Search

[1] In this paper we compare radiative transfer model calculations of the actinic flux in the UV spectral region with airborne measurements of the actinic flux, obtained during the Photochemical Activity and Ultraviolet Radiation (PAUR) campaign, which took place in the Aegean Sea, Greece, in June 1996, in order to assess the accuracy of the model in calculating photolysis rates, when the model input parameters are well defined from measurements. The model can simulate the total actinic flux (4 pi sr) in the UV-A region with an accuracy of 5% for all altitudes (0.1-12 km) in the cloud free troposphere, while in the UV-B the impact of the vertical distribution of ozone and aerosol can lead to differences, of about 5-10% at low altitudes up to 20% at higher altitudes. Next, the photolysis rates of J((OD)-D-1) and J(NO2) measured during the PAUR 2, in Crete, Greece (35.5degreesN, 23.8degreesE), in May 1999, at two altitudes (Gerani 30 m and Prases 1000 m), are compared with the respective model calculations in order to examine the effect of the alternating Sahara dust/maritime aerosol environments imposed to these photolysis rates, as well as to examine their differences due to the altitude difference. It is shown that high levels of tropospheric ozone and absorbing aerosols can cause a decrease in the photolysis rates of ozone near the surface, even under conditions of reduced total ozone content. This fact indicates that tropospheric ozone can be disproportionately important as a filter against UV-B radiation when most scattering of the radiation by air molecules and dust occurs in the troposphere. This behavior is not simulated accurately using a radiative transfer model constrained by observations of ozone and aerosol optical depths. The differences of the of the photolysis rates between the two altitudes as determined by the model and by the measurements, differ significantly for the Sahara dust event, indicating that during this event in the boundary layer, there is a mixture of desert and nonabsorbing aerosols.

A methodology for the absolute calibration of spectral measurements of direct solar ultraviolet radiation, performed with a Brewer spectrophotometer is presented. The method uses absolute measurements of global and diffuse solar irradiance obtained practically simultaneously at each wavelength with the direct-Sun component. On the basis of this calibration, direct-Sun spectra, measured over a wide range of solar zenith angles at a high altitude site, were used to determine the extraterrestrial solar spectrum by applying the Langley extrapolation method. Finally this spectrum is compared with a solar spectrum derived from the airborne tunable laser absorption spectrometer 3 Space Shuttle mission, showing an agreement of better than +/-3%.

The increasing international interest on solar ultraviolet radiation during the last decade stimulated large research activities about the methods and the instrumentation used to measure the solar irradiance in this spectral region. Spectroradiometers capable in recording the spectrum of the global solar UV irradiance reaching the earth’s surface have been recently deployed at various locations over the globe. The quality of the data provided by the existing instruments is constantly improving year-by-year and most of them are now capable to conduct measurements with an accuracy of about ±5%. However, the need for more reliable measurements, with an objective to achieve even ±1% accuracy, resulted in continuous efforts to reduce the errors and the uncertainties associated with the measurements of solar ultraviolet radiation.

The variability of solar irradiance in the UV-B spectral region is studied at four stations operating wellcalibrated spectroradiometers during the period 1991-96. It is confirmed that total ozone is the controlling factor in long-term changes of solar irradiance below 320 nm received at the ground. This result is supported by the similar long-term variability under clear sky and all sky conditions during the past five years. The study includes estimates of the amplitude of the annual cycle and estimates of long-term changes. An important result of this note is, that calculated changes in solar irradiance depend on the length of the observational period and that any speculations on its future changes should be treated always with caution. It is estimated that a 2.7 per cent change per year in solar irradiance at 305 nm at Thessaloniki is a rate that could possibly continue in view of the expected continuation of the ozone decline resulting to an increase of 27 per cent per decade, which is comparable to the amplitude of the annual cycle at that station.

In the frame of the Second European Stratospheric Arctic and Midlatitude Experiment an intensive program of spectral measurements of global solar UV radiation was organized at several European sites, extending from middle latitudes to the Arctic Circle, in March 1995. Before starting the intensive measurements, a 2-day intercomparison was performed at each site with a traveling spectrophotometer to ensure consistency and comparability between the instruments and their measurements during the period of the study. The collected data were used to investigate the variability of solar ultraviolet irradiances at high and middle latitudes during a period of considerably varying ozone levels. The ratio of the UV erythemal irradiance to UVA irradiance was used to exclude the interference of clouds and other wavelength independent factors. Finally, a simple statistical analysis of the data showed that the variability of UV erythemal dose due to clouds can be at least of similar importance to that induced by the variability of total ozone.

