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Ozone Monitoring Instrument (OMI), launched in July 2004, is dedicated to the monitoring of the Earth's ozone, air quality and climate. OMI is the successor of the Total Ozone Mapping Spectrometer (TOMS) instruments and provides among other atmospheric and radiometric quantities the total column of ozone (TOC), the surface ultraviolet (UV) irradiance at several wavelengths, the erythemal dose rates and the erythemal daily doses. The main objective of this work is to compare OMI data with data from ground-based instruments in order to use OMI products (collection 2) for scientific studies. The Laboratoire d'Optique Atmosphérique (LOA) located in Villeneuve d'Ascq (VdA) in the north of France performs solar UV measurements using a spectroradiometer. The site of Briançon in the French Southern Alps is also equipped with a spectroradiometer operated by Interaction Rayonnement Solaire Atmosphère (IRSA). The OMI total ozone column data is obtained from the OMI-TOMS and OMI-DOAS algorithms. The comparison between the TOC retrieved with ground-based measurements and OMI-TOMS data shows good agreement at both sites for all sky conditions with a relative difference for most of points better than 5%. For OMI-DOAS data, the agreement is generally better than 7% and these data show a significant dependence on solar zenith angle. Comparisons of spectral UV on clear sky conditions are also satisfying with relative differences smaller than 10% except at solar zenith angles larger than 65°. On the contrary, results of comparisons of the erythemal dose rates and erythemal daily doses for clear sky show that OMI overestimates surface UV doses at VdA by about 15% and that on cloudy skies, the bias increases. At Briançon, such a bias is observed if data corresponding to snow-covered surface are excluded.

The paper presents in detail a code developed to solve the vector equation of radiative transfer. It is based on the successive orders of scattering and on the expansion into Fourier series of the azimuth; it includes the polarizing effects of aerosol scattering and of reflectance by water or terrestrial surfaces. The code is available from the authors upon request, and this detailed description aims at helping potential users to adapt the code to their own problem. Some examples of applications are given.

We compare three different methods for determining an average ‘effective‧ UV albedo. These methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. The first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow-covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present the differences and the correlations between the various methods as well as the results for the different locations.

The Alps are one of the regions in Europe showing very high erythemal UV irradiance (UVery) which is a consequence of low aerosol levels, high altitude and snow-covered surfaces. Effective albedo plays a key role in understanding the radiative transfer over an Alpine terrain. The difficulty in determining the effective albedo is predominantly due to inhomogeneous surfaces in Alpine regions and a number of interrelated parameters. Most of these parameters can be corrected or normalized by simple methods. The effect of total ozone on different components of the erythemal UV irradiance and the influence of aerosols have been largely investigated and are determined by model calculations. After these atmospheric corrections or normalizations the albedo of the surrounding surfaces within a radius of about 25 km is the only remaining influence to be taken into account on the ratio between direct and diffuse UVery radiation. Satellite data from selected days allowed us to determine the fraction of snow coverage within this radius and to weight the effect of the surrounding surfaces on UVery radiation. With this information from space and data from local albedo measurements in the erythemal UV, it is possible to calculate effective albedo values for these selected days. A correlation between effective albedo and the ratio direct to diffuse was found to be linear for constant solar elevations. Hence this correlation allows us to determine effective albedo at any Alpine location with accurate measurements of direct and diffuse UVery radiation. It is shown from this investigation that the albedo effect can enhance diffuse UVery radiation by up to 57% and global UVery radiation by 30%, especially in spring.

Seven spectroradiorneters measured simultaneous surface UV irradiances at six different sites in the vicinity of Garmisch-Partenkirchen, Germany, during spring 1999. The measured clear-sky irradiance variability between the sites was analyzed with respect to altitude, aerosol optical depth, solar zenith angle, effective albedo, and tropospheric ozone. For conditions of low aerosol loading the increase of irradiance per 1000 m altitude difference was 9% at 400 nm. 20% at 320 nm, and 30% at 300 nm in this season. Effective albedo differences of 0.15, 0.29, and 0.65 were found between the snow-covered stations and the snow-free ground station with the higher effective albedo values determined at the two mountain stations. Clean continental aerosols with a single-scatter albedo of 0.95 were observed during this campaign. The measurements and the observed variations between the sites should enable more accurate modeling studies to be performed for an Alpine environment.

We present here the inversion algorithm implemented by the Laboratoire d'Optique Atmospherique, University of Lille, France, for the analysis of solar and lunar occultation data from the Stratospheric Aerosol and Gas Experiment III (SAGE III). The first flight of SAGE III is planned to be launched in late fall 2000 on the polar orbit spacecraft METEOR 3M. We present first the forward model algorithm for calculating atmospheric transmittances in four of the SAGE III channels: the solar ones, around 440 nm and 600 nm, and the lunar ones, around 413 nm and 660 nm. Then the inversion algorithm is introduced, accomplished in two sequential steps. The first one is the spatial inversion of the simulated slant optical thickness profile leading to the extinction coefficient profile. The second is the spectral inversion of the extinction coefficient at each altitude to separate gas and aerosol contributions. Lastly, error analysis is conducted by a Monte Carlo technique and discussed: the retrieved gas densities and aerosol extinction profiles favourably compare to the corresponding input profiles.

The UV spectroradiometer used at the University of Lille is presented, and its characterization and calibration are described. In order to detect possible shifts between calibrations, two different consistency tests are regularly performed on cloudless days. The erythemal dose rate obtained by integrating the weighted spectrum is compared to broadband measurements and modeling results. A Sun photometer provides the aerosol optical depth necessary as an input to the radiative transfer model. Results of these tests are presented for some cases; the general agreement is within 5%, which corresponds to the expected uncertainties.

