SURFRAD includes ancillary data (e.g., cloud cover, moisture) that affect the transfer of solar and thermal infrared radiation to and from the surface. An aerosol optical depth product has been recently added.
Aerosol optical depth is a measure of the extinction of the solar beam by dust and haze. In other words, particles in the atmosphere (dust, smoke, pollution) can block sunlight by absorbing or by scattering light. AOD tells us how much direct sunlight is prevented from reaching the ground by these aerosol particles. It is a dimensionless number that is related to the amount of aerosol in the vertical column of atmosphere over the observation location.
A value of 0.01 corresponds to an extremely clean atmosphere, and a value of 0.4 would correspond to a very hazy condition. An average aerosol optical depth for the U.S. is 0.1 to 0.15.
The MultiFilter Rotating Shadowband Radiometer
The MFRSR infers the solar beam intensity by making successive global and diffuse measurements and computing their difference. In this way it simulates measurements of a sun photometer. In processing the raw data, the cosine response of the instrument is accounted for, thus allowing for accurate calibration using the Langley method.
MFRSR channels are nominally 415, 500, 614. 670, 868, and 940 nm, although each MFRSR has unique measurement channels that may be up to 4 nm different than the nominal values. The actual measurement wavelengths are used in the SURFRAD algorithms. The 940-nm channel is not processed for AOD because of its high sensitivity to water vapor.