On the compatibility of Brewer total column ozone measurements in two adjacent valleys (Arosa and Davos) in the Swiss Alps

The Arosa site is well known in the ozone community for its continuous total ozone column observations recorded since 1926. Originally based on Dobson sun spectrophotometers, the site has been gradually complemented by three automatic Brewer instruments, in operation since 1998. To secure the long term ozone monitoring in this Alpine region and to benefit from synergies with the World Radiation Center, the feasibility of moving this activity to the nearby site at Davos (aerial distance of 13 km) has 5 been explored. Concerns about a possible rupture of the 90 years long record has motivated a careful comparison of the two sites since great attention to the data continuity and quality has always been central to the operations of the observatory at Arosa. To this end, one element of the Arosa Brewer triad has been set up at the Davos site since November 2011 to realize a campaign of parallel measurements and to study the deviations between the three Brewer instruments. The analysis of the coincident measurements shows that the differences between Arosa and Davos remain within the range of the 10 long term stability of the Brewer instruments. A non-significant seasonal cycle is observed, which could possibly be induced by a stray light bias and the altitude difference between the two sites. These differences are shown to be lower than the short term variability of the time series and the overall uncertainty from individual Brewer instruments and therefore are not statistically significant. It is therefore concluded that the world’s longest time series of the total ozone column obtained at Arosa site could be safely extended and continued with measurements taken from instruments located at the nearby Davos site without 15 introducing a bias in this unique record.