With the aid of CERES/TRMM (Clouds and the Earth's Radiant Energy System/Tropical Rainfall Measuring Mission) data, sampling problems that may impact the construction of anisotropic models have been investigated. We show the influence of the sun-orbit geometry, which causes a lack of measurements in forward and backward scattering directions at large viewing zenith angles, and how cloud properties retrieval algorithms as obtained from the Visible Infrared Scanner (VIRS) also affect sampling. We have also focused our attention on the dependency of the cloud cover retrieval algorithm with respect to the viewing zenith angle (VZA), showing that the shape of the cloud cover probability density function changes with VZA, becoming smoother, although this change in shape is only significant for VZAs larger than around 60 degrees. From 14 days of CERES/TRMM data, SW and LW anisotropic models have been built, and a performance analysis based on the error dependency with the observing geometry has been carried out. With scene identification in terms of cloud cover, a preferred region in the flux retrieval algorithm usually appears in the region of VZAs around 55 degrees, giving the possibility to obtain flux estimates with good accuracy and simple anisotropic models.

A new method is proposed for retrieving the photosynthetically active radiation (PAR) using remote sensing data (AVHRR) and a radiation transfer model (libRadtran). The approach applicable for clear and cloudy sky conditions takes into account atmospheric parameters affecting the radiation transfer through the atmosphere, namely, extinction by aerosols and clouds. Due to the high temporal variability of cloud distribution and cloud properties, a geostationary satellite as e.g. the future Meteosat Second Generation (MSG) with SEVIRI on board is proposed to derive inter-daily variation of cloud parameters. The upcoming SEVIRI sensor will deliver spectral information of clouds and atmosphere every 15 minutes. The spectral information content of SEVIRI is simulated by the existing polar orbiting sensor Advanced Very High Resolution Radiometer (AVHRR) on board of the NOAA satellites. Our first results are validated with ground truth data from the European Light Dosimeter Network (ELDONET). Up to 8 stations distributed over Europe are available for validation purposes. It is shown that for heterogeneous atmospheric conditions a good correlation exist between measured and estimated PAR values. Processing more AVHRR data over a longer time period and fine-adjustment of the algorithms combined with extended validation will consolidate our findings in the future.

In this paper the system employed at the Royal Meteorological Institute of Belgium (RMIB) within the Climate Monitoring Satellite Application Facility (CM-SAF) for the production of Top Of the Atmosphere (TOA) radiation budget components is described. One of the goals of the CM-SAF is to provide consistent TOA and surface radiation budget components and cloud properties at high spatial resolution and on an approximate equal area grid for a region that covers at least Europe and part of the North Atlantic Ocean. The TOA radiation products will be based on data from polar orbiting satellites for northern latitudes, and on data from MSG (METEOSAT Second Generation) for mid latitudes. The instruments used for the reflected solar and emitted thermal flux estimates will be GERB (Geostationary Earth Radiation Budget) and SEVIRI as the geostationary instruments and CERES (Clouds and the Earth's Radiant Energy System) for the non geostationary instruments. Daily means, monthly means and monthly mean diurnal cycles are to be provided. Until MSG fluxes will become available, fluxes from METEOSAT and CERES are used for development. At the TOA the three radiative flux components of incoming solar radiation, reflected solar radiation and emitted thermal radiation will be given. The daily mean GERB and CERES fluxes will be merged to produce a homogenized TOA flux product. The method used for the merging of the TOA fluxes and together with results using currently available input data are shown. The merging consists in the collocation of the two instruments, detection and the removal of the systematic dependencies of the flux estimates on scene type and viewing angles and regridding on a common grid.

The Geostationary Earth Radiation Budget (GERB) instrument has been launched this summer together with the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) on board of the Meteosat Second Generation (MSG) satellite. This broadband radiometer will aim to deliver near real-time estimates of the top of the atmosphere (TOA) radiative fluxes at the high temporal resolution due to the geostationary orbit. In order to infer these fluxes, a radiance-to-flux conversion based on Clouds and the Earth's Radiant Energy System (CERES) angular dependency models (ADMs) need to be performed on measured radiances. Due to the stratification of these ADMs according to some CERES scene identification (SI) features such as cloud optical depth and cloud fraction, the GERB ground segment must include some SI on SEVIRI data which mimic as close as possible the one from CERES in order to select the proper ADM. In this paper, we briefly present the method we used to retrieve cloud optical depth and cloud fraction on footprints made of several imager pixels. We then compare the retrieval of both features on the same targets using nearly time-simultaneous Meteosat-7 imager and CERES Single Satellite Footprint (SSF) data. The targets are defined as CERES radiometer footprints. We investigate the possible discrepancies between the two datasets according to surface type and, if they exist, suggest some strategies to homogenize GERB retrievals based on CERES ones.

Previous studies have shown that the albedo of clouds with inhomogeneous liquid water fields is lower than that of homogeneous clouds with the same average liquid water content. This can lead to biases in the retrieval of cloud properties from satellite images with pixel sizes significantly greater than the photon mean free path length. In this work we present a three-dimensional multifractal cloud model for use in radiative transfer calculations. The model is based on aircraft measurements of liquid water content taken during 98 flights over Tasmania, Australia. Monte Carlo radiative transfer is used to calculate the optical properties of clouds that were constructed according to this model. The reflectance of the cloud not only varies with the fractal parameters and mean liquid water content, but also with the area size over which it is averaged, i.e. the pixel size used. An "effective optical depth" is defined as the optical depth of a homogeneous cloud with the same reflectance as the 3D-multifractal cloud, and is parameterized as a function of the mean optical depth and the pixel size. This parameterization allows for fast radiation calculations in the remote sensing of cloud properties, by the replacement of an inhomogeneous cloud with a plane-parallel homogeneous one.

In this work we describe a methodology for the inclusion of first-order water vapor self-broadening effects in look-up-table based radiative transfer calculations. The methodology does not increase the complexity and size of look-up-tables which continue to exist in a 1-D setting, although the inclusion of a new variable (i.e., the water vapor concentration) would raise the dimensionality to 2-D. The scheme is discussed for modern infrared sensors whose spectral resolution falls in the range 0.1 - 2 cm^-1.

The Stratospheric Aerosol and Gas Experiment (SAGE) III is the fourth generation of solar occultation satellite instruments designed by NASA to measure vertical profiles of aerosol extinction and the molecular densities of gaseous species in the atmosphere. With an expanded spectral range compared with its predecessors, SAGE III has the capability to retrieve profiles of atmospheric temperature and pressure utilizing multi-spectral measurements of the oxygen A-band absorption feature near 762 nm. As part of NASA's Earth Science Enterprise, Earth Observing System, the first of two SAGE III instruments was successfully launched onboard the Meteor-3M satellite in December 2001. Given the inherent insensitivity of solar occultation experiments to long-term instrument degradation and the expected lifetime of the instruments (6+ years), the SAGE III instruments should provide a long-term record of self-calibrated, high vertical resolution temperature and pressure measurements. These measurements will be valuable for monitoring temperature trends in the stratosphere and mesosphere and for comparison studies with other temperature data sets.

