The 1997 North American interagency Intercomparison of Ultraviolet Spectroradiometers including narrowband filter radiometers - Statistical Data Included

Journal of Research of the National Institute of Standards and Technology, Jan, 2002 by Kathleen Lantz, Patrick Disterhoft, Edward Early, Ambler Thompson, John DeLuisi, Jerry Berndt, Lee Harrison, Peter Kiedron, James Ehramjian, Germar Bernhard, Lauriana Cabasug, James Robertson, Wanfeng Mou, Thomas Taylor, James Slusser, David Bigelow, Bill Durham, George Janson, Douglass Hayes, Mark Beaubien, Arthur Beaubien

3.1 Brewer Spectrophotometer

Two Sci-Tec Brewer spectrophotometers (Model MKIV) participated at the Intercomparison that measure total solar ultraviolet irradiance from 286.5 nm to 363 nm and total column [O.sub.3], [SO.sub.2], and [NO.sub.2] from both direct sun and zenith sky measurements at specific wavelengths. A right-angle prism directs light from one of several sources, either internal calibration lamps, the sky, or a Teflon diffuser, along the optical path. This path contains apertures, filters, and lenses that focus the light onto the entrance slit of a single-grating modified Ebert-type monochromator.

The exit slit focal plane of the monochromator contains six slits, five for selecting the wavelengths for determining the total column amounts and one for wavelength calibration. A slotted cylindrical slit-mask in front of the exit slit plane serves as the wavelength selector. The nominal bandwidth, set by the exit slits, is 0.6 nm. For the Model MKIV Brewer spectrophotometer, the diffraction grating operates in third order for UV spectral scans and [O.sub.3] and [SO.sub.2] measurements and second order for [NO.sub.2] measurements. Light from the exit slit passes through a lens and a filter before focusing onto the cathode of a photomultiplier tube (PMT). The photon pulses from the PMT are amplified, discriminated, and divided by the slit-mask cycle before being transmitted to the counter. For wavelengths shorter than 325 nm the MKIV model uses a [NiSO.sub.4] filter sandwiched between two Schott UG-1 1 filters, and a single UG-11 filter for longer wavelengths.

The wavelength of the monochromator in terms of micrometer steps of the instrument is determined at the factory from the wavelengths of Hg emission lines. The wavelength registration of the monochromator is periodically checked and adjusted throughout a day by scanning the micrometer forward and backward about the 302.3 nm line from the internal Hg calibration lamp. The EPA Network uses a set of lamps, housing, and power supply furnished by the manufacturer for stability checks. These are 50 W quartz-halogen lamps mounted horizontally 5 cm above the diffuser in a housing and operated at a constant 12 V.

Biospherical Ultraviolet Spectroradiometer

The Biospherical SUV- 150 11-002 Ultraviolet Spectroradiometer is a 150 mm f/4.4 Czerny-Turner double monochromator that employs a grating blazed at 240 nm with 2400 grooves per millimeter. The instrument has a nominal bandwidth of 0.7 nm and typically scans from 280 um to 600 nm in 0.2 nm steps, taking approximately 16 minutes to complete a scan. A schematic of the SUV-150 is given in Figure 3.1. The SUV-150 utilizes a quartz window with a vacuum-formed Teflon diffuser at the entrance port of an integrating sphere. The diffuser is heated to minimize ice and snow buildup. The monochromator is coupled to a 9-stage dynode R237 1 PMT mounted in a shielded temperature regulated semi-hermetic enclosure. The temperature is maintained within [ or -]1 [degrees]C by a thermoelectric heater/cooler, driven by a PD controller. The instrument is fully automated and weatherproofed for use in extreme conditions.