Aerosols and aerosol optical properties in Bergen
The solar radiation at the top of the atmosphere is attenuated before it reaches the earth's surface. The attenuation is due to extinction (scattering and absorption) by particles and molecules in the atmosphere. If the solar radiation at the top of the atmosphere is known, the radiation received at the earth's surface can be used to reveal the properties of the atmospheric aerosol.
Measurements of solar radiance at different wavelengths with a Cimel sun photometer stationed on the roof of the building housing the department of Physics and Technology, University of Bergen are made.
My objective is to find the spectral aerosol optical thickness, precipitable water vapour column, particle size distribution, and hence deduce the types of aerosols.
The solar radiances at the top of the atmosphere or the spectral calibration coefficients of the instrument are required before the instrument is used to make measurements for the retrieval the spectral aerosol optical thickness and precipitable water vapour column.
The calibration procedure requires a clear sky day where we assume a stable and constant atmosphere. Measurements are made every after about 30 - 45 minutes at different solar elevations and each time the solar zenith angle is recorded.
A Langley calibration procedure is then performed on the data to find the spectral calibration coefficients. With the spectral calibration coefficients known, a single direct sun radiance measurement can be used to retrieve the spectral aerosol optical thickness and precipitable water vapour column. Other optical properties of the atmospheric aerosol are retrieved using the spectral aerosol optical thickness and the sky radiance measurements.