Low UV Irradiation Research Articles

Incorporation of titanium and titanium–iron species inside a smectite-type mineral for photocatalysis

The present paper reports the modification of a clay mineral by the incorporation of TiO 2 (anatase) and iron–TiO 2 species to obtain delaminated solids with photocatalytic activity under very low intensity UV irradiation. The materials were characterised by X-ray fluorescence, X-ray diffraction, scanning electron microscopy, N 2 adsorption and IR spectroscopy confirming the effective insertion of TiO 2 in the clay mineral. The solids were catalytically tested in the total degradation of phenol in aqueous solution under mild conditions (25 °C and atmospheric pressure) showing that the photoactivity of the TiO 2/clay system can be enhanced or reduced by incorporating iron depending on the method of preparation. Simultaneous incorporation of Fe 3+ and Ti 4+ from cationic solutions forms TiO 2 nanoparticles in which the iron seems to be included in the anatase structure by isomorphic substitution with the consequent improvement of the photocatalytic activity.

OLEDs based on some mixed-ligand terbium carboxylates and zinc complexes with tetradentate Schiff bases: Mechanisms of electroluminescence degradation

Mixed-ligand terbium carboxylates—Tb(Carb) 3(TPPO) 2 (HCarb = HSal (salicylic acid), Hpobz (2-phenoxybenzoic acid); TPPO = triphenylphosphine oxide) and zinc complexes with tetradentate Schiff bases—ZnSB (H 2SB = H 2SAL1, H 2SAL2 (derivatives of salicylic aldehyde); H 2MO1, H 2MO2 (derivatives of o-vanillin)) were used as electroluminescent (EL) layers in organic light emitting diodes (OLEDs). These devices have undergone reversible and irreversible mechanisms of EL degradation. The reversible one is mainly associated with charge carrier trap filling. At the same time several mechanisms of irreversible degradation were observed under UV light irradiation, heating and ageing in ambient conditions. The degradation mechanisms of OLEDs under heating from 293 K to 320 K were related to the changes in the interface regions. The initial degradation was eliminated by (1) an application of alternating bias voltage; (2) a decrease of the substrate temperature and the velocity of the thermal evaporation of materials. The degradation of OLEDs was not observed under low UV irradiation that is promising for OLEDs operation under day light.

Novel UV-curable and alkali-soluble resins for light-shielding black matrix application

A carbon black (CB) photo resist, comprising CB, CB dispersant, photo-curable resin, photo-initiator, and solvent, has been developed in order to prepare a light-shielding black matrix (BM) in the liquid crystal display application. In order to prepare a BM with a high opacity property or optical density (OD), the effect of CB such as its particle and concentration on light absorption property was first evaluated, and the results showed that 45 wt% CB with a particle size of about 100 nm in BM could reach an OD value of 4 μm −1. Moreover, six different UV-curable and alkali-soluble resins (A1, A2, and A3; B1, B2, and B3) were synthesized as photo-curable resins. Structures of these resins were characterized by FTIR and GPC, in which concentrations of various functional groups, especially carboxylic acid and double bond, were calculated. Subsequently, their photo-initiated polymerization rate with or without CB were measured. Finally, it was found that through a proper selection of the newly synthesized resins to prepare a carbon black photo resist, a BM with an OD of 4 μm −1 and a good resolution of 10 μm was successfully prepared upon low UV irradiation energy of 50 mJ/cm 2.

Utilisation of an η3-allyl hydride complex, formed by UV irradiation, as a controlled source of 16-electron (η5-C5Me5)Rh(CH2CHMe)

Low temperature UV irradiation of solutions of (eta(5)-C(5)Me(5))Rh(CH(2)[double bond, length as m-dash]CHMe)(2) yields (eta(5)-C(5)Me(5))Rh(eta(3)-CH(2)CHCH(2))(H), which provides controlled access to the 16-electron fragment (eta(5)-C(5)Me(5))Rh(CH(2)[double bond, length as m-dash]CHMe).

