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The aim of this study is to predict the main aerosol properties in the atmosphere, Aerosol Optical Depth (AOD) and Angstrom Exponent (AE), with the aid of machine learning techniques and images from an All-Sky camera. Two different machine learning techniques have been used in this work: a random forest (RF) and an artificial neural network (ANN) with target values furnished by AERONET database. HDR images from the All-Sky camera sited in Burjassot (Spain) have been used. All of them have been taken in a clear-sky condition (without clouds) and with different aerosol depth. Selected images come out with a range from 0 to 0.5 of AOD at 500 nm as reference. The data in the groundbased station are available since the 10th of February of 2020 to the 31th March of 2021 in almost one year of samples. We have developed two ways of building signals combined with the two machine learning methods. Firstly, a signal generated from scattering angles in a single image which is obtained as the average of relative irradiance (RGB) using 100 random points in each scattering angle isoline, obtaining 29 values for each signal. Secondly, the signal has been generated in the same way but from zenith angles isolines of a single image. The main result obtained is that we improve significantly the state of art results of not calibrated images. For example, the red channel improves the percentage of predicted AOD values within the AERONET uncertainties from 62% to 90%-93% using an ANN and the zenith method.

All sky-cameras are devices with a very high potential in order to study atmospheric phenomena and were originally designed to obtain the cloud cover. However, methods based in different approaches produce significant differences in the results. State-of-art methods usually offer better performance, thanks to computer vision and machine learning (ML) techniques, than traditional algorithms based on channel ratios using both fixed and adaptive thresholds to classify the pixels of one image as cloud or cloud free. We have developed a cloud cover adaptive threshold algorithm base on Probability Density Function (PDF) of the Blue to Red Ratio (BRR), standing out in: simplicity; ease of implementation; compatibility with any sky-camera in terms of technical requirement and type of image acquisition. The goal of this study is to compare our algorithm with a most fashionable method based on Machine learning, discovering the pros and cons of each one and weather ultimately less can be more. The comparison has been done using a set of 1-year HDR imagery database, representing a wider range of atmospheric scenarios such as clear sky, cloudy, partly cloudy and different types of aerosol conditions and clouds such as cirrus, cumulus, stratum and nimbus. To stablish a quantitative comparison of both methods, a limited set of images has been chosen. The PDF method show better agreement than our ML implementation, with a better performance for all weather conditions, in comparison against our cloud cover database.

The radiative closure methodologies to obtain Cloud Optical Depth (COD) from Remote Sensing techniques have traditionally relied on one-dimensional (1D) assumptions. These assumptions might be far away from the radiation transport over a realistic three-dimensional (3D) atmosphere, especially in cloudy conditions, as the natural inhomogeneities of clouds are not conveniently represented and treated in 1D models. The differences between the 1D and 3D approaches manifests in the 3D effects: a) the plane-parallel albedo bias and, b) the horizontal transport effect. The plane-parallel albedo bias is usually addressed by means of the Independent Pixel approximation (IPA), that considers each pixel radiatively independent from the others. Nevertheless, the IPA neglects the horizontal transport, entailing bias in the retrievals. In this work, we use the advantages of 3D radiative transfer (RT) to analyze COD and parameterize the 3D biases in terms of the plane-parallel approach. Detailed 3D RT simulations using MYSTIC are performed over two Highly Resolved Large Eddy Simulations cloud fields of known optical thickness. The output radiance is analyzed by a 1D IPA inversion retrieval based on a radiative closure to obtain the COD. The comparison between the retrieved COD fields for diverse illumination conditions and the real COD allow us to study the 3D effects separately and evaluate the retrieval. Our results show radiation enhancement in cloud edges depending on solar, viewing and cloud geometries, that induces a COD underestimation. The 1D approach works well for overcast conditions and underestimates the COD in broken clouds scenarios.

We have estimated the radiative impact produced by an unusually extreme dust and wildfire episode on the performance of a photovoltaic (PV) plant. The dust and wildfire events were mostly active on 26–28 and 29–30 June 2012, respectively. We took advantage of the consecutiveness of both events to separate and derive empirically the radiative effect of dust and smoke aerosols. With this purpose, we employed measurements of aerosol load, radiation and PV power output from a collocated atmospheric station and PV plant located at Burjassot (Valencia, Spain). The empirical estimates were obtained by direct comparison with a summer background day, happened right before the two consecutive case studies. The whole episode is characterized by high aerosol optical depth (AOD) at 500 nm, reaching values up to 1 and 6 for dust and smoke, respectively. Our analysis shows an average daily energy loss of −132 kWh, that represents a fraction of 20% of the energy generated by the PV plant on the reference summer background day. A significant fraction of the energy lost was due to smoke, with a daily maximum of 43% and a daily average of 34%. In case of dust, the energy reduction was moderated, with a daily average of 6%. In terms of instantaneous power reduction, we have obtained peaks of 8 and 51% for dust and smoke, respectively. The energy reduction due to aerosols is strongly dependent on AOD. Further, we have found that the power loss per unit of AOD at 870 nm is useful to separate the radiative impact of different aerosol types. Essentially, the efficiency of smoke to reduce the energy generated by the PV plant is much greater than that of dust. This effect is mainly due to the different optical properties in the near infrared that result in a daily Power Aerosol Forcing Efficiency at 870 nm of −5.7 and −19.1 kW, for dust and smoke, respectively. Our results indicate that both the AOD and aerosol type drive the radiative impact of aerosols on the PV output, confirming that extreme aerosol events may drastically reduce the power generated by operative PV plants. This reduction may be as large as that reported for particle soiling. Therefore, considering both amount and type of aerosols in the PV power analyses is mandatory in order to obtain accurate forecast scenarios and improve the grid integration, especially during extreme episodes.

The atmospheric aerosols have an important role in the radiative forcing in the atmosphere. The solar radiation interacts with the aerosols, being absorbed or scattered in different directions, depending on the absorption and scattering coefficients. The scattering coefficient is highly dependent on the aerosol size, this being dependent on the relative humidity of air, if the aerosols are hygroscopic. The aerosol hygroscopic factor, f(RH), is the factor describing how the scattering coefficient depends on the relative humidity.To understand the relation between scattering coefficients of aerosols and the relative humidity of air, and thus improving our estimations of the radiative effect of urban aerosols, we recently started to measure the aerosol hygroscopic factor, extending previous data series of scattering and absorption properties obtained at our Burjassot site, in Valencia (Spain). Preliminary results already showed values of fRH (75%) between 1.13-1.31.In this study we are interested on analysing the effect of the air mass type on the aerosol properties at our site, mainly on the total scattering coefficients and the hygroscopic factor, so we can understand if the trajectory of the air masses carrying the aerosols influences their hygroscopic properties at our region.The area represented by our station is mainly of urban coastal character. The Burjassot site is located in the suburbs of Valencia city in Spain. The population of the metropolitan area of Valencia is about 1 million. The distance to the Valencia city centre is about 5 km southeast, and the distance to the seacoast is about 10 km east. The site is locally affected by the traffic pollution, but also affected by light industry and occasional agricultural or forest fires. The scattering coefficient measurements at ambient conditions were started at Burjassot site on the late 06’s, by means of a three channel TSI 3563 nephelometer. In 2017, an ACS 1000 (Aerosol Conditioning System, manufactured by Eco Tech Company) with a tandem of Aurora 3000 nephelometers for dry and wet channels were added, although technical problems with the wet channel prevented us to obtain useful results before 2019. Additionally, angular scattering coefficients are simultaneously measured at ambient conditions only, with an Aurora 4000 polar nephelometer, available since 2019.In the current analysis, we use a well-known trajectory computing model like HYSPLIT to determine the path of the air masses during the last 5 days before its arrival at our station. Then, the numerical backtrajectories are automatically analysed by the application of previously developed algorithms to derive the type of dominant air mass, and its origin. Finally, the scattering coefficients and hygroscopic factors are classified in relation to the air mass type, to understand how these aerosol properties are linked to the main air mass types that influence our region.This work is supported jointly by the Spanish Ministry of Economy and Competitiveness (MINECO) and the European Regional Development Fund (FEDER) under Projects CGL2017-86966-R and RTI2018-096548-B-I00.

