Automatic sun tracking photometer

The CE 318 automatic sun tracking photometer measures sun and sky radiance in order to derive total column water vapor, ozone and aerosols properties using a combination of spectral filters and azimuth/zenith viewing controlled by a microprocessor.

The sun photometer is composed of an optical head with a sun collimator without lens, and a sky collimator with lenses, an electronic box with two microprocessors for real time operation for data acquisition and motion control and a robot moved by step-by-step motors in zenith and azimuth planes.

The direct sun measurements are made in eight spectral bands requiring approximately 10 seconds. Seven interference filters at wavelengths of 340, 380, 440, 500, 670, 870, and 1020 nm are located in a filter wheel which is rotated by a direct drive stepping motor

Optical depth is calculated from spectral extinction of direct beam radiation at each wavelength based on the Beer-Bouguer Law.

The processed data provide a wide number of parameters and characteristics that are important for the comprehensive interpretation of the aerosol retrieval. The output includes both retrieved aerosol parameters (i.e., size distribution, complex refractive index and partition of spherical/non-spherical particles) and calculated on the basis of the retrieved aerosol properties (e.g. phase function, single scattering albedo, Angstrom exponent, spectral and broad-band fluxes, etc.). Accurate retrievals of SSA (with accuracies reaching 0.03) can be obtained for high aerosol loadings and for solar zenith angles >50 degrees.

The volume particle size distribution dV(r)/dlnr (μm3/μm2) is retrieved in 22 logarithmically equidistant bins in the range of sizes 0.05μm ≤ r ≤ 15 μm.