Study of Bacteriophage T4-encoded Dam DNA (Adenine-N6)-methyltransferase Binding with Substrates by Rapid Laser UV Cross-linking

DNA methyltransferases of the Dam family (including bacteriophage T4-encoded Dam DNA (adenine-N(6))-methyltransferase (T4Dam)) catalyze methyl group transfer from S-adenosyl-L-methionine (AdoMet), producing S-adenosyl-L-homocysteine (AdoHcy) and methylated adenine residues in palindromic GATC sequences. In this study, we describe the application of direct (i.e. no exogenous cross-linking reagents) laser UV cross-linking as a universal non-perturbing approach for studying the characteristics of T4Dam binding with substrates in the equilibrium and transient modes of interaction. UV irradiation of the enzyme.substrate complexes using an Nd(3+):yttrium aluminum garnet laser at 266 nm resulted in up to 3 and >15% yields of direct T4Dam cross-linking to DNA and AdoMet, respectively. Consequently, we were able to measure equilibrium constants and dissociation rates for enzyme.substrate complexes. In particular, we demonstrate that both reaction substrates, specific DNA and AdoMet (or product AdoHcy), stabilized the ternary complex. The improved substrate affinity for the enzyme in the ternary complex significantly reduced dissociation rates (up to 2 orders of magnitude). Several of the parameters obtained (such as dissociation rate constants for the binary T4Dam.AdoMet complex and for enzyme complexes with a nonfluorescent hemimethylated DNA duplex) were previously inaccessible by other means. However, where possible, the results of laser UV cross-linking were compared with those of fluorescence analysis. Our study suggests that rapid laser UV cross-linking efficiently complements standard DNA methyltransferase-related tools and is a method of choice to probe enzyme-substrate interactions in cases in which data cannot be acquired by other means.

Effects of low or medium-pressure UV irradiation on the release of intracellular microcystin

UV irradiation could be an alternative growth inhibition treatment against toxic Microcystis blooms in lakes. This study examined the effect of UV irradiation on the release of toxic intracellular microcystin. Conventional algicidal treatment (e.g. copper sulfate) was known to cause rapid release of intracellular microcystin and additional problems, but no studies have examined the case of UV treatment. A pure culture of Microcystis aeruginosa PCC 7806 was exposed to monochromatic low-pressure or polychromatic medium-pressure UV lamps. Irradiated pure culture suspension was subsequently incubated for 14 days under white light fluorescent lamps. During incubation, profiles of the number of cells, intracellular and extracelluar microcystin concentration were determined. After the UV exposure, the number of cells gradually diminished; the net log cell reduction after 6 days of incubation was 1.6 log or 2.0 log for 600 mJ/cm 2 of LP or MP UV irradiation, respectively. There were three findings for UV irradiation effects on the release of intracellular microcystin. First, because UV exposure can inhibit increases in the number of cells for about 6 days, it inhibited the increase of microcystin concentrations in water. Second, intracellular microcystin was gradually released into the surrounding water through a gradual reduction in the number of cells, preventing a rapid increase of microcystin concentration in water. Microcystin concentration in water was not significantly higher in UV-irradiated samples than non-UV-irradiated sample. Third, UV irradiation was able to degrade intracellular microcystin; it was reduced from 24.6 to 7.06 or 7.16 fg/cell by 600 mJ/cm 2 of LP or MP UV irradiation, respectively. This contributed to reduce increases in microcystin concentrations in water. UV treatment can inhibit Microcystis growth and reduce intracellular microcystin content without rapid release of intracellular microcystin.

Production of Hydrated Electrons from Photoionization of Dissolved Organic Matter in Natural Waters

Under UV irradiation, an important primary photochemical reaction of colored dissolved organic matter (CDOM) is electron ejection to produce hydrated electrons (e-aq). The efficiency of this process has been studied in both fresh water and seawater samples with both steady-state scavenger (S-SS) and time-resolved laser flash photolysis (LFP) methods. However, the apparent quantum yields (AQYs) of e-aq for the same samples using the two methods differ by as much as a factor of 100, necessitating a closer re-examination of how the process is measured. We developed a highly sensitive multipass LFP apparatus that allows detection of transient species at very low and variable UV irradiation intensities. Under single-photon conditions, we measured the AQY of e-aq from Laurentian fulvic acid as 1.3 x 10(-4), and set the upper limit for other CDOM samples at 6 x 10(-5), bringing the LFP results into agreement with those from S-SS methods. We also examined the ionization at elevated irradiation intensities and clearly demonstrated that multiphoton ionization occurs at intensities well below those usually used in LFP experiments, but well above those likely to occur at the earth's surface. This multiphoton ionization is probably the cause of the high AQYs reported by earlier LFP work. In addition, we also observed in real time other photochemical reactions, such as triplet quenching and bleaching, in the single photon regime.

