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Cost effective solar photocatalytic technology to water decontamination and disinfection in rural areas of developing contries (SOLWATER)


A specially designed titania photocatalyst was prepared by coating Ahlstrom non-woven paper, used as a flexible photocatalytic support, with Millennium PC500 anatase. Two types of photoreactors were designed for testing the photocatalytical behaviour of this catalyst in the presence of different organic pollutants. The reactors are: a solar multi-step cascade falling-film photoreactor(STEP)and a CPC solar with a coaxial support to place the catalyst. Several types of reactants were treated: 4-chlorophenol and gallic acid as a model organic pollutant; formetanate, a widely used pesticide in horticulture; a mixture of pesticides used in vineyards; and indigo carmine and Congo red, which are complex multifunctional dye molecules. Each reaction was performed simultaneously in a solar CPC slurry photoreactor and in the STEP photoreactor under identical solar exposure to better evaluate and validate the results obtained. The STEP solar reactor was found to be as efficient as the CPC for 4-chlorophenol and formetanate total degradation. In contrast, both dyes required longer treatment in STEP experiments. This new system, in which the final tedious filtration can actually be avoided, constitutes a good alternative to slurries. CNEA: The destruction of gallic, oxalic and citric acids and of catechol, was studied in detail. The surface interaction between the pollutant and TiO2 were characterized by FTIR-ATR and other techniques, and the kinetics of the process was followed by HPLC and other techniques. Total degradation is achieved through the formation of unknown intermediates, except in the case of oxalic acid. NTUA: The contrubtion of NTUA, in this part of the project, was to test and evaluate the CPC prototypes performance by carrying out experimental work for photocatalytic degradation of gallic acid (a target compound), and humic and fulvic substances. An important amount of experimental results from the monitoring of the CPC prototypes lead to the following conclusions: (a)CPC reactor with immobilized TiO2 is efficient for the photocatalytic treatment of DOM: Humic Substances (HS)and potential products of their degradation e.g. Gallic Acid (GA). (b) HS molecules are transformed photolytically up to 70-80% (requiring at least E=2-3 MJ/m2L). (c) HS molecules are efficient photosensitizers capable of self-catalysing their oxidation. (d) Immobilized TiO2 contributes to a further 10-20% to the HS phototransformation. (e) Complete mineralization of HS has not been achieved in the CPC and may be preliminarily estimated up to 30%. (f) GA Photolysis leads to a 40% degradation of the molecule. (g) GA Photocatalytic degradation by immobilized TiO2 may be complete. CIEMAT contribution to this result is based on the data obtained from photocatalytic degradation of gallic acid with TiO2 immobilized over Ahlstrom paper. The adsorption experiments with gallic acid in dark, adsorption rates up to over 80% were found. In dark experiments it was assumed that the adsorption is the only factor for gallic acid concentration decreasing. Most of the experiments are performed with an initial concentration of gallic acid of 20mg/l. This value has been used as classification value for humic acids in polytrophic lakes. It was proved that the photocatalytic degradation of gallic acid under solar radiation in the CPC solar photoreactor is faster than adsorption. According to previous experimental results obtained from photocatalytic degradation of E. Coli, the plane support reactor was found to be efficient than coaxial one in terms of light efficiency during the photocatalytic degradation of gallic acid. It was also demonstrated that highest initial concentrations of organic compound are faster removed. The degradation of 50 mg/L is at 65% removal while the results for 20 mg/L has shown already a fate of 90%.
