In early studies of the sun’s spectrum it was found that the atmosphere diminishes the intensity of solar radiation. In general the effect is larger in the blue or violet than at longer wavelengths. This is why a scene viewed through a red filter, such as red cellophane, appears to be so “clear”. It was further discovered in the nineteenth century that the diminishing effect of the atmosphere can actually be more complicated than a smooth blue-to-red gradient and that certain wavelengths of sunlight are strongly absorbed by individual trace gases in specific wavelength bands and, moreover, to complicate things even further the diminution or “turbidity” was found to vary from one day to another. Assessing this “diminution” nuisance became especially important in quantifying incoming solar radiation reaching the earth. The investigation of solar radiation reaching the earth became a subject of great interest because it was suspected that the sun may undergoes changes in its luminosity that would modulate climate and weather and the growing of crops.

We present further development of our analysis algorithm for Multi-Filter Rotating Shadowband Radiometer (MFRSR) data. The new additions include techniques allowing us to retrieve spectral aerosol single scattering albedo (SSA) and column amount of precipitable water vapor (PWV). The SSA retrievals employ MFRSR measurements of both direct normal and diffuse horizontal irradiances. We present a sensitivity study indicating dependence of SSA retrievals on optical thickness and other aerosol parameters. Influence on the retrievals of a possible error in separation of the direct and diffuse irradiances by the instrument is discussed. The algorithm has been tested on a long-term dataset from the local MFRSR network at the DOE Atmospheric Radiation Measurement (ARM) Program site in Southern Great Plains (SGP). Our results are compared to AERONET's almucantar retrievals of SSA from CIMEL sun-photometer co-located with the MFRSR at the SGP Central Facility. A constrained variant of the algorithm (assuming zero nitrogen dioxide column values) is used for this comparison and to study the influence of the uncertainty associated with this atmospheric gas on the retrieved aerosol absorption properties. Precipitable water vapor column amounts are determined from the direct normal irradiances in the 940 nm MFRSR spectral channel. HITRAN 2004 spectral database has been used to model the water vapor absorption, while a range of other databases (HITRAN 1996, 2000, ESA) is used in the sensitivity study. The results of the PWV retrievals for SGP's MFRSR network are compared with correlative measurements by Microwave Radiometers (MWR), GPS stations, AERONET, and MODIS satellite product. In the latter case an interpolation technique has been used to determine spatial structure of water vapor field from the network data and to create a 2D dataset comparable with satellite data.

Values for aerosol optical properties in the ultra-violet (UV) spectral range, including total ozone column (TOC), asymmetry factor (g) and seven wavelength channel (300-, 305-, 311-, 317-, 325-, 332- and 368-nm) aerosol optical depths (AOD) and aerosol single scattering albedos (SSA), were obtained via an optimal estimation algorithm from direct and diffuse ground-based irradiance measurements made by UV-MultiFilter Rotating ShadowBand Radiometers (UV-MFRSR), owned and operated by the United States Department of Agriculture's UV Monitoring and Research Program (USDA UVMRP). These instruments were deployed at three different sites during the 2006 Megacities Impact on Regional And Global Environment, Mexico City Pollution Outflow Experiement (MIRAGE-MEX) field campaign. The Tropospheric Ultraviolet-Visible (TUV) radiative transfer model was utilized as the forward model in the retrieval algorithm. Initial results of the aerosol optical properties at one site were analyzed and intercompared to results obtained from independent methods. Actinic fluxes were modeled using the retrieved atmospheric properties as inputs and the results compared to independent measurements. This data set, in conjunction with many others collected during the campaign, will help to address MIRAGE-MEX science objectives related to the aging of air pollution and the evolution of the radiative properties of gases and aerosols.

Direct and global solar radiation measurements carried out in Kathmandu valley (N, E 1350 masl) have been analyzed in this study. Observations were made using a microprocessor controlled portable aerosol meter Microtops II working in ultraviolet (340, 380 nm) and visible range (440, 500, 675 nm) under cloudless conditions. Aerosol optical depth at 380 nm was found to be 0.33, 0.48, 0.21 and 0.45 in summer, autumn, spring and winter season respectively. Monitoring of global solar radiation was done with a ground based ultraviolet radiometer (GUV). From the aerosol optical depth from Sun Photometer, Angstrom turbidity parameters were estimated by means of the least square fitting technique. The aerosol optical depth is also utilized to make a model fit for UV index and compare the GUV data. A decrease in humidity by about 70% is found to increase the UV index by about 22%. The atmospheric turbidity parameter (β) is found high in the morning and show decreasing trends from morning to late afternoon on average. The wavelength exponent (α) becomes smaller during noon indicating existence of higher proportion of tiny particles in lower atmosphere in day time. Finally an inversion of spectral optical depth measurements technique allowed estimating aerosol size distribution changing between unimodal to bimodal functions yearly.

The study is to analyze changes in monthly-averaged aerosol parameters derived from the SeaWiFS observations over East Asia from January 1998 through December 2004. All the SeaWiFS GAC Level 1 data (4 by 4 km spatial resolution data) that cover the Northeast Asian area were collected and processed by the standard atmospheric correction algorithm released by the SeaWiFS Project to produce daily aerosol optical thickness (AOT) and Angstrom exponent imageries. Cloud screening was applied if AOT at 490 predicted from the aerosol look-up tables embedded in the algorithm exceeded 0.7. From the daily composite images, monthly average AOT and Angstrom exponent values were extracted for each one of the six study areas chosen from the surrounding waters of Japan. The results showed that, although annual mean of AOT did not show any trend, +0.01-0.015 increase in Angstrom exponent in almost all study areas was observed over the study period. This increase is interpreted as 4-5% increase in submicron fraction (SMF), or the ratio of contribution of submicron aerosol particles to the total AOT, and will be interpreted as an increase of submicron particles due to the enhanced anthropogenic activities in East Asia.

The Earth surface reflectance is required for retrieval of many land parameters. In this paper we attempt a solution to the problem of retrieval of surface reflectance by exploiting the synergy of TERRA and AQUA MODIS data. The algorithm was applied to MODIS data both in Beijing and UK. The relative error of Aerosol Optical Thickness values derived compared with AERONET data is less than 10%. This method could be used over various ground surfaces, including high reflective surface.

In the context of reducing the impact of atmospheric pollution on public health in cities. Previous studies have shown that optical sensors aboard satellites may be sensitive to the level of pollution because of the relation between radiances and aerosol loading and especially particulate matter (PM). The purpose of this paper is to add to this evidence by studying cloud-free satellite images and ground measurements, and then to show that urban aerosols concentrations variations can be detected and quantified by the means of satellite images. We used the radiative transfer model (6S). We simulated the reflectance at pixel level in the Landsat-TM bands. The effects and contributions of parameters (H₂O content, O₃ content, albedo, aerosols and atmosphere optical thickness) are studied thanks to experimental design approach. Change in aerosol loading is the major contributor to change in reflectance. We thus demonstrate and quantify the sensitivity of reflectance to PM. We find that channel TM4 (~ 815 nm) of Landsat is the appropriate band. However, when taking into account the gain of the sensor, we recommend TM1. On TM1 image, a difference of 71 DN (digital number) represents a variation of 120 μg/m³. The minimal concentration variation detectable is around 2.6 μg/m³. Results from observations (ground measurements and DN on satellites images) are in good agreement with simulations. In particular, an empirical model that converts DN into [PM] confirms simulations.