We investigated the retrieval of aerosol properties and the extinction due to aerosols at the ozone and water vapor channels from simulated measurements at variations of the planned Stratospheric Aerosol and Gas Experiment (SAGE) III aerosol channels. The aerosol quantities surface area, volume, and effective radius are retrieved through the application of two distinct algorithms in the form of the randomized-minimization-search technique (RMST) and the constrained linear inversion (CLI) method. These aerosol quantities are important as inputs in climate, photochemical, and radiative forcing models and are useful in comparing diverse measurements. Ten analytical size distributions fitted to aerosol populations measured in situ are used with a Mie scattering code in conjunction with a Monte Carlo technique to simulate SAGE III measurements. These models consist of variations of prevolcanic and postvolcanic size distributions that exhibit various spectral shapes. Neither the complex components nor the uncertainties of the refractive indices are considered. We developed an objective scheme to estimate the systematic, random, and total uncertainties of each retrieved quantity that considers the contribution of the particles that lie outside the retrieved size range. Results, based on the 10 selected aerosol models, indicate that in the seven-eight SAGE III channel retrievals, both algorithms obtain estimated total errors in the range 8-50% for the surface area with an average total error (R*) of ∼25%; for the volume the range is 5-25% with an R* of ∼12%, and for the effective radius, the range is 6-36% with an R* of 20% though both inversion techniques are applied in different size ranges. The inversion of the six longest channels to study aerosol properties in both the lower stratosphere and the upper troposphere leads to RMST R* values of ∼32, ∼15, and ∼20% and CLI R* values of ∼48, ∼22, and ∼31% for the surface area, volume, and effective radius, respectively. In the seven wavelength retrievals, both algorithms retrieved the extinction coefficients at the unused channel to within their measurement uncertainties except at the 0.385 and 1.550 μm channels located at the tail ends of the SAGE III aerosol extinction spectrum. The calculated extinction due to aerosols at the water vapor channel at 0.940 μm and the ozone channel at 0.600 μm produced R* values of

The ultraviolet spectral irradiance has been measured in the Alps in Briancon (altitude 1300 m asl) in July 1996 and March 1997, with the spectroradiometer of the University of Lille. The erythemal dose rate has been measured simultaneously with a broadband instrument. The aerosol optical depth, necessary as input for modeling, was measured with a Sun photometer. On cloudy days, irradiance is highly variable, with values exceeding those of clear days when the direct solar beam is not obstructed by clouds. For clear days, measured values are compared with the results of a radiative transfer model, and the influence of altitude and ground surface reflectance is analyzed. For the same solar zenith angle, irradiance in UVA is 8–10% higher in Briancon than in Brussels, half of the difference being due to the higher altitude of Briancon. Comparison of winter and summer values shows an amplification of ∼10–15% in winter owing to the snow reflectance; it can be explained by an effective reflectance of 0.3–0.4.

A model for calculating snow bidirectional reflectance at any wavelength in the solar spectrum is presented. The method is based on the radiative transfer theory by using the adding/doubling method. The model includes polarization information which is quite new for snow and can help to interpret relevant remote-sensing data. The snow grain shape effects on the reflectance are studied and shown to be very important in the near and middle infrared. The snow grains are assumed to be either spherical or hexagonal ice particles; the single scattering properties are computed by the Mie and the ray-tracing theories, respectively.

We present a detailed discussion of the version 5 algorithms used to analyze the Polar Ozone and Aerosol Measurement II (POAM II) solar occultation data. As this version is primarily an ozone and aerosol retrieval, the scope of the paper is limited to these constituents. Forward model algorithms for calculating atmospheric transmittance in the nine POAM II spectral channels are presented, as well as detailed instrument simulation models used to generate synthetic data. Inversion of POAM II data is accomplished in two sequential steps, using the method of optimal estimation. A nonlinear spectral inversion is performed first, which simultaneously inverts the measured slant optical depth profiles in all channels to obtain the separate gas and aerosol optical depth components. Inversion of the slant path line of sight integral then yields altitude profiles of constituent density and aerosol extinction. Examples of both real and simulated retrievals are presented, and comparisons are made between retrievals obtained with the version 5 operational algorithm and an independent algorithm developed at the Laboratoire d'Optique Atmospherique, University of Lille, Lille, France. A detailed error analysis is also presented, providing estimates of the total random and systematic error budget for the ozone and aerosol retrievals. The results of this analysis indicate random errors of 5% or less in the ozone retrievals in the 10- to 50-km range. Random errors in the retrieved aerosol extinction are estimated to be 10 to 20% in the primary 1060-nm channel, increasing to 20 to 30% at 352 nm. Total systematic errors are estimated to be less than 5% for ozone and as much as 10 to 15% in the 1060-nm aerosol extinction and are primarily caused by systematic biases in the National Center for Environmental Prediction (NCEP) temperatures used in the analysis. Analysis of the retrieval averaging kernels shows that under present aerosol loading conditions, aerosol feedback into the ozone retrievals introduces biases of less than 2% at all altitudes.

