Shortwave Infrared Spectroradiometer for Atmospheric Transmittance Measurements

Abstract

The use of a shortwave infrared (SWIR) spectroradiometer as a solar radiometer is presented. The radiometer collects 1024 channels of data over the spectral range of 1.1–2.5 μm. The system was tested by applying the Langley method to data collected at a high altitude site on two consecutive days. Data processed for the 1.15–1.32-μm and 1.47–1.75-μm spectral intervals show temporal results similar to those obtained with a well- understood, visible, and near-infrared radiometer having 10 channels in the 0.38–1.03-μm spectral range. A modified Langley method was used for spectral regions where strong water vapor absorption invalidates the Langley method. It is estimated that the exoatmospheric intercept of the spectroradiometer was determined to better than 4% in nonabsorption regions between 1.15 and 1.75 μm and to better than 5% for a large portion of the 1.38-μm absorption band. These results, in addition to the agreement between the shortwave, and the visible and near-infrared radiometers, imply that the SWIR system operates 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 infrared. The exponent derived for this power law differed from that obtained by using only the visible and near-infrared by 6%. Aerosol optical depths in the shortwave infrared derived from the visible and near-infrared results differed from the measured values by 0.005 at an optical depth of 0.016 and wavelength of 1.66 μm.