# Ground-Based Monitoring of Stratospheric Ozone and Temperature Over Germany Since the 1960s

Wolfgang Steinbrecht, Voltaire A. Velazco, Ruud Dirksen, Lionel Doppler, Peter Oelsner, Roeland van Malderen, Hugo de Backer, Eliane Maillard Barras, René Stübi, Sophie Godin-Beekmann, et al.

► To cite this version:

Wolfgang Steinbrecht, Voltaire A. Velazco, Ruud Dirksen, Lionel Doppler, Peter Oelsner, et al. "Ground-Based Monitoring of Stratospheric Ozone and Temperature Over Germany Since the 1960s." Earth and Space Science, 2025, 12 (3), pp.e2024EA003821. 10.1029/2024EA003821. hal-04993818

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# Earth and Space Science

## RESEARCH ARTICLE

10.1029/2024EA003821

**Special Collection:**

Scientific Impact of Long-term
Observations of the Atmosphere
- In Honor of Walter Komhyr
(1931 - 2022)

**Key Points:**

* Starting in the 1960s, Hohenpeissenberg and Potsdam/Lindenberg have provided some of the longest ozone records in Europe

* Over Germany, stratospheric ozone has declined until 1995, and has increased only slightly since 2000, similar to the global changes

* The troposphere over Germany has warmed over the last 60 years, the stratosphere has cooled, and the tropopause has moved up

**Correspondence to:**

W. Steinbrecht,
wolfgang.steinbrecht@dwd.de

**Citation:**

Steinbrecht, W., Velazco, V. A., Dirksen, R., Doppler, L., Oelsner, P., Van Malderen, R., et al. (2025). Ground-based monitoring of stratospheric ozone and temperature over Germany since the 1960s. *Earth and Space Science, 12*, e2024EA003821. https://doi.org/10.1029/2024EA003821

Received 17 JUN 2024
Accepted 4 FEB 2025

**Author Contributions:**

**Conceptualization:** W. Steinbrecht

**Data curation:** V. A. Velazco, R. Dirksen, L. Doppler, P. Oelsner, R. Van Malderen, H. De Backer, E. Maillard Barras, R. Stübi, S. Godin-Beekmann, A. Hauchecorne

**Formal analysis:** W. Steinbrecht

**Investigation:** W. Steinbrecht

**Resources:** V. A. Velazco, L. Doppler, P. Oelsner, R. Van Malderen, E. Maillard Barras, R. Stübi, S. Godin-Beekmann, A. Hauchecorne

© 2025 The Author(s). Earth and Space
Science published by Wiley Periodicals
LLC on behalf of American Geophysical
Union.

This is an open access article under the
terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.

<figure><img src="image_2.png" /></figure>

## Ground-Based Monitoring of Stratospheric Ozone and Temperature Over Germany Since the 1960s

W. Steinbrecht¹ ID, V. A. Velazco¹ ID, R. Dirksen², L. Doppler² ID, P. Oelsner² ID, R. Van Malderen³ ID, H. De Backer³, E. Maillard Barras⁴ ID, R. Stübi⁴,⁶, S. Godin-Beekmann⁵ ID, and A. Hauchecorne⁵,⁶ ID

¹Deutscher Wetterdienst, Hohenpeissenberg, Germany, ²Deutscher Wetterdienst, Lindenberg, Germany, ³Royal Meteorological Institute, Uccle, Belgium, ⁴Federal Office of Meteorology and Climatology, MeteoSwiss, Payerne, Switzerland, ⁵LATMOS-IPSL, CNRS, Paris, France, ⁶Retired

**Abstract** Routine ground-based measurements of total ozone column (TOC), as well as ozone profile soundings started in the late 1960s in Germany. The resulting ozone and temperature records at Hohenpeissenberg and Berlin/Potsdam/Lindenberg show long-term changes similar to other stations in Central Europe, and to the changes seen globally. Following the increase of ozone depleting substances (ODS), stratospheric ozone has declined from the 1960s until the 1990s. Since about 2000, ozone has leveled or slightly increased, consistent with declining amounts of ODS. The stratosphere has been cooling and the troposphere has been warming, in agreement with general expectations due to increasing greenhouse gas concentrations. The clearest signs of recovering ozone are seen around 40 km altitude. Two factors contribute to this increase: the decrease of stratospheric chlorine loading and cooling of the upper stratosphere, which slows gas-phase ozone destruction cycles, and enhances the ter-molecular reaction producing ozone. Tropospheric ozone has increased substantially from the 1960s to the early 1990s. Since then, it has remained more or less constant, on a level higher compared to the 1960 and 1970s. Particularly low tropospheric ozone was observed in 2020, due to reduced precursor emissions during the COVID-19 related lockdowns. The atmospheric concentrations of greenhouse gases will likely continue to rise, while the concentrations of ozone depleting substances are expected to slowly decline. To see how the atmosphere responds, and to help understand future changes, continued monitoring will be required for many years to come, both over Germany and worldwide.

## 1. Introduction

The 1957/1958 International Geophysical Year (Chapman, 1959) may have been the initial spark for renewed attention to ozone measurements in the 1960s. Already in the 1930s in Oxford, Gordon Miller Bourne (G.M.D.) Dobson had developed a dedicated UV-spectrometer for precise measurements of total ozone columns (TOC), and had deployed these spectrometers in a network covering a large part of the World (Dobson, 1931, 1968). Arosa in Switzerland was one of the first stations, and F.W.P Götz from Arosa was cooperating with Dobson from 1926 until his early death in 1954 (Götz & Dobson, 1928; Götz et al., 1934; Staehelin & Viatte, 2019). In the late 1950s and early 1960s, Walter Komhyr started his career working with Dobson spectrometers in Canada and the USA. He greatly improved calibration and standard operating procedures (Komhyr, 1980), and established a calibration chain for a network of Dobson spectrometers (Komhyr et al., 1989). Walter's principles are still in use today in the Global Ozone Observing system of the World Meteorological Organization (Komhyr & Evans, 2008).

The second important instrument for ozone observations in the 1960s were balloon-borne ozonesondes. Allan Brewer, a student of Dobson in Oxford, and Milford had developed a wet-chemical sonde for routine measurements of the ozone profile (Brewer et al., 1960). A commercial version of this sonde produced by Mast Inc., the Brewer-Mast sonde, was routinely flown at a number of stations starting in the early 1960s. Walter Komhyr developed an improved dual Electro-Chemical-Cell (ECC) ozonesonde (Komhyr & Harris, 1971). With some upgrades, this ECC sonde is still the standard ozonesonde in use today (Smit et al., 2021; A. M. Thompson et al., 2011). Tests of Brewer-Mast, ECC, and other ozonesondes were carried out in Boulder in the early 1960s, in cooperation with the National Center for Atmospheric Research (NCAR), where H.U. Dütsch, a former student of F.W.P. Götz in Zürich, Switzerland, headed the Ozone Research Program from 1962 to 1964. In 1966, Walter Komhyr joined the new Atmospheric Physics and Chemistry Laboratory (APCL) in Boulder. Incidentally, APCL was headed from 1964 to 1980 by H. Weickmann, who from 1946 to 1949 had been director of the Meteorological
**Validation:** V. A. Velazco, R. Dirksen, L. Doppler, P. Oelsner, R. Van Malderen, H. De Backer, E. Maillard Barras, R. Stübi, S. Godin-Beekmann, A. Hauchecorne

**Visualization:** V. A. Velazco

**Writing – original draft:** W. Steinbrecht

**Writing – review & editing:**
V. A. Velazco, R. Dirksen, L. Doppler, R. Van Malderen, H. De Backer, E. Maillard Barras, S. Godin-Beekmann

<table><caption>**Table 1**
Main Instruments for Total Ozone Column (TOC) Measurements in Germany</caption><thead><tr><th>Instrument</th><th>Station</th><th>From</th><th>To</th><th>Main use</th></tr></thead><tbody><tr><td rowspan="2">Dobson #64</td><td>Potsdam</td><td>1969</td><td>1997</td><td>Regional standard</td></tr><tr><td>Hohenpeissenberg</td><td>1998</td><td>Now</td><td>Regional standard</td></tr><tr><td rowspan="2">Dobson #71</td><td>Potsdam</td><td>1964</td><td>2003</td><td>Routine TOC</td></tr><tr><td>Lindenberg</td><td>2003</td><td>Now</td><td>Occasional TOC</td></tr><tr><td>Dobson #104</td><td>Hohenpeissenberg</td><td>1968</td><td>Now</td><td>Routine TOC</td></tr><tr><td rowspan="3">Brewer MKII #10</td><td>Hohenpeissenberg</td><td>1984</td><td>Now</td><td>Routine TOC</td></tr><tr><td></td><td>1990</td><td>2007</td><td>Routine UV</td></tr><tr><td></td><td>1987</td><td>2003</td><td>Routine TOC</td></tr><tr><td rowspan="2">Brewer MKII #30</td><td>Potsdam</td><td>2003</td><td>Now</td><td>Routine TOC</td></tr><tr><td>Lindenberg</td><td>1992</td><td>Now</td><td>Routine TOC</td></tr><tr><td>Brewer MKIV #78</td><td>Lindenberg</td><td>1994</td><td>2003</td><td>Routine TOC, UV</td></tr><tr><td rowspan="2">Brewer MKIII #118</td><td>Potsdam</td><td>2003</td><td>Now</td><td>Routine TOC, UV</td></tr><tr><td>Lindenberg</td><td>2015</td><td>Now</td><td>Routine TOC</td></tr><tr><td rowspan="2">Brewer MKIII #226</td><td>Hohenpeissenberg</td><td>2022</td><td>Now</td><td>Routine UV</td></tr></tbody></table>

*Note.* Brewer instruments are also used for spectral ultraviolet measurements (UV).

