Measurement of Spectral UV Radiation

The Brewer Mark II instrument measures global spectral irradiance (mW/m2/nm) incident on the horizontal Teflon diffuser for wavelengths between 290 nm and 325 nm at a resolution of about 0.58 nm and a sampling interval of 0.5 nm (Kerr and McElroy, 1993). Later versions of the instrument have extended the wavelength range to include irradiance at wavelengths up to 365 nm (Bais, 1997; Grobner et al., 1998).

For the short scans (290 nm - 325 nm), the shortest operational exit slit (normally set a 306.3 nm for ozone measurements) is used. The grating is rotated via the micrometer stepping motor to locate radiation centered at the specified wavelength on slit 1. Samples progress from 290 nm to 325 nm and then back to 290 nm.

Radiation is sampled for about one second at each wavelength setting, and the grating is repositioned to the next wavelength setting 0.5 nm away. A complete scan takes about three minutes. The extended scan (290 nm - 365 nm) takes longer, but is usually sampled in one direction: from the shortest to longest wavelength. The extended scan also switches to the longest wavelength slit part way through the scan to allow coverage to 365 nm.

Measurement of spectral UV irradiance is an absolute measurement, which is intrinsically more difficult than the DOAS measurement method used for total ozone. Calibration and data processing therefore require additional attention in order to obtain high quality spectra that are accurate on an absolute scale. Data processing should include corrections for dark count, dead time, neutral density filters, stray light, temperature response, and cosine response as discussed in Section 6.4.

Spectral UV data measured by Brewer instruments are regularly reported to the WOUDC (http://www.msc-smc.ec.gc.ca/woudc/) as well as the European Ultraviolet Database (EUVDB; http://uv.fmi.fi/uvdb). Here the data are archived, checked for quality, and flagged should there be any problems. These data are readily available to the scientific community.

Calibration on an absolute scale is carried out using 1,000 W quartz lamps with known irradiance output traceable to a national standards institute, such as the National Institute for Standards and Technology (NIST) in the United States. Calibrated lamps are supplied with known emissivity as a function of wavelength with an estimated accuracy of about ± 2% on an absolute scale. Typically, several primary (supplied by a standards institute) and secondary lamps (calibrated by comparison to a primary) are used for periodic calibrations. Analysis of the long-term calibration records for some Brewer instruments has shown that the output stability and degradation with age varies significantly from lamp-to-lamp. Calibration results vary by as little as ± 1.5% for some lamps and as much as ± 4% for others. Newly calibrated primary lamps usually agree to within ± 2%; however, some degrade very rapidly with age and others have remained constant for many hours of operation over the period of several years.

For instrument calibration, the power to a lamp is accurately controlled to ensure that it is operated at the specified current and voltage. The lamp is placed vertically above the horizontal diffuser at an accurately determined distance inside a blackened housing or a dark room to block external radiation and to minimize radiation reflected from the lamp. From the lamp emissivity data and the geometric setup, it is possible to determine the irradiance at the location of the diffuser. Scans of the lamp are made and, using the known irradiance, the responsivity of the Brewer instrument as a function of wavelength is determined. The responsivity is determined for radiation arriving vertically downward from the zenith. Additional measurements must be made to determine the responsivity as a function of zenith angle. Correction for the departure of the responsivity from the cosine function can be applied to data.

Generally the lamp irradiance is significantly less than that seen with the instrument exposed to full sunlight. It is therefore important to ensure that the instrument responds linearly to radiation over the range that encompasses both calibration and measurement. The frequent testing of the linearity (as discussed in Section 6.4.2), and its application during data processing, ensure the linear response of the instrument and the accuracy of the data.

The pointing capabilities of the Brewer instrument also allow for the absolute measurement of direct irradiance provided absolute calibration is carried out for the direct sun port (Bais, 1997; Grobner and Kerr, 2001). This calibration is not done as part of a routine operation of field instruments. However, special methods have been developed to determine the responsivity of the direct measurement by using lamps viewed through the pointing prism, or by comparison of a direct measurement through the prism with a direct measurement on the diffuser (Kazadzis et al., 2005). Careful consideration must be made for neutral density filter transmission, instrument temperature, internal polarization, and stray light.

Absolute measurements of direct irradiance can be used to determine the absolute intensity of the solar spectrum using the Langley extrapolation method. Extrapolated measurements of the solar spectrum using well-calibrated Brewer instruments have shown agreement to within ± 3% compared with spectral measurements made from a satellite (Bais, 1997; Grobner and Kerr, 2001).

Brewer instruments have participated in many intercomparison campaigns for UV instruments. Instrument intercomparisons are often used as a check to quantify differences between individual instruments and various instrument types (e.g., Thompson et al. 1997; Bais et al., 2002). During intercomparisons, individual instruments are calibrated using their normal operating procedures and then used to measure global irradiance simultaneously with other calibrated instruments. With the participation of several instruments, all measuring the same signal, it is possible to determine measurement uncertainty from differences between instruments and to identify problem instruments. Intercomparisons have been used to standardize calibration and operational procedures and, overall, the quality of UV measurements has improved significantly with time as a result of knowledge gained through the intercomparisons.

Was this article helpful?

0 0

Post a comment