Mineral dusts, as the most important type of aerosols, play an important role in climate forcing and terribly affect human health, living in the vicinity of large persistent emission sources; especially located in the arid or hyper arid regions of Middle Eastern countries like Iran. In this paper, we systematically examine the TOMS satellite absorbing aerosol product (AI) over a 25-year period (1979-2004) for the evidence of local persistent dust sources, and simultaneously evaluate the most related synoptical parameters to dust emission derived from data records of more than 150 synoptical weather stations located all around the country. We also study two local prominent emission sources based on recent evaluation results, the first one is Sistan basin which is located between Iran and Afghanistan at ~31°N,61.5°E and the second are border wetlands which are located between Iran and Iraq at 31°N, ~ 46.5°E. Evaluations include the 50 year ground-based measurements in comparison with satellite remote sensing results based on data from MODerate Resolution Imaging Spectrometer (MODIS) on the Aqua satellite. Compared to ground-based measurements, satellite imagery, due to their large spatial coverage and reliable repeated measurements, provide us another important tool to monitor mineral dusts and their prominent emission sources.

The retrieval of surface emissivity and temperature from infrared radiances measured by an airborne hyperspectral sensor closely depends on the ability to correct the acquired data from atmospheric effects. In this paper we present a new atmospheric correction scheme based on sounding techniques and neural networks. A key problem of neural network is to select relevant entries and outputs. Therefore, a preliminary sensitivity analysis that takes into account atmospheric conditions as well as the surface emissivity and temperature variations is carried out. It shows that only the first three or four PCA coefficients of atmospheric profiles have a significant influence on the radiance measured in the 4.26 μm carbon dioxide and the 6.7 μm water absorption bands. But these coefficients allow to rebuilt temperature and water profiles with enough accuracy for the addressed problem. This lead us to develop two groups of neural networks, the first one to estimate the main PCA coefficients of temperature profile, and the second one to retrieve the related water PCA coefficients. The atmospheric profiles thus obtained are then used to derive the "ground" radiances. Eventually we evaluate the accuracy of surface temperature and emissivity obtained with the derived atmospheric profiles.

In this paper, variations with wavelength of aerosol optical properties which are optical thickness τ, single-scattering albedo ω₀ and asymmetry parameter g are modeled using polynomial functions in the case of dense biomass burning plumes in the spectral range [0.4 - 1.1 μm]. Optical properties are computed from Mie theory for various types of particles, size distributions and concentrations. In a first step, each optical property is fitted by polynomials with one, two and three parameters over the whole set of optical properties and then an error analysis is performed in order to choose the optimal number of parameters depending on wished accuracy. In a second step, the impact of modeling errors on top of atmosphere reflectance ρ^TOA is investigated depending on ground reflectance. The impact on ρ^TOA of ground reflectance variability under the smoke plume is also assessed. Calculations show that accurate modeling of spectral behaviour requires three parameters for τ and ω₀ and two parameters for g. It leads to simulations of ρ^TOA with an accuracy of about 0.001 which is compatible with the level of noise of current sensors. Using one less parameter for each optical property yields errors on ρ^TOA within 0.02.

On 29 March 2006, a total eclipse of the Sun was visible on the Greek island Kastelorizo (36.150°N, 29.596°E). An extended set of instruments was installed in order to measure the variability of different components of the radiation field during the eclipse. Seven spectroradiometers (two scanning double monochromators measuring especially in the UV range, 4 photo diode array instruments and one CCD-spectrograph for the UV and visible wavelength range) performed measurements during 28 and 29 March. A narrow band multi-filter radiometer and two broadband erythemal and UVA radiometers were operated with about 1 sec temporal resolution. Two sun-photometers were used to measure ozone column and aerosol optical depth. The weather conditions on March 28 were almost perfect, whereas on 29 March thin cirrus clouds were occasionally present in front of the sun. Details about the observed changes in the radiation field on the eclipse day are presented and compared with model calculations of the change in extraterrestrial solar irradiance. The results show an underestimation of the model calculations compared with measurements, with respect to the effect of the limb darkening on the spectral behavior of the solar irradiance during the eclipse. The absolute changes in the global and direct irradiance and their wavelength dependencies are discussed. Finally, the decrease in total ozone retrieved during the course of the eclipse from direct irradiance measurements is investigated with respect to the effect of the limb darkening and the influence of the diffuse radiation entering the field of view of the Brewer spectroradiometer.

Clouds and the Earth's Radiant Energy System instruments (scanners) on board the Terra and Aqua satellites have been used to meet variety of science data collection goals since their launch. Using a special programmable azimuth plan scan (PAPS) mode, a scanner collects radiation measurements of specified Earth targets or of a prescribed spatial orientation. Ongoing investigation of instruments' consistency has put some restrictions on scanning directions, partially alleviated by introducing a new elevation profile. Main objective of this paper is to explain scanning capabilities of the instruments under new conditions, and illustrate them with five different field campaigns of 2006.

The FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A-band) algorithm has been success-fully used in GOME and SCIAMACHY cloud retrieval for several years. It simulates the measured reflectance at TOA at 15 wavelengths at about 758, 761, and 765 nm. The reflectances are pre-calculated, and stored as a look-up-table. The FRESCO products are effective cloud fraction and cloud pressure, which are used in the cloud correction of trace gas retrievals from GOME and SCIAMACHY. Until now only O₂ absorption has been considered in FRESCO. In the new version of the FRESCO algorithm, called FRESCO+, single Rayleigh scattering is added in the reflectance database and the retrieval. Rayleigh scattering is mainly important for the cloud free part of the pixels. The new reflectance database has been tested with DAK (Doubling Adding KNMI) multiple scattering simulations of the O₂ A-band. Applied to GOME data, FRESCO+ gives a global average effective cloud fraction which is about 0.01 larger and a cloud pressure which is about 57 hPa higher than the current FRESCO version. The cloud pressure changes are largest for the less cloudy pixels. The FRESCO+ cloud height has been compared with ground-based cloud observations from the SGP/ARM site.

The Atmospheric Infrared Sounder (AIRS) has been operating since Sept. 2002 and is being used operationally by several weather centers. Routine retrieval processing is done by NASA. Daytime AIRS measurements in the 4.3um region show large (upto 12 K) brightness temperature shifts compared to nighttime observations. The daytime shifts result from the preferential absorption of solar radiance in the upper atmosphere by CO2 and other molecules. This energy is transferred to many of the 4 um CO2 bands, driving them into a state of Non-Local Thermodynamic Equilibrium (NLTE). We present comparisons of observations against the results of a Fast Model we developed for the AIRS instrument, that includes this effect. This algorithm is fast enough to be used for retrievals, and will be especially useful for the 4.3um R branch head channels that are used for temperature sounding.

Nitric oxide, which reacts catalytically to destroy ozone, can be produced in great abundance in the middle atmosphere during energetic particle precipitation triggered by solar storms. During the Antarctic winter, the strong polar vortex can rapidly transport nitric oxide downward, and this process has been identified as a mechanism that can link ozone recovery in the upper stratosphere with solar activity. As part of the Sun Earth Connection programme at the British Antarctic Survey (BAS), a new, state-of-the-art microwave radiometer is being developed in collaboration with the Max-Planck Institute (MPI) and the Norwegian Polar Institute (NPI) to simultaneously measure profiles of ozone and nitric oxide between 30 and 80 km deep within the Antarctic polar vortex. Operating in the 250 GHz spectral region, the semi-autonomous instrument will be coupled to moderate- and high-resolution chirp spectrometers to provide simultaneous spectra of the nitric oxide and ozone. In addition, a second local oscillator will be used to periodically examine carbon monoxide at 230.538 GHz to infer the vertical descent rate within the Antarctic vortex. Here, we present the science rationale for the observation programme as well as the instrument specifications, design and performance.