The launch of ADEOS in August 1996 with POLDER, TOMS, and OCTS instruments on board and the future launch of EOS-AM 1 in mid-1998 with MODIS and MISR instruments on board start a new era in remote sensing of aerosol as part of a new remote sensing of the whole Earth system (see a list of the acronyms in the Notation section of the paper). These platforms will be followed by other international platforms with unique aerosol sensing capability, some still in this century (e.g., ENVISAT in 1999). These international spaceborne multispectral, multiangular, and polarization measurements, combined for the first time with international automatic, routine monitoring of aerosol from the ground, are expected to form a quantum leap in our ability to observe the highly variable global aerosol. This new capability is contrasted with present single-channel techniques for AVHRR, Meteosat, and GOES that although poorly calibrated and poorly characterized already generated important aerosol global maps and regional transport assessments. The new data will improve significantly atmospheric corrections for the aerosol effect on remote sensing of the oceans and be used to generate first real-time atmospheric corrections over the land. This special issue summarizes the science behind this change in remote sensing, and the sensitivity studies and applications of the new algorithms to data from present satellite and aircraft instruments. Background information and a summary of a critical discussion that took place in a workshop devoted to this topic is given in this introductory paper. In the discussion it was concluded that the anticipated remote sensing of aerosol simultaneously from several space platforms with different observation strategies, together with continuous validations around the world, is expected to be of significant importance to test remote sensing approaches to characterize the complex and highly variable aerosol field. So far, we have only partial understanding of the information content and accuracy of the radiative transfer inversion of aerosol information from the satellite data, due to lack of sufficient theoretical analysis and applications to proper field data. This limitation will make the anticipated new data even more interesting and challenging. A main concern is the present inadequate ability to sense aerosol absorption, from space or from the ground. Absorption is a critical parameter for climate studies and atmospheric corrections. Over oceans, main concerns are the effects of white caps and dust on the correction scheme. Future improvement in aerosol retrieval and atmospheric corrections will require better climatology of the aerosol properties and understanding of the effects of mixed composition and shape of the particles. The main ingredient missing in the planned remote sensing of aerosol are spaceborne and ground-based lidar observations of the aerosol profiles.

The stratospheric aerosol layer in the Arctic winter was studied by three independent, height-resolving, optical techniques close in space and time on February 2, 1994, above northern Scandinavia. The balloon-borne Radiometre Ballon (RADIBAL) experiment measured altitude profiles of the radiance and polarization of scattered sunlight at two wavelengths: a ground-based lidar measured vertical backscatter ratio profiles at one wavelength and the satellite-borne Polar Ozone and Aerosol Measurement (POAM) II solar occultation instrument measured atmospheric extinction at nine wavelengths. From the RADIBAL data the mode radius and variance of the aerosol size distribution are derived as well as the particle refractive index. This size distribution is used to convert the lidar backscatter ratio to an extinction coefficient. The POAM 11 aerosol extinction coefficients are derived under the assumption that the spectral dependence of the aerosol optical depth follows a quadratic law at all altitudes. This quadratic dependence is used to deduce the mode radius and variance of the aerosol size distribution and to interpolate the POAM data to the wavelengths of the other two instruments. In the common altitude range of measurements, from 15 to 23 km, the derived aerosol extinction profiles agree within the instrument measurement errors and the temporal and spatial variability of the aerosol layer. The geographic area of measurements was outside the polar vortex on that day. The effective aerosol particle radius decreases slightly with increasing altitude from 0.40 μm at 16 km to 0.25 μm at 22 km and the effective variance ranges from 0.15 to 0.25. The mean refractive index is 1.44 at 850 nm, which is compatible with a 75%-sulfuric acid-water aerosol. The aerosol number densities decrease from 5 to 1 cm -3 over the 16- to 22-km altitude range and the surface area density decreases from 5 to less than 1 μm 2 cm -3 over the same altitude range.

Aerosols are a complex and important component of the atmosphere. These particles consist of various chemical compositions (homogeneous or inhomogeneous), shapes and sizes, and they affect human health, the environment, visibility, and atmospheric chemistry. A major concern is their influence on climate, directly by modifying the Earth’s radiation budget, and indirectly by modifying cloudiness, cloud properties, precipitation and atmospheric circulations. The aerosol influence does not depend only on their total amount, as it is the case for the gaseous components of the atmosphere, but on their chemical composition, shapes, and sizes.

This book gives a much needed explanation of the basic physical principles of radia5tive transfer and remote sensing, and presents all the instruments and retrieval algorithms in a homogenous manner. For the first time, an easy path from theory to practical algorithms is available in one easily accessible volume, making the connection between theoretical radiative transfer and individual practical solutions to retrieve aerosol information from remote sensing. In addition, the specifics and intercomparison of all current and historical methods are explained and clarified.

The simplest remote sensing methods rely on the observation of the extinction of radiation, as it is defined in Chapter 2, in Eqs 2.4 and 2.5 (Beer exponential extinction law), which we recall here as Eqs 4.1 and 4.2: $$I(S_2)=I(S_1)\, {\rm exp}(-\tau_e),$$ (4.1) where $${\tau_e}={\int\limits_{s_1}^{s_2}}{\sigma_e}(s)ds.$$ (4.2)

In Chapter 2, we analyzed the scattering and absorption properties of air molecules and aerosol particles. These properties create a specific signature of aerosol impact on radiation and this signature provides the basis for aerosol remote sensing. We have seen that this signature depends on the shape, size, and chemical nature of the particle, so that by measuring the result of an incident beam’s interaction with an aerosol particle, we can derive information about that particle. The problem becomes challenging because generally we have to deal with an ensemble of different particles, mixed within the air molecules, not just a single particle.

