Value Of Ω0 Research Articles

Peculiarities of ionic transport in LLTO solid electrolytes

AbstractThe Li3xLa2/3–xTiO3 (LLTO) (x = 0.1 Li+) ionic conductor was investigated by microwave spectrometer set‐up in the 106–109 Hz frequency range and by time‐domain transmission terahertz spectrometer in the 2·1011–3·1012 Hz frequency range. A relaxation dispersion of $ \tilde \sigma $ and $ \tilde \epsilon $ of LLTO ceramics was found in the microwave region and was attributed to the Li+ relaxation in bulk. The resonance dispersion of $ \tilde \sigma $ and $ \tilde \epsilon $ was found in the terahertz region with resonance frequency ω0. The value of ω0 correlates well with the value of attempt frequency f0, found from the Arrhenius plot of the relaxation frequency fR. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Wavelength dependence of dust aerosol single scattering albedo as observed by the Compact Reconnaissance Imaging Spectrometer

Observations by the Compact Reconnaissance Imaging Spectrometer (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) over the range 440–2920 nm of the very dusty Martian atmosphere of the 2007 planet‐encircling dust event are combined with those made by both Mars Exploration Rovers (MERs) to better characterize the single scattering albedo (ω0) of Martian dust aerosols. Using the diagnostic geometry of the CRISM emission phase function (EPF) sequences and the “ground truth” connection provided at both MER locations allows one to more effectively isolate the single scattering albedo (ω0). This approach eliminates a significant portion of the type of uncertainty involved in many of the earlier radiative transfer analyses. Furthermore, the use of a “first principles” or microphysical representation of the aerosol scattering properties offers a direct path to produce a set of complex refractive indices (m = n + ik) that are consistent with the retrieved ω0 values. We consider a family of effective particle radii: 1.2, 1.4, 1.6, and 1.8 μm. The resulting set of model data comparisons, ω0, and m are presented along with an assessment of potential sources of error and uncertainty. We discuss our results within the context of previous work, including the apparent dichotomy of the literature values: “dark” (solar band ω0 = 0.89–0.90) and “bright” (solar band ω0 = 0.92–0.94). Previous work suggests that a mean radius of 1.8 μm is representative for the conditions sampled by the CRISM observations. Using the m for this case and a smaller effective particle radius more appropriate for diffuse dust conditions (1.4 μm), we examine EPF‐derived optical depths relative to the MER 880 nm optical depths. Finally, we explore the potential impact of the resulting brighter solar band ω0 of 0.94 to atmospheric temperatures in the planetary boundary layer.

Depinning frequency in a heavily neutron-irradiated MgB2 sample

The magnetic-field-induced variations of the microwave surface resistance have been investigated in a heavily neutron-irradiated MgB2 sample, in which the irradiation has caused the merging of the two gaps into a single value. The experimental results have been analyzed in the framework of the Coffey and Clem model. By fitting the experimental data, we have determined the field dependence of the depinning frequency, ω0, at different values of the temperature. Although the pinning is not particularly effective, the value of ω0 obtained at low temperatures is considerably higher than that observed in conventional low-temperature superconductors.

DYNAMIC LIGHT SCATTERING STUDY OF REVERSE MICELLAR SYSTEMS FOR THE SYNTHESIS OF IRON-BASED NANOFLUIDS

Iron nanoparticles dispersed in hydraulic oil were prepared by mixing two microemulsions containing iron (II) sulfate and sodium borohydride at a temperature of 60°C. Six values of ω 0 = [water]/[surfactant] were used, namely 5, 10, 15, 20, 25, and 30. Dynamic light scattering measurements of the hydrodynamic radius of the reverse micelles showed that the average size, surfactant shell thickness and water core radius, increased with ω0. The micelle size distribution for ω0 values of 5, 10, and 15, was in the nanometer regime, while for ω0 values of 20, 25, and 30 it was in the micrometer regime. Scanning electron microscopy showed that the nanoparticle diameters were around 30 nm for the powders prepared using an ω 0 = 10. In addition, a comparison between the particle size distribution of the iron nanoparticles dispersed in isopropyl alcohol and the iron nanoparticles in the reverse micellar solution was made. It was shown that once the particles are cleaned and separated from the micellar solution, they agglomerate into particles that are about 1 μm in size.