Observatory Hohenpeissenberg (MOHP) in Germany. Dütsch returned to Switzerland in 1965, where he started regular ozone soundings with Brewer-Mast sondes in Thälwil in November 1966. In September 1968, the program was moved to Payerne, where it is still running today.

In Germany, W. Attmannspacher started regular Brewer-Mast ozone soundings at the Meteorological Observatory Hohenpeissenberg in November 1966, in cooperation with Dütsch, and after testing various ozonesonde types since 1964. In Berlin Tempelhof, the Free University under R. Scherhag had launched optical ozonesondes from 1964 on, but also switched to Brewer-Mast sondes in November 1966. These Berlin Tempelhof soundings ended in January 1973. They were re-started in 1975 by the East German Meteorological Service (MD), at its Meteorological Observatory Lindenberg (MOL-RAO) in Lindenberg (Tauche), about 60 km south-east of Berlin. In Potsdam, 30 km south-west of Berlin, total ozone columns had been measured already in the 1930s. In January 1964, the MD started continuous measurements with Dobson spectrometer #71 at Potsdam. In 2003, these total ozone column measurements also moved to Lindenberg. At Hohenpeissenberg, the West German Meteorological Service (Deutscher Wetterdienst, DWD) started regular total ozone column measurements with Dobson #104 in January 1968. See also Table 1.

The motivation for these early total ozone column and ozone profile measurements was to gain better understanding of air movements in the upper atmosphere, and to use the data for weather forecasting. Dobson and co-workers had noticed correlations between weather and total ozone column (Brönnimann, 2022; Dobson, 1931). A number of studies expanded on this in the 1950 and 1960s. The discovery of possible ozone depletion by chlorine from anthropogenic Chloro-Fluoro-Carbon gases (Molina & Rowland, 1974), and the discovery of the ozone hole above Antarctica (Farman et al., 1985), of course, changed the perspective on ozone measurements dramatically. By the late 1980s, there was enormous interest in long-term records of stratospheric ozone, to see how ozone had changed since the 1960s, and to look for ozone trends (WMO/UNEP, 1988).

Apart from Hohenpeissenberg, Thälwil/Payerne, and Potsdam/Berlin/Lindenberg, a few other stations in Europe had started regular ozone measurements in the 1960s, which continue until today, for example, Brussels-Uccle in Belgium, Belsk in Poland, or Hradec Králóve in Czechia. In Boulder, in the USA, the ozone records cover a number of years in the 1960 and 1970s, but continuous long-term monitoring re-started only in 1982. Canada, on the other hand, has largely continuous total column and profile ozone records dating back to the 1960s, at a number of stations. Under A. Brewer, who had moved to Canada, a new automatic UV spectrometer was developed in the 1970s (Brewer, 1973), to replace the manually operated Dobson spectrometers. Brewer spectrometers were deployed in the Canadian network, and have become the second mainstay of total ozone column measurements in the Global Ozone Observing system of the World Meteorological Organization. In Germany,
Brewer #10 has been measuring since 1984 at Hohenpeissenberg, and Brewer #226 since 2015. At Potsdam and Lindenberg, Brewers #30, #78, and #118 have been measuring since 1987, 1992, and 1994. Table 1 summarizes which Dobson and Brewer Spectrometers have been in routine operation at the German stations. More modern CCD-based UV Array-spectrometers (Gigahertz-Optik BTS) are now also deployed at Lindenberg and Hohenpeissenberg (Zuber et al., 2018, 2021), but their measurement records are still short and are not considered further here.

A major addition to the Hohenpeissenberg ozone measurements was a differential absorption lidar installed in 1987 (Geh, 1987; Werner et al., 1983). The lidar became operational in September 1987, and has since provided routine ozone profiles from about 12 km up to 48 km, as well as temperature profiles from about 27 km up to 60 km (Claude et al., 1994; Steinbrecht et al., 2009). The lidar extends the ozonesonde profiles to higher altitudes, and allows the routine intercomparison of two very different measurement techniques in their overlap range. Only a few stations worldwide have such a dual ozone lidar and sonde capability: Observatoire de Haute Provence in southern France, Maido/St. Denis on Reunion Island in the Indian Ocean, Mauna Loa/Hilo in Hawaii, and Lauder in New Zealand. In 2017, after 30 years of successful operation, the old lidar at Hohenpeissenberg was replaced by a new and more powerful system (Wing et al., 2021). With Dobson and Brewer spectrometers, the ozonesondes, and the lidar at Hohenpeissenberg, the German Meteorological Service (Deutscher Wetterdienst, DWD) is running a very comprehensive ground-based long-term ozone measurement program at its two observatories.

The purpose of this contribution to the Walter Komhyr special issue is to remind the reader of the historical context and the underlying measurement principles, which were used and improved by Walter. We also present the long-term ozone records in Germany, in context with comparable long-term records in Europe. We will not discuss the long-term ozone records generated by the Alfred-Wegener Institut at its Arctic Koldewey station in Ny-Ålesund and Antarctic Georg-Forster/Neumeyer stations (König-Langlo & Gernandt, 2009; Rex et al., 2000; von der Gathen et al., 1995; Wohltmann et al., 2020).

## 2. Instruments

We start with an overview of the main measurement principles and instruments for stratospheric ozone measurements used at Hohenpeissenberg and Potsdam/Berlin/Lindenberg.

### 2.1. Spectrometers for Total Column Ozone

The Dobson UV-spectrometer for measuring the total column amount of ozone was developed in the 1930s (Brönnimann, 2022; Dobson, 1931). It is still widely used today. A Dobson spectrometer discovered the Antarctic ozone hole (Farman et al., 1985). Starting in the 1980s, manual Dobson spectrometers were supplemented or replaced by automatic Brewer UV-spectrometers. For both spectrometers, the ozone column measurement is based on the strong ozone absorption of UV radiation at wavelengths below 315 nm. The Dobson uses a quartz-prism to separate two wavelengths of UV radiation coming from the sun. Inside the spectrometer, two slits in a rotating disc pass either one or the other wavelength. The light at both wavelengths is recombined by a second quartz-prism, and is focused onto a photomultiplier. The second prism also helps to reject straylight at out-of-band wavelengths. Typically, the photomultiplier receives different light intensities at the two wavelengths, which generates an alternating (AC) current. The trick in the Dobson spectrometer is to introduce a calibrated "gray wedge" into the light path at the longer, less absorbed wavelength. The wedge is positioned very precisely until the higher intensity at the longer wavelength is sufficiently reduced, and the AC current from the photomultiplier vanishes. The calibrated wedge position then provides a precise measurement of the light absorption by the ozone layer. The total ozone column is derived using the calibrated wedge position and the known ozone absorption coefficients (Komhyr, 1980; Komhyr & Evans, 2008). Typically, direct sun measurements are used at two wavelengths pairs: a shorter A pair and a longer D pair. Combination of both pairs reduces aerosol interference and provides more accurate total ozone columns. See also Voglmeier et al. (2024) for a detailed description.

The measurement principle of the Brewer UV-spectrometer is essentially the same (Brewer, 1973). However, wavelength separation is done by a grating, and instead of looking at the photomultiplier AC current from two wavelengths, the Brewer spectrometer sequentially measures the intensity at five different wavelengths, using a photomultiplier in photon counting mode. As with the Dobson, the total ozone column is then calculated from the measured intensities and the known ozone absorption cross-sections. The Brewer wavelengths are selected to reduce errors due to the temperature dependence of the ozone cross-sections, and to allow a measurement of the
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Langley plot of slant ozone column versus air mass for the calibration campaign of Dobson #64 on 26 September 2016 at the Izaña Observatory on Tenerife. Extrapolation of the measured data to zero air mass gives the extra-terrestrial calibration constant (ETC, here equivalent to 12.3 DU).</figcaption></figure>

total SO₂ column (which can interfere with the Dobson measurement). Over the years, different versions of Brewer spectrometers were developed: single monochromator types MkII and MkIV (with extended wavelength range up to 450 nm, allowing measurements of total NO₂ column), and the double monochromator MkIII for much better suppression of straylight and better UV-measurements down to the UV-edge near 290 nm, where solar UV radiation cannot reach the Earth's surface anymore.