Helsinki Testbed is a research project of mesoscale meteorology running from January 2005 till September 2007. Five commercial lidar ceilometers situated in and around Helsinki, Finland are involved in this measuring campaign. The enhanced single lens optical concept of these Vaisala CL31 ceilometers improves the ability to investigate surface inversions in the lowest 100 m of the atmosphere. Attenuated backscatter profiles report interval for the Helsinki Testbed campaign has been set to 16 s, range resolution is 10 m. At one of the five sites an additional ceilometer of the same type is installed, its attenuated backscatter profiles are in good correlation with the Helsinki Testbed unit. Wind and temperature profiles from a wind profiler with RASS option and from regular radiosoundings at two different sites are available for comparisons. Two different methods of mixing height assessment have been applied, a gradient method and the idealized backscatter method introduced by the Finnish Meteorological Institute (FMI) that fits ideal attenuated backscatter profiles with variable entrainment zones to measured profiles.

Mixing layer height (MLH) is a key parameter in many atmospheric boundary layer studies and processes. A Wavelet method is developed for the automatic determination of mixing layer height from backscatter profiles of an LD-40 ceilometer. Furthermore, a quality flag is introduced to identify unreliable MLH detections. The performance of the Wavelet MLH algorithm is analysed by comparing the results with MLH estimates from radiosondes, wind profiler and research lidar measurements. A correlation coefficient of 0.64 is found between ceilometer and radiosonde determinations when using only ceilometer MLH detections with good quality. A statistical analysis of the ceilometer MLH for a six year data set shows satisfactory results for availability and the results show the main characteristics of MLH, i.e. the diurnal and seasonal cycle. However, problems arise e.g. in case of multiple (well defined) aerosol layers, which renders the selection of the correct mixing layer top ambiguous. Furthermore, in spring and summer the detection of the MLH for deep (convective) boundary layer often fails. This is mostly due to the high variability of the aerosol backscatter signal with height which limits the range for MLH estimation in those conditions.

We report a summary of the experience and results from one-year unattended operation of an automatic backscatter lidar. The backscatter lidar is realized for measurements at altitudes of Planetary Boundary Layer (PBL) and the troposphere. Such lidar has been developed and tested to answer the necessity for operation at remote sites an/or during atmospheric measurement campaigns. The results are from one year of urban boundary layer height measurements in the city of Basel (Switzerland). During this one-year of operation the lidar was remotely controlled via Internet, including also the data transfer. Here we present examples of lidar measurements of diurnal cycles of the urban boundary layer development and its height. We also present cases of lidar measurements performed in clear and cloudy sky, where the lidar observations are compared with the visual control of the sky documented by automatic camera. The results and the experience are discussed in view of the application of such automatic lidar for long-term operation as part of aerosol lidar network.

We demonstrate the importance of surface reflectivity for the radiation field in polar regions by a combination of measurements and radiative transfer calculations. Results from measurements of spectral albedo, radiance and irradiance from 280 to 1050 nm at German Neumayer Station in Antarctica in summer 2003/2004 as well as measurements of UV irradiance during summer 1997/1998 at Australian Davis Station, Antarctica are presented. The impact of surface albedo inhomogeneity is investigated by 3-D Monte Carlo modelling. We found that high surface reflectivity in the ultraviolet and visible parts of the spectrum due to the snow covered surface in Antarctica modifies the radiation field considerably compared to mid-latitudes. A change of the spectral reflectivity, which happens as a consequence of climate change will have a large impact in the radiation properties in polar regions and vice versa.

Multiband filter radiometers (MBFRs) are extensively used in national networks for UV climate monitoring and information to the public about the potential risk of solar UV exposure. In order to provide an international, uniform expression of the Global UV index measurements, a harmonized calibration scale is needed. In this paper we present the results of the first international intercomparison of MBFRs held in Oslo in 2005. The purposes are to evaluate the UV-index scale of different radiometers and to provide a harmonized UV-index scale based on the radiometers individual directional and absolute spectral response functions. In total 43 MBFR radiometers and 4 high resolution spectroradiometers were assembled, representing UV-monitoring networks operated by institutions in US, Spain, Greece, Poland, Belgium, UK, Austria, Norway, Sweden and Finland. The radiometers are operating worldwide, with stations in the Antarctica and Arctic, North- and South-America, Africa, Europe, Middle-East and Nepal. All sky conditions were realized during the campaign period. The agreement between the users' own processed UVI and the reference is generally very good; within ±5% for 22 out of 26 data sets (75%) and ±10% for 23 out of 26 (88%). Solar zenith dependent discrepancies and drift in the users' UVI scales is seen, but the performance of most radiometers is generally very good. All the objectives planned for the intercomparison were fulfilled and the campaign considered a success.

The U.S. Central UV Calibration Facility (CUCF) at the National Oceanic and Atmospheric Administration (NOAA) of the Earth Systems Laboratory calibrates Yankee Environmental System (YES) UVB-1 broadband radiometers for the USDA UV Monitoring Program. The CUCF has three reference YES UVB-1 broadband radiometers that operate in the field at the CUCF's Table Mountain Test Facility (TMTF). These three reference broadband radiometers are run simultaneously against a reference U111 Spectroradiometer developed by Atmospheric Science Research Center (ASRC) at SUNY. The temporal stability will be shown of the erythema calibration factors of the CUCF reference YES UVB-1 radiometers under clear skies from 1994 until 2005. The USDA UV Monitoring Program has 51 UV broadband radiometers that are characterized and calibrated approximately once every 1-2 years by the CUCF starting in 1997. The average annual changes in the calibration are given for the 51 USDA YES UVB broadband radiometers.

The recent increase in the ultraviolet radiation that reaches the ground, mainly due to the decrease in stratospheric ozone, demands high quality measurements over the world. For this goal the use of broadband instruments to measure erythemal-weighted irradiance is widely extended due to their low cost and easy maintenance and use. Nevertheless considerable efforts in quality assurance and quality control (QA/QC) are requiredin order to obtain a homogenised eryhtemal radiation from different regional and national networks over whole Europe. The laboratories that provide the calibration to these networks must guarantee reliable methodologies. For this purpose, the WG4 (Quality Control) within the COST-726 European action (Long-term changes and climatology of the UV radiation over Europe) is promoting the inter-laboratory intercomparison and the adoption of common procedures. In this work the procedures for calibrating broadband radiometers adopted by the laboratory "El Arenosillo" in Huelva, Spain, are presented. The methodology used at INTA has been intercompared with the UV laboratory at PMOD/WRC (World Radiaton Centre) in Davos (Switzerland) and results are presented in this document.

'Norwegian Institute for Agricultural and Environmental Research' is the owner of a network of meteorological stations, established in 1987. 45 of the stations are measuring global radiation, hourly values. These time series are to be used as one of the input parameters for modelling UV-radiation. Also several other meteorological parameters of relevance for modelling UV-radiation are measured in this station network and discussed below. A discussion on quality of these data contains general considerations on the concept of quality, considerations on the methods for making measurements at the specific series from a few sites as well as specific quality considerations on several other available time series of meteorological parameters, like the albedo, precipitation, relative humidity of the air, temperature of the air etc. from the sites. Elements describing the sites are mentioned, like information on the horizon, the geographical coordinates etc. The availability of independent time series of measurements of ozone, cloudiness, precipitation, content of aerosol and vertical distributions of humidity to be used in models are also discussed. Modelling UV-radiation at the ground is defined as a complex problem of data integration using data from different sources. The main part of this paper contains discussion complex data integration.