Long-term effects of UV and visible irradiance on natural populations of a scytonemin-containing cyanobacterium (Calothrix sp.)

A natural warm spring stream with a cyanobacterial biofilm community composed almost entirely of Calothrix sp., a species rich in the UV-absorbing sheath pigment scytonemin, was used for 3 month summer experiments to test the following hypotheses: (1) UV radiation is necessary for the synthesis of high scytonemin content in sheaths. (2) High scytonemin content is required for uninhibited photosynthesis under high UV flux. Both of these hypotheses were answered affirmatively in earlier experiments with laboratory cultures of other cyanobacterial species. However, the tests of these hypotheses under natural conditions were necessary to confirm these conclusions, mainly because fluxes of UV and visible radiation and their ratios are very different under field conditions. Intact mats of Calothrix, siliceous substrate cleared of Calothrix, and artificial foam were treated for 3 months under filters transparent to UV radiation and visible light, and under filters that excluded only UV radiation. Neutral density screens were used in combination with filters to reduce the total irradiance over some sections of the stream. After 3 months, under all treatments, the predominant organism was still the same morphotype of Calothrix. Intact Calothrix mats produced high levels of scytonemin over the summer, except under low UV and low visible irradiance, while chlorophyll-a values per unit area remained relatively low and showed little change under all treatments. After 3 months, intact Calothrix mat and colonized mat with substantial scytonemin were not inhibited photosynthetically by UV irradiance. Only colonized populations exposed to ∼12% UV and ∼12% visible irradiance, and with very low scytonemin content, showed significant UV-inhibition. Thus, the correlation between UV protection and scytonemin presence was established experimentally for the first time under solar irradiance in a natural, essentially monospecific population of a cyanobacterium.

Chronopotentiometric Stripping Analysis Using Gold Electrodes, an Efficient Technique for Mercury Quantification in Natural Waters

This paper proposes a simple methodology for mercury quantification in natural water by stripping chronopotentiometry at constant current, using gold (film) electrodes constructed from recordable CDs in stationary cell. The proposed method allows the direct measurement of labile mercury in natural waters. To quantify total mercury, a robust and low cost UV irradiation system was developed for the degradation of organic constituents of water. The proposed system presents such advantages as excellent sensitivity, low cost, versatility, and smaller dimensions (portability for on-field applications) when compared with other techniques (ICP, GFAAS, fluorimetry) traditionally utilized for mercury quantification. A large linear region of responses was observed, situated over the range 0.02–200 μg L−1. Various experimental parameters were optimized and the system allowed quantifications in natural samples, with detection limit of 8 ng L−1 and excellent reproducibility (RSD of 1.4% for 48 repetitive measurements using a 10 μg L−1 mercury solution). Different metal ions were evaluated, including copper, as possible interferences on stripping mercury signals. Applications of the new method were demonstrated for the analysis of certified and groundwater samples spiked with a known amount of mercury and for the quantification of methylmercury in synthetic oceanic water, originally utilized for fishes contamination experiment.

Effect of solar radiation on two Antarctic marine bacterial strains

Two Antarctic marine bacterial strains were isolated, characterised and exposed to solar radiation during summer at Potter Cove (King George Island, South Shetland Islands, Antarctica). The effect of photosynthetically available radiation (PAR), ultraviolet-A (UV-A) and ultraviolet-B radiation (UV-B) on viability was studied using petri dishes covered with cut-off filters and providing the following treatments: dark, PAR, PAR+UV-A, PAR+UV-A+UV-B. UV-A and UV-B radiation have similar negative effects on the viability of the two studied strains under high UV irradiance but different sensitivities were observed under low UV irradiance. The regression analysis showed a logarithmic relationship between loss of viability and UV-B dose, with a threshold effect at 4.2 kJ m–2 (unweighted integrated dose) beyond which there was a >90% mortality. Hydrogen peroxide in Antarctic seawater under ambient UV radiation reached concentrations up to 1.45 µM. However, assays with one of the bacterial strains showed that there was no detectable effect of hydrogen peroxide over the range 0.5–50 µM, indicating that production of this reactive oxygen species was not directly involved in the observed loss of viability.

