Ozone is an omnipresent but rare component of the earth's atmosphere. At ground level in pure air, each cubic metre of air contains about 30 mm3 of ozone, which corresponds to a blend grade ratio of 30 ppb (parts per billion). While the direct effect of this gas in higher concentrations is harmful to living organisms, stratospheric ozone forms a vital protective shield against the sun's dangerous UV radiation. Changes in the ozone content in all layers of the atmosphere are, therefore, of great interest and should be measured reliably. In the early 1990s, a highly sensitive, compact and fast-responding ozone sensor was developed by Dr. Hans Güsten from the former Nuclear Research Center Karlsruhe (after its merger in 2006 part of the Karlsruhe Institute of Technology (KIT)) and Prof. Dr. Ulrich Schurath from the University of Bonn. For their work they were awarded the 1993 Helmholtz Prize in the field of "Physical Metrology in Medicine and Environmental Protection".
The novel sensor used chemiluminescence, which is generated by ozone in reaction with certain organic dyes. For this purpose, the ozone-containing air was sucked into the sealed lightproof interior of a sensor at high speed by means of a small fan. There, the air travelled past the sensor head which was made up of a 2.5 cm plate coated with silica gel. On this plate, the organic dye coumarin 47 (that emits in the blue spectral range) was absorbed. The ozone generated a faint bluish glow in a reaction with the dye, the so-called chemiluminescence, which was registered by a photomultiplier.
The sensor had a number of favourable characteristics. It was compact and had approximately the size of a cigar box, weighed less than 1 kg and consumed less than 20 watts, whereas common ozone detectors weighed 15 kg and consumed 100 watts. Moreover, the sensor was fast and sensitive: its response time was less than 0.1 s and its detection limit was less than 0.1 ppb, while standard ozone detectors had response times of 20 s and sensitivities of 1 ppb. Consequently, the novel sensor was able to temporally resolve rapid changes in concentration such as those that occur in ozone measurements in aeroplanes. The ozone sensor was also excellently suited for vertical soundings in the earth's atmosphere, a method where a sensor is sent up into the atmosphere with a balloon and floats down again on a parachute after the balloon has burst in the stratosphere at a height of 40 km. The ozone profiles recorded during its ascent and descent are almost identical.
In addition, the sensor was also suitable for monitoring ozone concentrations in the biosphere or at the workplace. It was therefore used to continuously measure the vertical ozone fluxes in a field of sunflowers and to monitor rapid changes in the ozone concentration in a welder's working area. The ozone sensor from Dr. Güsten and Dr. Schurath was used commercially, copied and further developed by other research institutes.
