Lately a number of studies related with UV irradiance estimates from satellite data based on the Ozone Monitoring
Instrument (OMI) have shown a high correlation with ground-based measurements but a positive bias in many locations,
the satellite derived UV being higher. One of the key factors that this bias has been attributed to is the boundary layer
aerosol absorption not taken into account in the current OMI UV algorithm. In this work we have used a correction
procedure based on climatological global aerosol absorption data taken from AeroComm aerosol initiative. This dataset
includes aerosol optical depth and aerosol single scattering albedo assembled by combining, ground-based aerosol
measurements from AERONET and information from several global aerosol models. The results of this correction were
compared with synchronous ground-based measurements from 9 UV monitoring stations. The results generally showed a
significantly reduced bias of 7-20%, a lower variability, and an unchanged, high correlation coefficient.
Modern polymeric materials possess an ever increasing potential in a large variety of outdoor objects and structures
offering an alternative for many traditional materials. In outdoor applications, however, polymers are subject to a
phenomenon called weathering. This is primarily observed as unwanted property changes: yellowing or fading, chalking,
blistering, and even severe erosion of the material surface. One of the major weathering factors is UV radiation.
In spring 2005, the Finnish Meteorological Institute with its research and industrial partners launched a five-year
material research project named UVEMA (UV radiation Effects on MAterials). Within the framework of the project, a
weathering network of seven European sites was established. The network extends from the Canary Islands of Spain
(latitude 28.5°N) to the Lapland of Finland (latitude 67.4°N), covering a wide range of UV radiation conditions. Since
autumn 2005, the sites of the network have been maintaining weathering platforms of specimens of different kinds of
polymeric materials. At the same time, the sites have been maintaining their long-term monitoring programmes for
spectrally resolved UV radiation. Within UVEMA, these data are used for explaining the differences between the
degradation rates of the materials at each site and for correlating the UV conditions in accelerated ageing tests to those
under the Sun.
We will present the objectives of the UVEMA project aiming at deeper understanding of the ageing of polymers and
more reliable assessments for their service life time. Methodologies adopted within the project and the first results of the
project will be summarized.
A method for estimating past UV radiation levels using measurements of total ozone, sunshine duration and
snow depth was applied to Abisko, northern Sweden, and Helsinki, southern Finland. The method has earlier
been applied to Sodankyla (Finland) and Davos (Switzerland). The performance of the method was, as in earlier
studies, found to be satisfactory with, for instance, a seasonal correlation coefficient as compared to measurements
of the order of 95%.
By extending the available ozone time series into the past using climatological values of the total ozone
column, we were able to make UV estimates all the way back to the early 20th century at both stations. Both
at Abisko and Helsinki, the produced time series of estimated daily erythemal UV doses shows increases over
the last few decades partly due to the diminution of the total ozone column, and partly due to variations in the
sunshine duration. Over the period 1950-1999, a statistically significant increasing trend of 2.2%/decade was
found for Abisko, whereas for Helsinki, a trend of 4.2%/decade (also statistically significant) was found for the
Monitoring of the terrestrial solar ultraviolet irradiance by using a radiometer is often considered as expensive
and laborious or the data collected as insufficient in spatial coverage and in some cases in its temporal
resolution, too. Therefore, alternative methods, all relying on modelling in one way or the other, have been
developed. They differ in which input they receive, either standard meteorological information, space-based
radiance measurements or ground-based irradiances from broadband or multiband UV radiometer or from
pyranometer. A comparison of performance is presented between three methods during a 15-month period.
The ground reference instrument is the Brewer Mk-III #107 spectroradiometer of the Observatory of
Jokioinen, Finland. Compared to the reference, the space-based method overestimates the UV irradiance at
noon by 14.6% and the pyranometer-based by 0.9% with root-mean-square differences of 35.5% and 10.4%,
respectively. Daily erythemal doses agree by 3.8% for the space-based and 0.4% for the pyranometer-based
method with a scatter of 16.5% and 4.6%, respectively. Spectral irradiances generated by the pyranometerbased
model agree within 0.4% on average with a standard deviation of 17%. A rough estimate on the cost of
each approach suggests that none of them is clearly superior to the others and the actual nature of the data
needed may be used in decision making concerning monitoring strategies.
The objectives of the COST action 726 are to establish long-term changes of UV-radiation in the past, which can only be derived by modelling with good and available proxy data. To find the best available models and input data, 16 models have been tested by modelling daily doses for two years of data measured at four stations distributed over Europe. The modelled data have been compared with the measured data, using different statistical methods. Models that use Cloud Modification Factors for the UV spectral range, derived from co-located measured global irradiance, give the best results.