The availability of a 15-year data set of radiative fluxes from the Earth Radiation Budget Experiment (ERBE) allows us to investigate the interannual variability of top-of-the-atmosphere (TOA) outgoing longwave radiation (OLR) and reflected shortwave radiation (SWR). Variance maps and empirical orthogonal function (EOF) analysis are used to describe temporal and spatial patterns of variability.

Aerosols direct and indirect effects on the Earth's climate are widely recognized but have yet to be adequately quantified. Aerosol particles scatter and absorb the radiation while at the same time acting as cloud condensation nuclei and thus entering the cloud formation process, influencing their microphysics and eventually the precipitation processes. Therefore the assessment of the aerosol optical properties is of greatest importance. Difficulties arise due to the very high spatial and temporal variability of aerosol concentration, which is the major cause of uncertainties in quantifying the atmospheric radiative forcing. A method to exploit the synergy between the polar orbiting Global Ozone Monitoring Experiment (GOME) onboard ERS-2 and the METEOSAT geostationary system was proposed, aiming at increasing the accuracy of the aerosol characterization and monitoring of the optical thickness. Results of the ongoing validation are presented for relevant transport events of desert dust and biomass burning aerosol over the Atlantic and Indian Oceans during year 2000. Retrieved aerosol optical properties are combined with radiative transfer calculations to assess the direct short wave aerosol radiative forcing in selected regions over the ocean, where strong aerosol events are detected. Retrievals are compared with space-time co-located measurements from the Clouds and the Earth's Radiant Energy System (CERES) TOA flux product.

The Atmospheric Infrared Sounder (AIRS) was launched in early May 2002. This new high-spectral resolution sounder is the first of a new generation of temperature and humidity sounders for numerical weather prediction and climate change studies. In addition, AIRS should be able to detect several minor gases, including ozone, carbon monoxide, methane and carbon dioxide. This paper presents a preliminary comparison between observed AIRS spectra and spectra computed from the ECMWF (European Center for Medium Range Forecasting) model fields. A key component of this comparison is the selection of clear fields of view, which we limited to night views over ocean, allowing the use of the relatively well known sea surface emissivity.

Global pollution aerosol monitoring is a very important climatic and environmental problem. It affects not only human health but also ecological systems. Because most pollution aerosols are concentrated in the atmospheric boundary layer where human, animal and vegetation live, global pollution aerosol stuides have been an important topic since about a decade ago. Recently, many new chemistry remote sensing satellite systems, such as NASA's Aura (EOS-CHEM), have been established. However, pollution aerosols in the atmospheric boundary layer cannot be detected using current remote sensing technologies. George Mason University (GMU) proposes to design scientific algorithms and technologies to monitor the atmospheric boundary layer pollution aerosols, using both satellite remote sensing measurements and ground measurements, collaborating with NASA and the United States Department of Agriculture (USDA)/Forest Services (FS). Boundary layer pollution aerosols result from industrial pollution, desert dust storms, smoke from wildfires and biomass burning, volcanic eruptions, and from other trace gases. The current and next generation satellite instruments, such as The Ozone Mapping and Profiler Suite (OMPS), Ozone Monitoring Instrument (OMI), Thermal Emission Spectrometer (TES), and High Resolution Dynamics Limb Sounder (HIRDLS) can be used for this study. Some surface measurements from USDA/FS and other agencies may also be used in this study. We will discuss critical issues for GPAM in the boundary layer using Earth observing satellite remote sensing in detail in this paper.

The paper presents the first laboratory measurement of the rate constant for quenching the CO₂(01¹0) state during collisions of CO₂ molecules with O atoms at temperatures realized near the Earth's mesopause. The measurement was carried out with a hollow-cathode glow discharge in the temperature range 206-358 K. The measured values are significantly smaller than those commonly used in solving the non-LTE CO₂ problem for the vibrational states of the mode ν₂ in the atmospheres of the Earth, Venus, and Mars. The measured temperature dependence of the rate constant is approximated by a simple relation, which is recommended for solving the above problem. The value of this rate constant is absolutely critical to remotely sense temperature, and hence also constituent densities, in the upper mesosphere and lower thermosphere of the Earth. The use of the new values of the rate constant significantly decreased the rate of cooling by the CO₂ 15-μm emission in the terrestrial lower thermosphere as compared to the previous estimates obtained for this rate. Over the most area of the Earth's surface, the maximum cooling rate occurs at an altitude of about 110 km and amounts to about 20 K/day.

Assuming large signal-to-noise ratio and using the rotationally resolved fundamental vibration-rotation band emission from NO near 5.3 μm we propose a scheme for remotely sensing temperature above the altitudes where the 15 μm emission from CO₂ becomes very weak. We also find that the rotationally resolved 5.3 μm emission can be used to remotely sense N(⁴S) atom, O₂, and O densities in the terrestrial thermosphere -- this being the only method for remotely sensing the first two species.

During the past four decades a variety of optical remote sensing techniques have revealed a rich spectrum of wave activity in the upper atmosphere. Many of these perturbations, with periodicites ranging from ~5 min to several hours and horizontal scales of a few ten's of km to several thousands km, are due to freely propagating buoyancy (or acoustic-gravity waves), and forced tidal oscillations. Optical observations of the spatial and temporal characteristics of these waves in the mesosphere and lower thermosphere (MLT) region (~80-100 km) are facilitated by several naturally occurring, vertically distinct nightglow layers. This paper describes the use of state-of-the-art ground-based CCD imaging techniques to detect these waves in intensity and temperature. All-sky (180°) image measurements from Bear Lake Observatory, Utah are used to illustrate the characteristics of small-scale, short period (< 1 hour) waves that are most frequently observed at MLT heights including a particular set of ducted wave motions, possibly associated with mesospheric bores. These results are then contrasted with measurements of mesospheric temperature made using a separate imaging system capable of determining induced temperature amplitudes of much larger-scale wave motions and investigating night-to-night and seasonal variability in mesospheric temperature.

We consider the example of the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment to deduce the sensitivity of an infrared emission limb sounder to gravity waves of different horizontal and vertical wavelength. The sensitivity studies show that gravity waves with horizontal wavelengths of the order of 100-200 km or longer can be detected. The deduced sensitivity factors are validated by comparing CRISTA and data sonde temperature profiles. Analysis of CRISTA temperature data reveals large gravity wave amplitudes in the stratosphere over southernmost South America. The horizontal structure is compared to model calculations. Global distributions are discussed with respect to convective sources, wind modulation, and Coriolis force modulation. It is shown that even the very dense spatial sampling of the CRISTA instrument is insufficient to fully resolve the horizontal structure of the waves which are seen in the vertical. Hence, increased spatial resolution of about 50 X 50 km or better is required to obtain all information the limb sounding technique is capable to provide.