Ship-borne measurements of spectral as well as biologically effective UV irradiance have been performed on the German research vessel Polarstern during the Atlantic transect from Bremerhaven, Germany, to Cape Town, south Africa, from 13 October to 17 November 2005. Such measurements are required to study UV effects on marine organisms. They are also necessary to validate satellite-derived surface UV irradiance. Cloud free radiative transfer calculations support the investigation of this latitudinal dependence. Input parameters, such as total ozone column and aerosol optical depth have been measured on board as well. The maximum daily dose of erythemal irradiance with 5420 J/m² was observed on 14 November 2005, when the ship was in the tropical Atlantic south of the equator. The expected UV maximum should have been observed with the Sun in the zenith during local noon (11 November). Stratiform clouds reduced the dose to 3835 J/m². In comparison, the daily erythemal doses in the mid-latitudinal Bay of Biscay only reached values between 410 and 980 J/m² depending on cloud conditions. The deviation in daily erythemal dose derived from different instruments is around 5%. The feasibility to perform ship-borne measurements of spectral UV irradiance is demonstrated.

The data from the Rotating Shadownband Spetroradiometer UV-RSS deployed at Table Mountain, Boulder Colorado since June 2003 are used to retrieve ozone column and aerosol Angstrom coefficients in the 300 nm-380 nm range. The retrievals are performed from Langley regressions and from direct normal instantaneous irradiance measurements. The results from retrievals are used to verify assumption on ozone absorption cross-sections and ozone vertical profiles. A comparison between UV-RSS retrievals and those from the collocated instruments like the UV-MFRSR, Dobson, ozone sondes and TOMS-&-OMI is performed.

The paper presents the results of the work aimed at the application of visibility (meteorological visual range) data for UV modeling in Poland for summer season. In the first step, the meteorological visibility data were processed in order to obtain the monthly mean visibility maps that correspond to aerosols distribution over Poland. The usefulness of such maps was then validated through the comparison with AERONET data as well as the results of the UV radiation transfer model calculations. The comparison with AERONET data shows that monthly mean values of aerosol optical depth, calculated from visibility data, are in a good agreement with ground measurements for most summer months. The monthly mean visibility maps were then used in the UV radiation transfer modeling. The results of the comparative analysis performed for three Polish stations: Leba, Legionowo, Zakopane are presented and discussed. It has been shown that use of monthly mean visibility values leads to an improvement in the UV calculations quality as opposed to the calculations done with a priori assumed aerosol conditions.

Solar spectral UV-monitoring data for 8 European sites with 5-10 years of data, and covering a latitudinal range from 41 degrees North to 67 North have been re-evaluated and resubmitted to the European UV-database (EUVDB) in Finland as part of the EU-project SCOUT-O3. All resubmitted spectra (420000) were quality checked, flagged, and corrected with respect to wavelength scale errors and spectral distortions using the SHICrivm software package. Additional data products provided by the software are standardized spectra, spectral atmospheric transmissions, and biologically weighted UV-irradiances for a wide variety of biological action spectra. The resubmitted spectra were shown to have improved based on the EUVDB quality flagging criteria. Spectral and effective irradiances were integrated and summed in a standardized way to obtain daily, monthly, and seasonal UV-doses. The measured summer sums varies from 478 kJ/m² for Thessaloniki to 228 kJ/m² for Sodankyla. Clouds reduced the exposure during summer time by 30% on average, in Bilthoven this was 35%, while in Thessaloniki only 17% was reduced. Using co-located ozone and pyranometer measurements results of a generic UV-modelling approach, derived in a specific low albedo and low surface elevation environment, are systematically compared to the UV-doses obtained for all sites. Generally, a good agreement is found, measured and modelled total UV-doses agree within a few percent with a standard deviation of 15 typically. Deviations with respect to the application in a high surface albedo and high altitude environment have been identified and handles to improve the modelling have been assigned.

The objectives of the COST action 726 are to establish long-term changes of UV-radiation in the past, which can only be derived by modelling with good and available proxy data. To find the best available models and input data, 16 models have been tested by modelling daily doses for two years of data measured at four stations distributed over Europe. The modelled data have been compared with the measured data, using different statistical methods. Models that use Cloud Modification Factors for the UV spectral range, derived from co-located measured global irradiance, give the best results.

In the geographic latitudes above 50° there is a period in each winter when the level of short-wave ultraviolet radiation (UVR) remains below the threshold needed for vitamin D synthesis in human skin. Vitamin D is produced with maximum efficiency at wavelengths around 297 nm. We proceed from the assumption that spectral irradiance at 306 nm is closely related to that at the wavelength 297 nm. The daily totals of the continuously recorded 306 nm spectral irradiance above the threshold value for vitamin D production have been studied during spring and autumnal transition periods as well as in summer. The "vitamin D winter" at the Tartu-Toravere Meteorological Station site (58.16'N, 26°.28'E, 70 m a.s.l.) lasted for 100-105 days from about Nov 6 to Feb 19 in cloudless weather and climatic total ozone conditions. During the closest 10 days to the conventional "vitamin D winter" in the most unfavorable conditions there could be no availability of the vitamin D synthesizing irradiance above its threshold level. In the most favorable conditions in separate cases small doses above the threshold could be available even about two weeks before the conventional end of the "vitamin D winter".

A method for estimating past UV radiation levels using measurements of total ozone, sunshine duration and snow depth was applied to Abisko, northern Sweden, and Helsinki, southern Finland. The method has earlier been applied to Sodankyla (Finland) and Davos (Switzerland). The performance of the method was, as in earlier studies, found to be satisfactory with, for instance, a seasonal correlation coefficient as compared to measurements of the order of 95%. By extending the available ozone time series into the past using climatological values of the total ozone column, we were able to make UV estimates all the way back to the early 20th century at both stations. Both at Abisko and Helsinki, the produced time series of estimated daily erythemal UV doses shows increases over the last few decades partly due to the diminution of the total ozone column, and partly due to variations in the sunshine duration. Over the period 1950-1999, a statistically significant increasing trend of 2.2%/decade was found for Abisko, whereas for Helsinki, a trend of 4.2%/decade (also statistically significant) was found for the period 1979-1999.

The method for modelling surface UV radiation consists in using a standard radiative transfer code (UVspec) and in exploiting various sources of information (satellite and ground) to assign values to the influencing parameters. In particular, METEOSAT or MSG is used to derive cloud optical thickness and surface UV albedo. In this way, a climatological data set of erythemal daily doses over Europe, starting in January 1984 has been generated. The possibility of providing the UV information in principle anywhere and anytime has been and is exploited in impact studies. The modelled UV data have supported the UVAC project on the influence of UV on the codfish population strength in the northern Atlantic. Data were delivered to external partners to support other impact studies such as UV effects on crop yield, exposure of schoolchildren or vitamin D deficiency assessments. At JRC, the method is used in a human UV exposure model to estimate doses received by an individual, according to his/her activities and behaviour. Results were also compared with ground measurements at a number of stations and included in studies of local UV conditions. With direct MSG data acquisition, a version of the processor was developed to generate near-real time UV index maps over Europe.