All Brewer and Dobson spectrometers require regular maintenance and calibration. The instruments have to be set to the correct wavelengths, for example, using spectral lamps. In addition, the ratio of instrumental transmission and solar irradiance at the different wavelengths needs to be determined. These values are usually called ETCs (extra terrestrial constants). Absolute calibration using the "Langley-plot" method is one way to determine the ETC: slant ozone column is measured following the sun during perfectly clear and stable days with constant overhead ozone and constant other atmospheric conditions. Figure 1 gives an example. The resulting data of measured slant ozone column versus different length light-paths through the atmosphere, also called air mass, are then extrapolated to zero light-path (= air mass zero). Air mass is the ratio of slant light-path to vertical light-path. Air mass equals 1, when the sun is directly above the Dobson. Air mass increases as the sun approaches the horizon. At zero air mass, slant ozone column must also be zero. Any offset of the extrapolated slant column at zero light-path (=air mass zero) then gives the ETC.

In Germany, and many other locations, perfectly clear days with constant ozone columns, atmospheric conditions, and low air masses (close to 1) are rare. Absolute "Langley" calibrations, therefore, are typically done at remote, low latitude and high altitude, island locations: Mauna Loa, Hawaii, or Izaña Atmospheric Observatory (IZO) on Tenerife. For the global network of Dobson and Brewer spectrometers, it is not feasible to bring all instruments to an absolute calibration campaign. Walter Komhyr and others thought of an alternative calibration method: side by side comparisons with an absolutely calibrated reference instrument. Again, during one or more days many simultaneous measurements are taken at various air masses by all instruments. The average offset between a network instrument to be calibrated and the reference instrument then provides the ETC for the network instrument.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Example for a network calibration, in this case the calibration of Dobson #121 by regional standard Dobson #64 in July 2023 at Hohenpeissenberg. The top panel shows the "initial" calibration of Dobson #121, as the instrument arrived damaged at Hohenpeissenberg. The bottom panel shows the "final" calibration after repair and alignment of the instrument. The numbers at the top of each plot give the average differences for the various air mass ranges. The gray shaded regions give the typical ±1% target accuracy for Dobson calibrations.</figcaption></figure>

An example of such a side-by-side network calibration is given in Figure 2. The instrument to be calibrated, Dobson #121, had been dropped sometime before the calibration campaign. The "initial" calibration in the top panel of Figure 2 quite clearly shows that Dobson #121 was not in good condition and, relative to the reference Dobson #64, provided large total ozone column deviations, which also varied with air mass. Dobson #121 was repaired and re-aligned, resulting in the much better agreement seen in the bottom panel of Figure 2, which already includes the newly determined ETC values. At this "final" calibration, Dobson #121 agrees within ±1% with the reference Dobson #64, over a wide range of air masses.

More than 100 Dobson instruments have been built and deployed all over the world. The principal setup did not change much after 1960. Organized calibration campaigns started in 1969 (Siofok, Hungary) followed by Belsk, Poland. These first calibration campaigns revealed discrepancies and instability of the calibration levels, with differences of up to ±10%. This prompted the decision to establish a global Dobson Calibration System, with one World Calibration Center, and 5 Regional Calibration Centers. In 1974, NOAA in Boulder, USA was nominated Official WMO World Dobson Calibration Center, along with regional centers in South Africa, Japan, Argentina, Australia, and Germany.

A long-term perspective on these calibrations and the overall consistency of the Dobson network is given in Figure 3. The left panel shows that large numbers of Dobson instruments have been calibrated at regular campaigns since the first campaign in 1968. Over the years, deviations at the "initial" comparisons became smaller, and the network instruments became more and more consistent with the reference. A much larger percentage of incoming instruments was already within the ±1% target set for the network. Before 1990, initial deviations exceeding 5% were quite common. Such large deviations did not occur any more after 1990. In the four last intercomparisons since 2020 more than 80% of the incoming instruments were already within the ±1% target.

Figure 3 indicates that systematic differences between two well calibrated Dobsons are less than 1%. This number does not, however, include systematic biases and uncertainties, which would be the same for both Dobsons, and would not become apparent in comparisons like Figures 2 and 3. On top of such systematic uncertainties, each individual Dobson measurement is also subject to random uncertainty, largely due to the limited precision
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Left panel: Scatterplot showing the deviation of network Dobsons from the reference instrument at Dobson calibration campaigns since the first campaign, 1968 in Siofok, Hungary. Each dot shows the "initial" deviation of a network instrument at one campaign. The red line shows average deviation and ±1 standard deviation of the data for each 10-year period. Right panel: percentage of network Dobsons that are already within the ±1% target at the "initial" comparison for each decade since 1960. The blue line shows the corresponding 10-year averages.</figcaption></figure>

(repeatability) of each measurement. This is particularly true for the manual and somewhat operator-dependent Dobson measurements, but also applies to automatic Brewer measurements. From the scatter of the individual data-points in Figure 2 the random uncertainty of Dobson data should be of the order of ±1%.

Other estimates for the random uncertainty (repeatability) of Dobson and Brewer measurements are given in Figure 4. There the climatological standard deviations of all ozone measurements taken on individual days by Dobson #104 and by Brewer #10 are plotted for the 12 calendar months, and as a function of air mass. If there were no real ozone variations during each day, the plotted values would give the typical random measurement uncertainty (repeatability). The figure indicates that this typical random uncertainty is less than about 0.7% for Dobson #104 and about 0.9% for Brewer #10. Note that systematic uncertainties, for example, due to the applied ozone cross-sections, the ETC calibrations, and other things, do not show up in Figures 2–4. Typically, the

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Climatological standard deviation of total ozone column measurements taken on individual days by Dobson #104 and Brewer #10 at Hohenpeissenberg, for the different calendar months, and as a function of average air mass. Vertical error bars, shown for a few selected months, give the typical variation from year to year (1σ). Horizontal error bars, also for selected months, show the range of air masses encountered in this calendar month (1σ). The dashed lines give a linear fit over the data from July to December, when true ozone variations over each day are small, and a large fraction of the standard deviation is due to the repeatability/precision of the measurement itself.</figcaption></figure>
systematic uncertainty of Dobson or Brewer measurements is estimated to be 1%–3% (Gröbner et al., 2021; Koukouli et al., 2016; Redondas et al., 2014; Voglmeier et al., 2024).

## 2.2. Ozonesondes

Since the late 1960s, balloon-borne ozonesondes have been key instruments for measuring ozone profiles, from the ground up to more than 30 km altitude (e.g., A. M. Thompson et al., 2011). The main types of ozonesondes used at Hohenpeissenberg and Berlin/Lindenberg are Brewer-Mast and Electro-Chemical-Cell (ECC) sondes. Both types rely on the same principle: A small pump forces ambient air through an electro-chemical cell, which is filled with a solution of potassium iodide (KI) in water (Brewer et al., 1960; Komhyr & Harris, 1971). Ozone in the pumped air will oxidize iodide ($I^-$) in the solution to iodine gas ($I_2$), and ultimately generate a current of two electrons ($e^-$) per ozone molecule. This current is measured and provides the ozone partial pressure, as the sonde ascends through the atmosphere (Smit et al., 2021).

In the Brewer-Mast sonde, the two electrodes of the electro-chemical cell are a platinum mesh cathode and a silver wire anode. The concentration of the potassium iodide solution is 0.1% KI (by weight) in water. In the ECC sonde, the electrodes are realized by two cells, linked by an ion-bridge: a cathode cell with a 0.5%–1% KI solution in water (depending on sonde version and manufacturer) and an anode cell with a saturated KI solution (130% or more, by weight). From the 1960s until today, several slightly different versions of ECC sondes were manufactured. Since 1993, also two different manufacturers have been producing ECC sondes. Brewer-Mast sondes, on the other hand, have undergone less change over the years. Until 1997, they were manufactured in the USA, since then in Costa Rica. Generally, ECC sondes are easier to use and more reliable, and provide more precise ozone profiles, especially in the troposphere (Kley & Smit, 1997; Smit et al., 2007). However, already from the beginning, Hohenpeissenberg developed a lot of experience in using Brewer-Mast sondes (Claude et al., 1987), and has achieved good and consistent results with this somewhat more demanding sonde type. A key requirement for Brewer-Mast sondes is a very thorough cleaning of all parts of pump, wet-chemical cell, and electrodes. Using sondes "out-of-the-box" will not work well for any type of ozonesonde, but it is particularly bad for Brewer Mast sondes. This may explain the lower performance of Brewer-Mast sondes in the Canadian network in the 1970s (Tarasick et al., 2002). For comprehensive and up-to-date detailed information about all aspects of operational ozone sounding see Smit et al. (2024).