TiO2 was immobilized by direct deposition or by sol-gel methods on small glass cylinders, porcelain beads, walls of plastic bottles (to be used in another similar project) and other convenient supports. So far, low-cost porcelain beads coated with a TiO2 acid suspension and glass Rashig rings seem to be the best materials. A very simple technique for immobilisation by immersion in a TiO2 suspension, previously acidified to pH 2.5, followed by calcination at 450 oC, was used. After a vigorous washing with a water jet, no detachment of the catalyst was observed at the end of the fixation process. The crystallographic nature of P-25 was not affected after attachment to the porcelain surface. The beads were submitted for 100 h to turbulent regimes corresponding to Re numbers of 100 and 400 in a recirculating system. The effect of the recirculation flow was carefully examined to evaluate the degree of detachment of the catalyst from the support and to assess how the treatment influences on the activity of the supported catalyst. For this, photocatalytic tests were performed on two model compounds, 4-chlorophenol and potassium dichromate in the presence of ethylenediaminetetraacetic acid, and results were compared with those of untreated beads. A good activity remained in the beads even after washing for more than 100 hours in a turbulent regime (both Re 100 and 400). The activity of particles detached from the support by ultrasonication was also compared with that of the pure precursor. In respect to Rashig rings, they were also active for solar photocatalytic degradation of 2,4-D, a very common herbicide used in Latin America. A TiO2 suspension (10% TiO2 (m/V) in water at pH 2 or a titanium isopropoxide solution in isopropanol (1:3) by means of the direct impregnation to 2/3 of a plastic bottle (horizontal lower part) yielded a film with large surface area and good adherence. Other sol-gel deposits and treatments gave materials with lower photocatalytic activity. It has been deposited titanium oxide on activated carbon in two forms as powder and as a pellet. This oxide was obtained using sol-gel process based on titanium isopropoxide and isopropyl alcohol. Nanocrystalline TiO2 anatase phase on activated carbon was obtained using an adequated thermal treatment of 325 oC by 5 h, optimized by X-ray diffraction. The resulting material was characterized by spectroscopic X-ray fluorescence, scanning electron microscopy with EDX, and Brunauer-Emmett-Teller (BET) surface area measurements. Activity measurements performed under lamp and solar irradiation have shown good results for the photo degradation of phenol in aqueous solution. For solar applications, a polyethylene terephtalate bottle containing the photocatalyts was filled up with the contaminated water and place few hours under the solar radiation of a couple of sunny days. The results indicate that 20 ppm of phenol can be removed by using this method. Pellets with different sizes and porosities can be fabricated. Photocatalytic degradation of phenol using TiO2 nanocrystals supported on activated carbon. Zinc oxide films were fabricated by a home made spray pyrolysis system equipped with an optical set-up ensuring the in-situ control of the film growth. 0.1 M of zinc acetate diluted in a mixture of ethanol and water was used as the precursor solution. The ethanol/water molar ratio, G, in the precursor solution was varied from 0 to 0.92. The deposition temperature and the pH of the precursor solution were kept at 350 oC and 4.5, respectively. X-ray diffraction patterns revealed that films were zincite-like with a grain size depending of the ethanol/water molar ratio in the precursor solution. The morphology of the ZnO films obtained from G vary in its roughness. Photo-electrocatalytic results indicated that there is a correlation of the partial molar volume of ethanol respect to water in the spraying solution with the roughness and photocatalytic efficiency of the ZnO films.
The solar photocatalytical inactivation of water containing bacteria can be successfully achieved by TiO2 and Ru(II)-complexes photo-assisted processes. Nevertheless, in the field of the drinking water treatment the final application has to provide to the users a source of safe and micro-organisms free water. In this sense, the complete disinfection has to be quantified not only by the concentration of microorganisms after the photo-assited proccess has finished but taking into account the possible regrowth effect of the microorganisms after light exposition. This fact can be atributed to the lack of killing capacity of the process and to the repairing mechanisms of the cells in dark after radiation. Therefore, the total disinfection (Final concentration = 0 CFU/ml) of a treated water has to be checked by regrowth controls kept in dak at certain times after the treatment. This is specially true for heterotrophic bacteria, a group including patogens like Pseudomonas aeruginosas, as they have been detected in photocatalytic treated water, in the CPC recicling system. After treatment, the water can be free of coliforms but having greater than initial heterotropic bacteria concentration due probably to the increased temperature that enhance biofilm growth in the dark parts of the CPC photoreactor. The presence of heterotrophic bacteria has been discarded in disinfected water using PET bottles that are fully illuminated in the presence of glass imobilized TiO2.
