Combined solar aureole and solar beam extinction measurements 1: Calibration considerations

A small field-of-view microprocessor-controlled radiometer has been developed for measuring the spectral radiance in the solar aureole region and on the solar disk itself. Calibration problems which include the influence of instrumental scatter on solar aureole measurements and the effects of optical instability on Langley plot data have been investigated. A simple technique for measuring the instrumental scatter component is presented and comparisons are made with actual aureole data. Simultaneous measurements of NO2 columnar abundance were performed in order to assess the effect of variable NO2 optical depth on Langley plot calibration constants in the blue region of the spectrum. The aureole radiance at a constant angular distance from the sun is used to infer optical stability as affected by aerosol content variation. The latter two studies were felt to be particularly relevant given that our measurements were made in the vicinity of a large urban center.