An important historical practice for ozone soundings is the "Dobson correction": the raw ozone profile measured by the sonde is scaled by a single factor, so that the total ozone column obtained by integrating the sounding data over altitude (plus an estimate for the ozone column above the burst-point of the balloon) is equal to the more precise total column measured by a UV-spectrometer (typically a Dobson or Brewer Spectrometer). The main effect of the Dobson correction is to minimize a possible bias of the sonde ozone profile over the 15–25 km altitude range, where most of the ozone resides. The correction does this at the expense of potentially biasing ozone values in the troposphere, or around 30 km. For Hohenpeissenberg Brewer-Mast sondes, the average correction factor is 1.082, that is, 8.2% are added to the raw ozone values from the sonde. For ECC sondes, the correction factor is usually quite close to 1.0, and the currently recommended practice is to not Dobson-correct ECC sonde ozone profiles at all (Smit et al., 2021).

Figure 5 shows the historical evolution of Dobson correction factors for Hohenpeissenberg, Berlin/Lindenberg, and Payerne. Clearly, substantial corrections, 15%–30% or more were applied to many Brewer-Mast sondes in the 1960–1980s. In contrast, the correction factors for ECC sondes are close to 1.0, at least for the annual means of Lindenberg and Payerne shown here. The Payerne ECC correction factors have a step-like increase around June 2013. This is due to the use of actual pump temperature data after that date. Although visible in the correction factors, the step is much reduced in the actual ozone profile data, because the Dobson total ozone column correction largely compensates for the changed pump temperature. The Payerne ozonesonde data used here are from the World Ozone and UV data center, and are Dobson corrected. In the Network for the Detection of Atmospheric Composition Change (NDACC) data archive, the Payerne ECC data are not Dobson corrected, as indicated by the negative sign of the correction factor (following the standard convention). Updated and homogenized Payerne ECC ozonesonde from the HEGIFTOM initiative (https://hegiftom.meteo.be/) will be uploaded to both data centers in the GEOMS-HDF data format. The HEGIFTOM data are not yet publicly available, are marginally different, and are not used here.
<figure><img src="image_7.png" /></figure>

**Figure 5.** Annual mean Dobson correction factors for ozone soundings at Hohenpeissenberg, Berlin/Lindenberg and Payerne. The annual mean is calculated from monthly means. The corresponding standard error of the mean ($\sigma/\sqrt{n}$) is plotted as uncertainty. For ECC Sondes, the Dobson correction factor is applied for the Payerne data used here (from the World Ozone and UV Data Center, WOUDC), and also for the Lindenberg data (also from WOUDC) before May 2014. Since May 2014, the Dobson correction is not applied any more at Lindenberg.

The correction factors for Hohenpeissenberg Brewer-Mast sondes (red line) are generally close to 1.08. They are almost always smaller than those at Berlin/Lindenberg (purple line) and Payerne (pink line). Hohenpeissenberg also shows less variation over time. Nevertheless, there are indications for step-like changes in the Hohenpeissenberg factors in 1976 and in 1994. In 1975/76, a small step might have also occurred in the Payerne Brewer-Mast correction factors. At Payerne, the step might be related to moving sonde pre-calibration from Zürich to Payerne in 1976. The Hohenpeissenberg step before 1995 coincides with the change from VIZ radiosondes and manually digitized data to the fully digital Vaisala RS80/Digicora radiosonde system in 1995, but also with the end of using water-activated batteries to power the ozonesondes, and with improved cleaning of the silver anode using a cyanide solution after 1993. The Payerne data also show a step around that time, which is attributed to major changes in standard operating procedures (pump lubrication, bubbler conditioning, cleaning). The earlier step around 1984 in the Payerne correction factors is likely caused by improved pump flow measurements using a Hastings flow-meter. As mentioned above, the Dobson correction to a large degree compensates such changes. So, despite a step in the correction factor, changes in the actual ozone profile values are small to negligible.

The Berlin/Lindenberg Brewer-Mast correction factors (purple line) show three distinct periods: In Berlin, from 1967 to 1972 orginal Brewer-Mast sondes, manufactured in the USA, were used. The correction factors were similar to those in Payerne. Lindenberg, from 1975 to 1992, used sondes similar to the Brewer-Mast, but manufactured in East Germany. After an initial learning curve, these sondes gave correction factors very similar to Hohenpeissenberg. This changed in 1987, when Lindenberg correction factors increased until the end of the East German sondes in 1992. Very likely, reduced quality during this third period was related to the huge economic and political changes in East Germany and the entire Eastern bloc in the late 1980s. After unification of Germany in 1990, Lindenberg switched to ECC sondes in 1992 (blue line), with correction factors generally close to 1.

Overall, Figure 5 indicates that Brewer-Mast sondes have been working well at Hohenpeissenberg and have been quite consistent with the more precise total ozone column data over the last 60 years. There is a small long-term trend in the correction factors, about -1% per decade. This gives an upper limit for erroneous trends in the Hohenpeissenberg ozonesonde data, for example, due to either application or non-application of the Dobson correction. Much larger variability in the Brewer-Mast correction factors is seen at Lindenberg and Payerne, where the switch-over to ECC has clearly improved the consistency between integrated sonde ozone profiles and
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Typical relative uncertainty (in percent) for individual ozone profiles from Brewer-Mast ozonesondes, with Dobson total ozone column correction (left panel), and from ECC ozonesondes, without Dobson correction (right panel). The thick blue lines show the overall uncertainty when flown with Vaisala RS80 radiosondes, the dashed red lines when flown with Vaisala RS41 radiosondes. The RS41 sondes have more accurate pressure/altitude registration, an important source of uncertainty above 25 km (cyan line for RS80, pink line for RS41). Purple lines show the uncertainty due to pump correction, another major source of uncertainty above 25 km, especially for the Brewer-Mast sonde. For comparison, the green lines show independent uncertainty estimates from twin-flights and JOSIE chamber experiments (see also Smit et al., 2007; Tarasick et al., 2016).</figcaption></figure>

the more accurate total ozone column data from the spectrometers. For the Payerne ECC soundings, the Dobson correction is applied for the data used here (from WOUDC). For the Lindenberg ECC soundings, the Dobson correction was applied before May 2014, but not any more after. This is now the preferred way, because the sonde data remain fully independent, and the correction factor is used only as a quality indicator.

Uncertainty and potential biases of ozonesondes have been investigated in many field and laboratory experiments. See Tarasick et al. (2021) and Smit et al. (2024) for recent overviews. Not all of these studies have provided consistent results, but generally the uncertainty of ozone profiles from ECC sondes is estimated around 3%–5% (Smit et al., 2021; Tarasick et al., 2016). The uncertainty of Brewer-Mast sondes is slightly worse, about 5% in the stratosphere and 5%–10% in the troposphere (Kley & Smit, 1997; Tarasick et al., 2016, 2021). Brewer-Mast sondes have larger uncertainty in pump efficiency, lower *KI* concentration and noisier currents, and need an external bias voltage to compensate the different electrochemical potential of silver anode and platinum cathode.

Following Tarasick et al. (2016, 2021), Figure 6 shows the uncertainties estimated for individual Brewer-Mast soundings with total ozone column correction (left panel), and for ECC ozone soundings without total ozone column correction (right panel). Up to about 25 km, the dominant sources of uncertainty are background current estimation, measurement noise, chemical stoichiometry and conversion efficiency, and pump flow rate at the ground. For Brewer-Mast sondes several of these factors of uncertainty are compensated by the Dobson correction, which has its own uncertainty. The Dobson correction reduces the overall uncertainty in the 15–25 km altitude region (where most of the total ozone column resides), at the cost of increased uncertainty below 15 km, and above 25 km. At the top of the profile, above 25 km, uncertainty in pump efficiency correction, especially for Brewer-Mast sondes, and uncertainty in pressure/altitude registration (due to limitations of the radiosonde pressure sensors) become the dominant sources of uncertainty, especially for older radiosondes with less accurate pressure/altitude sensors like the Vaisala RS80 or VIZ. Using new radiosondes, like the Vaisala RS41 (with more accurate pressure sensors and GPS altitudes), this part of the uncertainty becomes smaller.

A complex problem for both sonde types, not addressed here, are slow secondary reactions. They lead to background and hysteresis effects in the measured ozone, which are currently accounted for in an empirical average way only, through effective “pump” corrections (Komhyr, 1986; Komhyr et al., 1995). Only recently, Smit
et al. (2021), Vömel et al. (2020), and Smit et al. (2024) have suggested procedures to better account for these effects, which could provide more accurate ozonesonde data in the future.

