Light absorption from particulate impurities in snow and ice determined by spectrophotometric analysis of filters

Light absorption by particulate impurities in snow and ice can affect the surface albedo and is important for the climate. The absorption properties of these particles can be determined by collecting and melting snow samples and extracting the particulate material by filtration of the meltwater. This paper describes the optical design and testing of a new instrument to measure the absorption spectrum from 400 to 750 nm wavelength of the particles collected on filters using an "integrating-sandwich" configuration. The measured absorption is shown to be unaffected by scattering of light from the deposited particulates. A set of calibration standards is used to derive an upper limit for the concentration of black carbon (BC) in the snow. The wavelength dependence of the absorption spectra from 450 to 600 nm is used to calculate an absorption ?ngstrom exponent for the aerosol. This exponent is used to estimate the actual BC concentration in the snow samples as well as the relative contributions of BC and non-BC constituents to the absorption of solar radiation integrated over the wavelength band 300 to 750 nm.     

Propagation of elastic vibrations in snow

At rather low temperatures snow is characterized by three velocities of sound, two of which are related to propagation of longitudinal and transverse waves through a solid skeleton formed by ice crystals and the third of which is related to propagation of longitudinal waves through air in snow pores. The main laws governing propagation and absorption of these waves are determined. Analytical formulas that express the dependence of the coefficient of attenuation of waves on frequency are obtained.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 77, No. 6, pp. 124–130, November–December, 2004.     

Temperature Dependence of Absorption in Ice at 532 nm

A YAG laser was used to emit nanosecond pulses of light at 532 nm at depths from 1185 to 2200 m in Antarctic ice, corresponding to temperatures increasing from 229 to 249 K. From the timing distributions of photons arriving at phototubes at distances up to 100 m and at similar depths, the scattering and absorption coefficients were measured, and the temperature dependence of absorptivity at 532 nm was determined. Despite the absorptivity being many orders of magnitude lower at 532 nm than in the near ultraviolet and near infrared, the fractional increase of absorptivity, a(-1)da/dT = 0.01 K(-1), was the same in the visible, ultraviolet, and infrared. Analysis of published data at other wavelengths shows that a(-1)da/dT is ~0.01 K(-1) from 175 nm to ~1 cm, above which it increases strongly from 1 cm to 10 m. That temperature dependence applies only in regions not close to absorption bands.     

Radiative properties of snow for clear sky solar radiation

Spectral and integrated radiative properties (reflection, transmission, and the rate of heating) of finegrained wind-packed snow typical of subpolar regions are studied through a model taking into account surface reflection and volumetric multiple scattering. The surface reflection is modeled by a bidirectional reflectance distribution function applicable to powdered dielectric material. For the volumetric multiple scattering, the radiative transfer equation designed for strongly asymmetric scattering is solved. All multiple scattering parameters (single scattering albedo, various moments of the scattering phase function, and optical depth) are related to measurable physical characteristics (density, grain size, and the absorption coefficient of pure ice).Parameterized atmospheric spectral transmission coefficients for scattering and absorption by aerosols and gases are used to obtain the direct and diffuse components of solar flux, incident on the snow-cover. Calculated values of spectral and integrated visible and near infrared reflection and flux attenuation coefficients of snow are compared with observations. The rate of radiative heating at different depths within the snowcover is calculated from the net flux divergence. It is shown that the conventional method of calculating this rate using measured bulk extinction coefficients grossly underestimates the amount of heating within the top few millimeters. This study provides a better overall understanding of the radiative properties of snow under clear sky conditions in terms of the physical characteristics of the snowcover.     

Optical properties of deep ice at the South Pole: absorption

We discuss recent measurements of the wavelength-dependent absorption coefficients in deep South Pole ice. The method uses transit-time distributions of pulses from a variable-frequency laser sent between emitters and receivers embedded in the ice. At depths of 800-1000 m scattering is dominated by residual air bubbles, whereas absorption occurs both in ice itself and in insoluble impurities. The absorption coefficient increases approximately exponentially with wavelength in the measured interval 410-610 nm. At the shortest wavelength our value is approximately a factor 20 below previous values obtained for laboratory ice and lake ice; with increasing wavelength the discrepancy with previous measurements decreases. At ~415 to ~500 nm the experimental uncertainties are small enough for us to resolve an extrinsic contribution to absorption in ice: submicrometer dust particles contribute by an amount that increases with depth and corresponds well with the expected increase seen near the Last Glacial Maximum in Vostok and Dome C ice cores. The laser pulse method allows remote mapping of gross structure in dust concentration as a function of depth in glacial ice.     