The most destructive wildfire experienced in Spain since 2004 occurred close to Valencia in summer 2012. A total of 48.500ha were affected by two wildfires, which were mostly active during 29-30 June. The fresh smoke plume was detected at the Burjassot measurement station simultaneously to a severe dust episode. We propose an empirical method to evaluate the dust and smoke mixing and its impact on the microphysical and optical properties. For this, we combine direct-sun measurements with a Cimel CE-318 sun-photometer with an inversion methodology, and the Mie theory to derive the column-integrated size distribution, single scattering albedo (SSA) and asymmetry parameter (g). The mixing of dust and smoke greatly increased the aerosol load and modified the background aerosol properties. Mineral dust increased the aerosol optical depth (AOD) up to 1, while the smoke plume caused an extreme AOD peak of 8. The size distribution of the mixture was bimodal, with a fine and coarse modes dominated by the smoke particles and mineral dust, respectively. The SSA and g for the dust-smoke mixture show a marked sensitivity on the smoke mixing-ratio, mainly at longer wavelengths. Mineral dust and smoke share a similar SSA at 440nm (~0.90), but with opposite spectral dependency. A small dust contribution to the total AOD substantially affects the SSA of the mixture, and also SSA at 1020nm increases from 0.87 to 0.95. This leads to a different spectral behaviour of SSA that changes from positive (smoke plume) to negative (dust), depending on the dust and smoke mixing-ratio.

Simultaneous PM2.5, PM10 and columnar (ground and satellite based) AOD measurements have been analyzed at Burjassot site in the metropolitan area of Valencia (Spain) during the period 2007–2016. The site is representative of a south European city in the Western Mediterranean coastal area, influenced by local urban pollution but also from frequent Saharan dust events. First, multiannual statistics were performed to characterize the main aerosol burden characteristics. The averages and standard deviations resulted 18 ± 9 μg m−3, 25 ± 19 μg m−3, 0.15 ± 0.11, 0.23 ± 0.17, 0.19 ± 0.15 and 1.2 ± 0.3 for PM2.5, PM10, AERONET AOD, MODIS Terra AOD, MODIS Aqua AOD, and AERONET Angstrom exponent AE, respectively. The AOD is always referred to 550 nm. PM10 and AOD showed seasonal patterns with maxima in summer and minima in winter. However, PM2.5 and AE did not show such an evident seasonality. The relationship between surface and columnar measurements show a poor correlation (r down to 0.30) for daily values, although the correlation increases to r up to 0.90 for yearly averages. The relationship between PM and AOD becomes more consistent when the databases are binned in intervals of 0.05 AOD. Results for AERONET and MODIS AOD are very similar, although for daily averages is slightly worse for satellite than ground based measurements. In order to explain some seasonality effects found, the mixing layer height has been included in the analysis. Results show that the correlation is maximum when PM2.5 is used and the mixing layer height is greater than 1 000 m (r > 0.90).

We analyze the vertically-resolved radiative impact due to a dust storm in the Western Mediterranean. The dust plume travels around 3–5 km altitude and the aerosol optical depth derived by MODIS at 550 nm ranges from 0.33 to 0.52 at the overpass time (13:05 UT). The aerosol radiative forcing (ARF), forcing efficiency (FE) and heating rate profile (AHR) are determined throughout the dust trajectory in shortwave (SW) and longwave (LW) ranges. To do this, we integrate different satellite observations (CALIPSO and MODIS) and detailed radiative transfer modeling. The combined (SW + LW) effect of the dust event induces a net cooling in the studied region. On average, the FE at 22.4° solar zenith angle is −190.3 W m−2 and −38.1 W m−2, at surface and TOA, respectively. The corresponding LW/SW offset is 14% and 38% at surface and TOA, respectively. Our results at TOA are sensitive to the surface albedo in the SW and surface temperature in the LW. The absolute value of FE decrease (increase) in the SW (LW) with the surface albedo, resulting in an increasing LW/SW offset, up to 76%. The AHR profiles show a net warming within the dust layer, with a maximum value of 3.3 Kd-1. The ARF, FE and AHR are also highly sensitive to the dust optical properties in SW and LW. We evaluate this sensitivity by comparing the results obtained using two set of dust properties as input in our simulations: a) the prescribed dust model by Optical Properties of Aerosols and Clouds (OPAC) and; b) the dust optical properties derived from measurements of the size distribution and refractive index. Experimentally derived dust properties present larger SSA and asymmetry parameter in the SW than OPAC dust. Conversely, OPAC dust presents higher AOD in the LW range. These parameters drive the FE and AHR sensitivities in the SW and LW ranges, respectively. Therefore, when measured dust properties are used in our simulations: the ARF in the LW substantially reduces at surface and TOA (up to 57%); the absolute value of SW ARF is reduced by 15% at surface and an enhancement of 31% is observed at TOA; the AHR present less warming in the entire profile with deviations up to 53% within the dust layer, with respect to the results obtained using OPAC.

In-situ aerosol absorption properties measured in Valencia (Spain) for four years, from February 2011 to February 2015, have been analysed. Spectral absorption properties have been obtained using a seven-wavelength Aethalometer AE-31 which covers the range from UV (370 nm) to IR (950 nm). In order to obtain the absorption coefficients, compensation parameters have been calculated for the Aethalometer considering seasonal and spectral differences. For this multiyear measurement period, seasonal site-specific calibration parameters have been obtained. Furthermore, estimations of the absorption Angstrom Exponent (α abs ) have been calculated using the seven Aethalometer wavelengths. The averaged absorption coefficients (plus/minus the standard deviation) obtained for the seven channels range between 9 ± 4 Mm −1 at 950 nm and 33 ± 18 Mm −1 at 370 nm. These results are typical of a moderate polluted environment. The obtained absorption Angstrom Exponent (plus/minus the standard deviation) is 1.42 ± 0.08, which suggests the presence of brown carbon or black carbon coated by non-absorbing particles in our site. Seasonal and daily variations, together with the effect of both the boundary layer height and traffic, have been also studied. Strong seasonal differences in the absorption coefficient are found, mainly due to seasonal variation of the mixing layer height. On the opposite, the study of the diurnal variations of the absorption Angstrom Exponent proves that this parameter is more affected by traffic emissions than by the evolution of the mixing layer height.

