For more information about the Brewer Ozone Spectrometer and data, please contact Dr. Tom McElroy
A Brewer was first installed in the Canadian Arctic at Alert (82oN) in 1987. However the record was interrupted in 1996 and not restarted until 2004. Several Brewers are now operated at PEARL and at the Eureka weather station (80oN) from time to time, with the first data being collected in 1993. Data from Brewers are an important contribution to the long-term ozone-monitoring data set. Both single-monochromator and double-monochromator Brewers (the difference to be discussed later) are regularly operated at Eureka. The single Brewer that has been operating in the global network since the first commercial sales in 1982, and the double Brewer after 1992. A Brewer was installed at the Amundsen-Scott base, Antarctica (90oS) in February, 2008. So Canadian Brewers are operating almost from Pole to Pole (Figure 2).
The Brewer makes measurements in the ultraviolet region of the spectrum of sunlight. It measures in several modes. Direct-sun measurements (DS) are made by pointing the instrument automatically in the direction of the incoming solar radiation. Clearly there is a considerable period in the Arctic when there is no sunlight. However, the Brewer continues to measure in the winter night by measuring sunlight scattered from the surface of the moon. These measurements – focused-moon or FM observations – are useful for approximately two weeks out of each lunar month. Measurements can also be made by observing the light scattered down from the zenith sky – called zenith sky measurement (ZS) – which are analyzed using a special formula developed for the purpose [1].
History of the Brewer Development
The Brewer Ozone Spectrophotometer, developed at Environment Canada (EC) [2], is now deployed in more than 45 countries. The UV Index, also invented by J.B. Kerr, C.T. McElroy and D.I. Wardle at EC, was developed using data collected by Brewer instruments in Canada, and is reported in 25 countries. Canada has an extensive, national network of Brewers to monitor ozone and ultraviolet radiation. The network ranges from Toronto in the south to Alert at the north end of Ellesmere Island in the Arctic and from Saturna Island in B.C. in the west to Halifax, Nova Scotia in the East. More than 200 instruments have been manufactured since the first ones were made commercially in 1982.
The concept for an instrument, based on a diffraction grating, to replace the venerable Dobson Ozone Spectrophotometer was advanced by Alan Brewer while at the University of Toronto around 1970. Alan had developed an optical ray-tracing program while visiting MIT in the 1950s. As a post doctoral fellow, David Wardle used the software to design the precursor to the Brewer, in the form of a double monochromator, multi-wavelength spectrophotometer. The spectrum of ozone increases exponentially toward shorter wavelengths leading to a huge gradient in intensity when the spectrum of the sun is observed. This means that even small amounts of light spuriously scattered in the instrument to shorter wavelengths limits the ability of the instrument to properly determine the optical depth due to ozone and leads to non-linearity in the response of the system to ozone (too little ozone at large ozone paths).
However, in the early days of the instrument’s development, before lasers, it was very difficult to align an instrument as complex as a double monochrometer. Light has to pass through a dispersing instrument to produce a spectrum with a narrow spectral band selected by an exit slit. Then the light passes backward through an identical instrument to a final exit slit. For good throughput through the instrument, as a function of wavelength, the two halves of the device need to be aligned to be virtually identical. This was nearly impossible to do without laser sources. The decision was made to use only the single monochromator to measure ozone and to improve its stray light rejection with a filter, like the one used in the Dobson, that passed the UV region of interest and suppressed radiation from longer wavelength regions. This is the form of the instrument that was patented and that went into commercial production by a Canadian company in 1982. The decision to cut the instrument in half had an interesting side effect. It was modified to measure in the blue part of the spectrum to make the first differential, optical measurements of nitrogen dioxide in the atmosphere in 1972 [3].
However, the inability to make accurate ozone measurements at large atmospheric paths – small sun elevations – and useful spectral measurements at short wavelengths in the ultraviolet (wavelengths < 300 nm) argued strongly for a double monochromator. McElroy designed the modern double Brewer – which uses the optical frames of two single instruments – and had the first unit constructed in 1992. This instrument was unique in that the angle of the gratings in the two halves of the instrument are controlled independently so that the precision of alignment between the two halves of the instrument is reduced, since the control computer can determine a relative ‘dispersion’ equation to link the wavelength variation as a function of grating angle to optimize the throughput of the system. The spectral band pass is determined by the slits at the midpoint between the two halves of the instrument and the stray light properties are controlled by the final exit slit before the photomultiplier detector. The final exit slit is 1.5 times the image of the entrance slit, so that minor misalignments of the system do not result in variations in throughput.
