Comparing ground‐level spectrally resolved solar UV measurements using various instruments: A technique resolving effects of wavelength shift and slit width

Abstract

Spectrally resolved UV measurements are important for the study of biologically relevant UV in relation to changes in atmospheric parameters. The inter‐comparison of spectral instruments is essential as measurement techniques and calibrations are not standardized. The differences in slit functions cause large spectral variations when comparing the spectral readings directly. The method described, which compares spectral readings using different instruments, corrects for differences of wavelength calibrations and slit functions, and does not require knowledge of additional atmospheric parameters and UV‐transfer model calculations. The wavelength alignment has an accuracy of 0.02 nm over the wavelength interval from 300–400 nm, and a reproducibility of 0.01 nm. The robustness of the methods and reproducibility of results are shown in the evaluation of a seven day intercomparison campaign with three different scanning spectroradiometers.

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

“…The wavelength registration of the instruments during the intercomparison is performed either or both by using emission lines from an Hg lamp [24] and/or using the solar Fraunhofer structure [see Table 2]. The reported wavelength accuracy capable using the SHICrivm method is 0.02 nm with a reproducibility of 0.01 nm across the wavelength range 300-400 nm [7,8]. The wavelength accuracy depends on the wavelength accuracy of the extraterrestrial spectrum used in the analysis.…”

“…The following describes the data preparation and standardization of the spectra for differences in the instruments' slit-functions using two different methods to facilitate comparisons between measurements from the different instruments. The first method uses an algorithm that deconvolves and re-convolves to a 1-nm triangular slit function using a technique developed previously [7,8].…”

“…This is one of the methods used in the analysis of the spectral solar irradiance from the previous North American Intercomparisons. The second method uses an algorithm developed at RIVM in the Netherlands [7,8]. The method involves deconvolving the solar irradiance using a high-resolution extraterrestrial spectrum and then reconvolving the solar irradiance with a 1-nm triangular slit function.…”

“…The extraterrestrial spectrum is based on data from the Kitt Peak Observatory and Atlas 3 and has a resolution of better than 0.005 nm. The method has the additional advantage of correcting for possible wavelength shifts in the data by comparison of the structure in the solar spectra to the solar Fraunhofer structure as described in section 3.1 [7,8]. The algorithm has other features that help to identify outliers (e.g.…”

“…Instruments that have not participated in previous North American Intercomparisons (NSF_SUV, IMUK_BEN, NOAA_NIWA) have been characterized fully elsewhere. However, the wavelength accuracy was checked using an algorithm developed by H. Slaper and results are presented in this section [7,8].…”