Saharan dust outbreaks are one of the major aerosol sources in Southern Europe. Dust affects the radiative balance of the Earth–atmosphere system and impacts health. The assessment of Saharan outbreaks requires information about the vertical distribution of dust in the atmosphere, usually obtained from lidar measurements, such as the ones performed by the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) sensor. In this paper, we have used CALIOP level-1 daytime data in combination with aerosol optical depth (AOD) measurements from Aerosol Robotic Network (AERONET) ground-based photometers to obtain aerosol profiles during 23 dust outbreaks over Southern Europe. These profiles are used for two main purposes: analyzing the lidar ratio values and vertical distribution of dust and evaluating the performance of version 3 of CALIOP algorithms. The mean lidar ratio obtained from the synergic method ranged between 47.7 and 49.0 sr, depending on the criterion used to average CALIOP signal. The dust layers traveled at a wide range of altitudes, from surface level up to 7.6 km. We found that the AOD values retrieved from CALIOP level 2 (version 3) and AERONET were not correlated $({r} . In addition, we observed that the mean AOD obtained by satellite retrievals, between 0.16 and 0.18, was lower than the mean AERONET value, i.e., 0.33, by 50%. These differences in the AOD were found to be caused by two main reasons: misclassification of dense dust layers as clouds by CALIOP algorithms, mainly over 4 km, and limitations of CALIOP algorithms to detect weak aerosol layers, particularly between 1.5 and 2.5 km.

The research leading to these results has received funding from the H2020 program from the European Union (grant agreement no. 654109, 778349) and also from the Spanish Ministry of Industry, Economy and Competitiviness (MINECO, ref. CGL2013-45410-R, CGL2016-81092-R, CGL2017-85344-R, TEC2015-63832-P), the Spanish Ministry of Science, Innovation and Universities (ref. CGL2017-90884-REDT); the CommSensLab "Maria de Maeztu" Unity of Excellence (ref. MDM-2016-0600) financed by the Spanish Agencia Estatal de Investigación. Co-funding was also provided by the European Union through the European Regional Development Fund (ref. POCI-01-0145-FEDER-007690, ALT20-03-0145-FEDER-000004, ALT20-03-0145-FEDER-000011); by the Andalusia Regional Government (ref. P12-RNM-2409); by the Madrid Regional Government (projects TIGAS-CM, ref. Y2018/EMT-5177 and AIRTEC-CM, ref. P2018/EMT4329); by the University of Granada through “Plan Propio. Programa 9 Convocatoria 2013” and by the Portuguese Foundation for Science and Technology and national funding (ref. SFRH/BSAB/143164/2019). The BSC-DREAM8b and NNMB/BSC-Dust (now NMMB-MONARCH) model simulations were performed by the Mare Nostrum supercomputer hosted by the Barcelona Supercomputer Center (BSC). S. Basart acknowledges the AXA Research Fund for supporting aerosol research at the BSC through the AXA Chair on Sand and Dust Storms Fund, as well as the InDust project (COST Action CA16202). The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.ready.noaa.gov) used in this publication., An unprecedented extreme Saharan dust event was registered in winter time from 20 to 23 February 2017 over the Iberian Peninsula (IP). We report on aerosol optical properties observed under this extreme dust intrusion through passive and active remote sensing techniques. For that, AERONET (AErosol RObotic NETwork) and EARLINET (European Aerosol Research LIdar NETwork) databases are used. The sites considered are: Barcelona (41.38°N, 2.17°E), Burjassot (39.51°N, 0.42°W), Cabo da Roca (38.78°N, 9.50°W), Évora (38.57°N, 7.91°W), Granada (37.16°N, 3.61°W) and Madrid (40.45°N, 3.72°W). Large aerosol optical depths (AOD) and low Ångström exponents (AE) are observed. An AOD of 2.0 at 675 nm is reached in several stations. A maximum peak of 2.5 is registered in Évora. During and around the peak of AOD, AEs close to 0 and even slightly negative are measured. With regard to vertically-resolved aerosol optical properties, particle backscatter coefficients as high as 15 Mm−1 sr−1 at 355 nm are recorded at the lidar stations. Layer-mean lidar ratios are found in the range 40–55 sr at 355 nm and 34–61 sr at 532 nm during the event. The particle depolarization ratios are found to be constant inside the dust layer, and consistent from one site to another. Layer-mean values vary in the range 0.19–0.31. Another remarkable aspect of the event is the limited vertical distribution of the dust plume which never exceeds 5 km. The extreme aspect of the event also presented a nice case for testing the ability of two dust forecast models, BSC-DREAM8b and NMMB/BSC-Dust, to reproduce the arrival, the vertical distribution and the intensity of the dust plume over a long-range transport region. In the particular case of the February 2017 dust event, we found a large underestimation in the forecast of the extinction coefficient provided by BSC-DREAM8b at all heights independently of the site. In contrast NMMB/BSC-Dust forecasts presented a better agreement with the observations, especially in southwestern part of the IP. With regard to the forecast skill as a function of lead time, no clear degradation of the prognostic is appreciated at 24, 48 and 72 h for Évora and Granada stations (South). However the prognostic does degrade (bias increases and/or correlation decreases) for Barcelona (North), which is attributed to the fact that Barcelona is at a greater distance from the source region and to the singularity of the event., Funding from the H2020 program from the European Union (grant agreement no. 654109, 778349), Spanish Ministry of Industry, Economy and Competitiviness (MINECO, ref. CGL2013-45410-R, CGL2016-81092-R, CGL2017-85344-R, TEC2015-63832-P), Spanish Ministry of Science, Innovation and Universities (ref. CGL2017-90884-REDT), CommSensLab "Maria de Maeztu" Unity of Excellence (ref. MDM-2016-0600) financed by the Spanish Agencia Estatal de Investigación, Co-funding was also provided by the European Union through the European Regional Development Fund (ref. POCI-01-0145-FEDER-007690, ALT20-03-0145-FEDER-000004, ALT20-03-0145-FEDER-000011), Andalusia Regional Government (ref. P12-RNM-2409); by the Madrid Regional Government (projects TIGAS-CM, ref. Y2018/EMT-5177 and AIRTEC-CM, ref. P2018/EMT4329), Portuguese Foundation for Science and Technology and national funding (ref. SFRH/BSAB/143164/2019)

The solar background illumination has a strong effect on CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) measurements, leading to a decrease in the signal-to-noise ratio of the lidar signal. Because of this, CALIOP level 2 data algorithms might be limited in the retrieval of the properties of the aerosols in the atmosphere. In this work, we present a methodology that combines CALIOP level 1 data with AERONET (Aerosol RObotic NETwork) measurements to retrieve aerosol extinction profiles and lidar ratios in daytime conditions. In this way, we fulfill a two-fold objective: first, we obtain more accurate daytime aerosol information; second, we supplement column integrated measurements from AERONET sun photometers with information about the vertical distribution of aerosols. The methodology has been applied to Burjassot (39.30° N, 0.25° W) and Barcelona (41.39° N, 2.11° E) AERONET stations in the Mediterranean coast of Spain in the period from June 2006 to September 2011. We have found good agreement for the extinction profiles in several study cases of ground lidar measurements in Barcelona, coincident with CALIOP overpasses. Finally, the methodology has proved to be useful for the study of special episodes such as Saharan dust outbreaks.

