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. In Arosa, spectrophotometers 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. 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 determine the stratospheric and mesospheric ozone concentration between 20 and 70 kilometres altitude.
Measurement of ozone column
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.
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, one operator is required to take the measurement. An automatic version has recently been developed by MeteoSwiss.
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 for more than 25 years.
Measurement of the ozone profile
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.
Other systems for measuring the ozone profile
MeteoSwiss has two other methods for indirectly measuring ozone profiles via so-called remote sensing technic. These measurements necessitate relatively complex information processing and provide a profile with a spatial resolution between 7 and 15 kilometres and at altitude levels between 20 and 70 kilometres. The first method uses the SOMORA microwave radiometer, while the second method (”Umkehr” method) is based on measurements taken at sunrise and sunset with the Dobson and Brewer instruments.