Analysis of 4.3-μm CO₂ radiance data from the MSX (Midcourse Space Experiment) satellite has shown that gravity waves dominate the fluctuations of radiance at 4.3 μm for both earthlimb (above-the-horizon) and downlooking (below-the-horizon) lines-of-sight under a broad class of conditions. We review previous work on the spectra of known sources of gravity waves and on wave filtering mechanisms by M. J. Alexander and others, as well as the characteristics of gravity-wave power spectra. We then consider the power spectra of line-of-sight radiance fluctuations emitted and self-absorbed by an atmosphere perturbed by gravity waves, discussing the shape of the spectrum and the spectral slopes. We show examples of radiance spectra from gravity-wave-perturbed atmospheres that have two different slopes, with a steeper slope at large wavenumber, and discuss mechanisms that can account for this effect. The effect of latitude and season on the 4.3-μm fluctuations will also be considered.

The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) experiment was launched onboard the TIMED satellite in December, 2001. SABER is designed to provide measurements of temperature, constituents, and the key radiative and chemical sources and sinks of energy in the mesosphere and lower thermosphere (MLT). SABER measures Earth limb emission in 10 broadband radiometer channels ranging from 1.27 μm to 17 μm. Measurements are made both day and night over the latitude range from 52°S to 83°N with alternating hemisphere coverage every 60 days. In this paper we concentrate on retrieved profiles of kinetic temperature (T_k) and CO₂ volume mixing ratio (vmr), inferred from observed 15 μm and 4.3 μm limb emissions. SABER-measured limb radiances are in non-local thermodynamic equilibrium (non-LTE) in the MLT region. The complexity of non-LTE radiation transfer combined with the large volume of data measured by SABER requires new retrieval approaches and radiative transfer techniques to accurately and efficiently retrieve the data products. In this paper we present the salient features of the coupled non-LTE T_k/CO₂ retrieval algorithm, along with preliminary results.

On 1 March 2002 the Envisat research satellite has been launched successfully into its sun-synchronous orbit. One of its instruments for atmospheric composition sounding is the Michelson Interferometer for Passive Atmospheric Sounding, a limb-scanning mid-infrared Fourier transform spectrometer. Different scientific objectives of data users require different approaches to data analysis, which are discussed. A strategy on how to validate the involved algorithms and relevant strategies is presented.

On March 1, 2002 the space-borne limb viewing mid-infrared high-resolution Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) has been brought on board the ENVISAT satellite into a polar sun-synchroneous orbit. Although the level-1 testing and validation phase has not been finished, ESA has made available datasets of 4 early orbits to several groups involved in MIPAS calibration/validation; the groups have been given the opportunity for functionality tests of their analysis codes. We here present some example results of the IMK/IAA MIPAS level-2 processor the concept of which is presented in a companion paper. Temperatures retrieved along the orbits are compared to ECMWF data. Processing parameters as chosen in pre-launch studies, such as cloud identification thresholds, microwindow selection, or instrument characteristics, are discussed. Preliminary retrievals for various atmospheric conditions are shown.

It is of great interest to investigate the properties on the cloud optical, microphysical, and geometrical parameters, in particular, of low-level marine clouds which play crucial influence on the global climate system. Top height, base height, and geometrical thickness of cloud layer are considered here as cloud geometrical parameters. These parameters are very important to retrieve, because top and base heights are the factors which govern the strength of greenhouse effect through the thermal radiation from/to cloud layer, whereas the geometrical thickness is the key parameter for the estimation of gaseous absorption in cloud layer where multiple scattering process dominates. In this study, an algorithm was developed to retrieve simultaneously cloud optical thickness, effective particle radius, top height, and geometrical thickness of cloud layer from the spectral information of visible, near infrared, thermal infrared, and oxygen A band channels. This algorithm was applied to FIRE (First ISCCP Regional Experiment, 1987) airborne data which included the above four channels and targeted at the low-level marine clouds off the coast of California in summer. The retrieved results seems to be comparable to the in situ microphysical observation although further validation studies are required for the cloud geometrical parameters in particular.

Ground-based Atmospheric Emitted Radiance Interferometer (AERI) and Raman Lidar measurements are used to infer cirrus cloud absorption optical depth and effective particle size. Our methodology will be discussed, and results shown for a number of contrasting cloud cases. The high spectral resolution AERI measurements allow inversion of the infrared radiative transfer equation between gaseous absorption lines (e.g., regions of minimal atmospheric emission), referred to as microwindows, to derive the cloud infrared absorption optical depth. Spectral variation in the cloud optical depth yields information on particle size and shape. A best fit of absorption optical depth to the measured absorption optical depth in each microwindow is used to determine the effective radius of particles within the cloud. Results will also be compared to simultaneous upwelling aircraft measurements.

EarthCARE, a candidate Earth Explorer Core mission of ESA, aims to improve our knowledge of the impact of clouds and aerosols on the Earth's radiative budget. The satellite will carry two nadir sounding active instruments: a Cloud Profiling Radar (CPR) and a backscatter lidar. In addition, a multispectral cloud-imager, a Fourier transform spectrometer and a broadband radiometer complement the payload. The objective of the present study was to optimize the parameters of the CPR for retrieving accurate radiative profiles for highly layered cloud structures. Realistic cloud scenarios taken from ground-based experiments have been used for simulating the radar response to cloud layers. A radar simulator was developed initially for one-dimensional simulation of the radar echos. The cloud microphysical properties were retrieved using a model as a function of the reflectivity factor and temperature, based on information from in-situ measurements. An extensive parametric analysis was performed for various vertical resolutions and sensitivities which have direct impacts on the radar design and necessary resources on-board the satellite. The analysis demonstrated that the proposed radar characteristics will meet the top-of-the-atmosphere radiative flux density estimation accuracy of 10 W/m² as recommended by WCRP.

This paper presents the results of a case study using the dual-view ATSR-2 instrument to retrieve optical and microphysical parameters of cirrus clouds overlaying a variable water cloud. The proposed method utilizes the channels at 0.87 and 1.6μm in both nadir and forward views and the nadir view of the 3.7μm band. A lookup table is generated using a radiative transfer model, which is inverted using an evolutionary scatter search technique. An uncertainty analysis demonstrates the robustness of the retrieved ice cloud optical thickness and effective size, even in the presence of an optically thick water cloud below. A comparison with in-situ aircraft observations from the INCA campaign shows good agreement for these parameters.