An unprecedented extreme Saharan dust event was registered in winter time from 20 to 23 February 2017 over the Iberian Peninsula (IP). We report on aerosol optical properties observed under this extreme dust outbreak through remote sensing (active and passive) techniques. For that, EARLINET (European Aerosol Research LIdar NETwork) lidar and AERONET (AErosol RObotic NETwork) Sun-photometer Cimel CE 318 measurements are used. The sites considered are: Barcelona (41.38º N, 2.17º E), Burjassot (39.51º N, 0.42º W), Cabo da Roca (38.78º N, 9.50º W), Évora (38.57º N, 7.91º W), Granada (37.16º N, 3.61º W) and Madrid (40.45º N, 3.72º W). In general, large aerosol optical depths (AOD) and low Ångström exponents (AE) are observed. An AOD of 2.0 at 675 nm is reached in several stations. Maximum values of AOD675 of 2.5 are registered in Évora. During and around the peak of AOD675, AEs close to 0 are measured. With regard to vertically-resolved aerosol optical properties, particle backscatter coefficients as high as 1.5∙10−5 m−1 sr−1 at 355 nm are recorded at every lidar stations. Mean lidar ratios are found in the range 40–55 sr at 355 nm and 34–61 sr at 532 nm during the event inside the dust layer. Mean particle and volume depolarization ratios are found to be very consistent between lidar stations. They range 0.19–0.31 and 0.12–0.26 respectively. The optical properties are also found very stable with height in the dust layer. Another remarkable aspect of the event is the limited height of the dust transport which is found between the ground and 5 km. Our vertically-resolved aerosol properties are also used to estimate the performances of two dust models, namely BSC-DREAM8b and NMMB/BSC-Dust, in order to evaluate their forecast skills in such intense dust outbreaks. We found that forecasts provided by the NMMB/BSC-Dust show a better agreement with observations than the ones from BSC-DREAM8b. The BSC-DREAM8b forecasts (24 h) present a large underestimation during the event. No clear degradation of the prognostics is appreciated in 24, 48, 72 h except for the Barcelona station.

The study uses a Monte Carlo radiative transfer model to examine the sensitivity of the UV erythemal radiation (UVER) enhancement to broken liquid water clouds of the cumulus and stratocumulus type. The model uses monochromatic radiation at 310 nm corresponding approximately to the peak of the product between irradiance and the erythemal curve. All scattering, absorption, extinction coefficients, and spectral albedos are tuned to this wavelength. In order of importance, fractional cloud cover, the area of individual cloud patches, and cloud thickness exert a strong influence on the enhancement, with smaller contributions from cloud optical depth, cloud base height, and solar zenith angle. In order to produce realistic enhancements for our study area located in the Valencia region of Spain (39°30′N, 0°25′W), measurements were obtained from a Landsat image of the region in combination with a spectral Fourier transform model. The Monte Carlo model, as applied to the Fourier transform cloud distribution, produced satisfactory results compared to 1 year of measured UVER enhancement for the study region provided that fractional cloud cover was equal to or greater than 3/10. At smaller cloud fractions, the neglect of cloud patches less than 50 m × 50 m in area by the model created significant discrepancies.

We present the first multiyear set of simultaneous measurements of the global ultraviolet-B radiation and its two components: direct and diffuse. The measurements have been taken with four YES-UVB-1 radiometers: two radiometers to measure the diffuse radiation, one provided with a shadow band and the other with a shadow disk on a Sun tracker; a radiometer to measure the global horizontal radiation; and a Sun-tracking radiometer to measure the direct radiation with an especially designed radiance collimator. The diffuse minute-values measured with both instruments agree within a coefficient correlation of 1.00. The diffuse component represents at least 50% of the global UVB (ultraviolet-B) radiation. The minute values of global UVB irradiance obtained by adding the direct and diffuse components concur with the measured global irradiance. Therefore, the measurement of the direct irradiance enables the estimation of the diffuse component, and gives an insight into the factors that affect its value, especially aerosols.

This study examines the wavelength dependence of cloud optical depth. To accomplish this task two different wavelength bands of the solar spectrum were considered in the cloud optical depth retrieval which was conducted in Valencia, Spain. The first retrieval used global irradiance measurements in the UVER range taken from a YES-UVB-1 radiometer in combination with multiple scattering model estimates; while the second retrieval was obtained in the Broadband range, with measurements of global solar surface irradiance from a CM6 pyranometer and a multiple scattering model. Whilst the dependence of the cloud optical depth (τ) on the wavelength is small, the best result was displayed by the SBDART model with less than 2% deviation between two ranges and moderately worse results were obtained with the LibRadtran model. Finally, seasonal statistical data for optical depth are presented for 2011 and 2012.

This study examines the sensitivity of cloud optical depth (COD) for overcast conditions to radiation transmission using data collected in Valencia, Spain. These relationships are provided as simple empirical functions, therefore avoiding the need to apply complex model minimisation schemes to obtain COD. Comparisons are presented between COD obtained by a minimization method and several radiation transmission factors comprising a clearness index (kt), a modified version (kt'), a cloud modification factor (CMF) and its modified version (CMF'). Additionally, a statistical model of COD proposed by J.C. Barnard and C.N. Long (2004) is tested with our data. Statistical relationships between COD and these variables were developed for measurements in the ultraviolet Erythema Radiation (UVER) range as well as for broadband measurements covering the full solar spectrum. Measurements collected in 2011 were used to develop power and exponential relationships relating COD to the above transmission factors, and subsequently tested with independent data collected in 2012. In general, expressions relating COD to CMF perform better and exhibit a higher correlation than equivalent expressions relating COD to clearness indices, especially in the UVER range. The expression of Barnard and Long is potentially adequate for the estimation of COD for both UVER and broadband solar radiation in Valencia, but the regression coefficients need tuning for local conditions.

The aerosol optical depth (AOD) and Angstrom exponent (α) obtained from MODIS Terra and Aqua over the coastal urban site of Burjassot (Spain) during the period 2002–2011 have been compared with retrievals from AERONET (AErosol RObotic NETwork) and ESR (European Skynet Radiometers) in order to investigate the performance of MODIS algorithms over land and ocean, respectively. The comparison of the MODIS aerosol products C051 over both land and ocean has been performed for a window size of 50 km × 50 km centred on the monitoring site. The correlation coefficients obtained from the comparison of the AOD from MODIS with that from ground-based measurements are 0.85 and 0.87 over land for Terra and Aqua, respectively, and 0.90 over ocean for both satellites. For the comparison of α, the correlation coefficients are 0.25 and 0.30 over land and 0.73 and 0.66 over ocean for Terra and Aqua, respectively. Since our analysis shows that the MODIS algorithm over ocean characterizes better the aerosol parameters over the coastal urban site of Burjassot (Spain), this dataset has been used to perform a multi-year analysis of the aerosol properties in order to derive a local climatology over this site. The mean AOD at 550 nm is 0.16, with percentiles 25 and 75 of 0.07 and 0.21. These results indicate a relatively clear atmosphere in the area in spite of local anthropogenic and remote dust sources.