Aerosol Optical Properties and Particle Size Distributions on the East Coast of the United States Derived from Airborne In Situ and Remote Sensing Measurements

Abstract Airborne in situ measurements of vertical profiles and horizontal transects of aerosol optical and physical properties, obtained during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) field campaign off the East Coast of the United States during the summer of 2001, are presented. Most of the measurements were obtained in relatively clean air dominated by airflows that had passed over Canada and the northern Atlantic Ocean. Results from the 17 July and 2 August 2001 flights are presented; on these days, the aerosol loading was relatively high. In the lower troposphere, ω0 values at a wavelength of 550 nm were consistently above 0.93 throughout the field experiment, indicating the dominance of weakly absorbing aerosol. Particle number size distributions are presented and discussed for transects at altitudes ∼0.05–3.5 km above mean sea level. Particles with diameters (Dp) <0.1 μm made up the majority of the aerosol number. Accumulation mode particles dominate the number size, surface area, and volume distributions. The variability of optical and physical aerosol parameters was analyzed on horizontal scales of ∼1–4 km. There was little horizontal variability in the single-scattering albedo (ω0), aerosol optical depth (AOD), and accumulation mode size, but greater variability in particle number concentration. Comparisons of the airborne measurements with remotely sensed aerosol parameters, such as ω0 and effective particle radius (reff), derived from the Multiangle Imaging Spectroradiometer (MISR) aboard the Terra satellite were generally in good agreement. Overall, the MISR retrievals captured both the similarities and the differences between the properties of the aerosols measured on 17 July and 2 August.

Aerosol Properties and Chemical Apportionment of Aerosol Optical Depth at Locations off the U.S. East Coast in July and August 2001

Abstract Airborne in situ measurements of vertical profiles of the aerosol light scattering coefficient, light absorption coefficient, and single scattering albedo (ω0) are presented for locations off the East Coast of the United States in July–August 2001. The profiles were obtained in relatively clean air, dominated by airflows that had passed over Canada and the Atlantic Ocean. Comparisons of aerosol optical depths (AODs) at 550 nm derived from airborne in situ and sun-photometer measurements agree, on average, to within 0.034 ± 0.021. A frequency distribution of ω0 measured in the atmospheric boundary layer off the coast yields an average value of ω0 = 0.96 ± 0.03 at 550 nm. Values for the mass scattering efficiencies of sulfate and total carbon (organic and black carbon) derived from a multiple linear regression are 6.0 ± 1.0 m2 (g SO=4)−1 and 2.6 ± 0.9 m2 (g C)−1, respectively. Measurements of sulfate and total carbon mass concentrations are used to estimate the contributions of these two major components of the submicron aerosol to the AOD. Mean percentage contributions to the AOD from sulfate, total carbon, condensed water, and absorbing aerosols are 38% ± 8%, 26% ± 9%, 32% ± 9%, and 4% ± 2%, respectively. The sensitivity of the above results to the assumed values of the hygroscopic growth factors for the particles are examined and it is found that, although the AOD derived from the in situ measurements can vary by as much as 20%, the average value of ω0 is not changed significantly. The results are compared with those obtained in the same region in 1996 under more polluted conditions.

