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, part 2

The result is a daily time series of 500-nm AOD, as shown below for 28-Apr-2004 for the Sioux Falls station:

Cloud-screened AOD plot for Sioux Falls, 28 April, 2004

Because the MFRSR operates continuously, its data record includes measurements appropriate for computing AOD (i.e., data with a clear shot of the sun), as well as cloud-contaminated measurements. Therefore, a cloud screening algorithm is run on each daily AOD time series. In the example above, the blue points passed the cloud screen test and likely represent good AOD values, whereas the red points failed the cloud screen test. The plotted green dots represent the Angstrom exponent and follow the scale on the right. The Angstrom exponent is computed with the AODs for the 500-nm and 868-nm channels.