## 2.3. Ozone Lidars

A lidar for ozone profile measurements was installed at Hohenpeissenberg in 1986/87 (Geh, 1987; Werner et al., 1983). Operational measurements started in September 1987 (Claude et al., 1994; Steinbrecht et al., 2009). A newer and more powerful lidar system was installed from 2012 to 2016, and has been providing the routine measurements since 2018 (Wing et al., 2021). After 2 years of overlap, the old lidar was shut down in 2019. The old lidar was measuring ozone profiles from about 12 up to 45 km altitude, and temperature profiles from about 27 km up to 60 km altitude. With the new lidar these altitude ranges were extended up to 50 km for ozone, and up to 85 km for temperature.

As with the Dobson or Brewer, the underlying principle for an ozone lidar is the absorption of UV-light by ozone (McDermid, 1987; Mégie et al., 1977). While Dobson or Brewer spectrometers use the sun as light source, the lidar uses intense light pulses generated by lasers. The light pulses are sent straight up into the atmosphere. Some of the light is scattered by air molecules in the atmosphere. A small fraction of the scattered light is returned in the backward direction, down to the lidar receiver mirror on the ground. Emitted and back-scattered light travel at the speed of light, so the time between transmission and reception gives the altitude where the backscattering took place (150 m per micro-second). The returned intensity contains information about the atmosphere. Pulses are sent at an "on" wavelength absorbed by ozone (usually 308 nm), and an "off" wavelength, not absorbed by ozone (355 nm for the new lidar, 353 nm for the old lidar). Comparison of "on" and "off" intensities from different altitudes provides the light absorption by ozone between these altitudes. With the known ozone absorption cross sections, this provides the ozone number density profile (Godin et al., 1999; Leblanc, Sica, van Gijsel, Godin-Beekmann et al., 2016b).

The returned intensity at the non-absorbed "off" wavelength is proportional to air-density (more molecules also scatter more light). The resulting relative density profile can be integrated over altitude, which provides a relative pressure profile (assuming hydrostatic equilibrium, a good assumption up to about 100 km). Using the ideal gas law, the ratio of relative pressure and density (from integrated and measured intensity) then provides the temperature profile (Hauchecorne & Chanin, 1980; Leblanc et al., 2016). Reliable temperature values are obtained above 25-30 km, where additional light scattering by aerosol is usually negligible.

The Hohenpeissenberg lidar has been part of the Network for the Detection of Atmospheric Composition Change (NDACC, De Mazière et al., 2018) since 1994. Old and new lidar have been validated through side-by-side comparisons with the NDACC traveling standard lidar, and by comparisons with ozone profiles from satellites and local ozonesondes (Steinbrecht et al., 2009; Steinbrecht, Schwarz, & Claude, 1998; Wing et al., 2021). As shown in Figure 7, the lidar provides an overall uncertainty less than 5% for nightly mean ozone profiles between 20 and 40 km, rapidly decreasing to 20%-100% at 50 km. To a large degree the uncertainty is determined by the number of counted photons, which becomes very small at high altitudes. The increased uncertainty near 25 km is due to the switchover from reduced intensity near-range channels to full intensity far-range channels. Systematic limitations come from the uncertainty of ozone cross sections, about 2%. Below 20 km, uncertainty from having to account for extinction by Rayleigh scattering from air molecules becomes important, increasing ozone uncertainty up to 10%. The large decreases of ozone number density above 40 km and below 20 km contribute to the large increases of relative uncertainty at these altitudes. See also Leblanc, Sica, van Gijsel, Godin-Beekmann et al. (2016a) and Leblanc, Sica, van Gijsel, Godin-Beekmann et al. (2016b).

For temperature, overall uncertainty is less than 2 K below 40 km for the old lidar, and below 70 km for the new lidar. Figure 7 shows the substantial vertical variation of temperature uncertainty for the Hohenpeissenberg lidars. As for ozone, random photon counting uncertainty, due to the small number of photons returned from high altitudes, is the major contributor to the overall uncertainty. This part of the uncertainty can be reduced by averaging profiles, for example, over 1 month. The much improved uncertainty for the new lidar comes from many more counted photons, thanks to the use of a larger mirror, better optics, more sensitive photomultipliers, and for temperature especially, from using a separate laser which generates much more light at the reference wavelength (355 nm, not absorbed by ozone). See Leblanc, Sica, van Gijsel, Haefele, et al. (2016) for more details on the estimation of temperature uncertainty.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Left panel: typical relative uncertainty (in percent) for nightly mean ozone profiles from the old and new Hohenpeissenberg lidars. Right panel: typical absolute temperature uncertainty (in Kelvin) for the two lidars. The systematic uncertainty (black lines) follows Leblanc, Sica, van Gijsel, Haefele, et al. (2016) and Leblanc, Sica, van Gijsel, Godin-Beekmann et al. (2016b). It contains basically all sources of uncertainty, except photon counting statistics. This random source of uncertainty is shown by the lines in cyan (for the old lidar) and pink (for the new lidar). The dark blue and red lines (for old and new lidar) show the combined uncertainty, that is, the root-mean-square of random and systematic uncertainty.</figcaption></figure>

# 3. Time Series of Ozone and Temperature

## 3.1. Total Ozone Column Time Series

The long-term evolution of total ozone columns measured by Dobson and Brewer spectrometers at the two German stations since the late 1960s is shown in Figure 8, along with near-global means (60°S to 60°N) from ground-based and satellite data (Weber et al., 2022, scaled to match the station data). Generally, the German and near global time series look very similar: A long term decline from the late 1960s to the mid 1990s, and level or slightly increasing total ozone columns since the end of the 1990s. Ozone generally follows the (inverted) loading of the stratosphere with anthropogenic chlorine and bromine, depicted here by the pink line for equivalent effective stratospheric chlorine, EESC (Engel et al., 2018; Newman et al., 2007). Superimposed are decadal ozone variations associated with the 11 years cycle of solar activity (Ball et al., 2019; Maycock et al., 2016), and additional ozone depletion in the years after the large volcanic eruptions (Wilka et al., 2018). The previous ozone decline from the 1960s to the 1990s has been stopped. Since about 2000 ozone is on a slow path to ozone recovery, confirming the success of the 1987 International Montreal Protocol and its subsequent amendments. Values like in the 1960s, however, are not expected before the second half of this century.

A more detailed attribution of ozone variations at Hohenpeissenberg to different influencing factors is presented in Figure 9. The attribution uses multiple linear regression (MLR) to approximate the observed annual mean total ozone column time series by a linear combination of predictor time series. The MLR determines the weight for each predictor by minimizing the root-mean-square deviation between observed time series and the sum of the weighted predictor time series. This is the standard approach used in Steinbrecht et al. (2011), or the Scientific Assessments of Ozone Depletion (WMO/UNEP, 1988; WMO/UNEP, 2022).

The top panel of Figure 9 presents three slightly different ways to describe long term ozone trends in the MLR.

1. Using equivalent effective stratospheric chlorine (EESC) as a proxy in the MLR. This approach prescribes the shape of the long-term variation. In particular, the ratio of declining trend of ozone before ≈1997 and increasing trend of ozone after ≈1997 is prescribed. This approach does not allow independent trends before and after the EESC turn-around.

2. Using a "hockey-stick" trend as proxy in the MLR. While prescribing the general shape of the long-term variation, the "hockey stick" does allow different linear trends before and after 1997. The trend before 1997 and the trend after 1997 are free parameters, determined by the MLR.
<figure><img src="image_10.png" /></figure>

**Figure 8.** Annual mean total ozone columns above Hohenpeißenberg (black curve, scale on the left) and Potsdam/Lindenberg (dark blue curve), as well as worldwide annual means (cyan curve, 60°S to 60°N average, Weber et al., 2022, scaled). Potsdam Dobson data from Spänkuch et al. (1999) are used until 1994, and Lindenberg Brewer data after 1994 (shifted by -6.8 DU to account for systematic differences between Dobson and Brewer, Voglmeier et al., 2024). The pink curve for equivalent effective stratospheric chlorine (EESC, scale on the right, Engel et al., 2018; Newman et al., 2007) shows estimated chemical ozone destruction due to anthropogenic chlorine and bromine in the stratosphere. The green curve additionally includes ozone variations due to the 11-year cycle of solar activity and due to enhanced stratospheric aerosol loading after the major volcanic eruptions of Mt. Pinatubo and El Chichón.

3. Using a more complex dynamical linear model, DLM (Alsing, 2019; Laine et al., 2014). This does not prescribe any shape for the long-term ozone variation. Instead the DLM fits a smoothly varying long-term variation to the observed data.