The eruptions of the low latitude volcanoes El Chichon and Pinatubo have disturbed conditions in the stratosphere and the total ozone field. The spatial and temporal distribution of these disturbances is studied using monthly mean total ozone residuals after careful removal of the components of known oscillations (seasonal variation, QBO, El Nino/Southern Oscillation). The total ozone deficiencies attributed to the volcanic effect are found to range from -2% in the tropical region up to about -5% over middle and high latitudes, lasting for 6 or more months after the eruption. These deficiencies are larger than both the noise term and the anticipated error from the aerosol contaminated radiances.

Global spectral UV-B measurements performed at Reykjavik (64 degree(s)N), Iceland since November 1991 are used in this work to study the influence of changes in total ozone on the UV-B solar radiation reaching the earth's surface at high latitudes, under variable weather conditions. The measured UV-B levels during the winter-spring season of 1992 are compared with those of the same season in 1993, during which significantly low ozone values were observed. This comparison shows increases in solar UV-B fluxes at ground level of as high as 40%. Also, the UV-B variability during summer time, when prolonged daylight exists, is studied. The broad range of total ozone levels measured during the period of study allows the calculation of the UV-B amplification factor due to changes in total ozone at different solar zenith angles which were found to range between about 2 and 4.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This work presents evidence that ultraviolet (UV)-A solar irradiances show increasing trends at Thessaloniki, Greece, where air quality has been improving because of air pollution abatement strategies. In contrast, over Beijing, China, where air quality measures were taken later, solar brightening was delayed. It is shown that until the early 1990s, UV-A irradiances over Thessaloniki show a downward trend of –0.5% yr -1 , which reverses sign and becomes positive in the last decade (+0.8% yr -1 ). This brightening is related to a decreasing trend in local aerosol amounts. Both the negative rate of change (dimming) and the positive rate of change (brightening) are amplified in the UV-A solar irradiances, compared with the total solar irradiance, by a factor of 2.6. Satellite derived short-wave radiation over Beijing showed negative changes of –0.4% (1984–1991) and –0.1% yr -1 during 1994–2006. The negative trend in solar radiation continued even during 2000–2006. Satellite-derived aerosol optical depth (AOD) increased by +1.0% yr -1 during 2000–2006, in agreement with in situ measurements of increasing AOD. Therefore, a statistically significant change from dimming to brightening in Beijing could not be seen in the last decade, but it is expected to occur in the near future. DOI: 10.1111/j.1600-0889.2009.00425.x

The characterization of a charged-coupled device (CCD) spectrograph developed at the Laboratory of Atmospheric Physics, Thessaloniki is presented. The absolute sensitivity of the instrument for direct irradiance and sky radiance measurements was determined, respectively, with an uncertainty of 4.4% and 6.6% in the UV-B, and 3% and 6% in the UV-A, visible and near-infrared (NIR) wavelength ranges. The overall uncertainty associated with the direct irradiance and the sky radiance measurements is, respectively, of the order of 5% and 7% in the UV-B, increasing to 10% for low signals [e.g., at solar zenith angles (SZAs) larger than 70 degrees ], and 4% and 6% in the UV-A, visible, and NIR. Direct solar spectral irradiance measurements from an independently calibrated spectroradiometer (Bentham DTM 300) were compared with the corresponding CCD measurements. Their agreement in the wavelength range of 310-500nm is within 0.5% +/- 1.1% (for SZA between 20 degrees and 70 degrees ). Aerosol optical depth (AOD) derived by the two instruments using direct Sun spectra and by a collocated Cimel sunphotometer [Aerosol Robotic network (AERONET)] agree to within 0.02 +/- 0.02 in the range of 315-870 nm. Significant correlation coefficients with a maximum of 0.99 in the range of 340-360 nm and a minimum of 0.90 at 870 nm were found between synchronous AOD measurements with the Bentham and the Cimel instruments.

We present three different methods for the absolute calibration of direct spectral irradiances measured with a Brewer spectroradiometer, which are shown to agree to within 2%. Direct irradiance spectra derived by Brewer and Bentham spectroradiometers agree to within 4 3%. Good agreement was also found by a comparison of the aerosol optical depth and Angstrom exponent retrieved by the two instruments and a multifilter rotational shadowband radiometer. The spectral aerosol optical depth 300365 nm derived from six years of direct irradiance measurements at Thessaloniki shows a distinct seasonal variation, averaging to 0.3 at 340 nm in winter and 0.7 in summer. © 2005 Optical Society of America OCIS codes: 120.6200, 010.1110, 010.1290.