The KNMI/ESA Tropospheric EMission Internet Service (TEMIS) publishes daily forecasts of the clear-sky UV index for today and 8-days ahead and analyses of the daily UV dose of yesterday and before. Near-real time ozone satellite observations are used to account for the variations in UV radiation as a function of the overhead ozone column. The daily UV dose depends critically on the assumptions that are made for the cloud correction factor. Cloud cover fraction is the most uncertain parameter for day-to-day variations in the daily UV dose under non-overcast cloud conditions. The cloud optical thickness is the most important parameter determining variations in daily UV dose for overcast situations. A good cloud mask is essential to identify cloud-free scenes. Still further improvements in the daily UV dose can be made, including the inclusion of the diurnal variation in cloud optical thickness as well as corrections for aerosols.

In Europe (EU25) about half a million skin cancer cases are occurring per year and this is strongly associated with personal habits in relation to sun exposure and its UV component. Within the frame of the European GMES-Program (GMES=Global Monitoring for Environment and Security) the ESA-GSE Project PROMOTE addresses this problem by developing and implementing a UV information service that aims to reach as many as possible citizens of Europe (EU25). The overall PROMOTE UV service contains forecast and monitoring products. The underlying methods, the use of satellite data, the various UV products including related user interfaces, as well as accuracy aspects are described. One central ambition of the PROMOTE project is the close interaction between providers and users. Experiences that have been made and will be made during the different stages of the PROMOTE project contribute significantly to the further up-grading of the services.

In this study we have analysed the sensitivity of the erythemally effective radiation to uncertainties in measured total ozone content of the atmosphere (TOC).These uncertainties result from a restricted spatial resolution, a restricted temporal resolution or the restricted accuracy of measured TOC. Daily operational total ozone measurements from different instruments made over several years were applied. Measurements were gained space born by EPTOMS, ERS-2/GOME and TOVS and from the ground by Dobson and Brewer Spectrophotometers for the locations of Hradec Kralove (Czech Republic, 50°N), Nairobi (Kenya, 1°S) and Springbok (Rep. of South Africa, 30°S). The measurement uncertainties were analyzed by an inter-comparison of modeled erythemally effective UV radiation when using different sources of TOC. The evaluation of the uncertainties due to temporal delays was done in using TOC values with different temporal shifts. The influence of spatial gaps in TOC measurements was estimated separately in longitude and latitude up to distances of 1000 km around the measuring sites. From this analysis, requirements on the spatial resolution, temporal resolution and measuring uncertainties of total ozone measurements to calculate the erythemally effective UV radiation with a pre-selected accuracy can be derived in dependence of location and season.

To study the development of high air pollution episodes in a valley with urban areas, industry and transit traffic a highway emission study was performed in the Inn valley near Innsbruck, Austria. A DOAS consisting of a emitter/receiver unit and three retroreflectors was used for this study. One path was across the highway (120 m path length) in about 10 m altitude above highway level. Another path was set up in parallel to the highway and the third path was operated perpendicular to and away from the highway. The path across the highway was directly above the air pollution monitoring station Vomp which is only three meters away from the motorway. A measurement campaign was performed between October 2005 and February 2006 including an inter-comparison of the DOAS with in situ measurement devices for NO and NO₂ during one week at a site (near Schwaz) in some distance to the main emission sources. The concentrations of NO and NO₂ above the highway are clearly dominated by the traffic volume. Higher concentration values were found during week days than during the weekend. The concentrations above the highway are compared to those measured at the other DOAS paths. The daily differences in air pollution e.g. due to temporal variations of highway emissions (10 times higher during peak hours in the morning and afternoon compared to night hours) and meteorological conditions (wind directions) are investigated.

Airport air quality is influenced by traffic mainly. These are emissions from road traffic and aircraft. A measurement campaign on the airport Budapest was performed to investigate airport air quality and to identify major sources of air pollutants and to assess air quality for this airport. At four different locations, concentration of CO, CO₂, NO, NO₂ and PM10 as well as meteorological parameters were measured simultaneously. Measurement methodologies were classical in-situ techniques and open-path techniques (DOAS and FTIR). Highest concentrations were found during low wind speed conditions downwind of the airport. To quantify emissions on the airport, inverse dispersion modelling with a Bayesian approach was used on the basis of hourly averaged concentration measurements. Single emissions rates were highest for a car park, while for the whole campaign, aircraft emissions on the taxiway around terminal 2 are most important. Similar levels of emissions are reached for the car park and the freight area. Even though the most important source for NO_x on an airport, starting aircrafts, were not considered during this investigation, the results reveal, that dealing with air quality on airports, all sources of NO_x are important, and not only aircrafts.

Quantitative analysis of absorbance spectra to retrieve gas concentrations in open-path FTIR air monitoring is not always a straightforward task. Most of commercial software use classical-least-squared algorithms to retrieve the unknown concentrations. These codes usually work in real time and give appropriate results. However, sometimes these codes fail when the background reference spectrum presents absorption lines of the gas to be monitorized. This effect is frequent in some applications. Line-by-line approaches give satisfactory results because these codes solve the problem associated to the reference spectrum generating a synthetic reference background. The main drawback is that these algorithms do not work in real time, and need a skilled operator. In this work, we propose the use of artificial neural networks to analyze absorbance spectra in real time to retrieve the unknown concentrations in a simultaneous way. In addition, capabilities of the method to solve spectral overlapping will be studied. In this sense, simultaneous analysis of four atmospheric gases (CO₂, CO, H₂O and N₂O) will be included in this first version. The effectiveness of the method will be evaluated from the experimental point of view. Experimental open-path FTIR spectra (0.5 cm^-1 of spectral resolution) will be analyzed with the proposed method, as well as with CLS and LBL codes for comparison purposes. Moreover, in these experiments CO concentration has been measured by using standard extractive equipment and can be compared with the values provided by our method. Finally, some indications will be pointed to extend the method to other gases and spectral regions.

Automatic mixing layer height monitoring was performed by continuous ceilometer measurements in the Inn valley near Innsbruck, Austria. The Vaisala ceilometer LD40 was used which is an eye-safe commercial lidar and designed originally to detect cloud base heights and vertical visibility for aviation safety purposes. Special software for this ceilometer provides routine retrievals of mixing layer height from ceilometer data. Particular emphasis is given to the detection of thermally stable layers and inversions within the lower troposphere and their temporal development. Such elevated layers influence the diurnal variations of air pollution. A comparison was performed with parallel mixing layer height retrievals from a SODAR. In clear and cold winter nights sometimes several layers, which strongly influenced the air quality in that valley, could be detected with both instruments. In the absence of low clouds and precipitation ceilometers can estimate the mixing-layer-height fairly well. Ceilometer and SODAR partly complement each other.

Teide volcano, the Spanish highest peak (3716 m), is located in the central part of Tenerife, Canary Islands. An anomalous seismic activity was detected in and around Tenerife Island since middle of 2001 reaching a peak of seismicity in may 2004 by the National Seismic Network (IGN). Volcanic gases can provide a valuable information about this volcanic unrest at Tenerife; therefore, measurements of major and some minor volcanic gas components emitted by the fumarolic activity at the summit crater of Teide volcano were performed by means of optical remote sensing on September 2005. During this measurement campaign an UV-DOAS system and three op-TDL systems were used to monitor major and minor volcanic gas components in the open atmosphere inside the summit crater. The UV-DOAS system was used for monitoring SO₂ and CS₂, while three TDL-systems were used for monitoring CO₂, H₂S and HF. It could be demonstrated successfully, that the optical remote sensing instruments used (UV-DOAS and op-TDLs) were very appropriate to monitor continuously these volcanic gas components. In addition CO₂ flux measurements were performed at the summit crater of Teide volcano by means of well established methods. By multiplying CO₂ flux data by (gas)i/CO₂ ratios provided by optical remote sensing measurements, it is possible to estimate flux values for several detected volcanic gas components during this measurement campaign.

