A high resolution ultraviolet spectroradiometer and its application in solar radiation measurement

An ultraviolet (UV) spectroradiometer is refurbished and upgraded with a fore-optical module. In addition to measuring total UV irradiance, the UV spectroradiometer can measure solar direct beam and sky radiance at any preset azimuth and elevation angle. This double Czerny-Turner spectroradiometer, with an ion-etched holographic grating operating in the first order with 3600 lines per mm, enables wavelength scanning range from 290 nm to 410 nm, with a nominal bandwidth of 0.1 nm.  It can operate with a step-size of 0.0005 nm and a full width at half maximum of 0.1 nm. It has an out-of-band rejection ratio of approximately 10^–10. This high resolution spectroradiometer can be used as a reference instrument for UV radiation measurements and to monitoring atmospheric gases (O3, SO2, NO2). Recently laboratory work suggests that water vapor displays structured absorption features over 290-350 nm region with maximum and minimum cross-sections of 8.4×10-25 and 1.4×10-25 cm2/molecule. To investigate the water vapor absorption features in UV region in real atmosphere, we did a series of field observations by using this high resolution spectroradiometer. A residual analysis method is developed to analyze the absorption of atmospheric components and to retrieve the atmospheric optical depth. The residual spectra of multiple cases have spectral features similar to that of in-lab measured water vapor absorption in some wavelength regions, and the inferred ozone amount from residual analysis agrees with OMI retrievals. Through multiple case studies, the magnitude of residual optical depth from observed UV spectra is sensitive to the atmospheric water vapor amount. The greater the water vapor path, the larger the difference between observational spectra and calculated spectra without considering water vapor absorption.