We present the results of the aerosol measurements carried out over the Aegean Sea during the Photochemical Activity and Solar Ultraviolet Radiation campaign held in Greece during June 1996. Simultaneous observations performed with a lidar and a double-monochromator spectrophotometer allowed us to retrieve the optical depth, the Angström coefficient, and the backscatter-to-extinction ratio. The Sun photometric data can be used to improve quantitative aerosol measurements by lidar in the Planetary Boundary Layer. Systematic errors could arise otherwise, because the value of the backscatter-to-extinction ratio has to be supplied. Instead this ratio can be retrieved experimentally by use of an iterative solution of the lidar equation.

This work presents the climatology of the sulphur dioxide column over Thessaloniki derived from three years of observation with the Brewer spectrophotometer. The SO2 column averages 2.5 m atm‐cm and the shielding of UV‐B by SO2 is investigated and compared with the changes in shielding resulting from an anticipated ozone reduction. Columnar and surface SO2 concentrations over Thessaloniki appear to be typical of those in major industrial‐urban regions of the world. It is concluded that, for urban regions, predictions of changes in UV‐B as a result of man's activities must consider projections of changes in the SO2 column as well as ozone.

A programme of ground-based stratospheric and total NO2 column measurements was instituted at the Laboratory of Atmospheric Physics (40.5° N, 22.9° E) in August 1985. We present here the results of the first two years of measurements with a modified Canterbury filter photometer, details of which are given in the text. The stratospheric NO2 column, obtained at twilight during low local NO2 levels, shows the seasonal variation with monthly mean values of about 6×10-15 molec. cm-2 in the summertime to about 2.2×10-15 molec. cm-2 in the wintertime. These measurements compare well with measurements obtained with different instruments by other groups at similar latitudes (about 40° N) but in different places. Also, the asymmetry of the evening-to-morning stratospheric NO2 over Thessaloniki was found to be on the average equal to 1.58. Total NO2 column over Thessaloniki has a pronounced seasonal variation with amplitude of 0.68 matm. cm which can be explained partly from measured local NO2 sources which discharge in the mixing layer and partly from photolysis of the NO2 reservoir species.

This paper is the second part of a research programme concerning the modelling capabilities of accidental releases of heavier-than-air toxic gases. The existing theory, which includes the strength of the source and the subsequent development of the released cloud under representative environmental conditions, is described. Comparison of the ZZB-2 system predictions with field data from the Desert Tortoise and Lyme Bay V, ammonia and chlorine releases, shows excellent agreement at distances between ≈ 200 m and a few kilometres from the source. The correlation between observed and predicted cloud concentrations, was in all cases significant at a confidence level better than 95%.

Surface ozone data at several urban and one suburban station in Greece with variable record lengths, all show increasing trends during the period 1980–1987. Greece with its prolonged hot and sunny summers is particularly susceptible to photochemical production of ozone. The increasing summertime trends in monthly mean surface ozone values are statistically significant and, depending on station, they range from 8.5 ppb/year up to 13 ppb/year. The trends are larger in the suburban location than at urban sites. NOx ozone precursors also exhibit an increase of about 35% per year, which is about the same for all seasons. The large trends found in surface ozone, are among the largest in Europe.

Measurements with a tethered sonde ozone system are used to estimate the ozone sink at the ground in a semi-urban place in northern Greece. The estimate is based on measurements of the ozone removal, mostly by chemical contact with the ground, as the stable layer forms above the ground at night. An upper limit of the estimated ozone sink during this case study amounted to 1.1 · 1011 molecules cm−2 sec−1. Air quality and meteorological data for the region are also discussed.

Under the influence of the Digital Agenda initiative, governments and authorities across Europe adopt e-government tools in order to strengthen their efficiency and their ease of services provided to citizens. However, e-Environment services, as a category of e-Government services, are still underdeveloped or fragmented. This study presents the eEnviPer platform which offers an e-government tool for environmental permitting services to citizens. eEnviPer relies on and benefits from innovative technical approaches, such as clouds of public services and service-oriented architecture (SOA) to built open, flexible and collaborative e-government services, while at the same time lowering ICT costs. Five real life pilot cases were deployed in different European countries that prove and demonstrate the effectiveness and the benefits of the proposed platform.

The BREWER MARK II spectrophotometer is used to obtain simultaneous measurements of total ozone and UV flux at about 302, 306, 310, 313, 317 and 320 nm are used to estimate the ratio of the percent change in total ozone which is the UV magnification factor (UV-MF). The dependence of the monochromatic UV-MF on solar zenith angle and initial total ozone under representative enviromental quality conditions is presented.

Surface ozone concentration based on coloration of De James test-papers corresponding to daytime and nightime exposure was monitored daily at the Athens Observatory during the period 1901–1940. This neglected series was converted to ozone levels using hourly relative humidity (R.H.) data and the R.H.- ozone conversion chart of Linvill et al. (1980). The climatic fluctuations of the resulting long-term surface ozone series are presented in this report.