Mass balance of the Antarctic ice sheet

The Antarctic contribution to sea-level rise has long been uncertain. While regional variability in ice dynamics has been revealed, a picture of mass changes throughout the continental ice sheet is lacking. Here, we use satellite radar altimetry to measure the elevation change of 72% of the grounded ice sheet during the period 1992-2003. Depending on the density of the snow giving rise to the observed elevation fluctuations, the ice sheet mass trend falls in the range -5-+85Gtyr-1. We find that data from climate model reanalyses are not able to characterise the contemporary snowfall fluctuation with useful accuracy and our best estimate of the overall mass trend-growth of 27+/-29Gtyr-1-is based on an assessment of the expected snowfall variability. Mass gains from accumulating snow, particularly on the Antarctic Peninsula and within East Antarctica, exceed the ice dynamic mass loss from West Antarctica. The result exacerbates the difficulty of explaining twentieth century sea-level rise.     

On the application of Biot's theory to acoustic wave propagation in snow

A fluid-saturated, elastic, porous media model is used to describe acoustic wave propagation in snow. This model predicts the existence of two dilatational waves and a shear wave. In homogeneous, isotropic snow the two dilatational waves are uncoupled from one another but involve coupled motion between the interstitial air and ice skeleton. Dilatational waves of the first kind and shear waves are slightly dispersive and attenuated with distance. Dilatational waves of the second kind are strongly dispersive and highly attenuated. The model also predicts that the wave impedance for snow is close to that of air and that snow strongly absorbs acoustic wave energy.Available experimental phase velocity, impedance and attenuation data support the calculated results. Phase velocity measurements indicate three identifiable categories: fast dilatational waves (phase velocity ? 500 m/s), slow dilatational waves (phase velocity < 500 m/s) and shear waves. Wave impedance and attenuation measurements illustrate the low impedance, highly absorbing characteristics of snow. Additional impedance, attenuation and phase velocity data are required to further test and improve the model.     

Changes in ice dynamics and mass balance of the Antarctic ice sheet

The concept that the Antarctic ice sheet changes with eternal slowness has been challenged by recent observations from satellites. Pronounced regional warming in the Antarctic Peninsula triggered ice shelf collapse, which led to a 10-fold increase in glacier flow and rapid ice sheet retreat. This chain of events illustrated the vulnerability of ice shelves to climate warming and their buffering role on the mass balance of Antarctica. In West Antarctica, the Pine Island Bay sector is draining far more ice into the ocean than is stored upstream from snow accumulation. This sector could raise sea level by 1m and trigger widespread retreat of ice in West Antarctica. Pine Island Glacier accelerated 38% since 1975, and most of the speed up took place over the last decade. Its neighbour Thwaites Glacier is widening up and may double its width when its weakened eastern ice shelf breaks up. Widespread acceleration in this sector may be caused by glacier ungrounding from ice shelf melting by an ocean that has recently warmed by 0.3 degrees C. In contrast, glaciers buffered from oceanic change by large ice shelves have only small contributions to sea level. In East Antarctica, many glaciers are close to a state of mass balance, but sectors grounded well below sea level, such as Cook Ice Shelf, Ninnis/Mertz, Frost and Totten glaciers, are thinning and losing mass. Hence, East Antarctica is not immune to changes.     