Climate properties regulated by convection, like cloud cover and related distributions, are undersampled in asynoptic data from an orbiting platform. Following an overview of the information content of asynoptic sampling, we explore the consequences of undersampled cloud variance on (1) the construction of time-mean distributions and (2) the construction of synoptic maps, which describe the global organization of cloud and its evolution. The results are validated against true cloud cover in high-resolution Global Cloud Imagery that has been composited from 6 satellites simultaneously monitoring the earth. Undersampled diurnal variance leads to systematic errors in the time-mean distribution of cloud cover that exceeds 50% over tropical landmasses, where cloud cover undergoes a pronounced diurnal variation. The random component of undersampled variance can be treated successfully by rejecting small-scale incoherent variance. This reduces the error variance to 10% or less, enabling time series of synoptic maps to be constructed.

Characterization of the microphysical properties of non-precipitating stratus clouds including their suspended-water droplet size distribution and the cloud's liquid water content are estimated in this work. The dual wavelength ratio, DWR, and the differential extinction, DE, were computed at two millimeter frequencies, 33 GHz and 95 GHz, using UMass Cloud Profiling Radar System (CPRS) to estimate the drop size distribution. Data from radiosonde observations (Raob) is used as input in a recently calibrated model for estimation of the gaseous attenuation at K_a.-band and Liebe's model at W-band. Integrated specific humidity from a radiometer is used to constrain the radiosonde specific humidity. The radar reflectivity is corrected to take into account the effect of the wind speed, the difference of beamwidth at both frequencies and the difference in sampled range cells. Radar reflectivity and ancillary data are combined to obtain the differential extinction and the estimated cloud's liquid water density. Profiles of the processed data, such as DE, the DWR and the cloud's liquid water density are presented. Cloud's water density and radar reflectivity were used for the size distribution estimation of the suspended water droplets and the median drop diameter.

Various methods and techniques to estimate ice crystals radar response have been developed to study the structure of cirrus clouds. Most methods assume a spherical shape for the ice crystals. This assumption leads to mistakes on the parameter estimation related to the particles' size. In this work, we modeled the shape of ice particles found in cirrus cloud as measured by airborne instruments, specifically ice bullets. These can be found depending on the temperature and cloud altitude, isolated or in groups of two or more bullets, called bullet rosettes. The model of the bullets was developed using the parameters obtained by airborne measurements from the National Center for Atmospheric Research (NCAR) Video Ice Particle Sampler (VIPS). This is an airborne instrument that takes samples of the cirrus cloud particles sizes. With these sample parameters we created a bullet function in DDSCAT with the actual shape of the bullets. This software allows us to create irregular models of particles using the Discrete Dipole Approximation method. With this model we can analyze the backscattering produced by the bullet and rosette model or reflectivity and compute the total volume backscattering coefficient from the cirrus clouds. Various models of ice crystal habits are compared.

Statistical and spatial features are computed to detect clouds from MODIS Terra images. The presence of cirrus clouds is specifically studied. 1.38 μm band of MODIS is used. Cirrrus clouds that do not appear in visible imagery can be detected in this band. Although clouds appear to be uniform, they exhibit a texture pattern. This is evident from the texture measures computed for cloud pixels. Therefore, a supervised classification procedure that makes use of texture features is adopted. Second order gray level features, spectral features, wavelet features are computed. The non-cloud pixels and cloud pixels are identified, followed by classification of the cloud pixels. A comparison of the ability of the features is presented. To show the advantage of texture classification, the classified results are compared with results of cloud pixel identification from thresholding of pixel values. Images obtained from the MODIS sensor over the Caribbean region is classified. Wavelet features have been found to better characterize the cloud texture in the MODIS bands 17 and higher.

SF₆ is the most widely used atmospheric tracer in field tests held for performance evaluation of passive remote sensing systems. The most advanced systems are based on 2D arrays of MCT photovoltaic detectors operating in the LWIR (8-12μ) spectral band. Increasing the number of elements in an array is inevitably associated with a decrease in the detector responsivity cutoff wavelength. This might pose a significant difficulty in the use of SF₆ due to its sole narrow absorption band centered at 10.55μ, which falls within a spectral region of poor FPA sensitivity. A search for a more suitable tracer for advanced detector arrays has yielded that Trifluromethane (CHF₃) is the optimal alternative. The paper describes the considerations that led to its selection and surveys its main physical properties, safety, availability and costs. It also presents initial results from a field test that demonstrates its utility.

To retrieve aerosol properties from the radiance measured at the top of the atmosphere in clear sky conditions, the contributions of the surface and the various atmospheric constituents need to be separated. This is easiest done over dark surfaces, i.e. with a negligible reflectance that can relatively easily be accounted for. This principle has been used for the retrieval of aerosol optical depth (AOD) over water from satellite observations in the near-infrared. The AOD at wavelengths in the UV can be determined both over water and over land, using the same principle of a dark surface. For longer wavelengths, the dual view provided by ATSR-2 is used to separate the aerosol reflectance from the surface contribution. Single and dual-view algorithms have been developed by TNO-FEL and tested for the US east coast and over Europe. Currently the algorithms are extended with other aerosol types and tested versus data over the Indian Ocean (INDOEX area) and South Africa (SAFARI experiment). The initial results indicate that the AOD can be retrieved within reasonable limits. Apart from the ATSR-2, algorithms aimed at the determination of aerosol optical depth and composition are developed for AATSR and SCIAMACHY (ENVISAT) and OMI (EOS-TERRA).

Images from the Earth surface taken from satellites or airplanes need atmospheric correction including angle and aerosol effects to derive accurate surface properties. For low-flying airplanes which operates in heights between 1 and 2 km one has to distinguish between the atmosphere above and below the sensor. A critical task within the atmospheric correction is the correct consideration of the vertical distribution of aerosol particles. The atmospheric boundary layer can reach heights up to 3 km during a summer day in central Europe and it can change as much as 100% during a day. Long range transport of particles occurs normally in heights between 1 and 5 km. Both lead often to a considerable amount of particles above the airplane. It will be demonstrated that the effect of an inadequate aerosol vertical profile used within the atmospheric correction scheme on the derived surface reflectances is height dependent and wavelength dependent with much larger influences in the UV than in the long NIR wavelength range both due to decreasing Rayleigh and aerosol scatttering and increasing surface reflectance with increasing wavelength. The wavelength dependency leads to different NDVI and LAI indices. Furthermore the effect is modified by the intensity of the path radiance in relation to the surface reflected radiance with leads to a dependency on solar zenith angle and sensor zenith angle.