An analysis of the broadband solar irradiation, IT, and the erythemal UV irradiation, IUVER, has been performed using the measurements made from 2013 to 2015 at three sites located at altitudes over 1000 m a.s.l. In Northwestern Argentina (Salta, El Rosal, and Tolar Grande). The main objective of this paper is to determine a relationship between IT and IUVER, which would allow to estimate IUVER from IT in places with few IUVER measurements available, and especially in those where is important to establish adequate photoprotection measures given their dense population and location at high altitude. The relationship between the daily values of IUVER and IT has been fitted to a linear regression (IUVER = m IT + n), showing good correlation in the three measurement sites (R2 ≥ 0.77). Besides, the IUVER/IT ratio shows an increase with altitude of 0.32 ± 0.03 units per km, indicating a more significant influence of altitude on IUVER than on IT. Total ozone column also attenuates more IUVER than IT. In order to reduce the local nature of the relationship between IUVER and IT, the clearness indices kT and kTUVER have been used to obtain a multivariable linear regression of kTUVER as a function of the solar zenith angle, θz, and kT, which shows good correlation (R2 ≥ 0.89) for the three measurement sites. Fil: Utrillas, M. P.. Universidad de Valencia; España Fil: Marín, M. J.. Universidad de Valencia; España Fil: Esteve, A. R.. Universidad de Valencia; España Fil: Salazar, Germán Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; Argentina Fil: Suárez, H.. Universidad Nacional de Salta; Argentina Fil: Gandía, S.. Universidad de Valencia; España Fil: Martínez Lozano, J. A.. Universidad de Valencia; España

Resumen Antecedentes La tecnica del fototest evalua la sensibilidad de la piel a la radiacion ultravioleta (RUV) mediante la determinacion de la minima dosis de radiacion capaz de producir eritema (dosis minima eritematica [DEM]) y la respuesta anomala a UVA. No existen guias protocolizadas para la tecnica del fototest. Metodologia Estudio multicentrico de cohortes prospectivo. Un total de 232 voluntarios sanos fueron reclutados en 9 centros hospitalarios. El fototest se realizo con simuladores solares (SS) o lamparas fluorescentes de UVB de banda ancha (UVBBA). Cada sujeto recibio un total de 5 o 6 dosis progresivas de radiacion eritematica y 4 dosis de UVA. La lectura se realizo a las 24 h. Resultados La DEM media por fototipo fue de 23 ± 8, 28 ± 4, 35 ± 4 y 51 ± 6 mJ/cm2 (fototipos i a iv respectivamente) para los centros que utilizaron SS y de 28 ± 5, 32 ± 3 y 34 ± 5 mJ/cm2 cuando se utilizaron lamparas de UVBBA para fototipos del ii al iv . Se consideraron valores de DEM patologica 7, 19, 27 y 38 mJ/cm2, para los fototipos i al iv respectivamente cuando se emplearon SS y de 18, 24 y 24 mJ/cm2 para los fototipos ii - iv expuestos a lamparas de UVBBA. A dosis de hasta 20 J/cm2 de UVA no se observaron respuestas anomalas. Conclusiones Existe homogeneidad de resultados en los diferentes centros participantes, lo que permite estandarizar el metodo del fotodiagnostico para los diferentes fototipos cutaneos, asi como establecer las dosis umbral que definen una respuesta anomala a la radiacion ultravioleta.

Background: Phototesting is a technique that assesses the skin’s sensitivity to UV radiation by determining the smallest dose of radiation capable of inducing erythema (minimal erythema dose [MED]) and anomalous responses to UV-A radiation. No phototesting protocol guidelines have been published to date. Methodology: This was a multicenter prospective cohort study in which 232 healthy volunteers were recruited at 9 hospitals. Phototests were carried out with solar simulators or fluorescent broadband UV-B lamps. Each individual received a total of 5 or 6 incremental doses of erythemal radiation and 4 doses of UV-A radiation. The results were read at 24 hours. Results: At hospitals where solar simulators were used, the mean (SD) MED values were 23 (8), 28 (4), 35 (4), and 51 (6) mJ/cm 2 for skin phototypes i to iv, respectively. At hospitals where

The daily values of UVB irradiation (290–315 nm), IUVB, and the broadband total irradiation (300–2800 nm), IT, measured on a horizontal plane have been correlated for the period 2000–2008 at 16 measurement sites in Spain. The results have been compared with the daily experimental values registered at the same sites during the period 2009–2011. The coefficients of determination R2 obtained by applying a linear regression are higher than 0.88 for all sites and increase to 0.94 when using a quadratic regression. When all data are considered together, the values of R2 are 0.91 and 0.97 for the linear and quadratic regressions, respectively. Three different clearness indices, which are dimensionless parameters, have been also used in order to reduce the uncertainties of the estimations and to reduce the local nature of the coefficients: (1) the UVB clearness index, kTUVB = IUVB/I0UVB; (2) the modified UVB clearness index, k′TUVB and (3) the estimated UVB clearness index, k*TUVB. The results show a strong dependence on the solar zenith angle. The bi-dimensional regression of kTUVB as a function of the solar zenith angle and the clearness index, kT, substantially improves the correlation, with R2 values higher than 0.81 for all sites. Finally, a formula relating kTUVB and kT, valid for latitudes between 36°N and 44°N, has been obtained: kTUVB = 0.064 kT – 0.0011 θz – 1.24 E-05 ϕ + 7.6 E-05 λ + 0.063, where θz represents the solar zenith angle (in degrees), ϕ is the latitude (in degrees) of the site and λ is the distance to the sea (in km). We have compared the experimental values for the period 2009–2011 with those estimated using this expression. For this, the coefficient of determination (R2) is 0.87, with a MBD of –3.4%, a RMSE of 21% and a NRMSE of 8.7%.

Using a sky camera, episodes characterized by overcast low clouds in the Valencia region of Spain have been selected for analysis. One year of cloud optical depth data have been produced by inverting UV erythemal irradiance measured with a UVB-1 radiometer from Yankee Environmental Systems using the LibRadtran radiative transfer model. Measurement uncertainties, aerosol single scattering albedo and cloud properties are, in order of decreasing importance, the most important factors influencing the accuracy of cloud optical depth retrieval when using UV erythemal radiometers. Statistics for cloud optical depth show a highly skewed frequency distribution best described by a gamma distribution with highest frequency between optical depths of 10 and 15 and optical depths exceeding 100 at the extreme high end.

The aerosol radiative forcing for the period 2003–2011 has been analyzed in Burjassot, Spain, an urban coastal site on the western Mediterranean, using measurements of global solar irradiance and aerosol properties, as well as simulated data. The aerosol radiative forcing was calculated at 5 minute intervals, and then integrated over 24 h to obtain daily mean values for 200 complete days under cloud-free skies. The monthly mean values of the aerosol radiative forcing showed a clear variation during the year, associated to the annual variability of the aerosol optical depth at 500 nm, ranging from − 6 to − 29 Wm− 2 at the surface, and from − 1.5 to − 3.9 Wm− 2 at the top of the atmosphere, with mean values for the whole period of − 17 ± 10 Wm− 2 and − 2.2 ± 1.3 Wm− 2, respectively. The aerosol radiative forcing efficiency was found to be − 139 ± 3 Wm− 2 at the surface and − 19.2 ± 0.3 Wm− 2 at the top of the atmosphere. These negative values, which correspond to cooling effects, suggest a significant absorption of solar radiation by the atmosphere, showing the important role of atmospheric aerosols in this urban site of the Mediterranean.