The Earth’s atmosphere absorbs, scatters, and emits electromagnetic radiation. Although air molecules are the primary actors in these processes, aerosol particles are also present ubiquitously (see Chapter 1) and modify the radiation field. In fact, this modification constitutes the very physical basis of aerosol remote sensing. Whenever clouds are present, they have a much larger influence on radiation which largely overshadows the aerosol impact. Therefore, in aerosol remote sensing, one often has to limit observations to cloudless conditions and screen cloudy pixels.

The second Polar Ozone and Aerosol Measurement instrument (POAM II) is a spaceborne experiment designed to measure the vertical profiles of ozone, water vapor, nitrogen dioxide, aerosol extinction, and temperature in the polar stratosphere and upper troposphere with a vertical resolution of about 1 km. Measurements are made by the solar occultation technique. The instrument package, which has a mass of less than 25 kg, is carried on the Satellite Pour l'Observation de la Terre (SPOT) 3 spacecraft and has a design lifetime of 3–5 years. POAM II has provided data on the south polar ozone hole, north and south polar ozone phenomena, the spatial and temporal variability of stratospheric aerosols and polar stratospheric clouds, and has detected polar mesospheric clouds.

The influence of various cloud parameters and the interactions with the ground albedo and the solar zenith angle have been studied by means of model simulations. The radiative transfer model suitable for a cloudy atmosphere as well as for a clear atmosphere has been developed on the basis of the Discrete Ordinate Method. This study leads to a general understanding for cloudy atmospheres: in the presence of a uniform cloud, the cloud scattering is dominant to molecular and aerosol scattering, and it is also wavelength-independent; the ratio of transmitted irradiance in a cloudy atmosphere to that in the background clear atmosphere is independent of cloud height and solar zenith angle. That’s to say. the radiation downwelling out of a cloud is quite isotropic; it decreases approximately exponentially with the cloud optical depth at a rate related to the ground albedo; the reflected irradiance at the top of the atmosphere is dependent on cloud optical depth as well as on solar zenith angle, but not on ground albedo for clouds of not very thin optical depth.

The stratospheric aerosol layer is regularly monitored since 1979 by satellite experiments as Stratospheric Aerosol Measurement (SAM II) and Stratospheric Aerosol and Gas Experiment (SAGE I and II). The satellite spectral measurements of transmission permit one to obtain the spectral variations of extinction from which the aerosol size distribution may be derived. Several correlative measurements with a balloon-borne polarimeter, Radiometre Ballon (Radibal), were conducted during the post-El Chichon period to validate the satellite measurements. The radiance and polarization diagrams of the sunlight scattered by the aerosols, measured from the balloon, were analyzed within the primary scattering approximation, and the retrieved aerosol size distribution was compared with the satellite results. Following the eruption of Mount Pinatubo (June 15, 1991), two flights of Radibal were planned to allow a validation in case of large aerosol amount. Because of the importance of the multiple scatterings the usual inversion of the Radibal measurements was no more valid. We propose here an improved scheme to evaluate the coherence of both SAGE II and Radibal that gives excellent results.

Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV-visible spectrometers are now well developed in laboratories where stratospheric constituents are measured with this technique. An intercomparison between results from the different radiative transfer models used to calculate AMFs at twilight is presented here. The comparison was made for ozone AMFs at 510 nm and for NO2 AMFs at 440 nm. Vertical profiles were specified. Results are presented firstly for calculations in a pure Rayleigh atmosphere, then including background aerosols. Relative differences between calculated AMFs from different models cause relative errors in vertical columns of ozone and NO2 measured by zenith-sky spectrometers. For commonly used averages over solar zenith angles, these relative errors are ±2.3% in the vertical column of ozone and ±1.1% in the vertical column of NO2. Refinements to the calculations, suggested by the intercomparison, should reduce these errors to ±1.0% for ozone and ±0.5% for NO2.

POAM II is a space-borne instrument which uses the solar occultation technique to measure the vertical distribution of ozone, aerosols and polar stratospheric clouds, and other properties of the stratosphere and mesosphere. POAM II was launched aboard the SPOT 3 satellite in time to observe the dissipation of the 1993 Antarctic ozone hole. POAM data indicates that the Antarctic ozone hole dissipates from the top downward. It also supports the hypothesis that the Antarctic vortex is an effective containment vessel. However the strength of the containment appears to decrease markedly at altitudes below 18 km

The Stratospheric Aerosol and Gas Experiment II has one channel at 940 nm related to water vapor. Two inversion procedures were developed independently in order to obtain the water vapor profile: the Chahine method by the Langley Research Center, and the Mill method by the Laboratoire d'Optique Atmospherique. Comparisons were made between these two algorithms and some results are presented at midlatitudes (about 45 deg N) and tropical latitudes (12-25 deg S). They are compared with in situ frost point hygrometer data provided by balloon experiments from the Laboratoire de Meteorologie Dynamique. At +/- 0.5 ppmv, agreement between the inversion results and the experimental results was obtained in the altitude range from 18-19 to 26-27 km. Below 18-19 km and above 26-27 km the error is larger (sometimes 1 ppmv and more).

The use of scattering theory to infer atmospheric optical parameters requires the separation of absorption and scattering. It is demonstrated that a gradient flux relation exists that would provide the absorption (altitude) profile independently of scattering and irrespective of the state of polarization of the light field. The relation is derived for an atmosphere of plane-parallel or spherical geometry and for broad (continuum) and narrow (spectral line) frequency bands. The results are shown to hold, in particular, for the polarizations induced by both Rayleigh and Mie scattering in the field. Experimental setups are proposed for each of the cases considered of atmospheric geometry and frequency bandwidth. A final discussion considers the relevance of the present determination of the atmospheric absorption profile to the related problems of aerosol relative concentration, interpretation of radiometric and spectrometric data formed in the presence of scattering, clouds morphology, and radiative heat budget of the atmosphere.

