Retrieval of the ultraviolet effective snow albedo during 1998 winter campaign in the French Alps

A measurement campaign was carried out in February 1998 at Brian?on 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.     

Case study of modeled aerosol optical properties during the SAFARI 2000 campaign

We present modeled aerosol optical properties (single scattering albedo, asymmetry parameter, and lidar ratio) in two layers with different aerosol loadings and particle sizes, observed during the Southern African Regional Science Initiative 2,000 (SAFARI 2,000) campaign. The optical properties were calculated from aerosol size distributions retrieved from aerosol layer optical thickness spectra, measured using the NASA Ames airborne tracking 14-channel sunphotometer (AATS-14) and the refractive index based on the available information on aerosol chemical composition. The study focuses on sensitivity of modeled optical properties in the 0.3-1.5 microm wavelength range to assumptions regarding the mixing scenario. We considered two models for the mixture of absorbing and nonabsorbing aerosol components commonly used to model optical properties of biomass burning aerosol: a layered sphere with absorbing core and nonabsorbing shell and the Maxwell-Garnett effective medium model. In addition, comparisons of modeled optical properties with the measurements are discussed. We also estimated the radiative effect of the difference in aerosol absorption implied by the large difference between the single scattering albedo values (approximately 0.1 at midvisible wavelengths) obtained from different measurement methods for the case with a high amount of biomass burning particles. For that purpose, the volume fraction of black carbon was varied to obtain a range of single scattering albedo values (0.81-0.91 at lambda=0.50 microm). The difference in absorption resulted in a significant difference in the instantaneous radiative forcing at the surface and the top of the atmosphere (TOA) and can result in a change of the sign of the aerosol forcing at TOA from negative to positive.     

气溶胶光学厚度的双角度多光谱反演方法研究

给出了利用角度和光谱两方面信息进行气溶胶光学厚度反演的方法,依据反演方法要求研制了机载双角度多光谱大气辐射计,并通过航空模拟实验验证了用其进行空间对地气溶胶光学厚度反演的可行性。实验结果显示,利用双角度和多光谱的方法,可以摆脱传统方法对地面反射率数据的依赖,在一般区域能够实现实时空间对地气溶胶光学厚度的反演,从而显示出了该方法在对卫里光学遥感数据校正和研究全球气溶胶对气候影响上的潜力。     

Analysis of direct solar ultraviolet irradiance measurements in the French Alps. Retrieval of turbidity and ozone column amount

Direct ultraviolet spectral solar irradiance is regularly obtained by the difference between global and diffuse irradiances at the French Alpine station of Brian?on; 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 Dobson units; comparisons with TOMS (Total Ozone Mapping Spectrometer) overpass values shows agreement within the expected uncertainties of both instruments.     

Direct spectral measurements with a Brewer spectroradiometer: absolute calibration and aerosol optical depth retrieval

We present three different methods for the absolute calibration of direct spectral irradiances measured with a Brewer spectroradiometer, which are shown to agree to within +/- 2%. Direct irradiance spectra derived by Brewer and Bentham spectroradiometers agree to within 4 +/- 3%. Good agreement was also found by a comparison of the aerosol optical depth and Angstrom exponent retrieved by the two instruments and a multifilter rotational shadowband radiometer. The spectral aerosol optical depth (300-365 nm) derived from six years of direct irradiance measurements at Thessaloniki shows a distinct seasonal variation, averaging to approximately 0.3 at 340 nm in winter and approximately 0.7 in summer.     

A differential method of determining the principal optical parameters of an atmospheric aerosol in the ultraviolet band

Based on the well-known three-wave method of measuring small gaseous components of the atmosphere and the Angström formula, a differential method of determining the optical density of the aerosol constituent of the atmosphere in the ultraviolet band is suggested.     

Retrieval of aerosol profile variations from reflected radiation in the oxygen absorption a band

Through a Fredholm integral equation of the first kind, aerosol kernel functions relate the variations in radiance measured by satellites to the variations in the aerosol extinction profile and thus permit profile retrieval from radiance measurements by inversion of the set of radiative transfer equations for various spectral intervals. Previously [Appl. Opt. 36, 1328 (1997)] the kernel functions were evaluated for the red and near-infrared spectral regions outside molecular absorption bands. Here they are computed within the oxygen A band with 20-cm(-1) spectral resolution. It is shown that, even with such a relatively low spectral resolution, the new set of kernels is able to provide better information on and improved accuracy of the retrieved profile.     