Photodegradation of 1-nitropyrene in solution and in the adsorbed state

The photodegradation of the 1-nitropyrene (NPy) has been studied using conventional (Xe and medium pressure Hg lamps) and laser sources (XeCl excimer and Nd-YAG UV). Low energy monochromatic light sources were used to study the early stages of degradation (up to 50% conversion). The medium pressure Hg lamp was used for longer periods of irradiation (up to 6 h) and for greater degradation of NPy. The results of our work confirm that degradation occurs by radical mechanism. Aromatic hydroxymethyl, methoxy, hydroxy and nitroso derivatives of pyrene (Py) are created by low energy UV irradiation. After a massive UV irradiation, the Py aromatic system is destroyed and more polar low-molecular compounds are generated. The photochemical method described in this paper, based on irradiation by UV lamp, is therefore, suitable for degradation of fused benzene ring(s) and thus should also be effective for degradation of other nitrated polycyclic aromatic compounds.

Experimental transmission, pathogenicity and physical–chemical properties of infectious spleen and kidney necrosis virus (ISKNV)

The pathogenicity of infectious spleen and kidney necrosis virus (ISKNV) was examined by infection trials in the first known host species, the mandarinfish, Siniperca chuatsi, and in 20 other teleosts cultured in China. Mandarinfish and large-mouth bass, Micropterus salmoides, were highly susceptible to infection after intraperitoneal infection of ISKNV in spleen and kidney materials from infected fish. ISKNV did not induce mortality in grass carp ( Ctenopharyngodon idella), but histopathological studies revealed that they might be infected by both bathing and intraperitoneal infection. No deaths, clinical or histopathological signs were found in the 18 other species exposed by immersion or injection. Experimental infection studies showed that ISKNV infection in mandarinfish could be transmitted from infected to healthy fish through contaminated water. ISKN only occurred in mandarinfish cultured at temperature above 20°C. The resistance of the virus to treatment with disinfectants, high and low pH and UV irradiation, as well as storage at selected temperatures was assessed.

Future UV radiation in Central Europe modelled from ozone scenarios

Photobiologically and photochemically relevant UV radiation for the time around the years 2015 and 2050 is estimated by radiative transfer calculations using variable ozone content based on model simulations. The future cloud conditions are assumed unchanged. Assuming various emission scenarios of chlorfluorohydrocarbons (CFCs) and other trace gases, and taking into account future temperature development and changing atmospheric dynamic conditions, ozone values are simulated. On the basis of these data, three different scenarios of the future total ozone content over Central Europe are analysed, which represent from current knowledge, probable as well as optimistic (high ozone and low UV irradiance) and pessimistic (low ozone and high UV irradiance) conditions. According to these scenarios the future development of the UV radiation is expected not to follow the increasing trend of UV irradiation observed during the last three decades. The predicted changes are highly variable with season. During late winter and spring, the enhanced recent UV values will persist for the next decades. Till 2015 a further slight increase is predicted for springtime. In contrast, during summer and fall, the UV level is assumed to remain on the recent level. For 2050 a decrease to values close to that of an anthropogeneous nearly undisturbed ozone chemistry, as it was found around 1970, is predicted. In addition to average long-time variations of the UV irradiance, short-time increase may occur due to ozone minihole events or due to a large volcanic eruption. The latter can produce a marked increase in UV radiation for several months. During ozone minihole events, with maximum occurrence in spring, UV irradiance is typically increased for a few days. Such episodes must be taken into account additionally to the average UV development. They will occur also in the future and result in UV radiation increases against undisturbed conditions, which are similar to present minihole events. These differences are much larger than the average changes predicted for future ozone development.

Spatial Control of Ca2+ Signaling by Nicotinic Acid Adenine Dinucleotide Phosphate Diffusion and Gradients

Intracellular Ca(2+) is able to control numerous cellular responses through complex spatiotemporal organization. Ca(2+) waves mediated by inositol trisphosphate or ryanodine receptors propagate by Ca(2+)-induced Ca(2+) release and therefore do not have an absolute requirement for a gradient in either inositol trisphosphate or cyclic ADP-ribose, respectively. In contrast, we report that although Ca(2+) increases induced by nicotinic acid adenine dinucleotide phosphate (NAADP) are amplified by Ca(2+)-induced Ca(2+) release locally, Ca(2+) waves mediated by NAADP have an absolute requirement for an NAADP gradient. If NAADP is increased such that its concentration is spatially uniform in one region of an egg, the Ca(2+) increase occurs simultaneously throughout this area, and only where there is diffusion out of this area to establish an NAADP gradient is there a Ca(2+) wave. A local increase in NAADP results in a Ca(2+) increase that spreads by NAADP diffusion. NAADP diffusion is restricted at low but not high concentrations of NAADP, indicating that NAADP diffusion is strongly influenced by binding to immobile and saturable sites, probably the NAADP receptor itself. Thus, the range of action of NAADP can be tuned by its concentration from that of a local messenger, like Ca(2+), to that of a global messenger, like IP(3) or cyclic ADP-ribose.

