The Spectrometer

The Brewer instrument uses a modified Ebert grating spectrometer (Fastie, 1952) to disperse incoming radiation into a spectrum. The first optical element of the spectrometer is a tilted quartz lens. The purpose of the lens is to correct for optical aberrations, which are inherent in an Ebert spectrometer. Both of the lens surfaces correct for an aberration. An off-axis spherical concave surface of the lens compensates for coma due to the asymmetry of the Ebert spectrometer. The second surface is cylindrical convex and is used to correct for astigmatism.

The optical path continues to a spherical mirror where radiation is collimated and directed toward the grating. Radiation is dispersed at the grating and directed toward the spherical mirror where the spectrum is focused on the plane of the exit slits.

There are six exit slits that are approximately evenly spaced across the focal plane. The shortest wavelength slit is for wavelength calibration using a grouping of mercury emission lines near 302.1 nm or the single emission line at 296.68 nm.

The other five slits correspond to wavelengths that take advantage of the structure of both the ozone and SO2 absorption spectra at UV wavelengths. Figure 6.3 shows the wavelength positions of the six slits with the absorption spectra of ozone and SO2. The absorption spectrum of ozone differs from that of SO2 and, as a result, it is possible to quantify both absorbers (Kerr et al., 1981).

Ozone and SO; absorption coefficients

24 J

22 — 303.Onm 306.4nm 310. him 313.5nm 3 I6.8nm 320.0nm

0 ——i—i—]—r—i—i—i—|—r—i—r—r—|—r—i—r—i—p—i 305 310 315 320

Wavelength (tun)

Figure 6.3 Absorption coefficients for ozone and SO2 smoothed to the resolution of the Brewer instrument. Operational wavelengths of the Brewer Spectrophotometer (Ai to and the wavelength used for special applications (A0) are shown. The absorption features for SO2 are significantly different than those for ozone, making it possible to measure both constituents simultaneously. It should be noted that under polluted conditions, column SO2 is usually less than 5 Dobson Units (DU), and stratospheric ozone values typically range between 200 and 500 DU. The amount of UV-B absorbed by SO2 is therefore generally less than 5% that of ozone. However, absorption by SO2 can be significant under plumes from major volcanic eruptions (Kerr et al., 1982; Krueger, 1983). Absorption coefficients are those of Bass and Paur (1985) at - 45 °C, and the coefficients for SO2 are those of McGee and Burris (1987) at - 63°C

The slits are covered by a cylindrical mask with openings that are positioned by a stepping motor to open one slit at a time. In operation, the time required to switch from one wavelength to another is about 0.016 sec, and the sampling time at each wavelength position is seven times the switching time (about 0.115 sec). A sampling cycle progresses from a lower wavelength (e.g., wavelength 1) to an upper wavelength (say wavelength 5) and back. The range of the sampling cycle and the number of sampling cycles in a sample are programmable.

The cylindrical mask also contains a position that blocks all slits as well as opening two slits at a time. The purpose of the blocking position is to provide a quick sample of the instrument's dark count (i.e., the signal that the instrument registers without any radiation). Dark count is normally measured as part of the sampling sequence and is subtracted from the counts registered at each wavelength channel. The purpose of the double slit opening is to allow a quick measurement of the instrument's dead time. Dead time is measured by comparing the photon counts when both slits are opened with the sum of photon counts when the two slits are opened individually. This test is done quickly and automatically as part of the daily operational schedule.

An 1,800 line/mm holographic grating is used in the second order for the Mark II instrument and a 1,200 line/mm grating is used in the third order for the Mark IV instrument. The grating is mounted on a set of cross-springs that serves as a frictionless bearing to allow the control of rotation with virtually no hysteresis over the operational wavelength range. Rotation of the grating is controlled by a stepping motor which drives a micrometer acting at the end of a lever arm. The drive between the stepping motor to the micrometer is geared so that one motor step is equivalent to a shift of about 0.007 nm of the spectrum across the exit slit plane. For the existing design of the Brewer instrument, the mechanical limits of the grating rotation allow for a range of wavelengths spanning from about 285 nm (measured on the shortest slit) to 365 nm (measured using the longest slit). Extension of this wavelength range without loss of hysteresis and wavelength precision is difficult. The wavelength setting can be measured to about 0.1 micrometer step (Grobner et al., 1998; Kerr, 2002), but the wavelength positioning is limited to 1.0 micrometer steps.

The double monochromator (Mark EI) uses basically the same dispersion spectrometer as that of the Mark II and Mark IV with a 3,600 line/mm holographic grating used in the first order. The main difference is that a 45° mirror is placed ahead of the cylindrical slit mask to reflect radiation 90° downward onto the horizontal focal plane where radiation passes through the exit slits and into the recombining spectrometer, which is the mirror image of the dispersing spectrometer. Both gratings (holographic with 3,600 line/mm) for the Mark EI are individually controlled to allow automatic measurement and adjustment of the wavelength alignment of the two spectrometers. Further descriptions of the double monochromator regarding wavelength stability (Grobner et al., 1998) and spectral characteristics (Bais et al., 1996; Wardle et al., 1997) are reported in the literature.

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