The paper reviews the development of observations and modeling of the scattering and polarization of solar radiation in the Earth’s atmosphere. Application to remote sensing of aerosols is considered.

We present the results of a comparison of the total extinction altitude profiles measured at the same time and at same location by the ORA (Occultation Radiometer) and Stratospheric Aerosol and Gas Experiment II solar occultation experiments at three different wavelengths. A series of 25 events for which the grazing points of both experiments lie within a 2 degrees window has been analyzed. The mean relative differences observed over the altitude range 15-45 km are -8.4%, 1.6%, and 3% for the three channels (0.385, 0.6, and 1.02 microm). Some systematic degradation occurs below 20 km (as the result of signal saturation and possible cloud interference) and above 40 km (low absorption). The fair general agreement between the extinction profiles obtained by two different instruments enhances our confidence in the results of the ORA experiment and of the recently developed vertical inversion algorithm applied to real data.

Routine measurements of global and diffuse UV irradiances at Briançon station (1310 m a.s.l.) are used to retrieve the direct solar irradiance and the aerosol optical depth (AOD), for cloudless days. Data of three years (2003, 2004, 2005) are analyzed; the results confirm those of a preliminary analysis for 2001, 2002. The atmosphere is very clear in winter, with AODs between 0.05 and 0.1. The turbidity increases slowly in spring, starting end of February, with AODs around 0.2–0.3 in mid summer, some values reaching 0.4. A similar behaviour is observed for all years, with somewhat higher values in late summer for the year 2003.

The diurnal and annual variability of solar UV radiation in Europe is described for different latitudes, seasons and different biologic weighting functions. For the description of this variability under cloudless skies the widely used onedimensional version of the radiative transfer model UVSPEC is used. We reconfirm that the major factor influencing the diurnal and annual variability of UV irradiance is solar elevation. While ozone is a strong absorber of UV radiation its effect is relatively constant when compared with the temporal variability of clouds. We show the significant role that clouds play in modifying the UV climate by analyzing erythemal irradiance measurements from 28 stations in Europe in summer. On average, the daily erythemal dose under cloudless skies varies between 2.2 kJ m-2 at 70N and 5.2 kJ m-2 at 35N, whereas these values are reduced to 1.5–4.5 kJ m-2 if clouds are included. Thus clouds significantly reduce the monthly UV irradiation, with the smallest reductions, on average, at lower latitudes, which corresponds to the fact that it is often cloudless in the Mediterranean area in summer.

Ultraviolet spectral irradiances are systematically recorded in Briancon (at an altitude of 1310 m), in the French Southern Alps. Up to now, aerosols optical depth (AOD) had never been routinely measured in Briancon, so it has appeared of great interest to take advantage of the many spectra recorded to learn more about aerosols over the spectral range: 320 nm - 448 nm. To retrieve AOD, one only has to remove the contribution of ozone and molecule from the total optical depth, obtained from the direct irradiance. The method has been implemented for every spectra acquired during clear sky days selected among the period from July 2004 to March 2005. Data recorded show that at a given wavelength, AOD is roughly constant during the day. Lowest values for AOD at 12:00 U. T. are found to be around 0.05 at 440 nm and 0.15 at 320 nm, and may increase threefold for some days. AOD range between 320 and 448 nm is larger in summer than in winter. This agrees with the way α, the Angstrom parameter, varies from one season to the other. Unfortunately, one can only make a restricted use of α, as its determination over the ultraviolet range is subject to high AOD uncertainties. The main source of AOD error rests in the 10% of uncertainty on direct irradiance measured. This gives rises to relatively high uncertainties on AOD, which increase all the more when aerosols load in the atmosphere is low. In Briancon, renowned for its air goodness, true AOD values are likely to be reached within 45% - 50% as the best in a majority of cases.

Received 18 March 2005; revised 25 July 2005; accepted 26 September 2005; published 17 December 2005. (1) A differential absorption method, developed by the Laboratoire d'Optique Atmospherique (LOA), is applied to retrieve the total ozone column from UV global irradiance spectra under clear and cloudy sky conditions. Contrarily to clear sky, cloudy sky generates a high uncertainty in the retrieved ozone column. This study shows that under cloudy conditions the daily ozone mean is a rather good estimation of the true value. The standard deviation allows us to estimate the relative uncertainty of this mean value, i.e., about 7%. Results for all conditions from 3 years at Villeneuve d'Ascq are presented, as well as a comparison with TOMS (Total Ozone Mapping Spectrometer) data. The results are very similar to those obtained for clear days: In 75% of cases both values agree to within 5%; however, some cases present larger relative differences. Over the 3 years considered, there is a bias less than 3% (LOA > TOMS). This method is applied to five other European sites involved in the EDUCE (European Database for UV Climatology and Evaluation) project. Three of them are Brewer stations. The results of the comparison between the LOA-retrieved ozone and TOMS data show that a relative difference smaller than 5% is obtained in 63-80% of the cases, depending on site and year. Biases smaller than 3% are observed (LOA > TOMS). The comparison between the LOA-retrieved ozone and total ozone data from direct sun and zenith sky observations of the Brewer instruments shows better agreement. For more than about 80% of cases, except at one site, the relative difference is smaller than 5%, and the biases are smaller than 1%. The LOA method allows us to obtain a complementary data set and hence to provide time series of reliable measurements of total ozone column under all sky conditions.