Our research group has developed a singlet oxygen photosensitizing material made of porous silicone and an adsorbed polyazaheterocyclic Ru(II) coordination compound [tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II)]. Singlet oxygen is the lowest excited state of molecular oxygen. As a reactive oxygen species it has an electrophilic character, an energy excess of ca. 95 kJ mol-1 over its ground state and a long lifetime in condensed phase. It is known to inactivate bacteria efficiently since it is able to react with the proteins and lipids present in biological membranes. The key innovative features of this result are the following: -"Singlet molecular oxygen is efficiently produced by the Ru(II) photosensitizer immobilized on the polymer support. -"The reactive oxygen species has a long lifetime, which is an essential feature in order to allow singlet oxygen diffusion from the silicone material towards the external aqueous phase. -"The use of porous silicone displays the following advantages: high oxygen permeability (high oxygen solubility and large oxygen diffusion coefficient), the porosity of the polymer allows easy interaction with microorganisms to promote disinfection, the polymer shows good mechanical, thermal and chemical stabilities, and good optical properties as well. -"The use of Ru(II) photosensitizers shows the following advantages: strong light absorption in the visible range (400-550 nm), long excited state lifetimes (in the microsecond range) to allow collisional deactivation by molecular oxygen, and high probability of excited state quenching by oxygen leading to singlet oxygen. The photosensitizing material can be produced at large scale and tests that demonstrate the viability of this material for water disinfection with solar light have been performed with two solar photoreactors with different configurations. Efficient disinfection of gram(-) and gram(+) bacteria has been achieved with solar collectors containing less than 1 m2 of polymer illuminated area, allowing a daily production of 20-40 L of disinfected potable water. On site tests in developing countries such as Argentina, Peru and Mexico are currently underway to demonstrate the applicability of this technology to water disinfection in rural communities. Dissemination of these results among public institutions will facilitate the use of this technology and will maximize the benefits in terms of public health and life quality of population in developing countries.
The need of a low cost and robust system to measure the solar radiation, in the global and UV-A ranges and in the rural areas has been addressed with the construction of two kinds of radiometers based on cheap photovoltaic sensors. The electronic systems are based on a microcontroller, which calibrates each measured radiation with a patron measurement, saved in its memory. Calibrated measurements in W/m2 are shown using an electronic liquid crystal display. The radiometers system could be work as both, a completely autonomous or as a permanently connected to a portable computer system. a) As an autonomous radiometer system, it stores on its memory the data of long term measurement, the sampling time can be configured between 0.2s and 2hours. And it is able to record 4000 state changes. When the long term measurement is finished the data is transferred to a computer using serial RS-232 connection and an appropriated developed software. b) As a permanently connected radiometer system to a personal computer, its transfer automatically the measured data to a portable computer and solar radiation is ploted in real time.
During the SOLWATER project recent advances in solar water treatment (disinfection & decontamination) were confirmed and improved by using TiO2 and Ru(II) complexes as fixed catalyst. Ru(II) complexes are used as an alternative to chlorination or pasteurisation for potable water conditioning, meanwhile TiO2 photocatalyst is used as an unique way for oxidation of organic matter pollution dissolved in drinking water. Our firm has designed a new solar system for water treatment by coupling the contribution and research work of all partners (catalyst supporting, optics, disinfection and decontamination tests, handling recommendations, etc.) and the final result was a simple and robust equipment completely driven by solar energy. This equipment is able to produce potable water from polluted surface waters or wells in remote regions where no tap water and/or electricity are available. Also its design makes it an useful equipment for Scientists and Researchers working on water treatment issues. As a result of all these works our firm has got an important experience and skilled personnel in the engineering design of solar reactors for the use in different fields as disinfection, decontamination, wastewater treatment, fine chemical reactions and also in the design of new equipment for other solar topics as water heating. NTUA: Two types of CPC solar photocatalytic collectors (including the TiO2 paper) have been sent to NTUA from AOSOL: the coaxial and the fin-type. The support unit and all the necessary equipment (pump, piping, valves, flow meter etc) were constructed and assembled by NTUA, in order to set up the experimental treatment unit. The layout of the unit is according to the plans provided by ECOSYSTEM. The system can accommodate both types of CPC collectors by replacing easily the part of CPC collectors. From the engineering point of view, the system operates in closed circulating batch mode, for the batch treatment of 55 L of water. The unit consists of (a) a storage tank, which can be considered and modeled as a completely mixed reactor (although it is assumed that photocatalytic reactions do not take place in this as it is kept in dark) and (b) the CPC collector unit, which can be considered as a plug flow reactor. After their construction and assembling, the prototypes had been tested by NTUA for (a) Microbial disinfection of potable water and (b) Photocatalytic degradation of gallic acid (a target compound), and humic and fulvic substances. UCM: Two models of CPC solar photocatalytic collectors have been received from AOSOL (coaxial and fin-type). Both collectors included the TiO2 paper, that was exchanged for the photosensitizing material prepared at UCM to test the effect of singlet oxygen on bacterial disinfection). The supporting units and all the necessary equipment were constructed and assembled by UCM, in order to set up the solar demonstrator reactors. The layout of the units is according to the plans provided by ECOSYSTEM, with slight modifications. From the engineering point of view, both systems operate in closed circulating batch mode, for the batch treatment of up to 30 L of water. Each unit consists of (a) a storage tank (with vent hole), (b) CPC collector unit (coaxial: 4 out of 5 tubes with photosensitizing material; fin-type: 5 out of 7 tubes containing photosensitizing material), (c) pump, (d) radiometer, (e) 2 thermocouples (water inlet & outlet) and datalog, (f) PVC piping and valves and (g) stainless steel frame. After their construction and assembling, the prototypes had been tested by UCM for bacterial disinfection of potable water (Escherichia coli and Enterococcus faecalis at 100 and 10000 CFU/mL initial concentrations, respectively). Bacterial inactivation (reduction of the initial concentration by 2-3 orders of magnitude) has been achieved with an accumulated energy of 0.8 MJ/m2L in the visible region (350-700 nm) with both collector models and types of bacteria, although the fin-type collector seems to be more efficient.
