Determination of standards for a UV-B monitoring network
Safety, Environmental Protection
Laboratoire d'Optique Atmosphèrique
Department of Meteorology
Belgian Institute for Space Aeronomy
Aristotle University of Thessaloniki
Natural Environment Research Council
University of Cambridge
University of Tromsø
Koninklijk Nederlands Meteorologisch Instituut
Universitaet Fuer Bodenkultur Wien
ARISTOTLE UNIVERSITY OF THESSALONIKI HELLAS
University of Tromsø NORGE
NATURAL ENVIRONMENT RESEARCH COUNCIL UNITED KINGDOM
Koninklijk Nederlands Meteorologisch Instituut NEDERLAND
University of Cambridge UNITED KINGDOM
LEOPOLD-FRANZENS-UNIVERSITAET INNSBRUCK ÖSTERREICH
UNIVERSITAET FUER BODENKULTUR WIEN ÖSTERREICH
The overall objective of this project is to produce practical recommendations for the deployment of an integrated ultraviolet B(UVB) network throughout Europe.
The main results of these campaigns are summarised below 1. Good calibration procedures are a necessary first step for accurate and reliable measurements. Careful monitoring of the lamp current ensures consistency of the calibration source, the lamp-spectrometer geometry must be rigorously maintained, and stray light within the calibration room must be excluded. 2. The spectrometer geometry and mechanical stability must allow the calibration to remain valid between the fixed conditions of the calibration room and the different instrument orientations, spatial distributions of radiation and ambient conditions in the environment. In most cases this involves at least a temperature stabilisation of sensitive parts of the spectrometer. 3. Important characteristics of the ideal spectrometer are: (a) wavelength specification. This could be achieved to an accuracy of about 0.1 to 0.3nm in many of the instruments and is acceptable unless very accurate work in the short wavelength UV-B region of the spectrum is required. (b) slit function. This determines the near field stray light from adjacent wavelengths which is attributed to the nominated wavelength of a measurement. The slit function is important where the measured spectrum changes very rapidly with wavelength, as with solar UV-B. Measuring the slit function of the spectrometers allowed the effects of different slit functions on a measurement of the sun light to be calculated. Correcting for both slit function and wavelength specification improves the comparison between instruments, especially in the UV-B. (c) far-field stray light. Radiation from wavelengths outside the region of the slit function must be rigorously excluded. if not, it provides a background measurement which limits the sensitivity of the spectroradiometer. As the solar spectrum changes by three orders of magnitude across the UV-B regions, spectrometers need stray light rejection of the same order. In general the single monochromator and diode array instruments did not have sufficient stray light rejection for measurements at the shorter UV-B wavelengths. (d) cosine response. The fore optics of each spectrometer should have a cosine response but in practice this was often far from perfect. Asymmetry in the cosine response can lead to apparent diurnal asymmetry in the measured irradiances, and different cosine errors can add to discrepancies in response between instruments. Knowing the cosine response of an instrument allows corrections to be made, but these involve assumptions about the spectral distribution of the incident radiation. In conclusion, a group of UV spectrometers has been identified which could act together to form a UV network. Other instruments require some improvement in design or operational procedures, but could achieve the level of performance set by the core group. The experiences of the campaigners have led to a greater understanding of instrument and operational requirements, which will aid newcomers to this area of research and point the way to future improvements in hardware and methodologies. A transportable lamp system has been designed and built to maintain an independent check of calibrations when instruments are isolated at their home sites.
Correction of the stratospheric aerosol radiative influence in the POLDER measurements
M. HERMAN, B. LAFRANCE et TAT SOON YEO
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This paper presents the principles and performances of the stratospheric aerosol correction schemes for the Polarization and Directionality of the Earth's Reflectances (POLDER) spatial polarimeter measurements and the method used to derive, from the Stratospheric Aerosol and Gas Experiment II (SAGE II) data, the information about the aerosols that is needed for the correction. On the Advanced Earth Observing Satellite (ADEOS) platform since August 1996, POLDER performs multidirectional measurements, both of reflectance and of polarization in visible and near-infrared spectral bands. These new observational capabilities are used to observe clouds, lands, ocean surfaces, and tropospheric aerosols. These observations are weakly perturbed by the stratospheric aerosols, whose amount is currently low, but in the case of a major volcanic eruption, would increase strongly for few years. The possibility of such a situation has to be considered. Moreover, even near background conditions, the stratospheric aerosols perturb accurate retrieval of the ocean color and products deduced from the polarized light. That is why a systematic correction of their influence on the measured signal has been developed.