Direct ultraviolet spectral solar irradiance is regularly obtained by the difference between global and diffuse irradiances at the French Alpine station of Briancon; the data of years 2001 and 2002 are analyzed in this paper. Comparison with modeled values is used for cloud screening, and an average UV-A aerosol optical depth is used as an index of turbidity; it is found to be around 0.05 for the clear winter days and around 0.2 in summer. Langley plots are used to verify the instrument calibration; they confirm the expected uncertainty smaller than 5%. The ozone total column amount is estimated with an uncertainty between -3 and +5 Dobson units; comparisons with TOMS (Total Ozone Mapping Spectrometer) overpass values shows agreement within the expected uncertainties of both instruments.

[1] The enhancement of the UV global irradiance due to snow cover on the ground has been observed at the station of Briancon, in a high-altitude Alpine valley. The analysis relies on a three-dimensional (3-D) model, using an elevation map of the area. Without snow, comparison with the results of a 1-D model shows no detectable effect of topography, within the uncertainty of modeling (2–3%). The 3-D model relates the enhancement due to snow to the altitude of the snow line. The enhancement is shown to depend on the snow distribution around the site and not on the topography itself. The enhancement was measured at Briancon for nine cloudless days in winter 2002. As expected, it increases with the decrease of the snow line. In erythemal UV the enhancement reaches a maximum of about 22% in the beginning of March, in agreement with the results of the 3-D model, assuming a snow albedo of 0.3 above the snow line and below the tree line and 0.8 above the tree line. Retrieving an effective surface albedo is a very challenging problem. Very small uncertainties in enhancement (±2%) lead to large uncertainties (±0.05) in effective albedo. Using the snow distribution with a contribution function does not give good results when the snow line is high; this is explained by the low resolution of the map with the rapid variation of the contribution near the site.

[1] Situated at the very edge of the Aletsch Glacier and thus at the boundary between snow-covered and snow-free terrain, the High Alpine Research Station Jungfraujoch (Switzerland, 3576 m above sea level) represents an ideal site for experimental quantification of surface albedo effects on solar UV radiation. Monochromatic measurements of directional radiance distribution in the sky, performed with a 1° aperture lens optics at 310 nm and 350 nm under cloudless sky conditions, are compared with discrete ordinate model calculations of corresponding data over homogeneously snow-covered terrain, taking into account polarization effects for an aerosol-free atmosphere. The model agrees within ±5% with radiance measurements at the forward scattering direction of the principal plane toward southeast, where snow-covered terrain dominates. Toward northwest, the effect of snow-free terrain causes a significant reduction of UV sky radiance at the backward scattering direction of the principal plane relative to the modeled situation with homogeneous snow cover. Depending on the actual solar zenith angle, this reduction amounts to between 20% and 40%.

A measurement campaign was carried out in February 1998 at Briançon Station, French Alps (44.9 degrees N, 6.65 degrees E, 1,310 m above sea level) in order to determine the UV effective snow albedo that was retrieved for both erythemal and UV-A irradiances from measurements and modeling enhancement factors. The results are presented for 15 cloudless days with very variable snow cover and a small snowfall in the middle of the campaign. Erythemal irradiance enhancement due to the surface albedo was found to decrease from approximately +15% to +5% with a jump to +22% after the snowfall, whereas UV-A irradiance enhancement decreased from 7% to 5% and increased to 15% after the snowfall. Thesevalues fit to effective surface albedos of 0.4, 0.1, and 0.5 for erythemal, and to effective albedos of 0.25, 0.1, and 0.4 for UV-A irradiances, respectively. An unexpected difference between the effective albedos retrieved in the two wavelength regions can be explained by the difference of the environment contribution.

[1] The International Photolysis Frequency Measurement and Model Intercomparison (IPMMI) took place in Boulder, Colorado, from 15 to 19 June 1998, aiming to investigate the level of accuracy of photolysis frequency and spectral downwelling actinic flux measurements and to explore the ability of radiative transfer models to reproduce the measurements. During this period, 2 days were selected to compare model calculations with measurements, one cloud-free and one cloudy. A series of ancillary measurements were also performed and provided parameters required as input to the models. Both measurements and modeling were blind, in the sense that no exchanges of data or calculations were allowed among the participants, and the results were objectively analyzed and compared by two independent referees. The objective of this paper is, first, to present the results of comparisons made between measured and modeled downwelling actinic flux and irradiance spectra and, second, to investigate the reasons for which some of the models or measurements deviate from the others. For clear skies the relative agreement between the 16 models depends strongly on solar zenith angle (SZA) and wavelength as well as on the input parameters used, like the extraterrestrial (ET) solar flux and the absorption cross sections. The majority of the models (11) agreed to within about +/-6% for solar zenith angles smaller than similar to60degrees. The agreement among the measured spectra depends on the optical characteristics of the instruments (e.g., slit function, stray light rejection, and sensitivity). After transforming the measurements to a common spectral resolution, two of the three participating spectroradiometers agree to within similar to10% for wavelengths longer than 310 nm and at all solar zenith angles, while their differences increase when moving to shorter wavelengths. Most models agree well with the measurements (both downwelling actinic flux and global irradiance), especially at local noon, where the agreement is within a few percent. A few models exhibit significant deviations with respect either to wavelength or to solar zenith angle. Models that use the Atmospheric Laboratory for Applications and Science 3 (ATLAS-3) solar flux agree better with the measured spectra, suggesting that ATLAS-3 is probably more appropriate for radiative transfer modeling in the ultraviolet.