Reassuringly, all three approaches give similar results in the top panel of Figure 9. They confirm that anthropogenic chlorine and bromine (EESC) are indeed the main cause for the observed long-term ozone trends, and that ozone has started a slow recovery since the late 1990s. However, both the "hockey-stick" model and the DLM indicate lower ozone in recent years, than would be expected from the EESC curve. This lack of ozone increases is consistent with Ball et al. (2020) and WMO/UNEP (2022), is likely due to decadal variations in ozone transport (Brönnimann, 2022; Weber et al., 2022), but is still not well understood.

Results for the other proxies in the MLR are shown in the bottom panels of Figure 9. Typically, the associated ozone variations range from ±5 to ±10 DU, and are almost identical for the three cases above. In Figure 9, they are shown for case (2), "hockey-stick". The used proxies stand for the following influences.

1. Quasibiennial oscillation of stratospheric winds above the equator (QBO, represented by zonal wind at 10 and 30 hPa). The QBO affects stratospheric ozone in tropics and extra-tropics by modulating the transport of all trace gases in the stratospheric Brewer Dobson circulation (Baldwin et al., 2001; Chipperfield et al., 1994). The QBO also modulates temperature and thereby gas-phase and heterogeneous chemistry relevant for ozone (Zhang et al., 2021).

2. Arctic Oscillation, describing the large scale circulation over the North Atlantic and the Northern Hemisphere (Steinbrecht et al., 2011; D. W. J. Thompson & Wallace, 2000). It is connected with the stratospheric polar winter vortex, which strongly affects ozone.

3. The product of stratospheric aerosol optical depth and EESC, a proxy for additional ozone depletion and possible circulation changes due to major volcanic eruptions (Wilka et al., 2018), here especially Mt. Pinatubo in 1991 and El Chichón in 1982/83.

4. 10.7 cm solar flux, a proxy for the 11-year cycle of solar activity, which affects ozone production and transport globally (Ball et al., 2019; Maycock et al., 2016).
<figure><img src="image_11.png" /></figure>

**Figure 9.** Similar to Figure 8, but annual mean total ozone columns above Hohenpeissenberg only (black curve and dots in top panel). The top panel also shows long-term ozone trends estimated by (1) EESC (pink curve in top panel), by (2) fitting a hockey-stick curve (red curve), and by (3) a dynamic linear model (DLM, blue curve, with light blue shading for uncertainty range). The bottom panel shows ozone variations attributed to five proxies by the multiple linear regression (MLR) with a hockey-stick trend: quasibiennial oscillation of stratospheric winds above the equator (QBO, zonal wind at 10 and 30 hPa), Arctic Oscillation Index, product of stratospheric aerosol optical depth and EESC, and 10.7 cm solar flux. The gray line and circles in the top panel show the reconstructed time series from the MLR.

## 3.2. Ozone and Temperature Variations at Different Altitude Levels

The total ozone column, in Figures 8 and 9, sums ozone variations from all layers of the atmosphere. It is also the quantity most relevant for the penetration of UV-radiation to the Earth's surface. However, chemical and dynamical processes affecting ozone are quite different in different parts of the atmosphere (Brasseur & Solomon, 2005). In the upper stratosphere, photo-chemical reactions are fast and efficient, and dominate. In the lower stratosphere, photo-chemical relaxation times are long, and transport processes tend to dominate. In the troposphere, and especially in the atmospheric boundary layer, ozone is controlled quite differently from the stratosphere. For three of these altitude regions, Figures 10–13 show the long-term variation of ozone and temperature
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Annual mean deviations of ozone from the long-term climatological mean, for three layers of the atmosphere. The Hohenpeissenberg data are from ozone soundings (red circles and lines) and lidar (pink circles and lines). Berlin/Lindenberg ozonesonde data are shown in blue, Observatoire de Haute Provence lidar data (OHP) in purple. The near-global data (gray lines) are from NASA's SBUV/OMPS-MOD satellite data set (Frith et al., 2014). Surface ozone data for Hohenpeissenberg, Zugspitze, and Jungfraujoch (in green, cyan, light green) are from EBAS.</figcaption></figure>

over Germany, Europe, and worldwide. The 5 km level is typical for the free troposphere. 20 km is typical for the lower stratosphere, where the largest part of the total ozone column resides, and 40 km is a typical level for the upper stratosphere.

Consistent with the findings of WMO/UNEP (2022), the upper stratosphere (top panel, Figures 10 and 12) shows the largest ozone changes. Increasing chlorine until the end of the 1990s has resulted in substantial chemical ozone depletion. Since 2000, chlorine amounts have been declining, and ozone has increased in the upper stratosphere. This ozone recovery is enhanced by long-term cooling of the stratosphere (top panel, Figures 11 and 13), because lower temperatures slow the normal gas-phase cycles of ozone destruction, and make ozone production more efficient, resulting in increased ozone (WMO/UNEP, 2022).

The main cause for cooling of the upper stratosphere is the increasing concentration of CO₂ in the atmosphere (IPCC, 2023; WMO/UNEP, 2022). More CO₂ traps infra-red radiation in the troposphere, so less radiative energy reaches the stratosphere. Additionally, more CO₂ in the stratosphere can radiate more energy to space, which cools the upper stratosphere. Both effects combine to a long-term cooling of the stratosphere. In the Hohenpeissenberg data, substantial meteorological variations, for example, winters with and without sudden stratospheric warmings (SSWs), are super-imposed onto this long-term temperature decline.

The lower stratosphere also shows decreasing temperatures, both globally and at Hohenpeissenberg (middle panel, 20 km altitude, Figures 11 and 13). However, many factors combine in a complex way at these altitudes (Shine et al., 2003; D. W. J. Thompson & Solomon, 2009): CO₂ trends, varying concentrations of other
<figure><img src="image_13.png" /><figcaption>**Figure 11.** Same as Figure 10, but for annual mean deviations of temperature from the long-term climatological mean. The near-global data (gray lines) at 40 km are from rocket soundings (Dunkerton et al., 1998) and from SSU/AMSU satellite radiometers (Zou & Qian, 2016). At 5 and 20 km, the global data are from radiosondes (RATPAC, Free et al., 2005). In addition, temperatures above Hohenpeissenberg from operational meteorological analyses by the US National Center for Environmental Prediction (NCEP, from the NDACC database) are shown by the cyan lines. Before 1997, the NCEP data were collected only for the nights with lidar measurements (dashed cyan lines). The resulting sparse sampling explains the clear differences seen before 1997 at 5 and 20 km.</figcaption></figure>

radiatively active trace-gases, especially ozone and water vapor, volcanic eruptions, the 11-year solar-cycle, and last-not-least, decadal circulation changes.

The ozone evolution in the lower stratosphere (middle panel, 20 km altitude, Figures 10 and 12) is very similar to the evolution of total ozone columns. This is expected, because the largest contribution to the total column comes from the lower stratosphere. The main influences have already been described above, for example, in Figure 9.

Ozone and temperature in the troposphere (bottom panels, 5 km, Figures 10–13) behave very differently from their stratospheric counterparts. Tropospheric temperature follows global warming. At Hohenpeissenberg (and in Central Europe) surface and free tropospheric temperature have increased by more than 2°C from the late 1960s until today.

Tropospheric ozone shows large increases until the late 1980s. Since then, tropospheric ozone values have declined a bit, and have now stabilized on a more or less constant level. Recent years might indicate a slight increase. This evolution of tropospheric ozone is consistent with the general development of precursor substances (volatile organic compounds, nitrous oxides), which have increased substantially from the 1960 to the 1980s (Bowman et al., 2022). Since then, de-noxification of power-plants, catalytic converters for cars, and the breakdown of the East-European industries, have resulted in large decreases of ozone precursors. According to Figures 10 and 12 (bottom panels), tropospheric ozone ceased increasing in Central Europe in the late 1980s, or even went down.
<figure><img src="image_14.png" /><figcaption>**Figure 12.** Same as Figure 10 but showing additional ozone time series from neighboring stations. In the upper stratosphere (40 km, top panel), data from the microwave radiometers at Payerne and Bern are added. In the lower stratosphere and troposphere (middle and bottom panel) ozone data from sondes at Payerne and Uccle are shown.</figcaption></figure>

A notable feature in the tropospheric ozone time series (lower panels of Figures 10 and 12) are the low values observed at all stations in the year 2020. These low tropospheric ozone values were observed over much of the Northern Hemisphere in the summer of 2020 (Steinbrecht et al., 2021). They were caused by reduced ozone production due to the substantial emission reductions associated with the COVID-19 related worldwide lockdowns (Bouarar et al., 2021). An additional influence came from the large Arctic stratospheric ozone depletion in spring 2020 (Wohltmann et al., 2020).