The Large Scale Structure Peak as a Comoving Standard Ruler

Estimates of the curvature parameters Ω0 (density parameter) and Δ0 (cosmological constant) can be made geometrically by use of either a standard candle or a standard ruler. Just as supernovae of Type Ia appear to provide a good empirical standard candle, it now appears observationally justified to use the peak in the power spectrum of density perturbations at L ≍ 130±10h-1 Mpc as an empirical standard rod. It will be shown that voids of this size are traced by quasars in a homogeneous catalogue near the South Galactic Pole at z ˜ 2 and that the large scale structure peak of the catalogue constrains the value of Ω0 to 0.1 < Ω0 < 0.45 (68% confidence), independently of Δ0. Combination with the supernovae Ia data is sufficient to show that the observable Universe is almost flat. In other words, the combination of a standard ruler and a standard candle detected in two presently available data sets is sufficient to show that the Universe is nearly flat, independently of any microwave background data or any other data analyses.

Ω0 Concordance

The determinations of the mass density parameter Ω0 are examined with a particular emphasis given to the new cosmic microwave background (CMB) experiments. It is shown that the Ω0 and the Hubble constant H0 from CMB are quite consistent with those from other observations with the aid of the hierarchical structure formation models based on cold dark matter dominance with the cosmological constant that makes the universe flat. The concordance value of Ω0 is 0.25-0.45.

Self-Aggregation of Lipophilic Porphyrins in Reverse Micelles of Aerosol OT

The aggregation process of meso-tetra(4-aminosulfonylphenyl)porphyrin (PC0), of butyl and dodecyl derivatives (PC4 and PC12, respectively) in the presence of aerosol-OT (AOT) reversed micelles with different values of ω0 = [water]/[AOT] was investigated using steady-state extinction, fluorescence, and resonance light scattering (RLS) techniques. The scattering has been accounted for and the absorption spectra corrected from the extinction spectra. In homogeneous solutions, the porphyrins exist as a monomer showing the Soret band around 420 nm and emission bands at 650 and 716 nm. In reverse micelles, the spectra vary according to the chain length and the amount of solubilized water. Only PC12 is dissolved as a monomer at ω0 = 0, and PC4 shows some degree of aggregation. PC0 is present largely as an aggregate. At ω0 ≠ 0, a complete aggregation is observed for PC4 and PC12, whereas in PC0, some degree of disaggregation was detected. The extinction spectra of PC0 and PC4 in aggregate forms show a broad and red shifted Soret band indicating J aggregates. By contrast, the Soret band intensity of PC12 decreases and a new band around 376 nm appears with a broadening of the monomer Soret band which reflects H aggregates. RLS shows a strong signal for PC12 at red spectral region at high ω0 indicating J aggregate types. The fluorescence spectra showed the new band around 670 nm for PC0 and PC12, whereas in PC4, only blue shifted bands were observed with ω0 increase. The global fluorescence quantum yield of the PC12 aggregates is lower than the respective monomer. The aggregate sizes were followed by RLS measurements which showed an increase of intensity at around 440 nm. The aggregation dynamics is dependent on both the porphyrin concentration and ω0.

What do very nearly flat detectable cosmic topologies look like?

Recent studies of the detectability of the cosmic topology of nearly flat universes have often concentrated on the range of values of Ω0 given by current observations. Here we study the consequences of taking a range of bounds satisfying |Ω0 − 1| ≪ 1, which include those expected from future observations such as the Planck mission, as well as those predicted by inflationary models. We show that in this limit, a generic detectable non-flat manifold is locally indistinguishable from either a cylindrical or toroidal manifold, irrespective of its global shape, with the former being more likely. Importantly, this is compatible with some recent indications of the alignment of the quadrupole and octupole moments, based on the analysis of the first year WMAP data. It also implies that in this limit an observer would not be able to distinguish topologically whether the universe is spherical, hyperbolic or flat. By severely restricting the expected topological signatures of detectable isometries, our results provide an effective theoretical framework for interpreting cosmological observations, and can be used to confine the parameter spaces which realistic search strategies, such as the ‘circles in the sky’ method, need to concentrate on.