A homogeneous sample of ozonesonde observations covering the past 15 year was tested for the evidence of trends. The results show statistically significant trends in tropospheric ozone over middle and high latitudes throughout the records studied. No significant trends were found at stratospheric levels and in the ozone column above the 15 mb isobaric surface.

Measurements of S02 column at Thessaloniki (41°N) show a minimum in the late spring of 1983, approximately three months after the occurence of the maximum of the El Chichon aerosol in the stratosphere at these latitudes. It is suggested that the injection of the aerosol into the upper troposphere provides a scavenging mechanism for S02 and other trace gases.

Spectrally resolved measurements of solar ultraviolet (UV) radiation were made at adjacent mountain top and valley sites, vertically separated by 1 km. Data was collected over a period of 4 days in August 1993 providing simultaneously measured spectra for a range of zenith angles with both sites in direct sunlight and less than 2 octas of cloud cover. As there was no snow cover the local albedo was similar at both sites. Irradiances at the mountain site were 9 per cent higher than the valley at 370 nm, increasing gradually to 11 per cent at 320 nm. Across the UVB waveband the altitude effect increased dramatically to 24 per cent at 300 nm. The magnitude of the altitude effect was independent of zenith angle, within the measurement accuracy, for the range z = 30 degree-70 degree. These measured differences between the mountain and valley station can be interpreted by model calculations using the corresponding differences in air pressure, column ozone and aerosol content.

In the present study, a method is proposed for retrieving UV actinic flux density spectra at ground level from global and direct UV spectral irradiance measurements. The most important parameters to establish the relation between downwelling actinic flux density and global UV irradiance are the ratio of direct to global irradiance and the description of the angular distribution of the diffuse radiation field. Under clear skies, retrieved spectral actinic flux densities agreed with model calculations to within ±5% for a variety of aerosol conditions. The assumption that the diffuse radiation is isotropic leads to a wavelength-dependent overestimation of actinic flux densities from 10 to 15%, depending on solar zenith angle, especially in atmospheres with high aerosol content. Various sources of uncertainties, which are involved in the proposed methodology, are discussed and quantified.

During the solar eclipse of August 11, 1999, intensive measurements of UV solar irradiance and total ozone were performed at a number of observatories located near the path of the Moon's shadow. At the Laboratory of Atmospheric Physics (LAP) of the Aristotle University of Thessaloniki, Greece, global and direct spectra of UV solar irradiances (285–365 nm) were recorded with a double monochromator, and erythemal irradiances were measured with broadband pyranometers. In addition, higher-frequency measurements of global and direct irradiances at six UV wavelengths were performed with a single Brewer spectrophotometer. Total ozone measurements were also performed with Dobson and Brewer spectrophotometers at Hradec Kralove (Czech Republic), Ispra (Italy), Sestola (Italy), Hohenpeissenberg (Germany), Bucharest (Romania), Arosa (Switzerland), and Thessaloniki (Greece). From the spectral UV measurements the limb darkening effect of the solar disk was tentatively quantified from differences of measured solar spectral irradiances at the peak of the eclipse (near to limb conditions) and before the eclipse. Two blackbody curves were fit to the preeclipse and peak eclipse spectra, which have shown a difference in effective temperatures of about 165°K between the limb and the whole of the solar disk. The limb darkening effect is larger at the shorter UV wavelengths. The ratio of the diffuse to direct solar irradiances during the eclipse shows that the diffuse component is reduced much less compared to the decline of the direct solar irradiance at the shorter wavelengths. Moreover, a 20-min oscillation of erythemal UV-B solar irradiance was observed before and after the time of the eclipse maximum under clear skies, indicating a possible 20-min fluctuation in total ozone, presumably caused by the eclipse-induced gravity waves. This work also shows that routine total ozone measurements with a Brewer or a Dobson spectrophotometer should be used with caution during a solar eclipse. This is because the diffuse light increases by more than 30% with respect to the direct solar radiation, increasing more at the shorter wavelength side of the UV spectrum. This plausible mechanism introduces an artificial decrease in total ozone during solar eclipse of more than 30 Dobson units (DU), which is confirmed by all Brewer and Dobson measurements. Changes in total ozone cited earlier in the refereed literature have not been confirmed in the present study.