Figures 10 and 11 demonstrate fairly good agreement for ozone or temperature variations observed by the sondes at Hohenpeissenberg and Berlin/Lindenberg, or by the lidars at Hohenpeissenberg and Observatoire de Haute Provence (OHP). Figures 12 and 13 repeat the ozone and temperature time series from Figures 10 and 11, but show additional data from the long-term ozone soundings at Payerne and Uccle, and the microwave radiometers at Bern and Payerne. A summary of the ozone profiling instruments at the European stations used here is given in Table 2. All these stations and instruments show very similar year-to-year and long-term variations, giving good confidence in the quality of the data and in the reliability and representativeness of the observed variations and trends.

### 3.3. Changes in Tropopause Height

The tropopause separates the vertically mixed and generally ozone-poor troposphere from the ozone-rich and vertically stratified stratosphere. As shown in the previous section, the stratosphere has been cooling and the troposphere has been warming. Both factors have contributed to a long-term upward trend of tropopause height (Meng et al., 2021; Santer et al., 2003; Steinbrecht, Claude, et al., 1998). Ozone soundings provide ozone, temperature and also tropopause height.
<figure><img src="image_15.png" /><figcaption>**Figure 13.** Same as Figure 11 but showing additional temperature time series from neighboring stations. In the lower stratosphere and troposphere (middle and bottom panel) temperature data from balloon sondes at Payerne and Uccle are added.</figcaption></figure>

For our four European stations, Figure 14 shows the evolution of tropopause height (middle panel, defined by the standard WMO lapse rate criterion, where the lapse rate becomes smaller than 2 K per kilometer), tropospheric temperature (at 5 km, same as in previous Figures), and lower stratospheric ozone (at 17 km, similar to 20 km shown in previous Figures). A clear correlation is visible between 5 km temperature and tropopause height, as is an inverse correlation between tropopause height and ozone at 17 km. As before, all four stations show very similar time series (maybe with the exception of Uccle tropopause height in the 1970s). All stations show clear tropospheric warming, by about 0.3 K per decade. All stations also show an ozone decline by 15%–20% near 17 km from 1970 to 2020. In addition, at 3 of the 4 stations, the tropopause has risen by about 400 m over the last 50 years.

This is a substantial change of atmospheric structure and needs to be monitored in the future. Atmospheric dynamics play a key role in controlling ozone in the extra-polar lower stratosphere. Dynamical changes on multiannual time scales will, therefore, affect ozone trends and also tropopause height. They may well be a factor contributing to the lack of a clear indication for increases in lower stratospheric and total ozone columns in the last 20 years (Ball et al., 2020; Brönnimann, 2022; Chipperfield et al., 2018; Steinbrecht, Claude, et al., 1998; WMO/UNEP, 2022).

### 3.4. Ozone and Temperature Trends

Vertical profiles of ozone and temperature trends are shown in Figures 15 and 16. For ozone, it is necessary to separate between the ozone depletion period, from the 1960s until the mid 1990s, and the beginning recovery period, since about 2000. For temperature trends, where the continuing increase of greenhouse gas concentrations
<table><caption>**Table 2**
Long-Term Ozone Profile Records at the European Stations Used in This Study</caption><thead><tr><th>Instrument</th><th>Station</th><th>From</th><th>To</th></tr></thead><tbody><tr><td>Brewer-Mast sondes</td><td>Hohenpeissenberg</td><td>1967</td><td>Now</td></tr><tr><td>Lidar</td><td>Hohenpeissenberg</td><td>1987</td><td>Now</td></tr><tr><td>Brewer-Mast sondes</td><td>Berlin Tempelhof</td><td>1967</td><td>1972</td></tr><tr><td>Brewer-GDR sondes</td><td>Lindenberg</td><td>1975</td><td>1992</td></tr><tr><td>ECC sondes</td><td>Lindenberg</td><td>1992</td><td>Now</td></tr><tr><td>Brewer-Mast sondes</td><td>Zürich Thälwil</td><td>1967</td><td>1968</td></tr><tr><td>Brewer-Mast sondes</td><td>Payerne</td><td>1970</td><td>2002</td></tr><tr><td>ECC sondes</td><td>Payerne</td><td>2002</td><td>Now</td></tr><tr><td>Microwave</td><td>Payerne</td><td>2000</td><td>Now</td></tr><tr><td>Microwave</td><td>Bern</td><td>1995</td><td>Now</td></tr><tr><td>Brewer-Mast sondes</td><td>Uccle</td><td>1967</td><td>1996</td></tr><tr><td>ECC sondes</td><td>Uccle</td><td>1997</td><td>Now</td></tr><tr><td>ECC sondes<sup>a</sup></td><td>Haute Provence (OHP)</td><td>1991</td><td>Now</td></tr><tr><td>Lidar (ozone)</td><td>Haute Provence (OHP)</td><td>1986</td><td>Now</td></tr><tr><td>Lidar (temperature)</td><td>Haute Provence (OHP)</td><td>1978</td><td>Now</td></tr></tbody></table>

(mainly CO₂ and CH₄) plays the major role (Meng et al., 2021; Santer et al., 2003), it is not necessary to separate different trend periods.

Consistent with ozone trends over the 35°N to 60°N latitude band from Godin-Beekmann et al. (2022) and WMO/UNEP (2022), the European stations show clear ozone decreases before 1997, with the largest decreases in the upper stratosphere (−3 to −12% per decade), and in the lower stratosphere (up to −10% per decade). In addition, the European stations show substantial increases of tropospheric ozone over this period (around +12% per decade). After 2000, the only significant ozone trends are seen in the upper stratosphere, where the lidars at Hohenpeissenberg and Observatoire de Haute Provence show larger increases (up to +5% per decade) than the 35°N to 35°N latitude band (around +2% per decade) in Godin-Beekmann et al. (2022) and WMO/UNEP (2022). In the lower stratosphere and in the troposphere, trend uncertainties are quite large over this second period, and there are no significant trends. Station trends and latitude band trends, both observed and modeled, all agree within their uncertainties.

For the temperature trends in Figure 16, the stations (lidars and sondes), also show general agreement within their respective uncertainties. Above 25 km, Lindenberg sondes indicate a larger negative trend than the other sonde stations. The European station trends are in qualitative agreement with worldwide trends from Steiner et al. (2020), although the observed worldwide tropospheric warming and stratospheric cooling (cyan lines and shading) below 25 km are smaller than the trends in Europe. This quantitative differ-

<figure><img src="image_16.png" /><figcaption>**Figure 14.** Time series of annual mean anomalies of ozone at 17 km altitude (top panel), tropopause height (middle panel, WMO lapse rate criterion) and tropospheric temperature at 5 km altitude (bottom panel) for the four European ozonesonde stations. Simple linear trend lines fitted to the data are shown as well (thin lines).</figcaption></figure>

<footnote><sup>a</sup>Mentioned for completeness, but not used in this study.</footnote>
<figure><img src="image_17.png" /></figure>

**Figure 15.** Ozone trends as a function of altitude for the periods before 1997 (left panel) and after 2000 (right panel). Trends are from multiple linear regression including proxies for QBO, solar cycle, volcanic eruptions, and El Nino (slightly different set of proxies compared to the proxies used for total ozone column trends in Figure 9). Trends for stations and instruments from this study are shown by the thick colored lines, with shading for the uncertainty ranges for Hohenpeissenberg sondes (light red shading, ±2σ), and Hohenpeissenberg lidar (light pink shading, ±2σ). Uncertainty (±2σ) for the Lindenberg sondes is shown by the thin blue lines. For comparison, the cyan lines show observed trends and uncertainty ranges (±2σ) for the 35°N to 60°N latitude band from the recent LOTUS study (Godin-Beekmann et al., 2022; WMO/UNEP, 2022). The gray shading in the background shows the range of simulated trends from the Chemistry-Climate-Model-Initiative (also from WMO/UNEP, 2022).

<figure><img src="image_18.png" /></figure>

**Figure 16.** Temperature trends as a function of altitude for the entire period where data are available. Trends are from simple linear regression without additional proxies. Trends for stations and instruments from this study are shown by thick colored lines, with shading giving the uncertainty ranges for Hohenpeissenberg sondes (light red shading, ±2σ), Hohenpeissenberg lidar (light pink shading, ±2σ), and Observatoire de Haute Provence lidar (light purple shading). Uncertainty (±2σ) for the Lindenberg sondes is shown by the thin blue lines. For comparison, the cyan lines and shading show observed global trends and uncertainty ranges (±2σ) from Steiner et al. (2020), for radiosondes (below 20 hPa, 26 km), and for the combined SSU/AMSU satellite microwave sounders.

ence is expected, because tropospheric warming and stratospheric cooling are larger at higher northern latitudes (IPCC, 2023; Steiner et al., 2020).

## 4. Conclusions

In the 1960s, work on ozone measurements using spectro-photometers and balloon-borne sondes by Walter Komhyr and others laid the foundation for a global network of ground-based ozone observations. This network was instrumental in documenting ozone changes from the 1960s to the 1990s, caused by human activities, especially by the wide-spread production and consumption of ozone depleting substances. The network is still in operation today.