Retrieving the aerosol single-scattering albedo from the NO2 photolysis rate coefficient

A method for retrieving the aerosol single-scattering albedo ω0 in the ultraviolet region close to 400 nm within the atmospheric column is proposed based on a comparison between both NO2 photolysis rate coefficients JNO2 measured by an optical actinometer under cloud-free conditions and JNO2 calculated using a radiative transfer model. This methodology is based on the sensitivity of JNO2 to ω0 variations as quantified using the radiative transfer model with particular emphasis on the aerosol extinction vertical profile. The method has been applied to assess ω0 within the framework of the Indian Ocean Experiment (INDOEX) over the Goa area located on the west coast of India. In the first place, the period under study was between March 2 and March 13, 1999 when ground-based lidar measurements were available. It has been further extended between February 25 and March 23, 1999 by making appropriate assumptions on the mean altitude of the aerosol layer. Taking into account the uncertainties related to each parameter introduced in the calculation (optical thickness, Ångstróm coefficient, asymmetry factor, surface albedo) and the model used, resulted in a total absolute accuracy on ω0 calculated to be 0.06 for observations close to local noon, and up to 0.07 for observations performed within 3 h before and after local noon. Results are less accurate for other times. For the 12 days under study, the values of ω0 calculated at noon range from 0.89 to 0.93. They vary weakly throughout the day with further differences being observed from one day to another. Such small variations in ω0 are in very good agreement with results obtained by other methods within the framework of the same campaign.

Vertical profiles of light scattering, light absorption, and single scattering albedo during the dry, biomass burning season in southern Africa and comparisons of in situ and remote sensing measurements of aerosol optical depths

Airborne in situ measurements of vertical profiles of aerosol light scattering, light absorption, and single scattering albedo (ω0) are presented for a number of locations in southern Africa during the dry, biomass burning season. Features of the profiles include haze layers, clean air slots, and marked decreases in light scattering in passing from the boundary layer into the free troposphere. Frequency distributions of ω0 reflect the strong influence of smoke from biomass burning. For example, during a period when heavy smoke was advected into the region from the north, the mean value of ω0 in the boundary layer was 0.81 ± 0.02 compared to 0.89 ± 0.03 prior to this intrusion. Comparisons of layer aerosol optical depths derived from the in situ measurements with those measured by a Sun photometer aboard the aircraft show excellent agreement.

MODIS observation of aerosols and estimation of aerosol radiative forcing over southern Africa during SAFARI 2000

MODIS provides almost complete global coverage daily. Aerosol optical thickness (AOT or τaλ) and other aerosol parameters are retrieved over land and ocean at a spatial scale of 10 km (level 2), then aggregated to a global grid of 1° spatial resolution on daily, weekly, and monthly time scales (level 3). The SAFARI 2000 ground‐based measurements of AOT from the Aerosol Robotic Network (AERONET) group of Sun photometers were used to compare MODIS over‐land AOT based on a spatiotemporal statistical technique. At low aerosol loading (when τa470 < 0.6), MODIS τaλ values agree with those of AERONET at the blue wavelength (λ = 440, 470 nm). But, at higher wavelengths and with increasing aerosol loading, MODIS AOT values underestimate those of AERONET. There are also regional variations in validation accuracy. This AOT underestimation by MODIS during SAFARI 2000 is attributed to the application of a constant single‐scattering‐albedo (SSA or ω0) value of 0.90 globally for smoke aerosol retrieval. Recent studies based on long‐term observations with AERONET Sun photometers suggest that lower ω0 values of 0.88 and 0.84 at 440 nm and 670 nm wavelengths respectively would be more applicable over southern Africa (particularly over Zambia, where most of the measurements were based). A column climate model, with MODIS aerosol information as input, is used to calculate aerosol radiative forcing over the (southern Atlantic) Ocean part of the SAFARI 2000 region. For September 2000, the results show a forcing of −10 W/m2 at the top of the atmosphere and approximately −26 W/m2 at the terrestrial surface.