This study introduces an Earth observation (EO)-based system which is capable of operationally estimating and continuously monitoring the ultraviolet index (UVI) in Europe. The UVIOS (i.e. UV-Index Operating System) exploits a synergy of radiative transfer models with high performance computing and EO data from satellites (Meteosat Second Generation and Meteorological Operational Satellite-B), and retrieval processes (Tropospheric Emission Monitoring Internet Service, Copernicus Atmosphere Monitoring Service and the Global Land Service). It provides a near-real-time now-casting and short-term forecasting service for UV radiation over Europe. The main atmospheric inputs for the UVI simulations include ozone, clouds and aerosols while the impacts of ground elevation and surface albedo are also taken into account. The UVIOS output is the UVI at high spatial and temporal resolution (5 km and 15 minutes, respectively) for Europe (i.e. 1.5 million pixels) in real-time. The UVI is empirically related to biologically important UV dose rates and the reliability of this EO-based solution was verified against ground-based measurements from 17 stations across Europe. Stations are equipped with spectral, broadband or multi-filter instruments and cover a range of topographic and atmospheric conditions. A period of over one year of forecasted 15-min retrievals under all sky conditions were compared with the ground–based measurements. UVIOS forecasts were within ±0.5 of measured UVI for at least 70 % of the data compared at all stations. For clear sky conditions the agreement was better than 0.5 UVI for 80 % of the data. A sensitivity analysis of EO inputs and UVIOS outputs was performed in order to quantify the level of uncertainty in the derived products, and to identify the covariance between the accuracy of the output and the spatial and temporal resolution, and the quality of the inputs. Overall, UVIOS slightly overestimated UVI due to observational uncertainties in inputs of cloud and aerosol. This service will hopefully contribute to EO capabilities and will assist the provision of operational early warning systems that will help raise awareness among European Union citizens of the health implications of high UVI doses.

Eighteen radiative transfer models in use for calculation of UV index are compared with respect to their results for more than 100 cloud-free atmospheres, which describe present, possible future and extreme conditions. The comparison includes six multiple-scattering spectral models, eight fast spectral models and four empirical models. Averages of the results of the six participating multiple-scattering spectral models are taken as a basis for assessment. The agreement among the multiple-scattering models is within +/- 0.5 UV index values for more than 80 per cent of chosen atmospheric parameters. The fast spectral models have very different agreement, between ¤ 1 and up to 12 UV index values. The results of the empirical models agree reasonably well with the reference models but only for the atmospheres for which they have been developed. The data to describe the atmospheric conditions, which are used for the comparison, together with the individual results of all participating models and model descriptions are available on the Internet: http://www.meteo.physik.uni-muenchen.de/strahlung/cost/.

Using the longest available clear sky time series of spectral UV-B data (from November 1990 to present) at Thessaloniki, Greece (40°N), estimates are provided on the variability of UV-B on annual and longer time scales. Solar irradiance at two distinct wavelengths are used: the strongly ozone dependent 305nm and the weakly dependent on ozone 325nm. The results are based on Brewer measurements of spectral and erythemally weighted irradiances at 63° and 50° solar zenith angles. The long-term change in UV-B solar irradiance, which can be attributed solely to the observed ozone change of 4.5% per decade, is the order of 10% per decade at 305 nm, 63° SZA‥ These long-term change is checked against model calculations and changes imposed by the variability in air quality at Thessaloniki and is statistically significant. The peak-to-peak amplitude of the QBO in total ozone is 8% and its found to be associated with a 21% peak-to-peak amplitude relative to the mean for the solar irradiance at 305nm. Similar results are obtained with the erythemally weighted irradiance and are in agreement with results obtained from model calculations. The observed QBO in solar UV-B has many implications not only because of the QBO biorhythm imposed to many species but also because of its importance in atmospheric chemistry.

This study aims at providing experimental evidence, to support the hypothesis according to which the movement of the moon's shadow sweeping the ozone layer at supersonic speed, during a solar eclipse, creates gravity waves in the atmosphere. An experiment was conducted to study eclipse induced thermal fluctuations in the ozone layer (via measurements of total ozone column, ozone photolysis rates and UV irradiance), the ionosphere (Ionosonde Total Electron Content – ITEC, peak electron density height – hmF2), and the troposphere (temperature, relative humidity), before, during and after the total solar eclipse of 29 March 2006. We found the existence of eclipse induced dominant oscillations in the parameters related to the ozone layer and the ionosphere, with periods ranging between 30–40 min. Cross-spectrum analyses resulted to statistically significant square coherences between the observed oscillations, strengthening thermal stratospheric ozone forcing as the main mechanism for GWs. Additional support for a source below the ionosphere was provided by the amplitude of the oscillations in the ionospheric electron density, which increased upwards from 160 to 220 km height. Even though similar oscillations were shown in surface temperature and relative humidity data, no clear evidence for tropospheric influence could be derived from this study, due to the modest amplitude of these waves and the manifold rationale inside the boundary layer.