This study analyzes the atmospheric particles concentration at ground level during a severe pollution event that took place in Valencia (Spain) from 7 to 12 February 2011. The episode was related to a subtropical anticyclone system that affected the Iberian Peninsula and western Europe. The dominant anticyclone imposed an extended low gradient pressure system and a low mixing layer height in the region that caused the accumulation of pollutants over the western Mediterranean. Moreover, the stagnant conditions led to the formation of the local sea breeze, accompanied by an increase of relative humidity and the decrease of ambient temperature. Our study is mainly focused on the analysis of particulate matter levels (PM10 and PM2.5) and the concentration of black carbon (BC) measured at the Burjassot station, Valencia (Spain). BC and PM levels were found to steadily increase during the event (from 3.2 μg/m 3 and 33 μg/m 3 before the event to 4.9 μg/m 3 and 61 μg/m 3 for BC and PM2.5, respectively) and abruptly decreased to 1.3 μg/m 3 and 14 μg/m 3 , respectively, after a cold front swept the Iberian Peninsula, bringing fresh air from the Atlantic and increasing the mixing layer height, thus ventilating and diluting the accumulated pollutants. As a consequence, the EU limit of 50 μg/m 3 for PM10 size fraction was exceeded during days 7 to 12 February 2011 at Valencia, Spain.

Measurements of ultraviolet erythemal radiation (UVER) made during two years at three sites located at altitudes over 1000 m a.s.l. in Northwestern Argentina (Salta, San Carlos, and El Rosal) have been used to estimate and analyze the UV Index (UVI) and the cumulative doses at these locations. For the UVER irradiance, data of January (maximum values) and June (minimum values) have been analyzed as representative of the year for all locations. The UVI reaches extreme (> 11) values in > 20% of the analyzed days in Salta (1190 m a.s.l.), while these are reached in San Carlos (1611 m a.s.l.) and El Rosal (3355 m a.s.l.) in > 40% of the analyzed days. Finally, the cumulative doses over an average year have also been studied for each location. The doses received during austral summer and autumn are of the same order, and represent one third of the annual dose, while the doses received during austral winter and spring represent one sixth of the annual dose approximately. Fil: Utrillas, M. P.. Universidad de Valencia; España Fil: Marín Fernández, Maria J.. Universidad de Valencia; España Fil: Esteve, Anna R.. Universidad de Valencia; España Fil: Salazar, Germán Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; Argentina Fil: Suarez, Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; Argentina Fil: Castillo, Juan. Universidad Nacional de Salta. Facultad de Cs.exactas. Departamento de Física; Argentina Fil: Martinez Lozano, Jose A.. Universidad de Valencia; España

The Surface Processes and Ecosystem Changes Through Response Analysis (SPECTRA) Barrax Campaigns were validation campaigns developed in the framework of the SPECTRA mission in order to verify that the geophysical data products provided by satellite imagery are consistent with the measurements made by independent means. Two campaigns took place in Barrax, Spain, during the summers of 2003 and 2004. This paper presents the results of the characterization of the atmospheric composition from solar radiation, radiosoundings, and lidar measurements. Several potentially interesting situations involving atmospheric layers with different types of aerosols and water content are discussed. The presence of a residual layer capping the mixing layer during some days of the 2003 campaign and the arrival of a dust-rich air mass from the Sahara on the last two days of the 2004 campaign provide some relevant aerosol vertical profiles to test atmospheric correction algorithms. The study of the effects of these atmospheric situations on radiative transfer calculations is required in the development and validation of advanced atmospheric correction codes for the new generation of Earth Observation Systems. © 2007 IEEE.

The European Skynet Radiometers network (EuroSkyRad or ESR) has been recently established as a research network of European PREDE sun-sky radiometers. Moreover, ESR is federated with SKYNET, an international network of PREDE sun-sky radiometers mostly present in East Asia. In contrast to SKYNET, the European network also integrates users of the CIMEL CE318 sky–sun photometer. Keeping instrumental duality in mind, a set of open source algorithms has been developed consisting of two modules for (1) the retrieval of direct sun products (aerosol optical depth, wavelength exponent and water vapor) from the sun extinction measurements; and (2) the inversion of the sky radiance to derive other aerosol optical properties such as size distribution, single scattering albedo or refractive index. In this study we evaluate the ESR direct sun products in comparison with the AERosol RObotic NETwork (AERONET) products. Specifically, we have applied the ESR algorithm to a CIMEL CE318 and PREDE POM simultaneously for a 4-yr database measured at the Burjassot site (Valencia, Spain), and compared the resultant products with the AERONET direct sun measurements obtained with the same CIMEL CE318 sky–sun photometer. The comparison shows that aerosol optical depth differences are mostly within the nominal uncertainty of 0.003 for a standard calibration instrument, and fall within the nominal AERONET uncertainty of 0.01–0.02 for a field instrument in the spectral range 340 to 1020 nm. In the cases of the Ångström exponent and the columnar water vapor, the differences are lower than 0.02 and 0.15 cm, respectively. Therefore, we present an open source code program that can be used with both CIMEL and PREDE sky radiometers and whose results are equivalent to AERONET and SKYNET retrievals.

In this study, estimated data of the UV Index on vertical planes are presented for the latitude of Valencia, Spain. For that purpose, the UVER values have been generated on vertical planes by means of four different geometrical models a) isotropic, b) Perez, c) Gueymard, d) Muneer, based on values of the global horizontal UVER and the diffuse horizontal UVER, measured experimentally. The UVER values, obtained by any model, overestimate the experimental values for all orientations, with the exception of the Perez model for the East plane. The results show statistical values of the MAD parameter (Mean Absolute Deviation) between 10% and 25%, the Perez model being the one that obtained a lower MAD for all levels. As for the statistic RMSD parameter (Root Mean Square Deviation), the results show values between 17% and 32%, and again the Perez model provides the best results in all vertical planes. The difference between the estimated UV Index and the experimental UV Index, for vertical and 40° tilted planes, was also calculated. 40° is an angle close to the latitude of Burjassot, Valencia, (39.5°), which, according to various studies, is the optimum angle to capture maximum radiation on tilted planes. We conclude that the models provide a good estimate of the UV Index, as they coincide or differ in one unit compared to the experimental values in 99% of cases, and this is valid for all orientations. Finally, we examined the relation between the UV Index on vertical and 40° tilted planes, both the experimental and estimated by the Perez model, and the experimental UV Index on a horizontal plane at 12 GMT. Based on the results, we can conclude that it is possible to estimate with a good approximation the UV Index on vertical and 40° tilted planes in different directions on the basis of the experimental horizontal UVI value, thus justifying the interest of this study.

In this work, we present a simple model that allows us to easily obtain the character of the dominant air mass for any location in the Northern Hemisphere at any time. Specifically, in this article the model has been tested at Valencia, Spain, for the years from 2006 to 2009. The classification of the air masses is performed analysing their trajectories over pre-defined source regions in the Northern Hemisphere. Daily trajectories were simulated by the hybrid single-particle Lagrangian integrated trajectory model HYSPLIT and then classified by our model. In order to verify our classification results, those cases identified as African were compared with the Saharan dust intrusions reported for Valencia by the Ministerio de Medio Ambiente y Medio Rural y Marino MARM in collaboration with the Centro Superior de Investigaciones Cientificas CSIC. The model we present in this article can be applied in the selection of an optimum aerosol model for the atmospheric correction of remote sensing products.