The potential of microsatellite instrumentation is analysed in the context of volcanic plume monitoring. In October 1998 the Nyamuragira volcano (Dem. Rep. Congo, Africa) erupted releasing a significant amount of sulphur dioxide (SO₂). The Ozone Mapping Detector (OMAD) instrument on-board the FASAT-Bravo microsatellite observed this event as an anomaly in solar backscattered UV. The event was similarly detected by NASA's Total Ozone Mapping Spectrometer (TOMS) with higher wavelength resolution. The response of the two instruments was analysed using instrument models and the MODTRAN radiative transfer code. Good quantitative agreement was observed despite the inherent instrumental differences. Volcanic plume events have been observed before by large space-based instruments, however this is the first such detection by a small, low-cost instrument operating on a microsatellite. Using OMAD as a basis, a new miniaturised UV spectrometer is proposed with the aim of monitoring volcanic-SO₂ plumes in the UV region between 305-315 nm, with additional channels at 340 nm and 360 nm for potential aerosol retrievals and processing purposes. This new instrument will use a Silicon Carbide (SiC) detector-array, due to its solar-visible-blind response and its high detectivity. This instrument concept, if flown in a constellation of microsatellites, can augment and complement current missions.

The Atmospheric Infrared Sounder (AIRS) sounding suite, launched in 2002, is the most advanced atmospheric sounding system to date, with measurement accuracies far surpassing those of current operational weather satellites. From its sun-synchronous polar orbit, the AIRS system provides more than 300,000 all-weather soundings covering more than 90% of the globe every 24 hours. Usage of AIRS data products, available to all through the archive system operated by NASA, is spreading throughout the atmospheric and climate research community. An ongoing validation effort has confirmed that the system is very accurate and stable and is close to meeting the goal of providing global temperature soundings with an accuracy of 1 K per 1-km layer and water vapor soundings with an accuracy of 20% throughout the troposphere, surpassing the accuracy of radiosondes. This unprecedented data set is currently used for operational weather prediction in a number of countries, yielding significant positive impact on forecast accuracy and range. It is also enabling more detailed investigations of current issues in atmospheric and climate research. In addition to the basic soundings related to the hydrologic cycle, AIRS also measures a number of trace gases, the latest such product being the global distribution of carbon dioxide. We discuss some examples of recent research with AIRS data.

A 905-nm 5-kHz rep. rate diode-laser biaxial lidar ceilometer (cloud-height monitoring) prototype is presented. The prototype uses a low-cost Fresnel lens and a low-NEP avalanche photodiode (APD) opto-electronic receiver. The article presents the opto-mechanical engineering of both the system and main subsystems involved as well as the system energy link-budget. The reception subsystem is based on a low-cost Fresnel-lens telescope and collimating and focusing adjustable parts, which include a rectangular slit diaphragm to minimise background radiance. Equivalent focal length, background radiance rejection gain, confusion circle and imaged spot characteristics onto the photodiode surface are also formulated and discussed by means of a geometrical optics approach. The emission subsystem uses a beam expander to ensure eye-safety (maximum exposure levels) and ad-hoc mechanics to provide enough degrees of freedom for emission-reception overlap factor (OVF) adjustment. At this point, an overview of future alternative mechanical solutions for enhanced pointing accuracy and trade-offs among different laser diode-based solutions is presented. This part is complemented with OVF simulations of the prototype designed. Finally, preliminary test measurements at our premises in North Campus (UPC) are introduced as raw and rangecorrected processed signals.

For monitoring of urban atmospheric particles, simultaneous measurements of aerosols and suspended particulate matter (SPM) have been undertaken at Kinki University Campus, Higashi-Osaka, Japan, since 2004. The relationship between PM_2.5 concentrations and aerosol properties obtained from radiometry using a multi-spectral photometer located as a NASA/AERONET station is examined in this work. It is found a linear correlation between aerosol optical thickness and PM_2.5 concentrations for both ordinary days and days with dust events. This fact indicates that aerosol characteristics can be estimated from SPM data, and vice versa. Combining radiometric aerosol information with surface-level particulate mass measurements is useful in studying air quality and aerosol properties. It is of interest to mention that the largest dust event recorded among our long-term observations was detected during the spring of 2006.

This work proposes an approach to retrieve aerosol properties based on combination of multi-sensor data. Here both data provided by POLDER (Polarization and Directionality of Earth Reflectances) and GLI (Global Imager) mounted on Japanese satellite ADEOS-2(Advance Earth Observing Satellite-2), which was operating during April to October in 2003, are examined. POLDER sensor has such a unique facility as directional polarization measurements with three channels, and GLI provides high-resolution images over the wide range of wavelength from near ultra violet to the thermal infrared. This fact looks promising that combining both sensor data presents effective information of aerosols. A proposed algorithm for aerosol retrieval over the land, in addition to forgoing POLDER oriented procedure, involves following processes as: 1) classification of non-absorbing and absorbing aerosols based on GLI data in the near ultra violet and the violet channels. 2) a forgoing POLDER oriented procedure. 3) atmospheric correction for NDVI.

For the purpose of detecting clouds over large areas it is necessary to use satellite imagery. Although a variety of techniques for cloud detection and cloud height estimation exist, they often make assumptions concerning the radiometry and spectral coverage available to the sensor payload. This paper explores the use of registration shifts observed between dual-bank single-band image pairs from the DMC multi-spectral imager to detect and estimate the height of clouds. The Disaster Monitoring Constellation (DMC) comprises a network of five disaster-monitoring micro-satellites that have been built by Surrey Satellite Technology Ltd (SSTL). Each DMC satellite has a multi-spectral imager (MSI) consisting of 2 banks of 3 channels pairs. The proposed technique uses a narrow angle between imagers to discern altitude and is comparable to stereo imaging but able to distinguish absolute cloud height without reference to the ground surface, using satellite telemetry. Simulations have shown that with a sub-degree angle between imagers and appropriate sub-pixel level registration scheme, vertical accuracies in the order of a few hundred metres maybe extracted. Preliminary results using phase correlation registration to ensure sub-pixel accuracies between DMC imagery have helped confirm the viability of the technique and will be presented alongside simulations.

Exposure measurements of global solar ultraviolet radiation (UV) with two types of dosimeters are compared in this evaluation: polysulphone badges with plastic film that changes its transmission after UV-exposure, and dosimeters which function by UV-induced DNA-damage to dried bacteria spores. The dosimeters agree well and are sufficiently precise. They agree also with data of daily global solar UV by two services (the European Commission's Joint Research Centre "JRC" and the Swedish Meteorological and Hydrological Institute "SMHI") which provide maps with radiation transfer (RT) model calculations of UV-exposures on a horizontal surface based on actual weather and ozone parameters from satellites. For full day exposures the two model calculations agree with each other too, and with results from a UV-monitoring instrument. Results of personal UV-exposure measurements with the two types of dosimeters do not need to be corrected. The JRC-model was better suited to calculate percentages of global UV during outdoor stay due to it's higher spatial and temporal resolution.