The Effect of Merger Boosts on the Luminosity, Temperature, and Inferred Mass Functions of Clusters of Galaxies

In the standard cold dark matter model of structure formation, massive clusters form via the merger of smaller clusters. N-body/hydrodynamical simulations of merging galaxy clusters have shown that mergers can temporarily boost the X-ray luminosity and temperature of the merged cluster above the equilibrium values for the merged system. The cumulative effect of these "merger boosts" will affect the observed X-ray luminosity functions (XLFs) and temperature functions (TFs) of clusters. One expects this effect to be most important for the most luminous and hottest clusters. XLFs and TFs of clusters provide some of the strongest constraints on cosmological and large-scale structure parameters, such as the mean fluctuation parameter σ8 and the matter density divided by the critical density Ω0. Merger boosts may bias the values of σ8 and Ω0 inferred from cluster XLFs and TFs if virial equilibrium is assumed. We use a semianalytic technique to estimate the effect of merger boosts on the XLFs and TFs. The boosts from individual mergers are derived from N-body/hydrodynamical simulations of mergers. The statistics of the merger histories of clusters are determined from extended Press-Schechter (PS) merger trees. We find that merger boosts can increase the apparent number of hot, luminous clusters. For example, in a universe with Ω0 = 0.3 and ΩΛ = 0.7 at a redshift of z = 1, the number of clusters with temperatures T > 10 keV is increased by a factor of 9.5, and the number of clusters with luminosities LX > 5 × 1044 h-2 ergs s-1 is increased by a factor of 8.9. We have used our merger-boosted TFs and XLFs to derive the cosmological structure parameters σ8 and Ω0 by fitting PS equilibrium relations to local (z = 0) and distant (either z = 0.5 or z = 1) cluster samples. Merger boosts cause σ8 to be overestimated by about 20%. The matter density parameter Ω0 may be underestimated by about 20%, although this result is less clear. If the parameters of the fluctuation spectrum are derived from the observed TF or XLF (e.g., from a low-redshift sample), then this removes most of the boost effect on Ω0. However, larger merger boost effects may appear when cluster XLFs and TFs are compared to cosmological structure parameters derived by other techniques (e.g., cosmic microwave background fluctuations or the brightness of distant supernovae).

Composition fluctuations in a non-critical binary polymer blend studied by ultrasonic and light scattering experiments

A non-critical mixture of polyethylene glycol (PEG600, M = 600 g mol−1) and polypropylene glycol (PPG1000, M = 1000 g mol−1) was investigated by ultrasonic and light scattering experiments in the one-phase region. The mass fraction of polypropylene glycol in the non-critical mixture was yPPG = 0.365. The explored temperature and frequency range of the ultrasonic experiment was 0.1 ≤ T − TP ≤ 17.3 K and 0.4 MHz ≤ f ≤ 30 MHz (TP: phase separation temperature). The frequency dependence of the ultrasonic attenuation of the non-critical mixture shows a relaxation behaviour typical for composition fluctuations. The data can be analysed by the dynamic scaling theory of Bhattacharjee and Ferrell which was formally extended to the non-critical case using the concept of a pseudospinodal temperature. The characteristic time scale of the concentration fluctuations is described by a frequency ωD = 2D/ξ2 where D is the mutual diffusion coefficient and ξ is the correlation length. In the frame of the pseudospinodal concept the temperature dependence of the characteristic frequency is expressed by ωD = ω0εzν with ε = (T − TPS)/TPS (TPS: pseudospinodal temperature, ε: reduced temperature, ω0: critical amplitude, zν: critical exponent). The temperature dependence of the frequency ωD was determined by the ultrasonic spectra. From this data, using mean field exponents, a value of ω0 = 9.4 MHz was estimated which is comparable to that of the critical mixture. The description of the ultrasonic data with mean field exponents is justified by the results of dynamic light scattering.

