History of the Brewer
A major concern in many parts of the world is the amount of harmful ultraviolet (UV) radiation from the sun and sky that people are exposed to. ‘Holes in the Ozone layer’ are areas of stratospheric Ozone depletion and are not only confined to the North and South Poles. They are indicators of the general health of the atmosphere, and a reduction in Ozone means that more harmful UV reaches the ground.
To accurately measure stratospheric Ozone, and solar UV radiation, requires a sophisticated instrument. The Brewer Spectrophotometer is unique, designed specifically for operating automatically with high accuracy over long periods of time, in all climates and environments from the tropics to Antarctica.
The Beginning of Ozone measurement
Gordon Miller Bourne Dobson (1889 – 1976) was a British scientist who devoted much of his life to the observation and study of atmospheric Ozone, mainly at Oxford University. The results were to be of great importance in leading to an understanding of the structure and circulation of the stratosphere.
In order to make measurements of Ozone in the stratosphere he designed a spectrograph using the sun as the light source and photographic plates to capture the spectra. In 1924 he demonstrated the seasonal variation of Ozone, the maximum in the spring and the minimum in the autumn, and the close correlation between Ozone amount and meteorological conditions in the upper troposphere and lower stratosphere.
By the end of 1927 there were several spectrographs in use around the world. However, the inconvenience of the photographic technique led him to design and build the Dobson Photoelectric Spectrophotometer, which enabled the relative intensity at two wavelengths to be measured directly. It was completed in 1927 or 1928. Instrument number 1 is in the London Science Museum and the last instruments were made in the 1980s.
Ozone research became organised under the International Ozone Commission, which was set up in 1948. The Dobson Unit (DU) is the measurement quantity for total column of atmospheric Ozone.
History of the Brewer Spectrophotometer
Dr. Alan West Brewer (1915 – 2007) was a British-Canadian physicist who joined Professor Dobson at Oxford in 1948. Dobson retired in 1950 and Brewer moved to the University of Toronto in 1962.
The main problems with the Dobson Spectrophotometer are that it is manually operated, very large and heavy and can only measure Ozone. To address these issues Brewer and Dr. David Wardle, then a lecturer at the University of Toronto, began to develop a new automated instrument for measuring Ozone. Dr. Brewer retired in 1977.
David Wardle moved to Environment Canada and with Drs. Tom McElroy and James Kerr developed the Brewer Ozone Spectrophotometer to measure not only total Ozone but also sulphur dioxide (SO2) and ultraviolet (UV) radiation. Their original instrument (retrospectively, the Brewer MkI) was patented by Environment Canada, which still owns the Intellectual Property.
The Brewer is an automated, diffraction-grating spectrometer that provides near-simultaneous observations of the sun’s intensity at six wavelengths in the near UV range. These data are used to calculate Ozone and SO2 amounts and the aerosol optical depth. The thickness of the Ozone layer is determined by comparing the intensity of solar radiation at different wavelengths in the ultraviolet that experience very different absorption by ozone in passing through the atmosphere.
The instrument also makes measurements of the spectral UV radiation reaching the ground. The Brewer spectrophotometer remains the only ozone measuring spectrophotometer that can operate continuously outdoors and provide automatic observations of Ozone, UV radiation, SO2 and aerosol information.
In 1981, SCI-TEC Instruments of Saskatoon, Canada, started to make the first production instruments, the Brewer MkII. Later a quartz dome and diffuser were added to enable UV spectral scans of the global radiation from the sky in addition to the direct sun spectral scans.
In 1986 the Brewer MkIV was introduced. This had the ability to measure atmospheric Nitrogen Dioxide (NO2) instead of SO2. In 1990 the Brewer MkIV was given an extended wavelength range to measure both SO2 and NO2, which was of particular interest at a time of rising concern about ‘acid rain’ affecting forests and the biosphere in general. Starting in 1995, the United States Environmental Protection Agency (US EPA) set up a network of 22 MkIV Brewers, primarily for acid rain monitoring. However, measuring NO2 in the visible part of the spectrum somewhat compromises the Ozone and UV performance.
In 1988 the Brewer became the World Meteorological Organisation (WMO) Global Atmosphere Watch (GAW) standard for Stratospheric Ozone Measurement and a reference Brewer. ‘Triad’ was set up by Environment Canada in Toronto along with the World Ozone and Ultraviolet Radiation Data Centre (WOUDC).
The MkIII Brewer had been under development earlier and was finally introduced in 1992. It has two spectrometers in series for significantly improved UV response and measures UVA, UVB, UVE, UV Index, O3, SO2 and aerosol optical depth.
One Mk V ‘Red’ Brewer was built at the factory in 1996 for the Eureka Arctic base. This measures Ozone in the Infrared using the moon as the light source at high latitudes where the sun is often at low elevations or below the horizon.
In recent years the WMO recognised the Regional Brewer Calibration Centre for Europe (RBCC-E) at the Izaña Observatory on Tenerife. This centre has a triad of MkIII Brewers manufactured in Delft.
The Brewer MkIII is the only spectrometer currently in production sanctioned by the WMO for making total column Ozone measurements.
The Brewer in Delft
Sci-Tec Instruments in Saskatoon became Kipp & Zonen Inc. and Brewer production was transferred to Delft in 2004. The instrument is manufactured under license of Environment Canada exclusively by Kipp & Zonen in the Netherlands, and Environment Canada states: “Mark III Brewer Ozone Spectrophotometers are recommended by Environment Canada (EC) as the significantly superior model of Brewer instrument with which to measure ozone in the ultraviolet (UV) region of the spectrum.”
For this reason, and because of the significantly improved spectral measurement of UV radiation, (due to the double spectrometer) the Brewer MkIII is the only model now in production.
By the end of 2014 over 228 Brewers of all types will have been produced and most are still in operation. They are in over 40 countries around the world from the Tropics to the Arctic and Antarctic. About 80 stations regularly report to the WOUDC.
Kipp & Zonen provides a full range of calibration, spares, repairs and support services for all models of Brewer and continues to improve and update the Brewer MkIII.