Sensitivity of the light field under sea ice to spatially inhomogeneous optical properties and incident light assessed with three-dimensional Monte Carlo radiative transfer simulations

Light transmittance through sea ice is affected by surface cover and ice optical properties in the vicinity of the measurement. We present three-dimensional Monte Carlo simulations of light propagation in sea ice to derive upper bounds on the lateral spread of light. Our results give guidance on equipment design and on the possibility of using one-dimensional light transfer models to describe transmittance. Rules were derived for simple cases of optically homogeneous slabs. In the absence of absorption, 10% and 90% of the flux detected under optically thick, homogeneous ice are incident on the surface within a radius of less than 0.3 and 2.0 times the ice thickness, respectively. Any increase in optical thickness or absorption in the ice will reduce these radii. For example, the wavelength-dependent absorption of ice results in a 20% reduction at 700 nm. Optical anisotropy of the slab was also found to produce potentially significant spatial narrowing of the transmitted light field. In the case of direct sunlight, the photon path is displaced toward the sun relative to the location of the detector. This distortion can reach 1 m or more in optically thick, ponded ice but will be negligible under a surface scattering layer or snow cover. Case studies showed that transmittance of diffuse light in the vicinity of a semi-infinite surface obstruction could be approximated with exponential and error functions. An absorbing cylindrical perturbation of 0.05 m diameter in 1 m thick ice placed 1 m from the point of measurement will absorb less than 1% of the light otherwise registered by the detector. Many results for transmitted light were independent of the mean cosine of the scattering phase function.     

Determining the contribution of Antarctica to sea-level rise using data assimilation methods

The problem of forecasting the future behaviour of the Antarctic ice sheet is considered. We describe a method for optimizing this forecast by combining a model of ice sheet flow with observations. Under certain assumptions, a linearized model of glacial flow can be combined with observations of the thickness change, snow accumulation, and ice-flow, to forecast the Antarctic contribution to sea-level rise. Numerical simulations show that this approach can potentially be used to test whether changes observed in Antarctica are consistent with the natural forcing of a stable ice sheet by snowfall fluctuations. To make predictions under less restrictive assumptions, improvements in models of ice flow are needed. Some of the challenges that this prediction problem poses are highlighted, and potentially useful approaches drawn from numerical weather prediction are discussed.     

Thermophysical properties of ice,snow, and sea ice

The paper reviews and discusses data and information on the thermophysical properties of ice, snow, and sea ice. These properties include thermal conductivity, specific heat, density, thermal diffusivity, latent heat of fusion, thermal expansion, and absorption coefficient. The available data are shown graphically for convenience in conjunction with the recommended correlation equations.Paper presented at the Second U.S.-Japan Joint Seminar on Thermophysical Properties, June 23, 1988, Gaithersburg, Maryland, U.S.A.     

碳纳米管8～12 GHz电磁波衰减实验研究

用电磁波穿透方法实验研究了长度为2μm 和15μm , 直径分别为30 、60 和100 nm 的6 种碳纳米管8～12 GHz 电磁波衰减特性。结果表明, 直径为30 nm 时, 长度为15μm 的碳纳米管的电磁波衰减能力大于长度2μm的; 直径分别为60 nm 和100 nm 时, 长度为2μm 的碳纳米管的电磁波衰减能力均大于长度为15μm 的。在长度相同的情况下, 不同直径碳纳米管的电磁波衰减能力为: 60 nm > 100 nm > 30 nm。6 种碳纳米管的电磁波衰减能力为: Ф60 nm ×2μm > Ф60 nm ×15μm > Ф100 nm ×2μm > Ф100 nm ×15μm > Ф30 nm ×15μm > Ф30 nm ×2μm。实验还发现长径比较小的碳纳米管的电磁波衰减能力明显好于长径比较大的磁纳米管的电磁波衰减能力;但碳纳米管长径比与电磁波衰减能力之间并不存在简单的线性关系。      

