UVB-1 Broadband Calibration

Last Updated: January 21, 2015

We wish to acknowledge Kathleen O. Lantz, Cooperative Institute for Research in Environmental Sciences, University of Colorado; and NOAA/SRRB/ARL, Boulder, CO., for her assistance in preparing this section.

The UVB-1 broadband data measure the erythemal UV irradiance which is the total measured surface UV-B flux weighted by the erythemal action spectrum, i.e., that part of the UV-B spectrum responsible for sunburn on human skin (erythema) and DNA damage. The broadband instruments are calibrated assuming that the total ozone overhead at the site is 300 Dobson units (DU). This assumption can result in significant error if the true ozone value is either much higher or much lower than 300 DU. For example, if total ozone at the site is 200 DU, the erythemally-weighted irradiance at a solar zenith angle of 20 degrees will give erythema too low by approximately 15% unless a correction for the true ozone is applied. Coefficients that may be used to correct for ozone concentration depend of the solar zenith angle and have been measured at CUCF in the past. These coefficients are not currently updated due to funding changes at UVMRP. For more information regarding the original process for calibration of UVB-1 broadband radiometers, please refer to Lantz et al, 1999. Currently, a limited number of UVB-1 pyranometers are calibrated by CUCF and deployed at Mauna Loa Observatory (MLO) where they are used for standards for comparison

UVB-1 pyranometers are calibrated against the most recently CUCF calibrated instruments that have been deployed at MLO. Instruments are sent to MLO and deployed alongside two or more standards that are deployed at MLO on a semi-permanent basis. The values from all the instruments deployed at MLO are used in the calibration of any individual instrument. The contribution of any instrument in the calibration process is weighted by the inverse time since the last calibration of that instrument, measured in days. The resulting formula to obtain the weighted erythemal irradiance is:

Iwt is the weighted erythemal irradiance derived from the daily average erythemal irradiances from all k UVB-1 instruments at MLO and Ik is the daily average erythemal irradiance from the kth instrument. This scheme insures that the most recent calibrations are relied on most. Then a calibration scaling factor is determined for each instrument using its average erythemal irradiance

This factor is subsequently applied as a factor to adjust the calibration of the kth instrument until a new calibration factor is obtained.