The recently launched geostationary meteorological satellite METEOSAT-8 with the excellent temporal resolution as well as with well suited spectral channels of SEVIRI instrument is a useful source of the information about cloud cover. The paper presents the work aimed at the validation of the cloud mask derived from MSG/SEVIRI data with the use of the NWC SAF software. The total cloudiness amount obtained from satellite data for each Polish synoptic station was compared with the simultaneous surface observation. The analysis was performed for three hourly observations. The preliminary results for the day time conditions show a good coherence of the satellite cloudiness retrieval with ground observations. Mean difference between these two variables ranges from -0.7 octas to 1.7 octas depending on the illumination conditions. The largest differences were found for nighttime and twilight observations. However, during the night, in snow conditions, the cloudiness is grossly overestimated by the satellite.

Results of regular ground-based millimeter-wave (MM-wave) measurements of vertical ozone distribution (VOD) in the stratosphere and mesosphere over Moscow region for the last years are presented. The observations of the 142.175 GHz ozone line were done using low-noise heterodyne spectrometer. Tikhonov method was used for retrieval the ozone mixing ratio profiles from the experimental spectra. Descriptions of both the retrieval algorithm and the spectrometer are given. As seasonal changes in ozone as more short-term ozone variations with time scales from several days to several weeks were recorded. It was found that the most noticeable changes in the ozone layer occurred in cold seasons. Correlations between ozone content and other parameters of the atmosphere were established for different altitudes. It was shown that the VOD over Moscow is strongly influenced by large-scale atmospheric dynamics. Considerable deformations of the stratospheric ozone profiles were detected in winter months, when both decreased ozone content at altitudes of 25-45 km and local minimum near 30 km were observed many times. Appearance of the secondary (in the lower thermosphere around 90 km) and tertiary (in the mesosphere at altitudes of 55-75 km) maxima in the ozone profiles in night hours, and strong variations in the night ozone at the altitudes were measured. A comparison of the ground-based MM-wave data with results of satellite measurements by EOS MLS instrument has shown good coincidence of the ozone profiles obtained from the ground level and from space.

This paper presents a method for generating volumetric clutter for air surveillance radar simulation. Complex valued radar signal consists of magnitude and phase. In the presented simulation, radar clutter signal is created from magnitude and phase distribution and then filtered imitating the radar signal formation. Radar geometry can be integrated to the simulation by manipulating magnitude, phase, and phase difference distributions. Magnitude is affected by range bin size and distance from radar. Also weather condition and polarization effect on the signal. These can be controlled with adjustments to the distribution that the matrix is created from. This solution offers a simple way to create background to realistic radar simulation. Different distributions are used for signal magnitude and phase of various clutter sources. Typically, volumetric clutter source consists of many evenly sized scatterers. Preliminary phase, originating from randomly distributed particles, can be considered evenly distributed. Phase difference in long time, on the other hand, shows the radial movement of particles. Therefore, phase difference can be modeled, for example, with Gaussian distribution and magnitude with Weibull distribution, of course, depending on true environment. As an example, chaff is simulated with differing radial wind.

In this paper we estimate the signal-to-noise ratio (SNR) at the opto-electronic receiver output of both elastic and Raman lidar channels by means of parametric estimation of the total noise variance affecting the lidar system. In the most general case, the total noise variance conveys contributions from photo-induced signal-shot, dark-shot and thermal noise components. While photo-inducted signal-shot variance is proportional to the received optical signal (lidar return signal plus background component), dark-shot and thermal noise variance components are not. This is the basis for parametric estimation, in which the equivalent noise variance in any receiving channel is characterized by means of a two-component vector modeling equivalent noise parameters. The algorithm is based on simultaneous low-pass and high-pass filtering of the observable lidar returns and on weighted constrained optimization of the proposed variance noise model when fitting an estimate of the observation noise. A noise simulator is used to compare different noisy lidar channels (i.e. with different pre-defined noise vectors or dominant noise regimes) with the two-component noise vectors estimate retrieved. Both shot-dominant and thermal-dominant noise regimes, as well as a hybrid case are studied. Finally, the algorithm is used to estimate the SNR from lidar returns from tropospheric elastic and Raman channels with satisfactory results.

Atmospheric and surface thermodynamic parameters retrieved with advanced hyperspectral remote sensors of Earth observing satellites are critical for weather prediction and scientific research. The retrieval algorithms and retrieved parameters from satellite sounders must be validated to demonstrate the capability and accuracy of both observation and data processing systems. The European AQUA Thermodynamic Experiment (EAQUATE) was conducted mainly for validation of the Atmospheric InfraRed Sounder (AIRS) on the AQUA satellite, but also for assessment of validation systems of both ground-based and aircraft-based instruments which will be used for other satellite systems such as the Infrared Atmospheric Sounding Interferometer (IASI) on the European MetOp satellite, the Cross-track Infrared Sounder (CrIS) from the NPOESS Preparatory Project and the following NPOESS series of satellites. Detailed inter-comparisons were conducted and presented using different retrieval methodologies: measurements from airborne ultraspectral Fourier transform spectrometers, aircraft in-situ instruments, dedicated dropsondes and radiosondes, and ground based Raman Lidar, as well as from the European Center for Medium range Weather Forecasting (ECMWF) modeled thermal structures. The results of this study not only illustrate the quality of the measurements and retrieval products but also demonstrate the capability of these validation systems which are put in place to validate current and future hyperspectral sounding instruments and their scientific products.

Increasing user and service needs for Numerical Weather Prediction impose an Infrared Sounding (IRS) mission on Meteosat Third Generation platforms. A major objective of the IRS mission is to provide a vertical sounding capacity in the vicinity of clouds, above clouds and below semi-transparent clouds. Thus, cloud contamination has to be severely controlled at instrument or at system level. Pseudo-noise is defined as the measurement error generated by scene heterogeneity. Spectrally correlated, it affects the information content of the vertical sounding and thus the quality of high-level products. An IRS pseudo-noise estimation model has been developed in view of supporting the consolidation of observational requirements during the detailed instrument design and system specification process. Geophysical inputs to the model are a spatial atmosphere distribution and the corresponding high-resolution radiance spectra. Major instrument inputs are a PSF model and an ISRF model. The paper presents a mathematical specification of pseudo-noise components and their quantification upon current design assumptions for the two considered instrument concepts: dispersive and Fourier transform spectrometer. The pseudo-noise budget is established for meaningful geophysical scenarios, and the associated observational requirements are derived and discussed, providing useful orders of magnitude for further iteration of the trade-off between instrument specifications and system requirements.

The decrease in stratospheric ozone observed during last decades and the possible relationship with the observed increase UV-B radiation in biosphere has been extensively discussed in relevant studies. However, the detection of UV trends is difficult due to lack of long-term measurements of UV radiation at the Earth's surface, and the large variability introduced by changes in cloudiness, aerosols and surface albedo. Recently, several methods for reconstruction of UV radiation levels for the past at single sites have been proposed. In most of these studies, measurements of total ozone and empirical or model derived relations for the impact of clouds and surface albedo on UV transmittance have been used. In this study, a method for estimating erythemal (CIE) UV doses using measurements of total ozone and total solar radiation is presented for Thessaloniki, Greece (40.5°N, 23°E). Measurements of total solar radiation and UV erythemal dose rates for five years period (2000-2004) were used to estimate the effect of clouds and aerosols as a function of solar zenith angle (SZA). The method is then tested, when compared with measurements from previous years. The mean difference (%) between measured and modeled cloud modification factors for UV dose rates was calculated between -1.3% and 2.4% for all SZA groups.

