**Instrumentation and Measurement of Ground-Level Ultraviolet Irradiance and Spectral Composition in Estonia**

Kalju Eerme, Margit Aun and Uno Veismann

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59615

#### **1. Introduction**

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118 Solar Radiation Applications

Solar radiation sustains and affects all forms of life on Earth and is relevant to a variety of technological applications. The effects of solar radiation depend strongly on its spectral composition in which the influence of ultraviolet radiation (UVR) is the largest. Natural factors like geographical latitude, Earth-Sun distance, and solar zenith angle (SZA) strongly influence the UVB (wavelengths 280-315 nm) irradiance and to a somewhat lesser extent the UVA (315-400 nm) radiation. The exposure of organisms to UVR is characterized by annual and diurnal cycles of solar irradiance's availability and by the variance (anomalies) of biologicallyweighted irradiances within seasons.

Variations and trends in the availability and spectral composition of UVR are having various effects on atmospheric chemistry, plant health, litter decomposition and the carbon cycle, as well as on human health [1-10]. The effects of UV are mostly cumulative and depend on the spectral composition of received radiation energy. UVR is a globally important abiotic factor influencing ecosystem structure and functions in multiple ways [11]. The most influential UVB part of incident solar radiation is capable of breaking connections between atoms in organic molecules and exerts destructive effects on different materials. The net effect of UV radiation at the cellular level is a balance of damage and repair cellular key structures like DNA [5]. UVB radiation causes many biological and chemical processes, which are generally damaging to living organisms.

The impact of UVB radiation on vegetation changes with species and crops [6]. The effects of UVB radiation must be considered together with other climatic factors such as an increase in temperature and CO2 levels, which can modify the response to UVB radiation. Irradiation at

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wavelengths below 300 nm is extremely harmful to DNA. The ratio of irradiances in the UVB and UVA spectral ranges is one of the key factors for estimating its possible harmful influence.

Many important consequences can arise from the indirect effects of high UVB radiation through changes in the chemical composition and shape of the plants, or through changes in the abiotic environment. These indirect effects can include changes in the sensitivity of plants to being attacked by insects and pathogenic elements. The effects of UV radiation and those of other environmental factors at the ecosystem level are poorly known, as well as are those at molecular and organism levels.

Plants possess a number of defence systems against environmental stress factors in nature. Among such protection mechanisms is the altered synthesis of antioxidant substances as well as other secondary metabolites. Without repairs, the harmful photoproducts ultimately lead to cell death. To avoid this catastrophic effect, all organisms possess DNA repair systems that are able to recognize and remove UV photoproducts. The ultraviolet (UV) index is a standard vehicle for informing the public about the level of UV radiation reaching the Earth's surface and its potential harmful effects on human health. Although the received energy in the UVB band is only a small fraction of the extra-terrestrial solar radiation, it accounts for 80% of the harmful effects of sun exposure.

In most applications of the UVR data quantification of the received spectral doses a under‐ standing of the mechanisms of influence on the cell, organism and ecosystem levels are needed. The variations of total incoming solar radiation as well as spectral composition, especially in the UVB range, beside geographical factors depend strongly on atmospheric factors, such as clouds, total ozone, aerosols and precipitable water vapour. Spectral energy is necessary to estimate over days, parts of days and over longer time intervals.

Despite there being different broadband and narrowband UV sensors in use, spectral UV measurements are still considered the irreplaceable, ultimate reference in a variety of appli‐ cations. Spectral measurements allow the data to be applied to any biological process or chemical photoreaction with a known action spectrum. Weather conditions prescribing the availability and spectral composition of ground-reaching UV irradiance in key phenological phases of ecosystem development manifest significant year-to-year and longer-term periodic changes. These changes are reflected in ecosystem health and productivity. For sustainable agriculture and environmental management both the changes in quality of received irradiance and in ecosystem responses need to be investigated on a quantitative level. The present study financed by a programme of research and development of environmental technology is one of those attempts.

In the present chapter, the major features of systematic changes and the variability in groundlevel UVR at subpolar latitude are considered. The work is based on ground-level UVR spectra recorded at a research institute, Tartu Observatory, (58o .16'N, 26o .28'E, 70 m a.s.l.) since 2004 together with the auxiliary information on broadband solar radiation and weather conditions recorded by the Tartu-Tõravere meteorological station of the Estonian Environmental Agency at the same site. The homogeneous datasets of the broadband solar radiation and weather conditions for the site extend back to the beginning of 1955 [12-17]. The total number of broadband solar radiation and weather characterizing factors increased in the 1990s and 2000s. During the period of recording UV spectra the spectral dataset was well supplied with the necessary auxiliary information.