Aerosol optical properties were measured by NOAA's Airborne Aerosol Observatory over Bondville, Illinois, during more than two years using a light aircraft. Measured properties included total light scattering, backscattering, and absorption, while calculated parameters included aerosol optical depth (AOD), Ångström exponent, single-scattering albedo, hemispheric backscatter fraction, asymmetry parameter, and submicrometer mode fraction of scattering. The in-situ aircraft measurements are compared here with AERONET measurements and retrievals of the aerosol optical properties at the same location, although it is difficult to verify the AERONET retrieval algorithm at a site that is not highly polluted. The comparison reveals discrepancies between the aerosol properties retrieved from AERONET and from in-situ aircraft measurements. These discrepancies are smaller for the AOD, while the biggest discrepancies are for the other derived aerosol properties. Possible sources of discrepancy between the AOD measured by AERONET and the one calculated from the in-situ aircraft measurements are investigated. The largest portion of the AOD discrepancy is likely due to an incorrect adjustment to ambient RH of the scattering coefficient. Another significant part (along with uncertain nephelometer truncation corrections) may come from the possibility that there might be less aerosol below the lowest flight altitude or that the aircraft inlet excludes aerosol particles larger than 5–7 μm diameter.

Measurements of aerosol scattering properties obtained during the period from March 2006 to December 2010 at Valencia (Spain) have been analyzed. The total aerosol scattering (σsp) and backscattering (σbsp) coefficients were measured using a TSI Model 3563 three-wavelength integrating nephelometer. From the measurements of σsp and σbsp, it was possible to determine also the scattering Angstrom exponent (αs). For the entire measurement period, the mean values (±standard deviation) at 550 nm are: 80 ± 50 Mm−1 for σsp; 8 ± 5 Mm−1 for σbsp; and 1.6 ± 0.3 for αs. These results indicate a moderate polluted atmosphere characterized by fine particles. The daily variation for all seasons shows clear patterns in the scattering properties of the atmospheric aerosols, due to the traffic and the evolution of the boundary layer throughout the day. This influence of traffic was analyzed, observing different daily patterns for weekdays (Monday–Friday) and weekends (Saturday and Sunday).

UV erythemal irradiance (UVER) has been studied on a horizontal plane and on vertical surfaces with different orientations in Valencia, Spain. The evolution of the solar noon value and the maximum daily value over a year has been analysed from the instantaneous UVER taken every five minutes on the horizontal plane and on vertical surfaces with north, south, east and west orientations. The annual evolution of these values shows a sinusoidal form for all planes except for the vertical south plane with maxima in spring and autumn. The UV Index (UVI) on these planes was also determined. The percentage of coincidence of the UVI at solar noon and of the maximum daily UVER values, considering differences of 1 or 0 units, is 87% on the horizontal plane, 100% on the vertical north plane, 92% on the vertical south plane, 77% on the vertical east plane and 75% on the vertical west plane. We have compared the UVI of the vertical plane with the horizontal one by means of linear regressions. All planes show very good correlations except for the vertical south plane, which shows no correlation with the horizontal plane for the middle days of the year due to its particular geometry with respect to the Sun's apparent path. Finally, the annual cumulative UVER doses related to each phototype for the horizontal and vertical planes were calculated. It was noticed that the dose over a year on the horizontal plane doubles the dose on the vertical south plane, which is the one that receives the maximum dose. Copyright © 2011 Royal Meteorological Society

Low ozone level episodes (also known as mini-holes) have been analysed over Valencia, Spain, during the period from 1 January 2000 to 31 December 2007. In this study, 24 episodes, lasting 1 or 2 days each, were detected. Most of them can be explained dynamically by the existence of an anticyclone over the Azores, which sends air masses coming from the Caribbean over the Iberian Peninsula. A positive anomaly in the vertical temperature profile in the high troposphere and one negative anomaly in the medium and low stratosphere are also observed in most of the analysed episodes. The total ozone column decrease is accompanied by a rise in the UV erythemal radiation levels. This increase is quantified at 1.29 times the decrease in the total ozone column. This is known as the radiation amplification factor (RAF) of UV erythemal radiation. Copyright © 2010 Royal Meteorological Society

Ultraviolet erythemal radiation (UVER) values have been simulated for different values of column ozone and aerosol optical depth (AOD) using a multiple scattering model: SBDART. The results show that UVER decreased when atmospheric ozone and aerosols increased, with the ozone having greater influence than AOD. To quantify these dependencies the influence of aerosols on the simulated values of UVER has been analysed for different ozone values. This dependence was linear. Similarly, to quantify the influence of ozone on the UVER values this was simulated for fixed values of AOD while varying the column ozone. The expression of this dependence was found to be a second-order polynomial. We also analysed the dependence of the experimental UVER values on the total column ozone and the AOD at 500 nm. Daily values of UVER were used, normalized with respect to a reference value. It was seen that the modified clearness index, k*tUVER, decreased as the total column ozone increased and also as the 500 nm AOD increased. These linear relationships were obtained and were found to be of the same order for the different intervals of AOD and for those total column ozone intervals for which the greater number of experimental data were available. At the same time, the relations between the modified UVER clearness index and the ozone, as between the modified clearness index and the AOD, showed an annual pattern. Finally, an expression was found to represent the combined dependence of the modified UVER clearness index, k*tUVER, on ozone and aerosols. Copyright © 2009 Royal Meteorological Society

The values of three clearness indices have been measured for the city of Valencia: kt, the clearness index for the whole solar spectrum, ktUV the clearness index for the ultraviolet range, and the erythemal clearness index for the erythemal radiation ktUVER, corresponding to the solar radiation weighted by the erythemal action spectrum on human skin, normalized with respect to the solar constant corresponding to that spectrum. The measurements of ktUV reached values that were approximately 54% of that corresponding to kt, while those of ktUVER were much smaller. Some statistics of the erythemal clearness index have been calculated both for monthly as well as hourly means. The shape of the distribution in both cases was nearly normal, so standard deviation and variance were used to analyse the dispersion of the data. The ratio of the erythemal clearness index and the corresponding solar irradiance was also studied. When only the central hours of the day were considered the relation between the two was markedly linear, giving a correlation coefficient that reached 99%. Copyright © 2008 Royal Meteorological Society

Although ultraviolet B radiation (UVB), the solar radiation band between 280 and 315 nm, is only a small fraction of the extraterrestrial solar radiation, it accounts for 80% of the harmful effects of exposure to the sun. In this work values of erythemal solar irradiance (UVER) measured in Valencia on a horizontal plane in the period between June 2003 and June 2005 are analysed and compared with measures of UV (A+B) and global irradiances on a horizontal plane. UV erythemal (UVER) irradiance has been measured using a YES-UVB-1 broadband radiometer; UV (A+B) irradiance has been measured using a TUVR Eppley radiometer with spectral response between 290 and 385 nm and global irradiance on a horizontal plane has been measured with a Kipp and Zonen model CM-6 broadband radiometer. A study of a statistically representative index of UVER irradiance has been made by comparing with the results corresponding to UV (A+B) irradiance and those of global irradiance on a horizontal plane. A fit has been performed using all the irradiance values to estimate the UVER and UV (A+B) components as functions of the other ranges. We have calculated for each month the irradiation acumulated during an average day, for the UVER, UV (A+B) and global, the highest values was 4.1, 791 and 25 146 kJ/m 2 respectively. Copyright  2007 Royal Meteorological Society

The erythemal UV daily dose (EDD) and the local noon UV Index (UVI) obtained from the Ozone Monitoring Instrument (OMI), on board NASA’s Aura satellite, have been validated for the period 2005-2013 using ground based measurements at 5 different sites in the Mediterranean coast: Murcia, Valencia, Palma de Mallorca, Barcelona and Rome (where only measurements of the local noon UVI were available). Ground based measurements were made using YES UVB-1 radiometers in Murcia, Valencia, Palma de Mallorca and Barcelona, and a Brewer MKIV 067 spectrophotometer in Rome. The results of the validation showed good agreement between the satellite instrument and the ground based measurements, although the OMI values overestimate the ground based measurements, being the difference between both types of measurements maximum during the spring and summer, and minimum during autumn and winter. The evolution of the EDD shows a clear seasonal behavior for all measuring sites for both, ground based and satellite data, with maximum values in summer (June and July) and minimum values in winter (December and January). A high percentage of cases (>80%) showed minimum differences (0-1 UVI units) between the UVI obtained by OMI and the UVI obtained by ground based instruments for all measuring sites. In every measuring site, high (6-7) or very high (8-10) UVI values are reached for a high percentage of the days of the analyzed period, but very few extreme (≥11) UVI values are reached.

