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.

Computing Aerosol Optical Depth

Interpolated daily Vo calibration values are corrected back to an elliptical orbit, then combined with MFRSR measurements(V) using the linearized form of Beer's law (below) to compute a daily time series of total optical depth, Sigma Tau.

sigma tau equals natural log of V0 minus natural log V all divided by m

Effects of ozone absorption (tau sub o3) and molecular scattering (tau sub m) are removed from the total optical depth (sigma tau) to achieve aerosol optical depth (tau sub a).

tau sub a equals sigma tau minus tau sub O3 - tau sub m

Ozone absorption is computed from TOMS total ozone measurements over the station (obtained from a NASA web site), and molecular scattering is accurately computed used the measured atmospheric surface pressure at the SURFRAD station.