The World Health Organisation (WHO) and the World Meteorological Organisation (WMO) have jointly recommended that the UV Index (UVI) should be used to inform the public about possible health risks due to overexposure to solar radiation, especially skin damage. To test the current operational status of measuring and modelling techniques used in providing the public with UVI information, this article compares cloudless sky UVIs (measured using five instruments at four locations with different latitudes and climate) with the results of 13 models used in UVI forecasting schemes. For the models, only location, total ozone and solar zenith angle were provided as input parameters. In many cases the agreement is acceptable, i.e. less than 0.5 UVI. Larger differences may originate from instrumental errors and shortcomings in the models and their input parameters. A possible explanation for the differences between models is the treatment of the unknown input parameters, especially aerosols. Copyright © 2001 Royal Meteorological Society

Measurements of total ozone over Greece have been performed with NILU-UV multi-channel radiometers since 2005. In this study, the measurements at two stations, Thessaloniki and Athens, of the ultraviolet (UV) monitoring network are compared with ozone values derived from Brewer spectroradiometers at the same sites. The overall percentage difference on daily averages (including measurements for solar zenith angle below 70˚) is below (1 ± 3.5)% at both sites. The cloud modification factor (CMF), specified as the ratio of measured irradiance with a model-derived cloud-free value, is being used to examine the effect of cloudiness on the retrieved ozone abundances. During days with average CMF values above 0.7, the difference in total ozone measurements is within ± 5%. The use of momentary ozone values from the radiometers with this limitation in CMF reduces the difference of the average values to (0.0 ± 2.2)% for Athens and (0.21 ± 2.8)% for Thessaloniki.

Results from an intercomparison campaign of ultraviolet spectroradiometers that was organized at Nea Michaniona, Greece, July 1–13, 1997, are presented. Nineteen instrument systems from 15 different countries took part and provided spectra of global solar UV irradiance for two consecutive days from sunrise to sunset every half hour. No data exchange was allowed between participants in order to achieve absolutely independent results among the instruments. The data analysis procedure included the determination of wavelength shifts and the application of suitable corrections to the measured spectra, their standardization to common spectral resolution of 1 nm full width at half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instruments which were objectively selected and used as comparison norms for the assessment of the relative agreement among the various instruments. With regard to the absolute irradiance measurements, the range of the deviations from the reference for all spectra was within ±20%. About half of the instruments agreed to within ±5%, while only three fell outside the ±10% agreement limit. As for the accuracy of the wavelength registration of the recorded spectra, for most of the spectroradiometers (14) the calculated wavelength shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majority of the instruments was good and comparable to the commonly accepted uncertainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercomparison campaigns, held in 1995 in Ispra, Italy, and in 1993 in Garmisch-Partenkirchen, Germany. As a result of this series of intercomparison campaigns, several of the currently operating spectroradiometers operating may be regarded as a core group of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements. This work was conducted in the framework of the projec SUSPEN that was supported by the European Commission's Environment and Climate Programme, Contract ENV4-CT95-0056. The development OF THE SHICrivm software package was partly supported by the SUVDAMA project of the European Commission's Environment and Climate Programme, Contract ENV4-CT95-0177.

This paper provides the overview of an integrated, multi-disciplinary effort to study the effects of the 29 March 2006 total solar eclipse on the environment, with special focus on the atmosphere. The eclipse has been visible over the Eastern Mediterranean, and on this occasion several research and academic institutes organised co-ordinated experimental campaigns, at different distances from eclipse totality and at various environments in terms of air quality. Detailed results and findings are presented in a number of component scientific papers included in a Special Issue of Atmospheric Chemistry and Physics. The effects of the eclipse on meteorological parameters, though very clear, were shown to be controlled by local factors rather than the eclipse magnitudes, and the turbulence activity near surface was suppressed causing a decrease in the Planetary Boundary Layer. In addition to the above, the decrease in solar radiation has caused change to the photochemistry of the atmosphere, with night time chemistry dominating. The abrupt "switch off" of the sun, induced changes also in the ionosphere (140 up to 220 km) and the stratosphere. In the ionosphere, both photochemistry and dynamics resulted to changes in the reflection heights and the electron concentrations. Among the most important scientific findings from the experiments undertaken has been the experimental proof of eclipse induced thermal fluctuations in the ozone layer (Gravity Waves), due to the supersonic movement of the moon's shadow, for the first time with simultaneous measurements at three altitudes namely the troposphere, the stratosphere and the ionosphere. Within the challenging topics of the experiments has been the investigation of eclipse impacts on ecosystems (field crops and marine plankton). The rare event of a total solar eclipse provided the opportunity to evaluate 1 dimensional (1-D) and three dimensional (3-D) radiative transfer (in the atmosphere and underwater), mesoscale meteorological, regional air quality and photochemical box models, against measurements.

