MeteoSwiss uses various measuring devices to monitor the ozone concentration in the atmosphere above Switzerland. This task is essential since the ozone layer in the stratosphere filters out a large part of the damaging solar UV radiation.
To measure the ozone concentrations in the high atmosphere, MeteoSwiss uses different methods which cover different spatial and temporal scales. Spectrophotometers installed in Arosa until 2021 and in Davos measure the transparency of the atmosphere for ultraviolet solar radiation at different wavelengths. The total ozone amount in the air column above the station is calculated on the basis of these readings. Moreover, during sunrise and sunset these same spectrometers are also measuring profiles of ozone. In Payerne, the ozone profile is recorded directly (in situ) using ozone sondes attached to radio-sounding balloons. A microwave radiometer is also used in Payerne to continuously determine the stratospheric and mesospheric ozone concentration between 20 and 70 kilometres altitude.
The spectrophotometers are based on the measurements of the absorption of the solar radiation in the UV light range (300-330 nm) which allow the total ozone quantity contained in the atmospheric column above the station to be calculated. Two types of instrument are used here, the Dobson and the Brewer.
Dobson spectrophotometer and time series of ozone column observations by this type of instrument in Arosa/Davos since 1926.
The Dobson spectrophotometer measures the differential intensity of wavelength pairs of the solar radiation, with one being weakly absorbed and the other being strongly absorbed as they cross through the atmosphere. This allows the total ozone column to be derived. In order to mask disruptive effects, three different wavelength pairs are measured and combined. For the device depicted in the figure, a computer controls the sequence of operations necessary for measuring the ozone column. The very-long series of ozone measurements taken in Arosa/Davos measured since 1926 is based on this type of instrument.
The Brewer spectrophotometer measures the absolute intensity of the solar radiation for four wavelengths in the range from 310 to 320 nm. The ozone column can be determined on the basis of the weighted sum of these measurements. This device works automatically and is very reliable; it has been used in Arosa and Davos for more than 35 years.
Three times a week, MeteoSwiss measures the ozone profile using a radio-sounding balloon. The ozone sonde is based on the chemical reaction between the ozone (O3) and potassium iodide (KI) molecules part of a chemical solution contained in the small white capsules (see figure). The air is pumped into the inside of the Styrofoam sonde housing and then channelled into the capsule, which contains the low concentration potassium iodide solution. According to the principle of an electrochemical cell, the ozone concentration in the air can be measured on the basis of the electrical current between two electrodes. This device is light enough to be elevated using a radio-sounding balloon, allowing for a precise in-situ measurement of the ozone profile from ground level up to an altitude of 30-35 km.
In addition to the total ozone column, the Dobson and Brewer spectrophotomers measure two ozone profiles per day of good weather, during sunrise and sunset. The variation of the ratio of the intensity at two wavelengths, which feature stronger and weaker absorption by ozone, in function of the solar zenith angle allow to deduce ozone profiles from the surface up to 50 km at a resolution of 5 to 10 km (Umkehr method). The series of ozone profiles from Arosa/Davos started in 1956 and is the longest Umkehr time series of the world.
The microwave radiometer SOMORA (Stratospheric Ozone Monitoring Radiometer) measures ozone profiles by remote sensing. The volume mixing ratio of ozone in the stratosphere and lower mesosphere is observed with a vertical and temporal resolution of 8 to 15 km and 60 minutes, respectively. Developed by the Institute of Applied Physics of the University of Bern, the SOMORA MWR is continuously operated in Payerne since January 2000. MeteoSwiss is submitting the SOMORA data to the Network for the Detection of Atmospheric Composition Change (NDACC).
The vertical distribution of ozone in the stratosphere and lower mesosphere is deduced from the measurement of the spectral line of ozone at 142.17 GHz. The line intensity is proportional to the ozone concentration, its width is dependent on the ambient pressure at the location of the emission, i.e. on altitude. The ozone concentration is indicated in the unit of the number of ozone molecules per volume of air. Displayed as a series the 24 daily ozone profile observations clearly demonstrate the altitude-dependent annual cycle of the atmospheric ozone content above Payerne.
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