The application of Differential Optical Absorption Spectroscopy (DOAS) methodology to the zenith scattered light data collected with the GASCOD spectrometer developed at the ISAC Institute allow for the detection of stratospheric trace gases involved in the ozone cycle such as NO₂, OClO, BrO. The instrument was installed in December 1995 in the Italian Antarctic station at Terra Nova Bay (74°26'S, 164°03E', Ross Sea), after several tests both in laboratory and in Antarctic region, for unattended and continuous measurement in extreme high-latitude environment. The GASCOD is still working and producing very interesting data for the study of the denitrification processes during the formation of the so-called ozone hole over the Antarctic region. For the continuous NO₂ monitoring for whole the year, also during winter when the station is unmanned, the [407 - 460] nm spectral region is investigated. The results for Nitrogen Dioxide, obtained by application of DOAS algorithms to the data recorded during the year 2001, are presented. ERS-2 was launched in April 1995 into a near-polar sun-synchronous orbit at a mean altitude of 795 km. The descending node crosses the equator at 10:30 local time. GOME is a nadir-scanning double monochromator covering the 237 nm to 794 nm wavelength range with a spectral resolution of 0.17-0.33 nm. The spectrum is split into four spectral channels, each recorded quasi-simultaneously by a 1024-pixel photodiode array. The global spatial coverage is obtained within 3 days at the equator by a 960 km across-track swath (4.5 s forward scan, 1.5 s back scan). The ground pixel size of the measurements is 320 X 40 km². A comparison of GASCOD and GOME results for NO₂ total column is performed.

The use of multi-frequency radar's Doppler Spectrum to study different aspects of precipitation has demonstrated its utility as an accurate profiling rain-gauge method. Recent studies used this concept to retrieve the drop-size distribution and vertical air motion in rain using dual-frequency Cloud Profiling Radar System, which operates at 33 GHz (Ka-band) and 95 GHz (W-band). This study was performed for low to moderate rain-rates because the use of the Ka-band frequency limited the accuracy of the measurements for high rain-rates due to the attenuation this signal suffers while it passes through the cloud. In this work we use a non-attenuating frequency, 2.8 GHz, instead of the Ka-band, to obtain measurements over a wider dynamic range of rain conditions, extending the active rain-gauge concept to heavier rain-rates. The W-band signal provides accurate measurement of the vertical air motion in rain. The actual drop's shapes must be corrected for heavy rain in which case large non-spherical raindrops exist. Data will be processed as suggested by Firda et al., 1999 considering the drop's shape corrections. This research's goal is to develop IDL codes to align, process, and analyze the collected data to retrieve several cloud characterization parameters, such as drop size distribution and vertical air motion that would be used to study the inner processes of rain. Rain-rate approximations and the vertical air motion retrieval will be presented.

Atmospheric water vapor is important for many scientific domains because it is one of the most effective greenhouse gases in the atmosphere. In the context of Earth remote sensing the expected water vapor abundance is used mainly to remove one of the most important effects of atmosphere on the at-sensor radiance: the formation of absorption bands. In this paper we present a new algorithm for the retrieval of columnar (integrated) water vapor content based on a spectroscopic approach. In particular, the proposed technique analyses the H₂O absorption feature centred at about 940 nm. We also present some numerical calculation based on the MODTRAN 4 radiative transfer model and the HITRAN 2000 database as source of H₂O cross-section spectra. Some experimental results are derived from radiometrically calibrated images acquired by the AVIRIS sensor.

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is operating on board of the ENVISAT satellite and is acquiring for the first time high spectral resolution middle infrared emission limb sounding spectra of the Earth atmosphere from space. The measurement capabilities make it possible to determine every 75 sec. the vertical profile of several atmospheric trace constituents, during both day and night with an almost full coverage of the globe. This leads to a three dimensional measurement of the atmospheric composition. In order to handle the large data flow, an optimized code for the Level 2 near real time analysis of MIPAS data was developed by an international consortium of scientists under an ESA contract and was implemented in the ENVISAT Ground Segment. The code is designed to provide, in an automated and continuous mode, atmospheric vertical profiles of temperature and pressure, as well as of concentrations of O₃, H₂O, CH₄, HNO₃, N₂O and NO₂, in the altitude range from 6 to 60 km. The "commissioning phase," in which verification and validation of the instrument and of the analysis code are performed, is still in progress, but some preliminary results have been obtained. The first examples of the MIPAS near real time Level 2 data products, consisting of retrieved profiles and auxiliary data that characterize the measurement accuracy and resolution, are shown.

It is presented a neural network methodology to retrieve atmospheric parameters of meteorological interest such as temperature, water vapor and ozone profiles from upwelling high resolution infrared sensor spectra. Neural network approach has been developed on basis of the specification of the Infrared Atmospheric Sounding Interferometer (IASI), which is planned to be flown on the first European Meteorological Operational Satellite Metop in 2005. The performance of the neural network based inversion methodology has been evaluated by considering a suitable set of inversion exercises in which test cases are retrieved.

The Vaisala ceilometer CT25K is an eye-safe commercial lidar mainly used to report cloud base heights and vertical visibility for aviation safety purposes. Compared to ceilometers with bi-axial optics, its single-lens design provides a higher signal-to-noise ratio for lidar return signals from distances below about 600 m, thus increasing its abilities to examine the mixing layer. A CT25K ceilometer takes part in the environmental measuring campaign VALIUM at the Lower Saxony State Agency for Ecology (NLO) in Hannover, Germany, investigating the air pollution in an urban surrounding with various sensors. Lidar return signals are reported every 15 s with a height resolution of 15 m. This paper concentrates on the interpretation of these signals in respect of the aerosol backscatter of the atmosphere up to 30 m. Every 30 minutes the NLO reports PM10 and PM2.5 concentrations measured with in-situ sensors installed 20 m above the ceilometer. Humidity and precipitation monitor sensors help ruling out weather situations with water droplets contributing mainly to the ceilometer backscatter signal. Data collected between 01. 03. 2002 and 31. 07. 2002 show that during dry weather situations there is a correlation of more than 80% between the dust concentration and the aerosol backscatter, allowing a quantitative analysis of the atmospheric dust contents with a standard ceilometer. The ratio PM10/PM2.5 of in situ measurements is investigated also giving a regression function and a correlation coefficient.

Recent theoretical studies have demonstrated the potential of spaceborne high spectral resolution O₂ A-band spectrometers for retrieval of aerosol and cloud optical properties. High spectral resolution is the key to these retrievals because it permits a separation of the surface and atmospheric scattering components of the reflectance measurements. Although promising, this new approach poses numerous technical challenges related to instrument design, characterization, and calibration that have not been fully addressed by previous conceptual studies. To experimentally assess the capabilities of this promising new remote sensing application, the NASA Langley Research Center has developed the Langley Airborne A-Band Spectrometer (LAABS). This instrument will serve as a unique test bed to evaluate the impact of realistic instrument performance on A-band retrieval capabilities. After full characterization of the instrument through laboratory testing, a detailed forward model of the instrument's radiance measurements will be developed. The instrument model will be validated against actual measured spectra obtained from ground-based operations of the instrument. Airborne A-band spectra obtained during initial test flights of this instrument during the CLAMS field campaign in July 2001 will be analyzed to assess aerosol retrieval capability.

