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.
The SURFRAD MFRSR Channel Calibration Algorithm
MFRSR channels are calibrated using a linearized form of Beer's Law to produce Langley plots (right) from which an extrapolation to zero path length represents the calibration (Vo), or what the instrument would measure at the top of the atmosphere. Therefore absolute calibrations of the channels is not necessary for the AOD application.
A plot of the log of voltage measurements versus the path length (m) will produce a straight line if all measurements were collected with a clear view of the sun. The extrapolation to 0 m is the log of Vo--the calibration.
Therefore, only measurements made under pristine conditions are used for calibration Langley plots.
Global solar SURFRAD measurements marked with * in the daily time series above were identified as times of cloud-free skies. The afternoon clear-sky times are cross referenced with coincident MFRSR data to produce the calibration Langley plot on the right.
Blue points are MFRSR 500 nm measured MFRSR voltages that correspond to the afternoon cloud-free times on the plot to the left.
These automatically generated calibration Langley plots are constructed over one, or two-month periods and composited to identify a representative Vo for that period.
Choosing a Calibration
All Vo's in the two-month period that pass the statistical elimination are corrected to an circular orbit (because of the yearly variation in the earth-sun distance) and then averaged to a "representative" orbit-normalized calibration Vo for that period.
"Representative" Vo's for each channel (square symbols) are then plotted in time series over two year periods, as shown in the example below.
The sinusoidal lines are best fit functions used to interpolate spectral Vo's to specific days. Interpolated daily Vo values are corrected back to the earth-sun distance appropriate to the day being processed, and then combined with MFRSR measurements to produce daily time series of total optical depth, from which AOD is derived.
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, .
Effects of ozone absorption () and molecular scattering () are removed from the total optical depth () to achieve aerosol optical depth ().
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.
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:
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.