Light Propagation in Inhomogeneous Universes. III. Distributions of Image Separations

Using an analytical model, we compute the distribution of image separations resulting from gravitational lensing of distant sources, for seven COBE-normalized cold dark matter models with various combinations of Ω0 and λ0. Our model assumes that multiple imaging results from strong lensing by individual galaxies. We model galaxies as nonsingular isothermal spheres, whose parameters are functions of the luminosity and morphological type, and take into account the finite angular size of the sources. Our model neglects the contribution of the background matter distribution to lensing and assumes that lensing is entirely caused by galaxies. To test the validity of this assumption, we performed a series of ray-tracing experiments to study the effect of the background matter on the distribution of image separations. Our results are the following: (1) The presence of the background matter tends to increase the image separations produced by lensing galaxies, making the distributions of image separations wider. However, this effect is rather small and independent of the cosmological model. (2) Simulations with galaxies and background matter often produce a secondary peak in the distribution of image separations at large separations. This peak does not appear when the background matter is excluded from the simulations. (3) The effect of the background matter on the magnification distribution is negligible in low-density universes (Ω0 = 0.2) with small density contrast (σ8 = 0.4), but becomes very important as Ω0 and σ8 increase, resulting in a significant widening of the distribution. (4) Multiple imaging is caused primarily by early-type galaxies (elliptical and S0), with a negligible contribution from spiral galaxies. (5) Our analytical model, which has only two free parameters, is in good agreement with the results of ray-tracing experiments, successfully reproducing the distributions of image separations, and also the multiple-imaging probability, for all cosmological models considered. (6) The analytical model predicts that the distributions of image separations are virtually indistinguishable for flat, cosmological-constant models with different values of Ω0. (7) For models with no cosmological constant, the distributions of image separations do depend upon Ω0, but this dependence is weak. We conclude that while the number of multiple-imaged sources can put strong constraints on the cosmological parameters, the distribution of image separations does not constrain the cosmological models in any significant way and mostly provides constraints on the structure of the galaxies responsible for lensing.

Galactosynthesis: halo histories, star formation and discs

We investigate the effects of a variety of ingredients that must enter into a realistic model for disc galaxy formation, focusing primarily on the Tully–Fisher (TF) relation and its scatter in several wavebands. In particular, we employ analytic distributions for halo formation redshifts and halo spins, empirical star formation rates and initial mass functions, realistic stellar populations, and chemical evolution of the gas. Our main findings are as follows. (a) The slope, normalization and scatter of the TF relation across various wavebands are determined largely by the parent halo properties as dictated by the initial conditions, but are also influenced by star formation in the disc. (b) TF scatter in this model is due primarily to the spread in formation redshifts. The scatter can be measurably reduced by chemical evolution, and also by the weak anticorrelation between peak height and spin. (c) Multiwavelength constraints can be important in distinguishing between models that appear to fit the TF relation in I or K. (d) Assuming passive disc evolution, successful models seem to require that the bulk of disc formation cannot occur too early (z>2–3) or too late (z<0.2), and are inconsistent with high values of Ω0. (e) A simple, realistic model with the above ingredients, and fewer free parameters than typical semi-analytic models, can reasonably reproduce the observed z=0 TF relation in all bands (B, R, I and K), as well as the observed B-band surface brightness–magnitude relation. In such a model, the near-infrared TF relation at z=1 is similar to that at z=0, while bluer bands show a markedly steeper TF slope at high redshift, consistent with limited current data. The remarkable agreement with observations suggests that the amount of gas that is expelled or poured into a disc galaxy may be small (though small fluctuations might serve to align B-band predictions better with observations), and that the specific angular momentum of the baryons should roughly equal that of the halo; there is little room for angular momentum transfer. In Appendix A we present analytic fits to stellar population synthesis models.