Remote sensing of atmosphere is changing rapidly thanks to the development of high spectral resolution infrared space-borne sensors. The aim is to provide more and more accurate information on the lower atmosphere, as requested by the World Meteorological Organization (WMO), to improve reliability and time span of weather forecasts plus Earth's monitoring. In this paper a new channel selection strategy for water vapor is presented and analyzed both on simulated spectra and on real spectra.

The emission indices of aircraft engine exhausts to calculate precisely the emissions inventories of airports are not available up to now from measurements taken under operating conditions. To determine these data no installations nearby or behind the aircraft are possible at airports. That's why measurements by FTIR emission spectrometry were performed by the IMK-IFU with a spectrometer installed in a van and with total measurement time at one thrust level of about 1 minute to determine CO, NO and CO₂. The FTIR instrument telescope was aligned to the engine nozzle exit of standing aircraft. A DOAS and a FTIR spectrometer with globar were used for simultaneous open-path measurements of NO, NO₂, CO, CO₂ and speciated hydrocarbons behind the aircraft by the TUG-VKMB. Measurement results at the airports Frankfurt/Main, London-Heathrow and Vienna are presented. The methods are evaluated by comparing CO emission indices from passive measurements with open-path data. The measured emission indices of CO show slightly higher values than the International Civil Aviation Organisation data sheets but less values for NO_x emissions. A fruitful co-operation with the airlines AUA, BA and DLH as well as the airport authorities in Vienna and London-Heathrow supported this work which is financed from EC.

As part of the EU funded project AEROJET2, a number of gas turbine engine tests were performed in different facilities around Europe. At Farnborough, UK a Spey engine was used to test a suite of prototype optically based instrumentation designed to measure exhaust gas emissions without using extractive probe systems. In addition to the AEROJET 2 prototype instrumentation, a Bruker Equinox 55 Fourier transform infrared (FTIR) spectrometer was used to obtain infrared spectra of the exhaust plume both in emission and absorption mode. The Bruker FTIR spectrometer was fitted with a periscope system so that different lines of sight could be monitored in the plume in a vertical plane 25 cm downstream from the nozzle exit and 20 cm upstream of the center line of sight of the AEROJET 2 prototype instrumentation. DERA (now QinetiQ) provided exhaust gas analysis data for different engine running conditions using samples extracted from the plume with an intrusive probe. The probe sampled along a horizontal plane across the centerline of the engine 45 cm downstream of the nozzle exit. The Bruker spectrometer used both InSb (indium antimonide) and MCT (mercury-cadmium-telluride) detectors to maximize the sensitivity across the IR range 600-4000 cm^-1. Typically, CO₂ and H₂O IR signatures dominate the observed spectra of the plume. However, the engine tests showed that at low power engine conditions spectral features associated with CO around 2147 cm^-1 and with hydrocarbons could be observed at around 3000 cm^-1. In particular the presence of ethene (C₂H₂) was detected from observation of its characteristic in and out of plane vibration mode at 949 cm^-1. At high engine powers the presence of NO was detected at 1900.3 cm^-1. Species concentrations were calculated using a slab model for each line of sight compared against reference spectra. The engine plume was assumed to be symmetric about the centerline. On this basis, data from the extractive sampling gas analysis that had been obtained by traversing the probe across a horizontal plane through the centerline could be compared with non-intrusive measurements made by scanning vertically. Adjustments have been made to account for the 20 cm downstream offset in measurement planes of the probe and the spectrometer behind the nozzle exit.

One of the most actual problems in monitoring of the atmosphere is the recognition of chemical composition and microphysical characteristics of atmospheric aerosol impurities. In the present work the opportunities of lidar system based on multifrequency pulsed chain chemical DF laser (λ = 3.6 - 4.2 μm) are considered from this point of view. With the purpose of working-out of the requirements to the primary performances of lidar system, model experiments are executed as to the problem of recognition of aerosol impurities in the atmosphere with the use of DF-laser radiation lines. The obtained results and their analysis are presented in the paper.

We report in this paper lidar measurements of two events of transport of dust from the Sahara region to the Mediterranean Sea, observed in Lecce, Italy (40°20'N,18°06'E). The observations have been made in the framework of the EARLINET project, in which aerosol measurements are performed in all Europe by 21 stations on a coordinate basis. Lidar measurements are compared with informations available from other sources. General results for the lidar ratio are also presented.

Recent progress in sub-millimeter wave receiver technology gives the possibility to drastically improve the quality of limb sounding data by use of a superconductor-insulator-superconductor (SIS) mixer element. This receiver will detect molecular spectra with a signal-to-noise ratio one order of magnitude better than the conventional ambient--temperature Schottky receiver. SMILES (Superconductor Submillimeter-wave Limb-emission Sounder) is proposed by the Communications Research Laboratory and the National Space Agency of Japan, with technical support from the National Astronomical Observatory, and with scientific support from the University of Bremen, in order to demonstrate the new sub-millimeter wave technology in space, and to conduct the measurements of limb-emission sounding for a group of molecular species profiles. In order to anticipate the performance of the instrument, retrieval simulations are carried out. Synthetic measurements, as will be recorded by the SMILES instrument, are generated by the use of a forward model. These are then inverted, using an inversion model, in order to derive the variables of interest, such as molecular species profiles (e.g., O₃, ClO, HCl), atmospheric temperature profile, or a first order instrumental pointing correction (i.e., a pointing offset). The applied inversion algorithm is the Optimal Estimation Method (OEM). The advantage of the OEM is that it allows a formal error analysis needed for a general error characterization of retrieval performance. The error analysis takes into consideration the total statistical error, the measurement error, the vertical altitude resolution, and the correlation between the retrieved quantities. The altitude domain of a good measurement sensibility is defined by the measurement response.

