How Do We Know it Works?

The GLOBE Water Vapor Sun Photometers will be calibrated by transferring the calibration of a reference unit which itself will have been calibrated by comparing against other reference units co-located at the CART site of the Atmospheric Radiation Measurements (ARM) Program.  All units will be assembled from LEDs having the same characteristics.

The figure below shows comparison of the LED Water Vapor Photometer measurements compared with the Ames Airborne Turning Spectrometer (AATS), obtained at the CART site in Oklahoma.  The LED unit was calibrated against the Microwave Radiometer data.  The AATS instrument, although originally devised for aircraft observations, is used on the ground, looking up, similar to the LED instrument, and in fact, uses the same water vapor absorption band for measurements, except that the water vapor amount is derived directly from the spectral data.
 

 

What else can it measure?

The approach of comparing the energy at two different wavelengths or optical paths in which a material absorbs energy in one path or wavelength can be used to determine the abundance of other chemicals as well as aerosols in the atmosphere.  Indeed, GLOBE has an aerosol protocol that uses essentially the same device with LEDs sensitive to different wavelengths (525 and 635 nm).