Didier GILLOTAY (1) and Praveen K. PANDEY (2)
(1) Belgium Institute for Space Aeronomy (IASB-BIRA) (2) Belgian User Support and Operations Centre (B.USOC) 1180 Brussels, Belgium Corresponding email : Didier.firstname.lastname@example.org
The observation and monitoring of solar irradiance, in general, and UV (Ultra Violet) irradiance, in particular, is important due to its impact on human health and environment on regional and global scale. The UV irradiance, hence, the UV index (UVI), is affected by various atmospheric parameters, e.g., the solar zenith angle, the ozone overhead column and other atmospheric absorbers and scatters such as clouds and aerosols. In order to investigate the modifications induced by the UV irradiance on human environment, it is important to quantify the various factors affecting the UV irradiance, UV-B (280-315 nm) and UV-A (315-400 nm). In Particular, clouds are responsible for a great deal of the observed irradiance variability that in turn requires the examination of extensive observation data sets by detailed radiative transfer models using co-located meteorological and ozone measurements together with satellite based estimations. These factors are continuously monitored over Uccle, Belgium, since 1993 by means of ground based station producing spectral measurements of UV irradiance together with UVI computation. Today, six stations represent the contribution of Belgium in the European UV network. Our measurements give a good insight into the trend of UV irradiance and associated atmospheric parameters over Belgium. We present here some of the recent results from observations and radiative transfer modelling, e.g., the relation between the cloudiness, uncertainty in the cloudiness due to measurements and UVI, ozone trend and UVI, among others. It is seen that since 2000, despite of a stable ozone concentration, UV irradiance continues to increase. Modelling studies show that ozone and UVB trends are anti-correlated (1% of ozone reduction implies 2% of UVB increase). Aerosols reduction, as a consequence of improvement of air quality policy could be another important factor to explain the observed UV trend.