A backscatter lidar study of the aerosol stratification in PBL over the Rhine valley is presented in this work. The observations have been made from 6 till 14 September 1999 in the Special Observation Period of the Mesoscale Alpine Program at the site of Trubbach, Switzerland [47°04' N, 9°28' E, 490 m above sea level (asl)]. The lidar was operated alongside with a wind-temperature radar, a set of surface meteorological stations and radiosonde observations. The atmospheric conditions for the presented measurements are especially favorable for development of convective PBL. A daily series of range-corrected lidar signal and its gradient are given, showing the diurnal cycle of aerosol layers development in the PBL and lower troposphere over the valley. Averaged altitude distributions of the aerosol backscatter coefficient and the lidar signal gradient are presented and discussed. The results show that the aerosol distribution below the surrounding mountain peaks (approximately 2200 - 2400 m asl) have the diurnal pattern as following from the thermal wind cycle and the temperature inversions in the valley and is characterized by a highly dynamic aerosol stratification. Above this level, we observe a diurnally less variable stratification till altitudes of the highest Alpine peaks (approximately 4500 - 5000 m asl).

We report the progress in the development of a compact mid-infrared differential absorption lidar (DIAL) for ground-based and airborne monitoring of leaks in natural gas pipeline systems. This sensor, named Lidar II, weighs approximately 30 kg (70 lbs) and occupies a volume of 0.08 m³ (3.5 ft³). Lidar II can be used on the ground in a topographic mode or in a look-down mode from a helicopter platform. The 10-Hz pulse repetition rate and burst-mode averaging currently limit the airborne inspection speed to 30 km/h. The Lidar II laser transmitter employs an intracavity optical parametric oscillator. Wavelength tuning is accomplished through two mechanisms: a servo-controlled crystal rotation for slow and broad-band tuning and a fast piezo-activated wavelength shifter for on-line/off-line switching in less than 10 ms. The sensor operates in the 3.2-3.5-μm band with the primary focus on hydrocarbons and volatile organics. In the pipeline inspection work, the two main targets are methane and ethane, the latter chemical being important in preventing false positives. Initial results of Lidar II testing on actual pipeline leaks are reported. To supplement the mapping capabilities of Lidar II with range-resolved information, a short-range (less than 300 m) aerosol backscatter lidar is currently under development.

The paper concerns with calculative and experimental development of a remote electric-discharge DF-laser analyzer for subterranean methane monitoring up to its explosive concentrations. The analyzer is built using a lidar scheme when the probing emission is reflected by artificial or natural object located in a subterranean measurement path. Methane records in the air are made using the differential absorption technique. The required metrological parameters of the analyzer are reached with the multi-frequency laser atmospheric control. A block diagram of the methane analyzer is given together with procedures to probe mine media with many frequencies in the spectral range from 3.6 to 3.8 μm. The lidar model was tested under conditions simulating physical values of the subterranean air: (1) methane air content -- up to 3% vol.; (2) dust content -- about 10 mg/m³; (3) probing path length -- about 50 m.

To appreciate the radiative impact of clouds in the dynamics of the global atmosphere, it is important to deploy from space, from aircraft, or from ground, instruments able to describe the cloud layering and to document the cloud characteristics (namely liquid and/or ice water content, and the effective particle radius). In the framework of EarthCARE (ESA), that plans to associate a cloud radar and a lidar on the same spatial platform, RALI (RAdar-LIdar) airborne system is an interesting demonstrator. RALI combines the 95 GHz radar of the CETP and the 0.5 μm wavelength backscattering lidar of the SA. In order to derive the radiative and microphysical properties of clouds, a synergetic algorithm has been developed. It combines the apparent backscatter coefficient, β_a, from the lidar and the apparent reflectivity, Z_a, from the radar to infer properties of the particle size distribution. The principle of this algorithm is to apply in parallel the Hitschfeld-Bordan algorithm to the radar and the Klett algorithm to the lidar. Taken separately, these two algorithms are unstable, but by considering a mutual constraint, it is shown that a stable solution can be established. This solution formulates the retrieval of the true reflectivity and backscattering coefficient, to access microphysical and radiative parameters of clouds. This algorithm allows also to retrieve the variable N₀* parameter, which is a normalization parameter of the particle size distribution. This synergetic algorithm has been tested with simulated cases, and results of the algorithm applied on real data are validated by microphysical in-situ measurements.

Recent lidar measurements at Hampton, Virginia (37.1N, 76.3W) indicate that the current mid-latitude stratospheric aerosol level, after recovery from the 1991 eruption of Mount Pinatubo, is lower than the background level measure during the vocanically quiescent period in 1979. This suggests that perhaps the natural stratospheric aerosol background may be lower than previously thought. Volcanically inactive periods, such as the periods after the 1982 El Chichon eruption and again after the 1991 Pinatubo eruption, provide opportunities to study long-term trends in volcanic aerosol decay. And, the present state of very low aerosol loading in the stratosphere, provides opportunities to study and gain a better understanding of the natural background aerosol.

In this paper we study the accuracy of wind retrieval from lidar data by spectra accumulation method (SAM) when turbulent fluctuations of wind velocity are present. Simulation of wind retrieval from space borne scanning Doppler lidar data by this method we carry out based on the real data of German Weather Service (GWS). For simulation of small scale turbulent fluctuations of the radial wind velocity we use GWS height profiles of zonal and meridional components of mean wind, temperature and turbulent diffusion coefficients of momentum and heat. The error of retrieval of wind velocity and direction from lidar data is calculated in dependence on turbulent velocity fluctuations variance for different heights (signal to noise ratios). This study was supported by the Russian Foundation for Basic Research (Grant No00-05-64033).

A comparison of shortwave unfiltered radiances measured by CERES instruments operating on two different platforms, TRMM and Terra satellites, is presented. A focus of this paper is twofold: a summary of the recovery of data obtained by the CERES instrument (PFM) failing sensors at the end of its useful life in April and June of 2000, and validation of the recovered data. The data recovery is necessary as deteriorating electronics of the PFM polluted data with cross-talk and noise preventing data processing. It is shown that a pattern recognition is an effective strategy in the data clean-up. The validation is performed by comparing shortwave measurements obtained by the CERES instrument (FM1), on board the Terra satellite, to the recovered PFM data. Comparisons are made for the data collected when orbits of both satellites cross, and viewing geometries of the instruments match.

We present the results of a scale analysis of aerosol optical thickness (AOT) variability in time and space. The standard scale analysis methods based on variance spectra, structure functions and singular measures were applied to different datasets to determine AOT scaling properties in the scale range from 0.2 to 1200 km. We used data sets from the local network of Multi-Filter Rotating Shadowband Radiometers (MFRSRs) located at the DOE ARM program's Southern Great Plains (SGP) site. This network consists of 21 instruments arrayed across approximately 55,000 square miles in north-central Oklahoma and south-central Kansas. Our analysis demonstrates that datasets from the SGP network can provide definitive information on both temporal and spatial AOT variability. We compare AOT scale properties derived from MFRSR network data with those derived from MODIS satellite aerosol retrievals over SGP. We also use MODIS retrievals over larger land and ocean areas to study large-scale AOT variability. Our analysis suggests that the variability of AOT splits into three main scale regimes. Three-dimensional turbulent transport dominates small scales (0-30 km), the influence of 2D turbulence at scales larger than 30 km makes AOT more non-stationary, and finally, the influence of the AOT boundedness at scales larger than 100 km presses AOT behavior back towards stationarity. However, the scales used in our analysis were not large enough to observe the boundedness-induced asymptotic regime in AOT variability, where any further scale increase does not change variability properties.

