Ozone Time-Series at Arosa/Davos

Historical Background

In 1914, F. W. Paul Götz went for health reasons to Davos and worked at the “Observatorium Dorno”, the predecessor institution of the PMOD, for a few months in 1919 – 1920. He then moved to Arosa, where solar UV data were collected with an instrument based on a photoelectric cadmium cell, borrowed from Dorno (Staehelin et al., 2018).

In 1926, Götz founded the “Lichtklimatisches Observatorium” (LKO; see Figure 1) in his newly-built house, “Firnelicht”, which was equipped with a Fabry–Buisson type of solar spectrophotometer. In the same year, he began a collaboration with G. Dobson, an Oxford physicist and meteorologist. Arosa thus became one of six worldwide stations to house one of the first series of Dobson spectrophotometers. In the following decades, several Dobson instruments were used in Arosa. Several years after Götz’ death in 1954, the ETH Zürich took responsibility of the LKO under the auspices of H.U. Dütsch. Two Dobson instruments have been used in parallel (D015, D101; Figure 2) since 1967, while Umkehr measurements to calculate coarse vertical ozone profiles have also been conducted since 1956.

After Dütsch retired, MeteoSwiss became the responsible institution for the ozone measurements, which led to a significant extension of the instruments used in Arosa. A hitherto semi-automated Dobson (D051; Figure 3) was fully-automated to measure Umkehr profiles and three Brewer spectrophotometers were subsequently installed in 1988, 1991 and 1998 (e.g., Figure 4).

In about 2011, MeteoSwiss became aware of the critical situation at the LKO regarding personnel changes, and the question of the transfer of measurements to PMOD/WRC arose, which would also allow synergy effects between both observatories. The key question was whether the world’s longest, best-established and most-documented total ozone time-series could be transferred from Arosa to Davos, despite a horizontal distance between both locations of 13 km, and an altitude difference of 260 m (Davos 1585 m, Arosa 1847 m).

A detailed historical account of ozone measurements at the LKO in Arosa can be found in the publications by Staehlin et al. (2018) and Läubli (2019).

Ozone Time-Series

The agreement between MeteoSwiss and PMOD/WRC in 2018 ensured that the world’s longest total column ozone time-series (Figure 5) would continue in Davos as of 2019. As a first step, one of the Brewers (B072) from the Arosa triad was transferred to Davos in November 2011, where the double monochromator Brewer B163 had been measuring since 2007. At the same time, the LKO Dobsons were converted from their manual mode to fully automated instruments in 2012 (D051, D062) and 2014 (D101), a project that ran under the auspices of R. Stübi from MeteoSwiss. D051 was also occasionally used for total ozone measurements, thus becoming part of a Dobson spectrophotometer triad. In mid-January 2016, Dobson 101 was also transferred to Davos for parallel measurements. An important step was the slit-characterisation of this instrument at the Physikalisch-Technische Bundesanstalt (Braunschweig, Germany) within the ATMOZ EMRP project in spring 2017 to obtain more precise ozone absorption coefficients.

Both B072 and D101, as well as B163, were regularly taken back to Arosa for biennial comparisons with respect to the European reference Brewer from Izaña, and the European reference Dobsons from Hradec Kralove (2017) and Hohenpeissenberg (2018) in order to keep the instrument calibrations up-to-date.

Two literature publications (Stübi et al., 2017; Staehelin et al., 2018) analysed the long-term stability of the Brewer triad in Arosa, and the effects after five years (2012 – 2016) of parallel measurements of B072 in Davos versus the two other Brewers (B040, B156) in Arosa. The conclusion by Stübi et al. (2017) was:

“… that the ozone column series initiated at Arosa in 1926 would not be disrupted by a change of site. Local factors potentially influencing the measurements are below the measurement uncertainty and stay below the long-term stability of the Brewer instruments and within the uncertainties associated with the calibration procedures of the Brewer network“.

A similar study for the Dobson instruments is in preparation.

Following these encouraging results, PMOD/WRC signed a contract with MeteoSwiss to take responsibility of the operational aspects and quality assurance of the stratospheric ozone measurements in Davos. In autumn 2018, two more instruments (B156, D051) were transferred from Arosa to Davos. One Brewer (B040) and one Dobson (D062) will remain for a period of another 2½ years in Arosa so that a total overlapping period of 10 years between both sites is attained.

References

Media Links

Das versteckte Labor von Arosa
18.01.2016, NZZ

Warum die Forscherwelt nach Arosa blickt
20.09.2017, Swiss Television SRF

The Light Climatic Observatory Arosa
12.06.2019, MeteoSwiss

Ein Stück Umweltgeschichte zügelt
17.06.2019, Tages Anzeiger
Alternative Link