UV solar radiation has both beneficial (vitamin D photosynthesis) and
deleterious effects on humans (skin cancer, cataract...) and the
Routine spectral and integrated UV irradiance measurements at the surface
enable to study their variability versus atmospheric constituent
variability (ozone, clouds, aerosols).
Such measurements are conducted at Villeneuve d'Ascq by LOA since mid-1998
and a broadband radiometer, and since 2001 the
LOA's spectroradiometer is part of the NDACC (ex-NDSC). LOA is responsible
of another NDACC instrument at Observatoire de Haute Provence (OHP).
Compared with broadband measurements, spectral UV measurements present the
advantage to enable estimation of their biological effects (via action
spectra such as erythemal, ADN,...) and atmospheric effects (via photolysis
frequencies of photochemically active species). They also allow estimation
of the total ozone column and characterization of aerosols (for example
evaluation of the spectral optical thickness).
Broadband radiometers which are of easier use, measure integrated
irradiance (erythemal, UVB and UVA).
The UV spectral measurements began in Villeneuve d'Ascq with a
double monochromator Jobin-Yvon HD10
whose resolution is about 0.75 nm, which
measures the UV spectral irradiance in the 280-450 nm wavelength range,
with a 0.5 nm step. It performs measurements of the global (direct +
diffuse) radiation and, since 2003, of the diffuse radiation by means of a
disc masking the sun. Currently, measurements are conducted every 15 min,
from sunrise to sunset, alternating global and diffuse measurements.
Calibration is regularly performed (every 3 months) in a dark room using
lamps traceable to NIST and NPL, leading to a relative uncertainty of about
8% below 320 nm and about 5% above.
The total ozone column is retrieved from the spectra via a differential
absorption technique, with 3% uncertainty under clear sky conditions and 7%
under cloudy sky. Spectral aerosol optical thickness is retrieved in the
330-450 nm range from direct spectrum (global minus diffuse spectra) with
an absolute uncertainty equal to about 0.05.
Since March 2008 a new instrument equips the site: it is a double
monochromator Bentham DM300
, whose resolution is about 0.6 nm and which
performs similar measurements.
In October 2008, LOA has installed at OHP a spectroradiometer Bentham
DM300, replacing an instrument from IRSA (Université J. Fourier, Grenoble),
that had been operating in Briançon until December 2005 and that was also
part of NDACC.
In Villeneuve d'Ascq spectral measurements are completed by measurements
performed with two broadband instruments. The LOA's
from YES measures, since August 1998, the UV solar irradiance integrated over
the 270 à 352 nm spectral band. Its spectral response, close to the Diffey
erythemal action spectrum, exhibits a high sensitivity in the UVB and
decreases rapidly in the UVA. Its main advantages are a good temporal
stability and a minimal need of maintenance. It provides the erythemal flux
and the UVB fluxes integrated over the 280-315 nm and over the 280-320 nm
ranges with about 10% uncertainty.
Another broadband instrument is operating in Villeneuve d'Ascq since June
2007, it is a UV-S-AE-T Kipp & Zonen radiometer, providing the erythemal
flux and the UVA flux integrated over the 315-400 nm range.