An analysis is made of experimental ultraviolet erythemal solar radiation data measured during the years 2000 and 2001 by the Spanish UV-B radiation evaluation and prediction network. This network consists of 16 Robertson-Berger type pyranometers for evaluating solar erythemal radiation and five Brewer spectroradiometers for evaluating the stratospheric ozone. On the basis of these data the Ultraviolet Index (UVI) was evaluated for the measuring stations that are located either in coastal regions or in the more densely populated regions inland on the Iberian Peninsula. It has been checked that in most cases the maximum irradiance values corresponded to solar noon, although there were exceptions that could be explained by cloudiness. The maximum experimental values of the UVI were around 9 during the summer, though frequently passing this value at the inland measurement stations. The annual accumulated dose of irradiation on a horizontal plane has also been studied, as well as the evolution through the year in units of energy, standard erythemal doses and minimum erythemal doses, according to different phototypes.

From spectral solar irradiance extinction measurements in the visible range carried out at Sierra Nevada (2950 m a.s.l.) during July 1998, the characteristics of the aerosol column in the free troposphere have been measured. The results show that in stable atmospheric conditions, the column-integrated aerosol takes an optical depth (AOD) at 500 nm of 0.02 /spl plusmn/ 0.02, and an average radius in the accumulation mode of 0.15 /spl mu/m, with a likely Atlantic origin. In unstable situations, the AOD at 500 nm can increase by more than an order of magnitude, and the aerosols' origin is uncertain, since air masses can arrive from different sources at different altitudes.

Recent years have seen increasing public interest in issues related to energy saving and concern for the environment. It is important to highlight the work of public institutions in this respect. This was the motive that led the University of Valencia to finance a pilot project with the objective of studying useful initiatives for optimising energy consumption in accordance with the institution’s needs as well as the incorporation of innovative and more efficient technologies. The approach was to consider various aspects ranging from the analysis of the current energy consumption and the state of the installations, through the substitution of some energy inefficient components, to the study of the possible installation of a photovoltaic solar powered electricity generation station connected to the network. Also purely technical questions on economic efficiency should not be forgotten as this could lead to a reduction in energy consumption and the optimisation of the current energy consuming equipment, since generally, it is this question that limits the possible energy saving actions.

In this paper we present the results obtained from ground-based measurements using a CIMEL sunphotometer during selected clear days corresponding to the winter and summer periods of 2002 in Valencia, an urban coastal Mediterranean site. From the direct solar extinction data the spectral aerosol optical depth (AOD) has been retrieved. This spectral AOD has been used to obtain the size distribution function from the King inversion algorithm. The results show the great dependence of the optical aerosol characteristics on the dominant winds in this area. This location is subject to winds which have either a land or Mediterranean origin which frequently mask the aerosols coming from the Atlantic or north Africa, respectively.

During the past 2 years automatic measurements of UVB solar irradiance have been made sstematically at ground level at a number of points in Spain. Values for both the ultraviolet erythemal radiation and the ultraviolet index (UVI) have been obtained from these measurements. In this paper we present a first analysis of these values for two locations on the Spanish Mediterranean coast—Valencia and Barcelona—an area where a large part of the summer tourism of the country is concentrated. During the summer the UVI only reached ten on three occasions, though a value of nine was reached repeatedly (both classified as extreme risk by the World Health Organization). It has been shown that, in 90% of cases, the maximum daily value of UVI deviates from the value at solar 12 : 00 by, at most, one unit. The annual accumulated doses are expressed as a function of the minimum erythemal doses corresponding to each of the four usual skin phototypes. Copyright © 2002 Royal Meteorological Society

The authors present a preliminary study of some optical properties of atmospheric aerosols over the area of Valencia, Spain, a coastal Mediterranean city. Measurements of spectral direct irradiance in the 300-1100 nm range were taken simultaneously at three sites: rural-continental, rural-coastal, and urban-coastal, all located within a 50 km radius of the city of Valencia. The irradiance measurements were obtained using three Li-cor 1800 spectroradiometers provided with radiance limiting tubes with field of views (FOVs) of 4.7/spl deg/. The measurements were made under clear sky conditions during a field campaign carried out in the summer of 1998. In order to avoid the uncertainties associated with the determination of the water vapor content and the other atmospheric constituents, the analysis of the spectral aerosol optical thickness (AOT) values was limited to the 400-670 mm spectral band. From the values of the spectral AOT, both the Angstrom coefficients and the aerosol size distributions were obtained. The results show the great dependence of the optical aerosol characteristic on the direction of the prevailing winds (maritime or continental) in this area.

UVSPEC and SMARTS2 models have been used to estimate the UV spectral irradiance values in Valencia, Spain, and the results of these estimations have been compared with experimental measurements of direct irradiance at normal incidence obtained using an Optronic OL 754 in the 300-400 nm range. The relative RMSD and MBD parameters have been used to estimate the deviations of the modelled values with respect to the experimental values. The results indicate that the deviations are smaller when the SMARTS2 model is used with the appropriate aerosol coefficients.

The spectral values of the aerosol optical thickness τ a λ in the 400–670 nm band have been determined from 500 solar direct irradiance spectra at normal incidence registered at Valencia (Spain) in the period from July 1993 to March 1997. The τ a λ values obtained from experimental measurements have been compared with the boundary layer aerosol models implemented in the radiative transfer codes ZD-LOA and LOWTRAN 7. For the ZD-LOA code, the continental and maritime models have been considered and for the LOWTRAN 7 code the rural, maritime, urban and tropospheric models have been used. The obtained results show that the aerosol model that best represents the average turbidity of the boundary layer for the urban area of Valencia (Spain) is the continental model when the ZD-LOA code is used and the urban model when the LOWTRAN 7 code is used.

This paper presents the results of an analysis of 6 years of measurements of UV and broad band irradiation values in Valencia, Spain. Hourly and daily integrated UV irradiance, ITUV, measured by a TUVR Eppley radiometer, and global irradiance IT from a Kipp-Zonen CM-11 pyranometer, were highly correlated, with ITUV/IT percentages varying from 2.9% to 3.5% for hourly values and from 2.9% to 3.4% for daily values. If a general linear relation ITUV=mIT is considered, the correlation coefficient r is always greater than 0.96 for hourly values and 0.91 for daily values. However, the relation between ITUV/IT and the clearness index kT is poorly correlated, although improved results, with less dependence on a specific location, can be observed using the kTUV and kT clearness indices. For a general linear relation, kTUV=mkT, the correlation coefficient r is always greater than 0.89 for hourly values and 0.86 for daily values. Copyright © 1999 Royal Meteorological Society