles at Arosa/Davos, Switzerland, Atmos. Chem. Phys., 22, 14283–14302, https://doi.org/10.5194/acp-22-14283-2022, 2022.

References

Frith, S.M., Kramarova, N.A., Stolarski, R.S., McPeters, R.D., Bhartia, P.K., and Labow, G.J.: Recent changes in total column ozone based on the SBUV Version 8.6 Merged Ozone Data Set, J. Geophys. Res. Atmos., 119, 9735– 9751,doi:10.1002/2014JD021889, 2014.

Szeląg, M. E., Sofieva, V. F., Degenstein, D., Roth, C., Davis, S., and Froidevaux, L.: Seasonal stratospheric ozone trends over 2000–2018 derived from several merged data sets, Atmos. Chem. Phys., 20, 7035–7047, https://doi.org/10.5194/acp-20-7035-2020, 2020.