Temperature dependence of the Brewer ultraviolet data

Abstract

Abstract. We characterize the temperature dependence of the U.S. Environmental Protection Agency/University of Georgia network of Brewer spectrophotometers as used to measure solar ultraviolet (UV) radiation. The instruments used in this study are operated in partnership with the National Park Service at 14 national park sites and in 7 urban areas. The daily and seasonal measurements of UV radiation provided by the instruments can be affected by changes in the internal instrument temperatures at the sites. These effects can lead to errors on the order of _+ 10% in the resulting spectral data and of the same order of magnitude for CIE-weighted UV. Fortunately, the temperature dependence for each instrument can be quantified and the data corrected, improving the accuracy to values closer to the levels attainable with high-quality calibration and operation. The temperature dependence of the Brewers is found to vary significantly among the different instruments. A 0.8% per degree Celsius dependence can result in temperature effects as large as 12% at sites where temperatures can vary by 15øC in 1 day. These effects can result in a _+5% error in the spectral irradiance. The errors to the spectral irradiance vary seasonally in a manner that is not random: in the warmer summertime the temperature dependence of the instruments can cause the irradiances to be underestimated, while during the colder winters the effect will be to overestimate UV amounts. In the part of the spectrum above 325 nm, the temperature dependence is generally independent of wavelength. Below 325 nm the temperature effects vary as a function of wavelength over a range of values and are generally largest at the shortest wavelengths. Because changes in temperature from one calibration to the next can affect an instrument's response, understanding the temperature effects is necessary to ensure that artificial trends are not introduced into the Brewer data records.

Monitoring of UV spectral irradiance at Thessaloniki (1990–2005): data re-evaluation and quality control

“…These measurements are corrected regularly for the effects from changes in temperature, but, because the ozone algorithms are based on radiance ratios, temperature related changes to the instrument response are not critical, as long as the changes are independent of wavelength (Weatherhead et al, 2001). However, in the spectral irradiance measurements the temperature sensitivity of the Brewer is expected to be more apparent.…”

“…The advantage of this method is that it provides a lot of information on the history of the temperature coefficients, which appear to vary with time. The main disadvantage is the lack of information on possible temperature effects on the operation of the internal standard lamps, including the electronic circuit that drives these lamps (Weatherhead et al, 2001).…”

“…In addition, a series of post processing procedures have been developed aiming at the improvement of the spectral solar UV measurements' accuracy. These, include methods for correcting the measurements for angular response errors (Seckmeyer and Bernhard, 1993;Bais et al, 1998;Feister et al, 1996), for wavelength scale errors (Huber et al, 1993;Slaper et al, 1995;Slaper and Koskela, 1997), for temperature dependence (Weatherhead et al, 2001;Capellani and Kochler, 1999), for stray light rejection (Davies, 1996;Bais et al, 1997a) and for spikes (Meinander et al, 2003).…”

2003 North American interagency intercomparison of ultraviolet spectroradiometers: scanning and spectrograph instruments.

“…8(a) to 8(b) [27]. They also correct for the temperature dependence of the responsivity that typically is 0.4 -0.1%/°C [28,29]. However, the temperature difference between the two responsivity measurements is only 0.1°C and this cannot explain the difference observed from Fig.…”

“…The stability of the lamps used for the CUCF calibration are given in section 4 and are less than 1% during the intercomparison. Other uncertainties include the temperature dependency in the responsivity and the correction of the data [28]; recent studies have looked at the temperature dependence of PTFE diffusers [32,33]. Uncertainties in spectral solar irradiance from well-maintained spectroradiometers have been estimated at ±6.3% -12.7% from the UVA to 300 nm [23].…”