IPCC third report says that we have still a lot of uncertainties to predict global warming even using latest GCMs. Regarding atmospheric radiation, uncertainty of the radiative forcing is still large, which is mainly caused by aerosols, clouds, and water vapor interacting among them. National Space Development Agency of JAPAN (NASDA) and Communications Research Laboratory (CRL) started Phase-A study with European Space Agency (ESA) in the EarthCARE project. The objectives of EarthCARE project are to observe vertical and horizontal distributions and physical characteristics of aerosols and clouds from a satellite, and also to measure the precise Earth radiation budget simultaneously. Finally we will be able to evaluate physical processes of clouds and aerosols regarding the radiative budget and forcing. The EarthCARE satellite carries 5 sensors, namely Cloud Profiling RADAR (CPR), Atmospheric LIDAR (ATLID), Multi-Spectral Imager (MSI), Broad Band Radiometer (BBR) and Fourier Transform Spectrometer (FTS). The result of the pre-Phase A study shows the synergy observation benefits using some compensative combinations of sensors, such as CPR/ATLID for clouds, ATLID/MSI for aerosols, BBR/FTS for the radiation budget. NASDA and CRL are studying FTS and CPR, respectively. CPR is a 94GHz RADAR using 2.5m diameter reflector with Doppler measurement mode. The sensitivity is -38dBZ. The vertical and horizontal resolution is 100 m, 1 km, respectively. FTS is a Michelson interferometer of which spectral measurement range is from 5.7 μm to 25 μm with 0.5 cm^-1 unapodized spectral resolution. FOV is 10 km by 10 km. EarthCARE is planned to be launched in 2008 for 2 years mission. Phase-A study will continue until the end of 2003.

Images from GOES satellite were used to develop a method to search for sites suitable to astronomical observations in the infrared. An area of study located in the Peruvian Andes was chosen, with altitudes above 2500 m. Forty-three images from the GOES meteorological satellite in channels 3, 4 and 5 were used. The GOES images, spanning an 11-day period, in each channel, were combined to produced images expressing the surface visibility in each channel. Atmospheric turbulence could be estimated from the variation of visibility over six-hour periods, with one image per hour. As criteria to classify sites on the Andes, we combined information on altitude, visibility of the surface in the infrared, the amount of water vapor in the atmosphere, and atmospheric turbulence. Results of this new method showed that the region of Moquegua, in South Peru, is to be preferred in surveys for astronomical sites. Comparisons with results from other investigators, which used other approaches, indicated that this methodology can produce valid results and can be applied to studies covering larger periods. The general results of this study indicate that the method is valid and can effectively be used as an important resource in surveys for infrared astronomical sites.

Integrated air quality management requires considering many information classes simultaneously, including environmental quality data, health impact pathway models, economic analyses, the respective regulatory framework, and the priorities of the concerned stakeholders. Integrated air pollution assessment in particular includes accurate representation of the pollution distribution in time and space, identification of the main emission sources, and evaluation of the possible alternatives for coping with the observed environmental burden. The current state of the art in air quality assessment, monitoring and management comprises analytical measurements and atmospheric transport modeling. Earth observation from satellites may provide an additional information layer through the calculation of synoptic air pollution indicators, such as the tropospheric aerosol optical thickness. This paper outlines a paradigm for efficient data and model fusion for the integrated assessment of the health impact due to airborne chemicals. The information management techniques employed and the problems due to the multidisciplinary nature of the phenomena analyzed are highlighted. Selected examples from using this methodology for the assessment of air quality in the European Union are given.

High spatial resolution (HSR) satellite observations, while not frequent enough to follow air pollution's dynamic fluctuations, can provide spatially resolved information related to urban air quality. More specifically, HSR satellites can provide independent "spatial measurements" on the columnar aerosol optical thickness in the visible (AOTV). When normalized to ground level, AOTV can be correlated to fine aerosol concentrations, and when monitored over long or representative periods it could be used as an air-quality indicator to bridge the gap between "point measurements" by ground-based sampling, and "spatial estimations" by atmospheric modelling. We briefly review in this paper the methods we developed to map AOTV over urban areas from HSR satellites; we then describe qualitative AOTV validation procedures for the case of Athens. We finally present preliminary quantitative results from a pilot application where we compared data on air quality acquired using the three tools (i.e., satellite observations, atmospheric modelling and ground measurements) over two polluted European sites. This comparison showed good agreement between satellite-derived AOTV, on the one hand, and ground-level aerosol precursor concentrations and modelling-derived pollutant flow patterns on the other. These preliminary results encouraged an in-depth investigation of the benefits from the complementary use of these three techniques for integrated air-quality monitoring. During four pilot campaigns foreseen in the framework of the ICAROS NET project, we plan to collect detailed atmospheric data and run numerical models in conjunction with the satellite passages.

The newly developed Nd:YAG portable 3-D-scanning lidar from the Universitat Politecnica de Catalunya (UPC, Technical University of Catalonia, Barcelona, Spain) was used to improve our knowledge of the aerosols properties in the Barcelona area where an important number of pollution and saharan dust events can be observed all year round in the atmosphere. The system simultaneously operated at the 1064-nm and 532-nm elastic wavelengths, and was used in its scanning mode from 15 degrees to 70 degrees from zenith with 5 degree steps. A variational method was used to invert the multi-angular profiles and to retrieve the aerosols optical thickness and backscatter coefficient at each wavelength without making any assumption on the aerosol type. At the same time, the ratio of the backscatter profiles was used to retrieve the profile of the Angstrom coefficient in backscatter. The backscatter-to-extinction ratio could not be calculated directly but various values of this parameter were used in Klett method until backscatter coefficient profiles could match the one retrieved with the variational method (at least in some altitude regions). Very good agreement (differences less than 20%) was observed in the 0.3 - 2.5 km region with a value of 0.030 sr^-1, whereas no agreement could be achieved above where supposedly mixed aerosols were initially observed. The lidar profiles closer to the zenith at both wavelengths allowed to calculate a new Angstrom coefficient in backscatter that is compared to the one retrieved by the variational method. The comparison showed good agreement in the lower layers and thus validated the backscatter-to-extinction ratio profiles used in the Klett method. However, the difficulties encountered to invert the lidar signal above an altitude of 1.6 km show that non negligible inhomogeneities of the atmosphere were present in each line of sight, proving the dense and fast-moving aerosol load over the Barcelona basin.