UV curable polyurethane-based microspheres

Radiation curable polyurethane-based microspheres were prepared by modifying conventional procedures for preparing anionic polyurethane dispersions. Vinyl groups were introduced to the side chains and the ends of the polyurethane main chains. Photoinitiators and multifunctional acrylate oligomers were incorporated into each microsphere. The curing behavior of the films obtained from these microspheres under UV were studied. The MEK resistance of UV cured films without multifunctional acrylate oligomers was raised when the content of vinyl groups is above a certain threshold value. The effect of the incorporation of multifunctional acrylates oligomers on MEK resistance was observed. This effect was remarkable at low UV irradiation intensity. The curing behavior of the films was followed by real time FT-IR spectroscopy. The accelerating effect of the incorporation of multifunctional acrylates became striking particularly at the early stage of curing process. Multifunctional acrylates were found to be effective in raising the extent of the conversion of the double bonds. The curing behavior under UV irradiation was compared with that under EB irradiation.

Hydrogen peroxide production rates in clean and polluted coastal marine waters of the Mediterranean, Red and Baltic Seas

The main aim of this study was to assess the hydrogen peroxide (HP) production rates (HPPR) related to anthropogenic pollution in coastal waters by laboratory and field experiments. HPPR's were assessed by simultaneous measurements of HP concentrations, cumulative solar UV irradiation and dissolved organic matter (DOM) fluorescence in the seawater samples at clean and polluted sites in the Mediterranean, Red and the Baltic Seas. The natural HP concentrations at all sites (8–100 nM) fall within the normal range recorded elsewhere, and follows a diurnal pattern. The polluted stations in the Mediterranean and Baltic Seas showed higher HPPR (3.2–16.6 nM m 2 W −1 h −1) than the clean stations, while in the Red Sea no significant differences were found because the station that was considered a priori polluted was actually rather clean (2–3 nM m 2 W −1 h −1). Laboratory experiments demonstrated that HPPR was positively linearly correlated to DOM fluorescence, however, this relationship was not found in the natural seawater samples examined in the field. The lack of relationship between HPPR and DOM in the field was attributed to enzymatic breakdown of HP as observed in dark decay experiments. HP dark decay rates were highest in polluted stations, probably due to larger bacterioplankton populations in these samples. Moreover, the HP dark decay rates were much lower in filtered than in non-filtered samples. Sun incubation of filtered (0.2 μm) seawater samples were performed to assess whether elimination of particles would yield higher HPPRs than in non-filtered samples. In the Baltic Sea (low UV irradiation) HPPR was higher in the filtered samples, while at sites with relatively high UV irradiation (Mediterranean and Red Seas), filtration of seawater did not stimulate HPPRs, probably due to UV-damage to planktonic microorganisms.

Growth and Structure of Spherulites in Polymer-Dispersed Liquid Crystals Fabricated by Photopolymerization-Induced Phase Separation

Spherulites appear during the formation of a polymer-dispersed liquid crystal (PDLC) under certain fabrication conditions. High UV irradiance results in a sample dominated by liquid crystal droplets while low UV irradiation results in the growth of spherulites, possibly because the latter condition creates fewer droplets, which is preferable for spherulitic growth. Spherulites that are nucleated adjacent to each other maintain their circular shape during growth until they make contact with each other, and then form a straight line boundary while maintaining radial growth. Direct observation of the inner aspect of PDLCs by means of Nomarsky differential interference microscopy and scanning electron microscopy (SEM) has revealed that the three-dimensional structure of a spherulite is plate like and that the boundary line seen in polarization microscopy contains no structure within the resin matrix, indicating that the body of a spherulite consists mainly of liquid crystal. A model of merging spherulites is proposed in this paper.