Stratosphere ; Stratospheric aerosol ; Atmospheric correction ; Radiative properties ; Polarimetry ; Satellite observation ; Performance ;
Determination of the wind speed threshold for the emission of desert dust using satellite remote sensing in the thermal infrared
O. CHOMETTE, M. Legrand, B. MARTICORENA et Béatrice Marticoréna
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The Infrared Difference Dust Index (IDDI), derived from images obtained from the Meteosat 10.5- to 12.5-μm channel, describes the dust distribution over the Saharan-Sahelian region. This IDDI, associated with the 10-m wind speed reanalyses from the European Centre for Mediurn-Range Weather Forecasts (ECMWF), reveals whether or not the observed dust is associated with emission from an underlying source. This result allows one to determine the wind speed thresholds for dust emission from targets located in the western, central, and eastern Saharan-Sahelian region, by means of satellite remote sensing. Threshold values determined for seven targets are presented. A comparison is carried out between such values and direct determinations obtained through the description of the soil texture and surface roughness of these targets. The agreement between these quite independent determinations is conclusive, with an average difference of 0.3 m s-1 and a rms difference of 0.35 m s-1.
Atmospheric dust ; Desert ; Wind velocity ; Satellite observation ; Image analysis ; Space remote sensing ; Ground surface ; Roughness ; Texture ; Sahara ; Sahel ; Meteosat satellites ; Africa ;
Global observation of anthropogenic aerosols from satellite
F.-M. BREON, J.-L. Deuzé, M. HERMAN, A. MARCHAND, F. NADAL, Goloub PHILLIPPE et Didier TANRE
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This paper provides the first climatology from satellite of aerosol loading, both over land and oceanic surfaces. Over land, aerosol loading is retrieved using a new remote sensing technique based on the polarization signature of scattered solar radiances. The product is sensitive to particles that are within the accumulation mode, smaller than ≃ 0.5μm. Those are mainly produced by anthropogenic sources, like smoke and urban/industrial aerosols. A rough identification of aerosol origin is obtained from the concomitant detection of fires from spaceborne observations. The combination of aerosol load and fire identification suggests that biomass burning is the main source of small particles in the atmosphere and that, at the regional scale, other anthropogenic activities have a significant impact limited to China and India.
Long-range transport ; Sea surface ; Satellite observation ; Anthropogenic factor ; Vegetation fire ; Radiance ; Polarization ; Space remote sensing ; Ground surface ; Climatology ; Aerosols ;
Uncertainties in assessing radiative forcing by mineral dust
Y. Balkanski, Yves Balkanski, Olivier Boucher, T. CLAQUIN, T. Claquin et M. Schulz
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The assessment of the climatic effects of an aerosol with a large variability like mineral dust requires some approximations whose validity is investigated in this paper. Calculations of direct radiative fordng by mineral dust (short-wave, long-wave and net) are performed with a single-column radiation model for two standard cases in clear sky condition: a desert case and an oceanic case. Surface forcing result from a large diminution of the short-wave fluxes and of the increase in down-welling long-wave fluxes. Top of the atmosphere (TOA) forcing is negative when short-wave backscattering dominates, for instance above the ocean, and positive when short-wave or long-wave absorption dominates, which occurs above deserts. We study here the sensitivity of these mineral forcings to different treatments of the aerosol complex refractive index and size distribution. We also describe the importance of the dust vertical profile, ground temperature, emissivity and albedo. Among these parameters, the aerosol complex refractive index has been identified as a critical parameter given the paucity and the incertitude associated with it. Furthermore, the imaginary part of the refractive index is inadequate if spectrally averaged. Its natural variability (linked to mineralogical characteristics) lead to variations of up to ± 40% in aerosol forcing calculations. A proper representation of the size distribution when modelling mineral aerosols is required since dust optical properties are very sensitive to the presence of small particles. In addition we demonstrate that LW forcing imply a non-negligible sensitivity to the vertical profiles of temperature and dust, the latter being an important constraint for dust effect calculations.