A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has been developed and validated. The expanded uncertainty of solar irradiance measurements with this reference spectroradiometer, based on the described methodology, is 8.8% to 4.6%, depending on the wavelength and the solar zenith angle. The accuracy of the spectroradiometer was validated by repeated site visits to two European UV monitoring sites as well as by regular comparisons with the reference spectroradiometer of the European Reference Centre for UV radiation measurements in Ispra, Italy. The spectral solar irradiance measurements of the Quality Assurance of Spectral Ultraviolet Measurements in Europe through the Development of a Transportable Unit (QASUME) spectroradiometer and these three spectroradiometers have agreed to better than 6% during the ten intercomparison campaigns held from 2002 to 2004. If the differences in irradiance scales of as much as 2% are taken into account, the agreement is of the order of 4% over the wavelength range of 300-400 nm.

[1] There have been relatively few studies that have attempted to separate the effects of the different factors affecting the short- and long-term changes of UV radiation. The effect of ozone is fairly easily represented, but that of other factors (surface albedo, aerosols, clouds) is more complex. In this paper, we present a methodology that can account for the other effects as well. We have used this methodology to study what part of the short- and long-term variability of measured spectral UV data from Sodankyla, Finland (67°N), and Thessaloniki, Greece (40°N), is explained by each factor. It was found that the effect of ozone on the short-term variability of monthly mean irradiance can be almost as high as 100%, whereas on average it is about 35%. The corresponding impacts by clouds are typically smaller, 40% and 12%, respectively. During May the albedo-related effect is strongest at Sodankyla, being 21% at its maximum and 7% on average at monthly levels. The amplitude of the variability caused by ozone is much stronger at Sodankyla than at Thessaloniki, so a longer time series at the former place is needed to detect any possible long-term trend. In the Thessaloniki time series there is no significant ozone-related increase. In the summer data from Thessaloniki, however, there is a long-term increase, which is mainly caused by cloudiness. It was found that it is crucial to check the spectral data for any possible wavelength shift, if the long-term variability at a single wavelength is studied. Moreover, it was demonstrated that if the irradiance measurements of a given solar zenith angle (SZA) range are selected, the use of even the rather narrow band of 2° can introduce an effect that hampers trend detection. In other words, it is important to remove the effect of SZA and normalize all the measurements to some constant SZA value.

We present different methods for in-field elevation calibration of MAX-DOAS (Multi AXis Differential Optical Absorption Spectroscopy) instruments that were applied and inter-compared during the second Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI-2). One necessary prerequisite of consistent MAX-DOAS retrievals is a precise and accurate calibration of the elevation angles of the different measuring systems. Therefore, different methods for this calibration were applied to 12 instruments from 11 groups during the campaign and the results were inter-compared. This work first introduces and explains the different methods, namely far and near lamp measurements, white/bright stripe scans and horizon scans, using data and results for only one (mainly the MPIC) instrument. In the second part, the far lamp measurements and the horizon scans are examined for all participating groups. Here, the results for both methods are first inter-compared for the different instruments and secondly, the two methods are compared amongst each other. All methods turned out to be well-suited for the calibration of the elevation angles of MAX-DOAS systems, with each of them having individual advantages and drawbacks. Considering the results of this study, the uncertainties of the methods can be estimated as ± 0.05° for the far lamp measurements, ± 0.1° to ± 0.3° for the horizon scans, and around ± 0.1° for the white stripe and near lamp measurements. When comparing the results of far lamp and horizon scan measurements, a spread of around 1° in the elevation calibrations is found between the participating instruments for both methods. This spread is on the order of a typical field of view (FOV) of a MAX-DOAS instrument and therefore, affecting the retrieval results. Further, a consistent (wavelength dependent) offset of 0.31° and 0.40° between far lamp measurements and horizon scans is found, which can be explained by the fact that, despite the flat topography around the measurement site, obstacles such as trees might mark the visible horizon during daytime. The observed wavelength dependence can be explained by surface albedo effects. Lastly, the results are discussed and recommendations for future campaigns are given.

