SHORTWAVE INFRARED SPECTRORADIOMETER FOR ATMOSPHERIC TRANSMITTANCE MEASUREMENTS

The use of a shortwave infrared (SWIR) spectroradiometer as a solar ra diometer is presented. The radiometer collects 1024 channels of data o ver the spectral range of 1.1-2.5 mu m. The system was tested by apply ing the Langley method to data collected at a high altitude site on tw o consecutive days. Data processed for the 1.15-1.32-mu m and 1.47-1.7 5-mu m spectral intervals shovel temporal results similar to those obt ained with a well-understood, visible. and near-infrared radiometer ha ving 10 channels in the 0.38-1.03-mu m spectral range. A modified Lang ley method was used for spectral regions where strong water vapor abso rption invalidates the Langley method. It is estimated that the exoatm ospheric intercept of the spectroradiometer was determined to better t han 4% in nonabsorption regions between 1.15 and 1.75 mu m and to bett er than 5% for a large portion of the 1.38-mu m absorption band. These results, in addition to the agreement between the shortwave, and the visible and near-infrared radiometers, imply that the SWIR system oper ates well as a solar radiometer. The spectral optical depths from one day were used to determine a power-law aerosol size distribution using data from both the visible and near-infrared, and the shortwave infra red. The exponent derived for this power law differed from that obtain ed by using only the visible and near-infrared by 6%. Aerosol optical depths in the shortwave infrared derived from the visible and near-inf rared results differed from the measured values by 0.005 at an optical depth of 0.016 and wavelength of 1.66 mu m.