Aerosols ; Atmospheric dust ; Desert ; Radiative properties ; Effect on climate ; Clear sky ; Numerical simulation ; Refraction index ; Particle size distribution ; Sensitivity analysis ; Uncertainty ;
Remote sensing of aerosols over land surfaces from POLDER-ADEOS-1 polarized measurements
F.-M. BREON, Francois-Marie Breon, C. DEVAUX, J.-L. Deuzé, M. HERMAN, B. Lafrance, B. Lafrance, F. MAIGNAN, A. MARCHAND, F Maignan, F. NADAL, F. NADAL, G. PERRY, Goloub PHILLIPPE et Didier TANRE
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The polarization measurements achieved by the POLDER instrument on ADEOS-1 are used for the remote sensing of aerosols over land surfaces. The key advantage of using polarized observations is their ability to systematically correct for the ground contribution, whereas the classical approach using natural light fails. The estimation of land surface polarizing properties from POLDER has been examined in a previous paper. Here we consider how the optical thickness δ0 and Ångström exponent a of aerosols are derived from the polarized light backscattered by the particles. The inversion scheme is detailed, and illustrative results are presented. Maps of the retrieved optical thickness allow for detection of large aerosol features, and in the case of small aerosols, the δ0 and a retrievals are consistent with correlative ground-based measurements. However, because polarized light stems mainly from small particles, the results are biased for aerosol distributions containing coarser modes of particles. To overcome this limitation, an aerosol index defined as the product AI = δ0α is proposed. Theoretical analysis and comparison with ground-based measurements suggest that AI is approximately the same when using δ0, and a is related to the entire aerosol size distribution or derived from the polarized light originating from the small polarizing particles alone. This invariance is specially assessed by testing the continuity of Al across coastlines, given the unbiased properties of aerosol retrieval over ocean. Although reducing the information concerning the aerosols, this single parameter allows a link between the POLDER aerosol surveys over land and ocean. POLDER aerosol index global maps enable the monitoring of major aerosol sources over continental areas.
Space remote sensing ; Aerosols ; Ground surface ; Polarization ; Polarized light ; Optical thickness ; Particle size distribution ; Monitoring ; Satellite observation ; Backscattering ; ADEOS satellite ;
Particles of human origin extinguishing natural solar irradiance in climate systems
Meteorology, Resources of the Sea, Fisheries, Environmental Protection, Forecasting
Laboratoire d'Optique Atmosphèrique
Laboratoire des Sciences du Climat et de l'Environnement
Institute for Chemistry (otto Hahn Institute)
Institute for Meteorology
Centre National de la Recherche Scientifique
Commission of the European Communities
University of Crete
UNIVERSITY OF CRETE HELLAS
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE FRANCE
UTRECHT UNIVERSITY NEDERLAND
COMMISSION OF THE EUROPEAN COMMUNITIES ITALIA
NATIONAL RESEARCH COUNCIL OF ITALY ITALIA
PHOENICS is a global modelling project to study the direct climate effect of multi-component mixed troposphere aerosols. The potentially great climatic importance of aerosols urgently requires improvement of the estimates of the climate effect of aerosols and better evaluation of the associated uncertainties. Innovative size-resolved simulations of the distribution and properties of the mixture of all major aerosol components will be performed with a global 3-dimensional atmospheric general circulation model to assess the direct effect of aerosols. Several of the main uncertainties associated with this effect will be quantified and reduced by model improvement, comparison to selected observations and optimal use of satellite data. The impact of European emissions on the European and global environment and climate, and the influence of other world regions on Europe will be assessed focusing on the role of the Mediterranean.