On galaxy cluster sizes and temperatures

We show that the distribution of the sizes and temperatures of clusters can be used to constrain cosmological models. The size–temperature (ST) distribution predicted in a flat Gaussian cluster-abundance-normalized model agrees well with the fairly tight ST relation observed. A larger power-spectrum amplitude σ8 would give rise to a larger scatter about the ST relation as would a larger value of Ω0 and/or long non-Gaussian high-density tails in the probability density function. For Gaussian initial conditions, the ST distribution suggests a constraint The ST relation is expected to get tighter at high redshifts. In the process, we derive a simple formula for the halo formation–redshift distribution for non-Gaussian models. We also suggest that the discrepancy between the naive zero-redshift ST relation and that observed may be owing, at least in part, to the fact that lower-mass clusters form over a wider range of redshifts. An Appendix derives an equation for the formation–redshift distribution of haloes.

A Serendipitous Galaxy Cluster Survey withXMM: Expected Catalog Properties and Scientific Applications

This paper describes a serendipitous galaxy cluster survey that we plan to conduct with the XMM X-ray satellite. We have modeled the expected properties of such a survey for three different cosmological models, using an extended Press-Schechter formalism combined with a detailed characterization of the expected capabilities of the European Photon Imaging Camera (EPIC) camera on board XMM. We estimate that, over the 10 yr design lifetime of XMM, the EPIC camera will image a total of ≃800 deg2 in fields suitable for the serendipitous detection of clusters of galaxies. For the presently favored low-density model with a cosmological constant, our simulations predict that this survey area would yield a catalog of more than 8000 clusters, ranging from poor to very rich systems, with around 750 detections above z = 1. A low-density open universe yields similar numbers, though with a different redshift distribution, while a critical-density universe gives considerably fewer clusters. This dependence of catalog properties on cosmology means that the proposed survey will place strong constraints on the values of Ω0 and ΩΛ. The survey would also facilitate a variety of follow-up projects, including the quantification of evolution in the cluster X-ray luminosity-temperature relation, the study of high-redshift galaxies via gravitational lensing, follow-up observations of the Sunyaev-Zeldovich effect, and foreground analyses of cosmic microwave background maps.

Characterization of the optical properties of biomass burning aerosols in Zambia during the 1997 ZIBBEE field campaign

The physical and optical properties of biomass burning aerosols in a savanna region in south central Africa (Zambia) were analyzed from measurements made during the Zambian International Biomass Burning Emissions Experiment (ZIBBEE) during August‐September 1997. Due to the large spatial extent of African savannas and the high frequency of occurrence of burning in the annual dry seasons, characterization of the optical properties of the resultant biomass burning aerosols is important for the study of atmospheric radiative processes and for remote sensing of both surface and atmospheric properties in these regions. Aerosol Robotic Network Sun‐sky radiometer spectral measurements of direct Sun observations and directional sky radiances were utilized to infer spectral aerosol optical depths (τa), aerosol size distributions, and single‐scattering albedos. During the primary ZIBBEE study period, which coincided with the peak period of biomass burning in the region, there was a high correlation between the measured τaand the total column water vapor or precipitable water vapor (PWV), suggesting transport of smoke aerosol from regions with higher PWV. Size distribution retrievals of the biomass burning smoke show that the accumulation mode dominated and a comparison with smoke from Amazonia (Bolivia) shows a shift toward smaller particles for African savanna smoke. This may be the result of differences in mode of combustion (flaming versus smoldering), fuel type and moisture content, and the aging processes of the aerosol. The single‐scattering albedo (ω0) of the aerosols were retrieved using several approaches, yielding average values of ω0at ∼550 nm during ZIBBEE varying from ∼0.82 to ∼0.85, thus showing good agreement within the retrieval uncertainty of ∼0.03 of these methods. In general, ω0was relatively constant as a function of aerosol loading, with very little change occurring for τaat 440 nm ranging from 0.7 to 1.7. African savanna smoke exhibits significantly higher absorption than smoke from Amazonian forested regions and also a greater rate of decrease of ω0with increasing wavelength. Variations in the spectral change of the Angström wavelength exponent were also investigated with respect to the degree of aerosol absorption and changes in the accumulation mode size distributions.