A High Resolution Ultraviolet Spectroradiometer And Its Application In Solar Radiation Measurement

A recent laboratory study suggests that water vapor displays structured absorption features over the 290-350 nm region, with maximum and minimum cross-sections of 8.4 x 10(-25) and 1.4 x 10(-25) cm(2)/molecule at room temperature (Pei et al. 2019, ; Du et al., 2013, ). To observe water vapor absorption features in the ultraviolet (UV) region in the atmosphere, a United States Department of Agriculture reference spectroradiometer was upgraded with a new fore-optical module, enabling it to measure direct solar beam and sky radiance at given azimuth and elevation angles. This double Czerny-Turner spectroradiometer enables wavelength scanning from 290 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 for monitoring atmospheric gases such as O-3, SO2, and NO2. A series of field observations were made using this spectroradiometer in the University at Albany campus. A residual analysis method is developed to analyze absorption by atmospheric components and to retrieve atmospheric optical depth. The residual optical depth was calculated by subtracting the optical depths of Rayleigh scattering, aerosol extinction, and absorption of typical atmospheric gases such as O-3, SO2, and NO2 from the retrieved total optical depth. Multiple case studies show that residual optical depth from the observed UV spectra is sensitive to the atmospheric water vapor amount. The greater the water vapor path, the larger the magnitude of residual optical depth. The ozone amount was inferred from the residual analysis; it is comparable to the satellite measurements. For example, in a case with water vapor path of 13 mm on October 24, 2019, the inferred ozone amount from residual analysis is 2.7% lower than retrievals from the Ozone Monitoring Instrument-Total Ozone Mapping Spectrometer.