Retrieval and monitoring of aerosol optical thickness over an urban area by spaceborne and ground-based remote sensing

We used an instrumental synergy of both ground-based (sunphotometer) and spaceborne [POLDER (polarization and directionality of the Earth's reflectances) and Meteosat] passive remote-sensing devices to determine the aerosol optical thickness over the suburban area of Thessaloniki, Greece, from April 1996 to June 1997. The POLDER spaceborne instrument measures the degree of polarization of the solar radiance reflected by the Earth-atmosphere system. Aerosol optical thickness (AOT) retrieval needs an accurate estimate of the contribution of the ground surface to the top of atmosphere's polarized radiance. We tested existing surface reflectance models and fitted their parameters to find the best model for the Thessaloniki area. The model was constrained and validated by use of independent data sets of coincident sunphotometer and POLDER measurements. The comparison indicated that the urban AOT over Thessaloniki was retrieved by the POLDER instrument with an accuracy of +/-0.05. From analysis of Meteosat data we found that approximately 40% of the days with high AOT (>0.18) are associated with African dust transport events, all observed in the period March-July. Excluding dust events, the 15-month AOT averages 0.12 +/- 0.04. During the 15-month period that the study was conducted, we observed aerosol pollution peaks with an AOT of >0.24 on 15 of the 164 days on which measurements were possible.     

Multiaccess interference in a non-line-of-sight ultraviolet optical wireless sensor network

The concept of exploiting both the scattering properties and the absence of solar radiation in the "solar blind ultraviolet" spectral range for achieving a non-line-of-sight (NLOS) communication link for wireless sensor networks has been discussed in scientific literature. We address the issue of the multiaccess interference (MAI) that would be encountered in a simple and low-cost sensor network operating on the above NLOS principle, for different sensor node densities and traffic levels, and use a Poisson model for the sensor node distribution. A metric for evaluation and comparison of sensor node distribution scenarios is derived and used to discuss the performance limitations of NLOS wireless sensor networks operating in the solar blind ultraviolet spectrum. Guidelines for NLOS wireless sensor network design are outlined taking into consideration the cumulative effect of interference from distant sensor nodes, the expected number of hops, and the trade-off between node redundancy and node isolation. The significant contribution of network traffic control to system operability is demonstrated.     

Multiwavelength imaging of defects in ultraviolet optical materials

Laser-induced damage in bare glass substrates and thin films has long been widely acknowledged as a localized phenomenon associated with the presence of micrometer and submicrometer scale defects. The scanning of both optical absorption and scattering allows us to discriminate between absorbing and nonabsorbing defects and can give specific information about the origin of the defects. We investigate the spectral properties of defects in thin films and fused-silica surfaces. Absorbing and scattering defects are studied at different wavelengths in the ultraviolet, visible, and infrared ranges. Absorbing defects are shown to be highly wavelength dependent, whereas we have observed significant correlation between scattering defects.     

Retrieval of aerosol refractive index from extinction spectra with a damped harmonic-oscillator band model

A new method for the retrieval of the spectral refractive indices of micrometer-sized particles from infrared aerosol extinction spectra has been developed. With this method we use a classical damped harmonic-oscillator model of molecular absorption in conjunction with Mie scattering to model extinction spectra, which we then fit to the measurements using a numerical optimal estimation algorithm. The main advantage of this method over the more traditional Kramers-Kronig approach is that it allows the full complex refractive-index spectra, along with the parameters of the particle size distribution, to be retrieved from a single extinction spectrum. The retrieval scheme has been extensively characterized and has been found to provide refractive indices with a maximum uncertainty of approximately 10% (with a minimum of approximately 0.1%). Comparison of refractive indices calculated from measurements of a ternary solution of HNO3, H2SO4, and H2O with those published in J. Phys. Chem. A 104, 783 (2000) show similar differences as found by other authors.     

Development of a backscattering type ultraviolet apertureless near-field scanning optical microscope

A backscattering type ultraviolet apertureless near-field scanning optical microscope (ANSOM) for the correlated measurement of topographical and optical characteristics of photonic materials with high optical resolution was developed. The near-field Rayleigh scattering image of GaN covered with periodic submicron Cr dots showed that optical resolution around 40 nm was achievable. By measuring the tip scattered photoluminescence of InGaN/GaN multi quantum wells, the applicability of the developed microscope for high resolution fluorescence measurement was also demonstrated.     

