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How do I calculate UV-B and UV-A from GUV data?

UV-B and UV-A irradiance can be calculated from GUV measurements in a similar way as the UV-Index: as a linear combination of GUV channels (305, 313, 320, 340, and 380 nm):

UV-B (or UV-A) = a1 E305 + a2 E313 + a3 E320 + a4 E340 + a5 E380

The coefficients a1 to a5 can be determined via a multilinear regression of UV-B (or UV-A) against spectral irradiance E305 ... E380 using either data of a spectroradiometer or radiative transfer model calculations. For the coefficients provided below, we used UV spectra measured between April 2006 and July 2007 by the UV spectroradiometer at Palmer Station. From these spectra, spectral irradiance at 305, 313, 320, 340, and 380 nm were extracted and correlated against UV-B and UV-A irradiance determined by integrating the same spectra over the UV-B and UV-A wavelength ranges.

We note that UV-B and UV-A is not unambiguously defined. Some people define UV-B as the integral of 280–315 nm (which is more appropriate), some use 280–320 nm. The choice of the upper wavelength makes a big difference because of the dramatic change of the solar spectrum at Earth's surface in this wavelength range. Likewise, UV-A is sometimes defined as 315–400 nm and sometimes as 320–400 nm. Here the difference is small as the wavelength interval of 315–320 nm contributes only very little. We provide coefficients for all four wavelength ranges below.

Our analysis showed that UV-B and UV-A irradiance can be calculated from only one or two GUV channels with excellent accuracy. The resulting coefficients ai, and the square of the correlation coefficients, R2, are provided below. Note that spectral irradiances, Ei, have to be provided in units of µW/(cm2 nm) for the calculation to be correct. The resulting UV-B or UV-A irradiance is in units of µW/cm2.

• UV-B (290–315 nm) calculated from measurements at 305 and 313 nm:
UV-B = 8.91  E305 + 5.13 E313
R2=0.9999
•
• UV-B (290–320 nm) calculated from measurements at 305 and 313 nm:
UV-B = -1.373   E305 + 14.6 E313
R2=0.9992
•
• UV-B (290–320 nm) calculated from measurement at 313 nm:
UV-B = 14.33 E313
R2=0.9992
•
• UV-A (315–400 nm) calculated from measurements at 340 and 380 nm:
UV-A = 32.57  E340 + 42.86 E380
R2=0.9993
•
• UV-A (315400 nm) calculated from measurement at 380 nm:
UV-A = 70.615 E380
R2=0.9969
•
• UV-A (320400 nm) calculated from measurements at 340 and 380 nm:
UV-A = 30.27  E340 + 43.15 E380
R2=0.9994
•
• UV-A (320400 nm) calculated from measurement at 380 nm:
UV-A = 69.94 E380
R2=0.9972