Multi-channel solar ultraviolet radiometer
Cloudiness is among the largest sources of variance in the daily integrated UV irradiance at the surface. Therefore, in order to detect trends in solar UV-B radiation due to trends in total ozone amount a UV monitoring network should measure both UV-radiation, total ozone amount and cloud transmission.
NILU has developed a multi-channel radiometer for measurement of solar ultraviolet radiation. The instrument measures the irradiance (direct plus diffuse radiation) in the UV-B region (280nm - 315 nm) and in the UV-A region (315nm - 400nm) in 5 channels. The instrument contains a built-in data logger which can store up to 3 weeks of 1 minute average readings from all channels, as well as the temperature, which is measured close to the detectors. The instrument can also be set to store a 1 minute average data every 15 minutes only. In this mode the storage capacity of the data logger is one year.
A modem or a PC can be connected directly to the instrument for transfer of data. Eight instruments have been used in a UV monitoring network in Southern Chile since April 1995.
The following versions of the instrument are avaliable:
UV-5: 5 channel radiometer. Without temperature stabilization.
UV-5T: 5 channel radiometer. Heated and stabilized at 40 degrees C. This prevents the building of snow and ice on the diffuser.
UV-5S: 5 channel radiometer. Without temperature stabilization. Electrical power provided by solar cells/battery.
Sensitivity to ozone and cloud cover
Changes in the UV radation due to variations in total ozone abundance and cloud cover can be recorded with the instrument. The UV-B channels are sensitive to variations in total ozone and variations in cloudiness. UV-A channels, however, are sensitive to variations in cloudiness but not sensitive to ozone variations.
The 308nm level (UV-B) increased by 50% from March 29 to March 30 due to a 20% decrease in the total ozone abundance. The sky was clear on both days and no change was observed in the UV-A channels.
Measurements from March 29 and March 30, 1995 at NILU illustrate the sensitivity to ozone variations. The integrated ozone amount was 417 Dobson Units (DU) on March 29 and 329 DU on March 30, i.e. a 20% decrease. The measured increase in UV radiation from March 29 to March 30 was 100%, 50% and 27% in the 305nm, 308nm, and 313 nm channels, respectively. No changes were observed in the UV-A channels since the sky was clear on both days and since UV-A wavelengths are known to be insensitive to ozone.
By using a radiative transfer model combined with such irradiance measurements, biologically effective UV-doses, total ozone amount and cloud transmission can be determined.