Analysis of an optical depth converter used in a three-dimensional integral imaging system

An optical depth converter that uses a lens array pair is analyzed theoretically and experimentally. We present a theory of depth conversion and explain the effects of the system parameters in the optical depth converter by using wave-optical analysis. Ray-optical analysis is applied to the investigation of the tendencies of the system parameter effects. We also show that the optical depth converter can be used for the three-dimensional screen in projection-type integral imaging systems.     

Theoretical analysis of optical characteristics of the alpha spodumene in ultraviolet region

It is presented an analysis of some important optical characteristics of the natural spodumene crystal, based on the first-principles calculations of its electronic structure and complex dielectric tensor. The optical absorption spectrum is interpreted in terms of electronic band structure for incident radiation energy up to 35 eV. The orientation of the three orthogonal principal optical axes is determined relative to the crystallographic axes, and expressed as function of the incident radiation wavelength. Reflectivity and electron energy loss are calculated along the three principal axes. All calculated optical properties are found to be highly anisotropic.     

Influence of depth of intermediate layer on optical power distribution in W-type optical fibers

For different depth and width of the intermediate layer, a power flow equation is used to calculate spatial transients and steady state of power distribution in W-type optical fibers (doubly clad fibers with three layers). A numerical solution has been obtained by the explicit finite difference method. Results show how the power distribution in W-type optical fibers varies with the depth of the intermediate layer for different values of intermediate layer width and coupling strength. We have found that with increasing depth of the intermediate layer, the fiber length at which the steady-state distribution is achieved increases. Such characterization of these fibers is consistent with their manifested effectiveness in reducing modal dispersion and improving bandwidth.     

Nephelometer optical response model for the interpretation of atmospheric aerosol measurements

A numerical model evaluating the response of a typical integrating nephelometer is described. The model incorporates the actual scattering geometry as well as the effects of a finite light source, detector size, and a nonideal Lambertian diffuser. An angular scattering weighting function is introduced to provide a tractable approach in numerical calculations and easy application. Using established size distribution ensembles associated with a few representative aerosol types, we compare the calculated response of a real nephelometer with that of an ideal, or perfect, nephelometer. The results indicate that, frequently, the nephelometer-produced aerosol-scattering coefficient is of the order of 10-20% too small; but for some naturally occurring aerosols, the difference may be as large as 40-50%. For a multiple-wavelength nephelometer, the response model can be employed to estimate the expected error in the aerosol-scattering coefficients directly from the measurements themselves.     

Equivalence of pyrheliometric and monochromatic aerosol optical depths at a single key wavelength

The atmospheric aerosol optical depth (AOD) weighted over the solar spectrum is equal to the monochromatic AOD at a certain wavelength. This key wavelength is ~0.7 mum, which is only slightly influenced by air mass and aerosol content. On the basis of this result, simple relations are proposed to predict monochromatic AOD from pyrheliometric data and vice versa. The accuracy achieved is close to ?0.01 units of AOD at ~0.7 mum, estimated from simultaneous sunphotometer data. The precision required for the estimation of the precipitable water-vapor content is approximately ?0.5 cm.     

Sensitivity of surface reflectance retrieval to uncertainties in aerosol optical properties

We formulate a procedure to investigate the sensitivity of surface reflectances retrieved from satellite sensor data to uncertainties in aerosol optical properties. Aerosol optical characteristics encompassed in the study include the aerosol optical depth, the Junge parameter (i.e., spectral dependence), and the imaginary part of the refractive index (i.e., aerosol absorption). The study includes both clear and hazy atmospheric conditions, wavelengths of 0.550 and 0.870 μm, three solar zenith angles, and five viewing geometries. Key results are presented graphically in terms of accuracy requirements on the aerosol property under consideration for a 5% uncertainty in predicted surface reflectance.     

稀疏植被地区气溶胶光学厚度反演

研究了稀疏植被地区地表反射率的精确确定方法,构建了精确确定稀疏植被地区地表反射率的地面光谱模型,实现了该类地区气溶胶光学厚度的卫星数据反演.分析表明,所构模型的地表反射率确定误差在0.015以内.通过对气溶胶光学厚度反演的不确定性分析,确定稀疏植被地区的气溶胶光学厚度的卫星反演误差在0.15以内,并使用MODIS数据反演了北京市区及周围地区的气溶胶光学厚度.用太阳分光光度计测量的气溶胶光学厚度对MODIS数据的气溶胶光学厚度反演结果进行了验证.