In Germany, routine measurements of tropospheric and stratospheric ozone profiles, as well as total ozone columns started at Hohenpeissenberg and Potsdam/Berlin in the late 1960s. These stations, and slightly later Lindenberg, have contributed substantially to the global ozone observing network. Hohenpeissenberg has been hosting the Regional Dobson Calibration Center for WMO's Regional Association VI since 1999 (in cooperation with the Czech Hydrometeorological Institute at Hradec Králóve), and has been providing data and expertise to the regular ozone assessments (WMO/UNEP, 1988; WMO/UNEP, 2022). Lindenberg is lead center of the Global Research Upper Air Network (GRUAN, https://www.gruan.org/). The German and Central European long-term ozone and temperature data show changes similar to the global changes documented in the International Assessments.

* Following the increase of ozone depleting substances in the atmosphere, stratospheric ozone has declined from the 1960s until the 1990s. Thanks to
the ban of ozone depleting substances (ODS) by the international Montreal Protocol and their slowly declining concentrations since the late 1990s, the ozone data at Hohenpeissenberg and Lindenberg confirm that ozone levels are reacting, and have leveled or slightly increased since the ODS peak.

* The clearest signs of ozone recovery since 2000 are seen in the Hohenpeissenberg lidar data at about 40 km altitude. According to WMO/UNEP (2022) two factors contribute about equally to the observed increase: a.) the decrease of stratospheric chlorine, b.) cooling of the upper stratosphere due to increasing levels of CO₂.

* The stratosphere has been cooling and the troposphere has been warming at the stations in Germany and Central Europe, in agreement with general climate change due to increasing greenhouse gases.

* Tropospheric ozone has increased substantially from the 1960s to the early 1990s. Since then, tropospheric ozone has remained more or less constant, on a level higher than in the 1960s. Particularly low tropospheric ozone was observed in 2020, consistent with the reduced emissions of precursors during the COVID-19 related lockdowns.

Today and in the foreseeable future it will remain necessary to monitor and document changes to our atmosphere, to follow the recovery process expected for the ozone layer in this century, and to see how climate change affects stratosphere and troposphere in the future. It is also necessary to provide ground-based references for the validation and calibration of global satellite measurements. Vertical profiles from limb-observing satellites will become much rarer in coming years, as key instruments are ending their lifetime. This means that ground-based measurements and profiles, like the German ones described here, will remain important, and will need to fill some of the expected gaps.

Compared to the fast ozone decline due to increasing ODS from the 1960s to the 1990s, the expected ozone recovery trends will be substantially smaller and the interaction of contributing factors will be more complex (WMO/UNEP, 2022). This increases the need for data quality and precision from global networks, both ground- and space-based. Based on the foundations laid by pioneers like Walter Komhyr, the ground-based total ozone column and ozonesonde networks have proved their value. Hopefully they will continue and be instrumental in observing the expected recovery of the ozone layer, an international success story, where the Montreal Protocol has successfully addressed a global environmental problem.

## Acronyms

<table><tr><td>TOC</td><td>total ozone column, the integral of ozone concentration over altitude, from the ground to the top of the atmosphere</td></tr><tr><td>DU</td><td>Dobson Unit, the usual unit to measure total ozone columns. 100 Dobson Units correspond to a 1 mm thick layer of gaseous pure ozone at 1013 hPa and 273.15 K</td></tr><tr><td>UV</td><td>Ultraviolet radiation</td></tr><tr><td>ECC</td><td>Electro Chemical Cell, used to measure ozone concentration</td></tr><tr><td>ETC</td><td>Extra-Terrestrial Constants, calibration parameters for Dobson and Brewer Spectrometers, accounting for differences of incoming solar radiation and instrumental transmission and efficiency, at the wavelengths used for ozone measurements</td></tr><tr><td>MOHP</td><td>Meteorological Observatory Hohenpeissenberg</td></tr><tr><td>MOL</td><td>Meteorological Observatory Lindenberg</td></tr><tr><td>MOL-RAO</td><td>Meteorological Observatory Lindenberg, Richard Aßmann Observatory</td></tr><tr><td>OHP</td><td>Observatoire de Haute Provence</td></tr><tr><td>IZO</td><td>Izaña Atmospheric Observatory on Tenerife</td></tr><tr><td>DWD</td><td>Deutscher Wetterdienst, German Meteorological Service</td></tr><tr><td>MD</td><td>Meteorologischer Dienst, former East German Meteorological Service</td></tr><tr><td>NASA</td><td>National Aeronautics and Space Administration (of the United States)</td></tr><tr><td>NOAA</td><td>National Oceanic and Atmospheric Administration (of the United States)</td></tr></table>
<table><tr><td>NCEP</td><td>National Center for Environmental Prediction (of the United States)</td></tr><tr><td>SBUV</td><td>Solar Backscatter UltraViolet, series of satellite-borne UV spectrometers measuring ozone profiles and total columns</td></tr><tr><td>OMPS</td><td>Ozone Mapping and Profiler Suite, satellite-borne UV spectrometers measuring ozone profiles and total columns</td></tr><tr><td>WMO</td><td>World Meteorological Organization</td></tr><tr><td>UNEP</td><td>United Nations Environmental Program</td></tr><tr><td>IPCC</td><td>Inter-governmental Panel on Climate Change</td></tr><tr><td>NDACC</td><td>Network for the Detection of Atmospheric Composition Change</td></tr><tr><td>GRUAN</td><td>Global Research Upper Air Network</td></tr><tr><td>HEGIFTOM</td><td>Harmonization and Evaluation of Ground-based Instruments for Free Tropospheric Ozone Measurements, A Focus Working Group in the Tropospheric Ozone Assessment Report, Phase II</td></tr><tr><td>RATPAC</td><td>Radiosonde Atmospheric Temperature Products for Assessing Climate</td></tr><tr><td>WOUDC</td><td>World Ozone and Ultraviolet Data Center</td></tr><tr><td>EMEP</td><td>European Monitoring and Evaluation Program</td></tr><tr><td>EBAS</td><td>EMEP database</td></tr><tr><td>ODS</td><td>Ozone Depleting Substances</td></tr><tr><td>EESC</td><td>Effective Equivalent Stratospheric Chlorine (an Bromine), mixing ratio of summed chlorine and bromine in the stratosphere, weighted by their ozone depletion efficiency</td></tr><tr><td>QBO</td><td>Quasi Biennial Oscillation of wind in the equatorial stratosphere</td></tr><tr><td>MLR</td><td>Multiple Linear Regression, standard method fitting prescribed proxy time series to an observed time series</td></tr><tr><td>DLM</td><td>Dynamic Linear Model, a more general Monte Carlo method fitting a dynamic model to an observed time series</td></tr></table>

Data Availability Statement

All Hohenpeissenberg sonde, lidar, and total ozone data are available at https://www-air.larc.nasa.gov/pub/NDACC/PUBLIC/stations/hohenpeissenberg/ from the Network for the Detection of Atmospheric Composition Change (NDACC, De Mazière et al., 2018). The NDACC database (https://www-air.larc.nasa.gov/missions/ndacc/data.html) also provided the microwave data from Bern and Payerne, the Observatoire de Haute Provence lidar data, and the NCEP temperature profiles. The ozone sonde data from Hohenpeissenberg, Lindenberg, Payerne and Uccle, and the total ozone column data from Hohenpeissenberg and Lindenberg, were obtained from the World Ozone and UV Data Center (WOUDC) at https://woudc.org, the Hohenpeissenberg data directly at https://woudc.org/archive/Archive-NewFormat/OzoneSonde_1.0_1/stn099/ and https://woudc.org/archive/Archive-NewFormat/TotalOzone_1.0_1/stn099/. Ozonesonde data are also available from the NDACC database. Surface ozone data were taken from the EBAS database at https://ebas nilu.no/. The SBUV-MOD near global satellite ozone data are available at https://acd-ext.gsfc.nasa.gov/Data_services/merged/. The SSU/AMSU near global satellite temperature data are available at https://www.star.nesdis.noaa.gov/smcd/emb/mscat/. The RATPAC sonde data are available at https://www.ncei.noaa.gov/products/weather-balloon/radiosonde-atmospheric-temperature-products.
Acknowledgments

The authors gratefully acknowledge the
unwaivering long-term support of the
ozone programme by DWD. W.
Attmannspacher had the vision to start the
programme in the late 1960s. Over the
years, many technicians and scientists put a
lot of effort into making good
measurements and providing high quality
data. To name just a few, the authors thank
M. Adelwart, H. Claude, M. Heinen, M.
Kirchner, U. Köhler, F. Schönenborn and
S. Steiner at Hohenpeissenberg, and W.
Baum, U. Feister and D. Spänkuch at
Lindenberg. Open Access funding enabled
and organized by Projekt DEAL.

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