Singlet molecular oxygen is a reactive oxygen species which can be efficiently generated via quenching of the excited state of a sensitizer by ground state molecular oxygen in the so-called photosensitization process. Singlet oxygen can be used for different applications, such as organic synthesis, dye bleaching, disinfection processes and photodynamic therapy. Production of singlet oxygen by solid-supported sensitizers is often employed because the photosensitizer can be readily removed at the end of the process. In order to design the most adequate system for each application, it is essential to carry out a precise quantification of the singlet oxygen production by the supported photosensitizers. In practice, this involves measurement of the singlet oxygen production quantum yields (PHIdelta). While this issue is well solved in homogeneous media due to availability of various standards, it is far from established in heterogeneous systems. Our research group has proposed methylene blue (MB) dyed Nafion films as a convenient reference system to quantify singlet oxygen production in solid samples. We have thoroughly characterized the production of singlet oxygen generated by MB photosensitization in films of Nafion ionomer and we have compared our results with those of two well known standards: MB in methanol and tris(2,2'-bipyridyl)ruthenium(II) in acetonitrile. We have determined singlet oxygen production quantum yields of MB in dry Nafion (PHIdelta = 0.24) and in methanol-swollen Nafion (PHIdelta = 0.49). After the characterization of the MB/Nafion system, we have concluded that Nafion films loaded with MB are homogeneous, reproducible and stable systems suitable to be used as a reference for PHIdelta determination in solid phase.
The efficiency of solar disinfection by heterogeneous photocatalysis was measured. The disinfection was performed with sol-gel immobilized TiO2 films over glass rings. Spring water from a mexican drinking water source naturally polluted with coliform bacteria was exposed to sunlight in plastic bottles in solar collectors and the disinfection effectiveness was measured. Total and fecal coliforms quantification was performed by means of the definite substrate method in order to obtain the efficiency of each technique. An important part of this study has been to determine the bacterial regrowth in water after disinfection by heterogeneous photocatalysis. The disinfection with TiO2 turned out to be more efficient than the current method of SODIS for total coliforms as well as for fecal ones. In a sunny day (more than 1000 W/m 2 irradiance), the disinfection with immobilized TiO2 needed fifteen minutes of irradiation to reduce the fecal coliforms content to zero and thirty minutes for total coliforms, less than half times required using SODIS. Bacterial regrowth of total coliforms was observed for SODIS disinfection, more frequently than for fecal coliforms. In contrast, when using the catalyst TiO2 no regrowth was detected, neither for total nor fecal coliforms. The disinfection process using TiO2 keeps water free of coliforms at least for seven days. The demonstration opens the possibility of application of this simple method in rural areas of developing countries. Other possible use is in disinfection of water in short periodes of time in zones where the distribution systems are collapsed by extreme weather phenomena.The SOLWATER prototype was tested in the disinfection of wild strains, from a Well in Jiutepec, Mexico. Four hours of sun irradiation were enough to inactivate fecal and total colifoms but not the more resistent Pseudomonas aeruginosa strain. Tests carried out with the SOLWATER prototype using well water from Los Peryera, Tucuman, Argentina, demonstrated that wild strains of total coliforms, fecal coliforms and enterococcus fecalis were destroyed effectively. The time needed to disinfect 20 L of recirulating water varied depending on the available sun energy. Even in partially cloudy days, destruction was complete in ca. 4 h. No regrowth is observed. On the other hand, pseudomona aeruginosa is more resistent. Results suggest that this strain may be sensitive to the process, but only after the other strains have been destroyed; this assertion needs confirmation. It should be noted that these experiments use a combination of heterogeneous photocatalysis and photosensitization. The CIEMAT contribution in this results consist of seeveral sets of experimental works. A part of this is performed using water suspensions of TiO2 (Degussa P25) in a CPC reactor and another it is made with supoorted TiO2 on Ahlstrom paper matrix. As result of this