[1] This paper presents the inversion algorithms that we propose to apply to the Stratospheric Aerosol and Gas Experiment (SAGE) III measurements for retrieving the aerosol and gas components of the Earth atmosphere. SAGE III is an upgraded version of SAGE II: The main improvements are the larger number of channels, including several resolved channels and a channel centered near 1.55 μm, and a higher vertical resolution. The species retrieval from the transmission measurements can be achieved via the inversion algorithm that we developed at Laboratoire d'Optique Atmospherique for SAGE II with only minor changes, in which all the channels are handled altogether for species separation by using an iterative method. For SAGE III, ozone and nitrogen dioxide can also be retrieved independently from two specific resolved channels by using their spectral signature over a small wavelength range; this second inversion leads only to the ozone and nitrogen dioxide concentrations and to the aerosols at two wavelengths. Since these later O3 and NO2 retrievals are shown to be of better quality, it is further possible to use them in the first algorithm, in a combined analysis, for aerosol separation in the other channels. The performances of these three inversions have been compared for various atmospheric models, accounting for some probable measurement errors. The three methods perform very well in the case of nominal data. When errors are added to the measurements, the combined analysis leads generally to better retrieval for O3 and NO2, while there is almost no improvement for the aerosols. For random measurement error, the retrieved profiles exhibit no bias with respect to the input profiles, except at high altitude. The accuracy of the results is shown to be dependent on the species and on the atmospheric model.

Albedo inversion techniques are investigated in this work. Several methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. One first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow- covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present sensitivity analysis, the differences and the correlations between the various methods as well as the results for the different locations.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A detailed discussion on the inversion algorithm developed by the Laboratoire d'Optique Atmospherique, University of Lille, France, for the analysis of the Stratospheric Aerosol and Gas Experiment III (SAGE III) solar occultation data is presented. The scope of the paper is limited to nitrogen dioxide, ozone and aerosol retrieval. The forward model algorithm for calculating atmospheric transmittances in the two SAGE III solar channels, at 440 nm and at 600 nm, is presented. Then the inversion algorithm is introduced, accomplished in two sequential steps: the first one is the spatial inversion of the simulated slant optical thickness profile to obtain the extinction coefficient profile and the second is the spectral inversion of the extinction coefficient at each altitude to separate gas and aerosol contributions by using a least squares method over the spectral signatures. Error analysis is also discussed: the results of this analysis indicate regression error of 1% or less in the aerosol retrieval, of 0.1% or less for the ozone retrieval and of about 0.1% for the nitrogen dioxide retrieval. The paper also discuss on the correlation between the relative differences and the relative contribution of the considered constituents in the total extinction coefficient and then can determine the inversion algorithm's performances.

The Stratospheric Aerosol and Gas Experiment (SAGE) III is planned to be launched in summer 1998. It will provide aerosol extinction coefficients from solar occultation measurements at more wavelengths than SAGE II. This paper describes two methods used for the retrieval of some aerosol characteristics of great interest for the climate modeling and for the study of heterogeneous chemistry: effective radius, effective variance, surface area density and volume density. The first technique consists in a King inversion scheme and the second relies on a least squares fit on the extinction measurements. The two methods are applied to simulated extinction measurements at SAGE III wavelengths to investigate the ability of retrieving the aerosol characteristics in case of unimodal or bimodal log-normal size distributions. The contribution of channel 1.550 micrometer is estimated and uncertainties are also determined. The results derived from the two techniques are consistent in unimodal and bimodal cases for the four quantities: the least squares fit method is much faster but leads to larger uncertainties, in bimodal case the King method allows a better retrieval.

A balloon borne instrument BALLAD (balloon limb aerosol detection) has been developed at the LOA (Laboratoire d'Optique Atmospherique). It scans the Earth's limb at three wavelengths (450, 600, and 850 nm) from the float altitude between 30 - 35 km when the sun is low above the horizon; polarization is also measured in the 850 nm channel. A flight has been performed from the southwest of France on October 13, 1994 during the phase II of the SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment) campaign. An analysis of the reflectances at 850 and 450 nm without polarization is presented.