How does the Brewer measure ozone and sulphur dioxide?
The Brewer optical design is based on the Ebert-Fastie spectrometer. In the classic design, light from a narrow entrance slit, on which the light to be measured is imaged, is collimated by a reflection from a spherical mirror located at one-half its radius from the slit. The collimated beam is reflected by the mirror toward a diffraction grating mounted on the instrument’s optical axis, coincident with the mirror normal, which send a diffracted beam to the second half of the same mirror to produce an image at the exit plane where a second slit captures the wavelength of interest. The symmetry of the system suggests that optical aberrations in the system should be very small. As it turns out, this is not the case because the diffraction grating has to be operated at an angle to the optical axis of the instrument to select the wavelength to be measured. This results in the beam leaving the grating being of a different width than the one from the entrance slit that landed on the grating. The aberrations caused in one half of the instrument are not precisely cancelled by the other half and this results in residual astigmatism and coma, which degrades the spectral resolution of the instrument.
A solution to this problem was found using a computer ray tracing process that allowed the design of a compensating lens located just behind the entrance slit, that has allowed the instrument to produce a very high quality image of the entrance slit at the exit plane. In addition, it was found that a desirable, flat image plane over a range of wavelengths could be produced by slightly, separately adjusting the distance to the mirror surface of the entrance slit and exit plane. This allowed the development of a high-quality spectrum over a range of wavelengths on the flat image plane, normal to the instrument optical axis. This was desirable since the aim was to provide multiple exit slits that could be opened and closed sequentially by a mechanical chopper to allow the multiplexing of one detector to measure all wavelengths, rather than rotating the diffraction grating with its mechanical limitations and relatively slow operation. The result is a very high relative measurement accuracy of the observed wavelengths, even under changing observing conditions and changes in the detector absolute sensitivity.
Molecules have variations in their absorption spectra as a function of wavelength that are unique to the molecule. The relative change in light intensity as a function of wavelength, caused by that absorption, allows the measurement of the molecule using sunlight, skylight and moonlight. To make ozone (O3) and sulphur dioxide (SO2) measurements with the Brewer, five different wavelengths are isolated with precision exit slits and light is allowed to reach each of the slits in turn using a drum-like mask with over-size openings rotated by a stepping motor. In addition to the five wavelength positions, one extra is used to monitor the dark signal by having a mask position with no opening. Measurements are made from the dark position, 0, to slit 5, and then in the reverse order. By measuring in both directions, linear changes in intensity that may occur during the measurement are cancelled out. The entire cycle of 12 measurements takes place in about 1.2 seconds and 32 cycles (~38 s) are co-added to produce a measurement. In the O3 and SO2 measurement mode, the wavelengths measured are 303.2, 306.3, 310.0, 313.5, 316.8 and 320.0 nm, with the shortest wavelength contributing the SO2 information. Some sample Brewer ozone data are shown in Figure 3.
The measurements are mathematically reduced to suppress the influence of absorbers other than O3 and SO2, to eliminate the influence of absolute intensity, and suppress the effect of small shifts in wavelength. Information about the Brewer can be found at the manufacturer’s website [4].
References
[1] Fioletov, V.E., C.A. McLinden, C.T. McElroy, and V. Savastiouk, New method for deriving total ozone from Brewer zenith sky observations, J. Geophys. Res., 116, D08301, doi:10.1029/2010JD015399, 2011.
[2] Kerr, J.B., C.T. McElroy, and D.I. Wardle, Grating Ozone Spectrophotometer, United States Patent, 4,652,761, 16 pp, March 24, 1987.
[3] Brewer, A.W., C.T. McElroy, and J.B. Kerr, Nitrogen dioxide concentrations in the atmosphere, Nature, 246, n. 5429, 129-133, 1973. [4] http://kippzonen-brewer.com/about-brewer/technical-specs/
The Author
Tom McElroy is a former senior scientist from Environment Canada (EC) and a co-inventor of the Brewer Ozone Spectrophotometer and the UV Index. While at EC he was a strong promoter of building the Eureka observatory rather than constructing a facility in Resolute Bay. The reason for this was the unique placement of the site in the high Arctic and the high elevation of the laboratory site which gets it above the low-level ice fog and most of the water vapour which otherwise hinders infrared spectral and LIDAR observations at sea level. After leaving EC in 2011 he took up an Industrial Research Chair and worked as a professor at York University until June 2018. He remains a Professor Emeritus and Senior Scholar at York.