work, the CPC solar photoreactor has been demonstrated to be efficient for bacteria disinfection by solar photocatalysis with TiO2 slurries and supported TiO2 during treatment periods of 30-60 min. Bactericidal deactivation by sunlight in a CPC solar collector occurs whether or not the catalyst is present. The total photocatalytic deactivation of pure E. Coli suspensions is a consequence of the combined effect of sunlight and the oxidant species generated in the TiO2 in suspensions and or by supported TiO2. However, while sunlight deactivates E. Coli suspensions, it does not completely deactivate them, since bacteria regrowth was detected. This method of disinfection has to be improved by the photocatalytic action of TiO2 under solar radiation to kill bacteria completely, which was proven successful and efficient enough to persevere in work in photocatalytic applications for drinking water disinfection. The irradiated area in the CPC collector plays a key role in the bacteria inactivation by solar irradiation. There is also a synergistic effect of the experiment runtime and the irradiated collector surface due to stress from flow rate on the bacteria suspensions. Compared to the supported TiO2 as a photocatalyst, the slurry TiO2 behaves more efficient for bacteria deactivation. Under our experimental conditions, the disinfection rate is independent of catalyst concentration, although not as high as those reported in the literature. Results found with supported TiO2 on fibreglass paper (Ahlstrom©) are promising.
A system was developed in which water to be disinfected in recirulated through glass tubing exposed to solar radiation in a CPC type geometry. The results demonstrate that photonic flux adds to heating caused by IR radiation and to fluid flow effects in order to inactivate bacteria.
Recent advances on water disinfection treatments propose singlet oxygen, a reactive oxygen species, as a bactericidal species which can be generated photocatalytically with solar light in aqueous solution and also in heterogeneous phase. Singlet oxygen can therefore be used as an alternative to other typical chemical reagents employed in water disinfection treatment. Its electrophilic character, energy excess (ca. 95 kJ mol-1) and high bimolecular rate constants with biomolecules such as proteins (6 x 107 M-1s-1) and lipids (1 x 105 M-1s-1) makes singlet oxygen a promising bactericidal agent vs. water borne microorganisms. Our research group has developed a new family of singlet oxygen photosensitizers based on polyazaheterocyclic Ru(II) complexes with different structural features. They display singlet oxygen production quantum yields in the 0.2-1 range in homogeneous phase and those containing the 4,7-diphenyl-1,10-phenanthroline ligand or its derivatives show very efficient generation of singlet oxygen. We have carried out experiments with aqueous samples containing E. coli or E. faecalis in the presence of RDP2+ [tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II)] or RSD4- [tris(4,7-diphenylsulfonate-1,10-phenanthrolinyl)ruthenate(II)] photosensitizers, in the dark or under Vis irradiation. No toxic effects were observed for E. coli or E. faecalis after 5 h incubation in the dark with RSD4- 10-6 M or with RDP2+ 5 x 10-9 M (due to its very low solybility in water). On the other hand, under Vis irradiation, no disinfection was observed for E. coli or E. faecalis after 4 h irradiation with RSD4- 5 x 10-9 M, while a decrease of 5 orders of magnitude in bacteria concentration was observed in the case of RDP2+ photosensitizer. Disinfection assays in homogeneous phase were also performed using suspensions of E. coli in water with RDP2+ or RSD4- at concentrations up to 5.0 x 10-8 M. Significant disinfection (two orders of magnitude decrease in viable bacteria concentration) were obtained after 6 hours irradiation in the presence of the cationic photosensitizer. With the anionic photosensitiser, using the same experimental conditions, low disinfection (ca. 15% decrease in the viable bacteria concentration) has been observed. No efficient disinfection was observed with RSD4- photosensitizer in homogeneous phase while it was clearly observed with RDP2+ (even at nanomolar concentration due to its scarce water solubility). This result can be explained if the anionic character of biological membranes and also of RSD4- photosensitizer is taken into account, which limits the interaction between the microorganism and the sensitizer molecules. On the other hand, the cationic photosensitizer RDP2+ interacts electrostatically with the anionic membrane of the microorganism, leading to efficient inactivation.
