Indicators of the ozone recovery for selected sites in the Northern Hemisphere mid-latitudes derived from various total column ozone datasets (1980–2020)
Authors: Krzyścin Janusz 
Published: October 2022

Abstract. We propose a method to examine the current status of the ozone recovery attributed to ozone-depleting substances (ODS) changes in the stratosphere. The total column ozone (TCO3) datasets used are based on the ground-based (by the Dobson and/or Brewer spectrophotometer) measurements, satellite observations (from the Solar Backscatter Ultraviolet, SBUV, and Ozone Mapping and Profiler Suite, OMPS, instrument), and output of reanalyzes (Multi-Sensor Reanalysis version 2, MSR2, and Modern-Era Retrospective Analysis for Research and Applications, version 2, MERRA2). The TCO3 time series are calculated for selected sites in the NH mid latitudes (35° N–60° N), which are station locations with long term TCO3 observations archived at the World Ozone and Ultraviolet Radiation Data Centre (WOUDC). The TCO3 monthly means (1980–2020) are averaged over the April–September period to obtained TCO3 time series for the warm sub period of the year. Two types of the averaged TCO3 time series are considered: the original one and that with removed natural variability by a standard multiple regression model. TCO3 time series were smoothed by the Locally Weighted Scatterplot Smoother (LOWESS) and the Super Smoother (SS). The smoothed TCO3 values in 1980, 1988, 1997, and 2020 are used to build Ozone Recovery Indices (ORIs) in 2020. These are key years in the equivalent effective stratospheric chlorine (EESC) time series, i.e., the EESC minimum year when the stratosphere was only slightly contaminated by ODS, the year (before the ESSC maximum) with the EESC value equal to that at the end of the TCO3 data, the EESC maximum year in the Northern Hemisphere mid-latitudes, and the end of TCO3 data. The first proposed ORI, ORI1, is the normalized difference between the TCO3 values in 2020 and 1988. The second one, ORI2, is the percentage of the recovered TCO3 in 2020 since the ODS maximum. Following these definitions, the corresponding reference values, 0 % for ORI1 and 51.7 % for ORI2, are obtained from the EESC time series. The ozone recovery phases are classified comparing the current ORI values and their uncertainty ranges (by the bootstrapping) with the reference values. In the analyzed TCO3 time series, for specific combinations of datasets, data types, and the smoother used, we find faster (ORI1>0 % or ORI2>51.7 %), slower (ORI1<0 % or ORI2<51.7 %) recovery in 2020 than that inferred from the EESC change, and a continuation of TCO3 decline after the EESC peak (ORI2<0 %). Using the original datasets, the strong signal of the slower TCO3 recovery can be found in five stations: Nashville, Toronto, Hohenpeissenberg, Hradec Kralove, and Belsk. This is also in the series with removed proxy effects for those stations excluding Nashville. A continuation of ozone decline after the turnaround in ODS concentration is found both in the original and non-proxy time series from WOUDC (Toronto), SBUV&amp;OMPS (Toronto, Arosa, Hohenpeissenberg, Uccle, Hradec Kralove, and Belsk), and MERRA2 data (Hohenpeissenberg, Hradec Kralove, and Belsk).