The stratospheric ozone depletion during the last two decades, the increase of UV-B irradiance levels at the ground and the possible impact on the biosphere has led scientists to develop and use instruments of high accuracy for UV measurements. During the last two years, 9 UV stations have been established in different environments in Greece and Cyprus, with the aim to establish a long-term monitoring network. The instruments of the network (NILU-UV multichannel filter radiometers) can provide measurements of irradiance in the UV and the visible part of the solar spectrum. In this study, first results from the calibration measurements and the quality assurance procedures are presented. The stability of the maximum of spectral response and the full width at half maximum was measured within 0.5 nm. Lamp tests were performed and downward drifts up to 40% in UVA channel sensitivity were observed. Calibration factors derived from lamp measurements could provide measurements of UV dose rate and total ozone with quite good agreement when compared with standard ultraviolet instruments.

Monitoring of the terrestrial solar ultraviolet irradiance by using a radiometer is often considered as expensive and laborious or the data collected as insufficient in spatial coverage and in some cases in its temporal resolution, too. Therefore, alternative methods, all relying on modelling in one way or the other, have been developed. They differ in which input they receive, either standard meteorological information, space-based radiance measurements or ground-based irradiances from broadband or multiband UV radiometer or from pyranometer. A comparison of performance is presented between three methods during a 15-month period. The ground reference instrument is the Brewer Mk-III #107 spectroradiometer of the Observatory of Jokioinen, Finland. Compared to the reference, the space-based method overestimates the UV irradiance at noon by 14.6% and the pyranometer-based by 0.9% with root-mean-square differences of 35.5% and 10.4%, respectively. Daily erythemal doses agree by 3.8% for the space-based and 0.4% for the pyranometer-based method with a scatter of 16.5% and 4.6%, respectively. Spectral irradiances generated by the pyranometerbased model agree within 0.4% on average with a standard deviation of 17%. A rough estimate on the cost of each approach suggests that none of them is clearly superior to the others and the actual nature of the data needed may be used in decision making concerning monitoring strategies.

For climatological studies it is most important to have data from long time series observations. Typical UV measurement series are not longer then 15 years. The UV reconstruction algorithms pretend to enhance UV data series, using the solar radiation measurements, available even since 1950. In the presented method, the UV (erythemal) reconstruction algorithm is based on temporal variability analysis. The proposed procedure with Fast Fourier Transform (FFT) computes power spectrum of the global irradiance (GR) transmission for all or for the selected part of the diurnal cycle. The similar GR transmission power spectra are found from the available series of data. It has been shown, that for the data with similar power spectra in GR transmittance, the power spectra in the UV transmittance are as well similar. So, it is possible to calculate the all-sky UV irradiance, using the UV transmittance and the radiation transfer model for clear-sky. This algorithm is particularly useful to estimate the effect of clouds on the downwelling shortwave irradiance (without direct cloud observations). The algorithm can be applied for UV reconstruction, using information on total solar irradiance, total ozone and aerosols. As the second approach the wavelet method has been applied. The Streamer and Fastran radiation transfer models have been used for testing the proposed UV reconstruction algorithms. The testing has been performed on available data from Arosa site, for year 2002, within COST 726 Action.

There has been a growing interest in the connection of vitamin D and solar ultraviolet-B (UVB) radiation because numerous studies have shown that there is an anti-correlation between the vitamin D level and many types of cancer and various diseases. Additionally there is the well known anti-correlation with osteoporosis and its progress, a disease that affects approximately in 30% of all post-menopausal women. Information about the actual effectiveness of solar UVB radiation in producing vitamin D could therefore a very helpful tool for health care. Such information should base on well estimated parameters either measured or modeled. Therefore, we investigated the possibilities in measuring, modeling and forecasting the vitamin D effective radiation. Measurements were made with two different broadband devices. One is a hand-held radiometer which was designed as a personal dosimeter and another is a device which is in use world wide to measure the erythemally effective UV radiation. Further on we introduce a world wide forecast of the vitamin D effective UV radiation for clear skies together with its validation. Input parameters for the forecast are date and time, geographical position and altitude, total ozone, low aerosol content and neglected albedo.

Long-term ground-based UV radiometers and satellite UV spectrometers have been utilized for detecting trends in UV radiation and for establishing its climatology. The aim of this work is to compare noon erythemal (CIE) UV irradiance data from NASA Total Ozone Mapping Spectrometer (TOMS) with ground-based measurements from a Brewer spectrophotometer. The Brewer instrument is located at the Atmospheric Sounding Station "El Arenosillo" (ESAt) in Huelva, Southwestern Spain. ESAt, with a high number of cloud-free days per year, is particularly suitable for atmospheric-radiation studies and satellite validation. The period of study covers the years 2000 to 2004. The effect of clouds and aerosols on the satellite vs ground-based bias is evaluated under different atmospheric conditions regarding aerosol load and cloudiness. It is found that under all sky conditions TOMS overestimates the noon CIE irradiance about 8%. This bias is even higher (about 12%) for cloud-free days, showing statistically significant correlation with the aerosol optical depth at 440 nm as measured by a co-located CIMEL-AERONET sunphotometer. However, for thick clouds (high TOMS reflectivity) the bias becomes negative. Regarding aerosols, the bias increases as the aerosol load increases, showing the highest values during dust events. All these facts suggest the need to correct the TOMS UV products mainly under dust conditions.

An Antarctic UV-monitoring network established in 1999 as a Spanish-Finnish-Argentinian co-operation consists of multiband filter radiometers located at Belgrano, Marambio, and Ushuaia. To provide with quality controlled and assured calibrated groundbased Antarctic UV data, bi-weekly lamp tests were used on every site and visits of travelling reference instruments on two of the sites. Along the six years of operation, the sensitivity in some of the instrument channels was found to drift up to 61%. In both stations, always the same channels showed the best stability or worst instability. The rigorous quality assurance programme ensured that reliable time series of solar data could be produced, however. The most recent Antarctic ozone depletion period of 2005/2006 was studied by comparing OMI satellite-based erythemally weighted daily doses with the measured polynomial corrected data for August 2005-March 2006 for Ushuaia and Marambio. The root mean square (RMS) of difference between the groundbased and satellite-retrieved daily doses was on monthly basis smaller for Ushuaia (19 - 28 %) than for Marambio (17-58 %), possibly due to e.g. bigger heterogeneity of the ground albedo, and variability of the cloudiness. Our final task of combining the polynomial corrected lamp calibration factors and the traveling reference calibration factors, to produce the final calibrated Antartic UV data, is discussed, too.

Currently there is still low spatial coverage of ground-based instruments measuring UV irradiance (spectral or broad band) and the length of time over which reliable UV observations have been made is mainly around 10 years. Solar spectral irradiance (from 290 to 325 nm at 0.5 nm wavelength step) has been measured at the stations of Rome (41.9°N, 12.5°E, 75 m a.s.l.) and Ispra (45.8°N, 8.6°E, 240 m a.s.l.), by means of Brewer single monocromator spectrophotometer since 1992. In this study a climatological characterization based on the time series of UV index (UVI) is presented. The mean of maximum UV indexes is (7.2±0.2) at Ispra and (8.9±0.4) at Rome under clear sky conditions. Low exposure category (UVI<2) is persistent at Ispra and high exposure category (6