This result shows the capability for the drinking water disinfection of the solar photocatalysis with TiO2 and Ruthenium complexes -Ru(II)- in a solar photoreactor. Both systems are proven to be successful if used separated. Nevertheless, the combined effect of both disposed one after the other (the order is not important, as it has been proven) yields not significative efficiency improvement, since no synergetic effects could be detected in the experimental series performed. Immobilized Ru(II)-complexes disinfection results Ru(II)-complexes within this project are specifically designated for the drinking disinfection experiments. In the experiments with Ru(II)-complexes immobilized on polymer strips the E. Coli concentration already decreased 4 decimal powers within 60 min. Comparing the Ru(II)-complexes performance as a photosensitizer for bacteria disinfection with a blank experiment (only light, no catalyst), it becomes clear that Ru(II)-complexes involved processes have a strong impact in bacteria deactivation. From achieved results, has been observed that the bacteria concentration in the disinfection experiment with Ru(II) decreases fast, while the concentration of the blank experiment almost keeps being the same. Ru(II) versus immobilized TiO2-P25 on Ahlstrom paper When comparing disinfection capacities of Ru(II)-complexes immobilized on polymer strips and TiO2 P25 immobilized on Ahlstrom (KN47-type) paper matrixes, both in the plane support reactor, the Ru(II) photosensitized disinfection starts faster. That means that the reaction constant is higher and the disinfection performance is better. Effects of TiO2 with Ru(II) in series This part analyses the possible synergetic effect of Ru(II) together with TiO2. To examine this point, the two different catalysts were connected in series to assess the performance of hybrid catalysed systems.It can be observed that the experiment with only Ru(II)-complex has the best disinfection activities, followed by the experiment with KN47 with P25 TiO2 coating. Up to date, a small difference exists to the disinfection results of the experiment with Ru(II) and KN47 with P25 TiO2 coating in series. Therefore, no synergetic effects could be detected in the experimental series performed.
With the development of solar photocatalytic technology, the need for an adequate evaluation of the ultraviolet solar resource has emerged. Depending on the type of technology, different kind of information is required for the proper dimensioning of the systems (radiation on a horizontal plane, tilted plane, diffuse, direct, reflected). With the aim to develop a map of the solar UV resource in Argentina for photocatalytic applications, a simple solar UV direct radiation model for clear days is presented and the results are compared with measured values by the latitudinal ultraviolet measuring network [1]. The model considers solar extraterrestrial ultraviolet radiation filtered by the absorption of ozone and air molecules. The value of the ozone total column was obtained from the information available at the TOMS webpage [2] and the absorption by air molecules was incorporated using the equations for the corresponding cross sections presented in Refs [3] and [4]. The ultraviolet network comprises 4 measuring stations distributed throughout the country that record values at 4 wavelengths: 305, 320, 340 and 380 nm. From this comparative study, it is possible to evaluate the importance of other factors that are not included in the model such as the absorption and scattering by aerosols, water vapor and other gases, as well as diffuse radiation. Information for the year 2002 was analyzed for the station of Jujuy (Lat: 24.1 South, Long: 65.01 West). The mean annual difference in the daily integral for the Jujuy monitoring station indicates that the model overestimates the 305 nm band by (14.2 ± 13.8) %, and underestimates the 320, 340 and 380 bands by (34.9± 4.6) %, (5.3 ± 4.2) % and (3.0 ± 3.0) % respectively. These results show that it is possible to asses the daily integral UV-A radiation on clear days with the modeled wavelengths and the corresponding correction factors. [1] V. Luis Orce, E. Walter Helbling, (1997) Latitudinal UVR-PAR measurements in Argentina: extent of the ozone hole, Global and Planetary Change 15 p. 113-121 [2] [3] S. Madronich, (1993) The atmosphere and Uv-B Radiation at ground level, Environmental UV Photobiology, New York Press. [4]

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