Effects of low molecular weight photosensitizer and UV irradiation on gas permeability and selectivity of polyimide membrane

A polyimide membrane, 6FDA–TMPD, was prepared from 5,5′-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofurandione (6FDA), and 2,4,6-trimethyl-1,3-phenylenediamine (TMPD). The effect of ultraviolet irradiation (UV irradiation) using a high-pressure mercury lamp on the gas transport properties of the 6FDA–TMPD membrane and membranes containing a photosensitizer was explored. Comparison between UV treatment and thermal treatment was also examined. The gas selectivities in all membranes were increased due to the irradiation. Especially in the H 2/N 2 and H 2/CH 4 separations, the selectivities effectively increased without a significant reduction in the H 2 permeability. These increases were considerable in the membrane having no photosensitizer which was expected to be an initiator for the cross-linking reaction. There was little effect of cross-linking, which was formed by the irradiation, on the gas permeation properties. Photosensitizers prevented drastic change in the membrane morphologies due to the irradiation.

Long-term effects of UV and visible irradiance on natural populations of a scytonemin-containing cyanobacterium ( Calothrix sp.)

A natural warm spring stream with a cyanobacterial biofilm community composed almost entirely of Calothrix sp., a species rich in the UV-absorbing sheath pigment scytonemin, was used for 3 month summer experiments to test the following hypotheses: (1) UV radiation is necessary for the synthesis of high scytonemin content in sheaths. (2) High scytonemin content is required for uninhibited photosynthesis under high UV flux. Both of these hypotheses were answered affirmatively in earlier experiments with laboratory cultures of other cyanobacterial species. However, the tests of these hypotheses under natural conditions were necessary to confirm these conclusions, mainly because fluxes of UV and visible radiation and their ratios are very different under field conditions. Intact mats of Calothrix, siliceous substrate cleared of Calothrix, and artificial foam were treated for 3 months under filters transparent to UV radiation and visible light, and under filters that excluded only UV radiation. Neutral density screens were used in combination with filters to reduce the total irradiance over some sections of the stream. After 3 months, under all treatments, the predominant organism was still the same morphotype of Calothrix. Intact Calothrix mats produced high levels of scytonemin over the summer, except under low UV and low visible irradiance, while chlorophyll-a values per unit area remained relatively low and showed little change under all treatments. After 3 months, intact Calothrix mat and colonized mat with substantial scytonemin were not inhibited photosynthetically by UV irradiance. Only colonized populations exposed to ∼12% UV and ∼12% visible irradiance, and with very low scytonemin content, showed significant UV-inhibition. Thus, the correlation between UV protection and scytonemin presence was established experimentally for the first time under solar irradiance in a natural, essentially monospecific population of a cyanobacterium.

New maximum UV irradiance levels observed in Central Europe

Simultaneous measurements of UV irradiance have been made from two adjacent sites (47.5°N, 11.1°E) in Garmisch-Partenkirchen (730 m a.s.1.) and the Zugspitze (2964 m a.s.1.). New maximum erythemally weighted irradiance levels were observed in Garmisch-Partenkirchen, Germany during March 1996 and June 1995. The March episode was associated with a polar stratospheric cloud while the June episode was associated with an unusually low total ozone column (for June) and broken clouds. The June maximum may be especially significant because it was preceded by a minimum of UV irradiance caused by heavy cloud cover, thus the absolute change in the UV irradiance was very extreme. Even though the maximum irradiance levels increased, these measurements show that the monthly mean spectral UV irradiation over Germany in 1995 was not significantly different compared to previous years. This is important because an increase in the number of extreme fluctuations from low UV irradiation followed by very high UV irradiation may be much more dangerous for the biosphere than a small gradual increase in average UV dose, because natural adaptation mechanisms may not be able to cope with these extreme fluctuations. The monthly erythemally weighted irradiation is between 25 and 90% higher on the Zugspitze than on the lower Garmisch-Partenkirchen site. The difference between the two sites cannot be characterized by a single number because the average monthly ratio of the irradiances was very variable with respect to both time and wavelength. The variability in the differences between the two sites indicates that the differences are caused by a combination of several factors including Rayleigh scattering, cloud effects, air pollutants (e.g. tropospheric ozone), aerosols and albedo. These results show that although UV irradiation measurements from one site (e.g. Garmisch-Partenkirchen) can be used in the assessment of biological effects in the immediate neighbourhood, data from a single site are not sufficient to extrapolate even to different altitudes near the measurement site and certainly should not be used to extrapolate conclusions to the atmospheric chemistry of the global troposphere.