The state of the stratospheric ozone layer and the temperature structure of the atmosphere are largely controlled by the solar spectral irradiance (SSI) through its influence on heating and photolysis rates. This study focuses on the uncertainties in the photolysis rate response to solar irradiance variability related to the choice of SSI data set and to the performance of the photolysis codes used in global chemistry-climate models. To estimate the impact of SSI uncertainties, we compared several photolysis rates calculated with the radiative transfer model libRadtran, using SSI calculated with two models and observed during the Solar Radiation and Climate Experiment (SORCE) satellite mission. The importance of the calculated differences in the photolysis rate response for ozone and temperature changes has been estimated using 1D radiative-convective-photochemical model. We demonstrate that the main photolysis reactions, responsible for the solar signal in the stratosphere, are highly sensitive to the spectral distribution of SSI variations. Accordingly, the ozone changes and related ozone-temperature feedback are shown to depend substantially on the SSI dataset being used, which highlights the necessity of obtaining accurate SSI variations. To evaluate the performance of photolysis codes, we compared the results of eight, widely used, photolysis codes against two reference schemes. We show that, in most cases, absolute values of the photolysis rates and their response to applied SSI changes agree within 30%. However, larger errors may appear in specific atmospheric regions because of differences, for instance, in the treatment of Rayleigh scattering, quantum yields or absorption cross-sections.

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA., La lista completa de autores que integran el documento puede consultarse en el archivo., Departamento de Aeronáutica, Facultad de Ingeniería, Instituto Argentino de Radioastronomía, Facultad de Ciencias Astronómicas y Geofísicas

We present a methodology for correcting the global UV spectral measurements of a Brewer MKIII spectroradiometer for the error introduced by the deviation of the angular response of the instrument from the ideal response. This methodology is applicable also to other Brewer spectroradiometers that are currently in operation. The various stages of the methodology are described in detail, together with the uncertainties involved in each stage. Finally global spectral UV measurements with and without the application of the correction are compared with collocated measurements of another spectroradiometer and with model calculations, demonstrating the efficiency of the method. Depending on wavelength and on the aerosol loading, the cosine correction factors range from 2% to 7%. The uncertainties involved in the calculation of these correction factors were found to be relatively small, ranging from ~0.2% to ~2%.

In September 2002, the first INSPECTRO campaign to study the influence of clouds on the spectral actinic flux in the lower troposphere was carried out in East Anglia, England. Measurements of the actinic flux, the irradiance and aerosol and cloud properties were made from four ground stations and by aircraft. The radiation measurements were modelled using the uvspec model and ancillary data. For cloudless conditions, the measurements of the actinic flux were reproduced by 1-D radiative transfer modelling within the measurement and model uncertainties of about ±10%. For overcast days, the ground-based and aircraft radiation measurements and the cloud microphysical property measurements are consistent within the framework of 1-D radiative transfer and within experimental uncertainties. Furthermore, the actinic flux is increased by between 60-100% above the cloud when compared to a cloudless sky, with the largest increase for the optically thickest cloud. Correspondingly, the below cloud actinic flux is decreased by about 55-65%. Just below the cloud top, the downwelling actinic flux has a maximum that is seen in both the measurements and the model results. For broken clouds the traditional cloud fraction approximation is not able to simultaneously reproduce the measured above-cloud enhancement and below-cloud reduction in the actinic flux. © Author(s) 2005. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.

Accurate spectral measurements of solar ultraviolet radiation are essential to an understanding of the photochemical and biological effects of ozone depletion. The analysis of spatial and temporal variations in ultraviolet fluxes will depend on the collation of spectra from many independent laboratories. However, results from diverse instruments operated in isolation may not be consistent with each other. To investigate the compatibility of different designs of spectroradiometer, a blind trial of six distinct instrument types was carried out at a suburban site in Greece. Comparisons were performed in daylight, and with tungsten lamps indoors. Excellent agreement was obtained in the relative spectral response of the instruments, but their absolute lamp calibrations varied, and did not generally agree with results from the daylight experiments. Simultaneous spectral scans by all instruments revealed discrepancies attributable to stray light, bandwidth, and cosine response, which would not otherwise...

Surface UV irradiances were measured at two different sites in Greece during June 1996 under noncloudy conditions. The measured UV irradiances are simulated by a radiative transfer model using measured ozone density and aerosol optical depth profiles. The absolute difference between model and measurements ranges between −5% and +5% with little dependence on wavelength. The temporal and solar zenith angle dependence in the difference between model and measurement suggests that part of this difference may be explained by assumptions made about the aerosol single-scattering albedo and phase function. Simulated spectra including aerosols are compared with calculated spectra excluding aerosols. It is found that for otherwise similar atmospheric conditions the UVB irradiance is reduced with respect to aerosol free conditions by 5% to 35% depending on the aerosol optical depth and single-scattering albedo. For the campaign period, changes in the aerosol loading gave larger variations in the surface UV irradiances than the changes seen in the ozone column.