The Stratospheric Aerosol and Gas Experiment II (SAGE II), that was launched in October 1984, has monitored the stratospheric aerosol layer after the Pinatubo's eruption. Two flights of the balloon-borne experiment RADIBAL (RADIometer BALloon) were performed in June 1992 and May 1993 in coincidence with SAGE II events. Because of the large aerosol loading, the inversion of the balloon measurements (consisting in radiance and polarization diagrams) was impracticable. A code taking into account the multiple scatterings has then been used to calculate theoretical diagrams for an aerosol model deduced from SAGE II data. The obtained diagrams have been compared satisfactorily to the experimental ones.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Polar Ozone and Aerosol Measurements II (POAM II) has been developed by the Naval Research Laboratory and launched end of September 1993 on the french satellite SPOT 3. The instrument observes solar occultations at 9 wavelength channels. The inversion algorithm allows the retrieval of the extinction vertical profiles and the separation of the species contributing to this extinction: air molecules, aerosols, ozone, water vapor and nitrogen dioxide; furthermore the retrieval of aerosol extinction at 4 wavelengths provides an information on the aerosol size distribution. An inversion algorithm has been developed at the LOA and preliminary comparisons with the NRL algorithm are presented. Two balloon-borne instruments have been flown from Kiruna (Sweden) in coincidence with POAM II observations: one of the instrument (RADIBAL) measures the radiance and polarization diagrams in the near infrared during the balloon ascent; the other instrument (BALLAD) observes the earth's limb at 3 wavelengths from the balloon ceiling altitude. Both instruments are complementary and provide the aerosol profile and the aerosol size distribution. Preliminary comparisons with POAM II data are presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Polar Ozone and Aerosol Measurements II (POAM II) has been developed by the Naval Research Laboratory and launched end of September 1993 on the french satellite SPOT 3. The instrument observes solar occultations at 9 wavelength channels. Two channels are devoted to the retrieval of the water vapor profile, one at 935 nm in the water vapor absorption band, and a nearby channel at 920 nm, almost free of water vapor absorption. The two channels are used in a differential mode to separate the aerosol extinction from the water vapor absorption. The major difficulty to retrieve a water vapor profile from the transmission data, is due to the line structure of the absorption spectrum. Line-by-line models are used as basic benchmarks, and the GEISA and HITRAN models are compared. However the line-by-line models are much too complex to be run at each step in an inversion algorithm; a simple parameterization has been sought, following the method used for SAGE II. Results are presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The analysis of European correlative experiments of the stratospheric aerosol layer is performed for three periods. These independent experiments are lidar measurements obtained at Garmisch-Partenkirchen, balloon measurements of the reflectance and of the polarization achieved at Aire sur l'Adour with RADIBAL experiment and transmission measurements provided by the SAGE II experiment. Comparisons of the aerosol slant optical thicknesses are performed, and the agreement is generally good. Two different schemes are used to retrieve two parameters of the size distribution from the polarization and from the transmission spectral measurements. The discrepancies that appear in a few cases for the radius comparison are analyzed.

[1] Spectral UV global irradiance has been measured regularly during the year 2000, in the French Alps, using two UV spectroradiometers. The total ozone column amount is retrieved from the ratio of irradiances around 305 and 340 nm, using look up tables based on modeling data. During cloudless days, ozone values from direct sun observations agree to within 5 DU. These ground based values compare reasonably well (to within about 4%, or 12 DU) with TOMS ozone data; this is consistent with the uncertainty of both methods which is estimated to 5 DU for TOMS, and 5–7 DU for clear sky ground based measurements. Generally the ground based ozone values seem to be slightly higher than the TOMS ozone values. For a few cases, the difference between TOMS and ground based values, can reach 10 to 15%. Different possible explanations of the discrepancies are considered.

A measurement campaign was organized in March 1999 in the Bavarian Alps as part of the European project, Characteristics of the UV Radiation Field in the Alps (CUVRA), to analyze the effect of altitude, aerosols, and snow cover on ground-level UV spectral irradiance. We present the results of simultaneous measurements of aerosol optical depth (AOD) made at various sites on two cloudless days in March 1999. The two days exhibited different aerosol conditions. Results derived from spectral measurements of UV irradiance are compared with data from filter radiometer measurements made at discrete wavelengths extending from the UV to the near IR. The different methods generated values for the AOD that were in good agreement. This result confirms that one can use either method to retrieve the AOD with an uncertainty of approximately 0.03-0.05. On 18 March, high turbidity was observed at low altitude (400-nm AOD approximately 0.5 at 700 m above sea level), and the AOD decreased regularly with altitude; on 24 March, the turbidity was much less (0.11 at 700 m above sea level). On both days very low AODs (0.05-0.09) were measured at 3000 m above sea level. The spectral dependence of the AOD is often parameterized by the angstrom relationship; the alpha parameter is generally difficult or impossible to retrieve from spectral measurements because of the relatively narrow wavelength range (320-400 nm), and only one of the spectro-radiometers used during the campaign permits this retrieval. In most cases, during this field campaign, alpha was found by filter sunphotometers to be 1.1-1.5.

A new method for correcting the wavelength misalignment in measured UV spectra is presented. It is based on a comparison between measured irradiances and irradiances computed from a radiative transfer code for a set of given atmospheric and solar conditions (250 < Dobson units < 450, 30 degrees < solar zenith angle < 75 degrees ). Results of tests run with spectra recorded on a clear-sky day by two spectroradiometers in a French UV spectral network station are analyzed. Applying the method once reduces shift to less than 0.05 nm. The smoothing included in the method enables detection of aberrant irradiance values and then completion of an initial quality control of measured spectra. A technique for assessing the instruments' slit function is also presented. The key algorithms needed to build a computer code based on this method are given.

A three-dimensional Monte Carlo code is used to compute the ultraviolet zenith sky radiance; the code is validated by comparison with a successive-orders-of-scattering code. The amplifications of global irradiance, diffuse irradiance, and zenith radiance that are due to multiple reflectances between a snow-covered ground surface and the atmosphere are compared. For an inhomogeneous Lambertian surface, the contribution of the site environment is analyzed; it depends slightly on the atmospheric turbidity and on the surface reflectance distribution. However, in most cases one can expect approximately 12–15% of the reflected photon contribution to come from within 1 km about the observation site, 25–30% come from areas from 1 to 5 km from the site, 43–47% from 5 to 30 km, and still 10–15% reflected at larger distances. An average contribution function is proposed and used to compute an effective reflectance, which permits retrieval of the sky radiance within 2–4% with a one-dimensional model.