Abstract
Abstract. The longest ozone column measurement series are based on the Dobson sun spectrophotometers developed in the 1920s by Gordon B. W. Dobson. These instruments still constitute an important part of the World Meteorological Organization's global network due to their optical qualities and ruggedness. The primary drawback of this instrument is the effort needed for its manual operation. In industrialized and some less developed countries, most stations have made the choice to replace the Dobson by the automated Brewer sun spectrophotometers, but some are still relying on the Dobson instrument. One of them is the Arosa station where both instrument types are run in parallel. Here, an automated version of the Dobson instrument was developed and implemented recently. In the present paper, the results of the analysis of simultaneous measurements from pairs of Dobson instruments that were either collocated at Arosa or Davos or operated one at each location are presented for four distinct time periods: 1992–2012 – manual vs. manual operation of collocated Dobson instruments (MMC); 2012–2013 – manual vs. automated operation of collocated Dobson instruments (MAC); 2012–2019 – automated vs. automated operation of collocated Dobson instruments (AAC); 2016–2019 – automated vs. automated operation of distant Dobson instruments (AAD). The direct comparison of two instruments using the standard operation procedure during the MMC period gives a metric necessary to validate the automated version of Dobson instruments. The direct comparison of two collocated instruments using the standard manual operation procedure reveals random differences of coincident observations with a standard deviation of ∼ 0.45 % and monthly mean differences between −1.0 % and +0.8 %. In most cases the observed biases are not statistically significant. The same analysis of two automated Dobson instruments yields significantly smaller standard deviation of ∼ 0.25 % and biases of between −0.7 % and 0.8 %. This demonstrates that the repeatability has improved with the automation, while the systematic differences are only marginally smaller. The analysis of the AAD period of coincident measurements from the distant sites Arosa and Davos reveals a small positive bias (not significant) compatible with the 250 m altitude difference. The description of the automated data acquisition and control of the Dobson instrument is presented in a separate paper (Stübi et al., 2020).
Highlights
In 2017, the celebration of 30 years of the Montreal Protocol (Albrecht and Parker, 2019) was a reminder of this important worldwide agreement to ban the use and the production of substances harmful for the ozone layer in industrial processes as well as their release to the atmosphere (Solomon, 1999)
Stübi et al.: Automated Dobson total ozone measurements the ozone hole has favored the development of well organized dedicated monitoring networks based in particular on the Dobson and Brewer instruments
The method used to separate the mid- to long-term systematic biases between instruments and the short-term random variations associated with each instrument were first presented in Stübi et al (2017a). This method allowed us to reduce by half the overall global bias range from typically IPR97.5th–2.5th ∼ 3 % (Table 2) down to IPR97.5th–2.5th ∼ 1.5 % (Table 3)
Summary
In 2017, the celebration of 30 years of the Montreal Protocol (Albrecht and Parker, 2019) was a reminder of this important worldwide agreement to ban the use and the production of substances harmful for the ozone layer in industrial processes as well as their release to the atmosphere (Solomon, 1999). The automated Dobson instruments require only occasional presence on-site essentially for lamps tests Following this transition to automated operation, the comparison of the two sites Arosa and Davos started, with a view to continue the world’s longest total column ozone series based on Dobson observations in Davos. The present analysis is a response to the GCOS principles for the change of Dobson measuring technique (manual to automated) followed by a change of instrument location (Arosa to Davos). The adherence to these principles assures the continuity of the Arosa series with measurements at Davos and the homogeneity of the combined record.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