VIS–NIR overtone bands of snow: Photoacoustic spectroscopy

Photoacoustic (PA) spectrum of natural dry snow was obtained in the wavelength range of 200–1100 nm, using an indigenously developed PA Spectrometer (Kapil, J.C., Joshi, S.K., Rai. A.K., 2003. Insitu Photoacoustic Investigations of some optically transparent samples like ice and snow. Review of Scientific Instruments 74 (7), 3536–3543), working in a temperature range of room temperature to − 40 °C. Fundamental frequencies (v₁, v₂, v₃) as well as overtone frequencies of O–H vibrations in a snow crystal were identified and the corresponding combinational frequencies were assigned in the UV–VIS–NIR region of electromagnetic spectrum. The PA spectrum of snow thus obtained was compared with the PA spectra of distilled water and hexagonal ice (ice-Ih), in the same wavelength region. Bathochromic shifts (942→974→983 nm) in the third overtone frequency (3v₁) of fundamental O–H vibrations in the H₂O molecule were observed when the phase changes as vapor→liquid→solid. These shifts have been explained in the context of increased contents of hydrogen bonding (skeleton of H-bonds) in the denser phases (liquid and solid) as compared to the rare phase (vapor). The effect of temperature on the creation or breakage of H-bonds in snow crystals has been demonstrated by the relative shifting of the absorption maxima. For monitoring the effect of H-bonding leading to the intricacy in the crystallography of snow, vibrational absorption bands were analyzed from the relative shifting associated to the absorption maxima of snow and snow-melt water at two different wavelengths (535 and 826 nm) near the phase transitions. Also, our investigation reveals that ΔE_j (change in vibrational energy) are symmetrically aligned about the Δv-axes (change in vibrational quantum number) at phase transitions.     

Investigation of nonlinear dynamics in CdS by time-resolved nondegenerate pump–probe system with phase object

The time-resolved nondegenerate pump–probe system with phase object is employed for investigation of nonlinear absorption and refraction dynamics in CdS. The 532?nm laser beam with 21?ps duration is used as the excitation and the laser beams of 600 and 680?nm with 10?ps duration from optical parametric generation are used for probing. The experimental results at both probe wavelengths show free-carrier absorption and large free-carrier refraction along with two-photon absorption and bound electronic optical Kerr effect. By numerically fitting the experimental data based on the nondegenerate pump–probe theory, the nondegenerate two-photon absorption coefficient, the nondegenerate Kerr coefficient, the free-carrier decay time, the free-carrier absorptive cross-section and free-carrier refractive coefficient at different wavelengths are all determined.     

Attenuation of lateral propagating light in sea ice measured with an artificial lamp in winter Arctic

The attenuation property of a lateral propagating light (LPL) in sea ice was measured using an artificial lamp in the Canadian Arctic during the 2007/2008 winter. A measurement method is proposed and applied whereby a recording instrument is buried in the sea ice and an artificial lamp is moved across the instrument. The apparent attenuation coefficient μ(λ) for the lateral propagating light is obtained from the measured logarithmic relative variation rate. With the exception of blue and red lights, the attenuation coefficient changed little with wavelength, but changed considerably with depth. The vertical decrease of the attenuation coefficient was found to be correlated with salinity: the greater the salinity, the greater the attenuation coefficient. A clear linear relation of salinity and the lateral attenuation coefficient with R² = 0.939 exists to address the close correlation of the attenuation of LPL with the scattering from the brine. The observed attenuation coefficient of LPL is much larger than that of the vertical propagation light, which we speculate to be caused by scattering. Part of this scattered component is transmitted out of the sea ice from the upper and lower surfaces.     

A spectrophotometer for measuring liquid attenuation lengths at wavelengths between 400 and 600 nm

We describe the design and operation of a 1 m long spectrophotometer for measuring liquid attenuation lengths using light having wavelengths between 400 and 600 nm. By utilizing a blue light emitting diode and a lens focusing system, we are able to measure attenuation lengths from less than 1 m to above 30 m.     

Direct determination of levoglucosan at the picogram per milliliter level in Antarctic ice by high-performance liquid chromatography/electrospray ionization triple quadrupole mass spectrometry

A method for the direct determination of levoglucosan at the picogram per milliliter level in less than 1 mL of Antarctic ice has been developed. Chemical analysis is performed by high-performance liquid chromatography with triple quadrupole tandem mass spectrometric detection. Levoglucosan, a specific molecular marker for biomass burning, is identified by negative ion electrospray mass spectrometry using m/z 161/113, 161/101, 161/85, and 161/71 as monitoring ion transitions. Contamination problems were carefully taken into account by adopting ultraclean procedures during sampling and sample pretreatment phases. The limit of detection is 3 pg mL(-1) (0.3 pg absolute amount injected); the repeatability ranges between 20% and 50% at a concentration of 20 and 9 pg mL(-1), respectively. This methodology allowed the direct determination of levoglucosan in a 1 mL sample of Antarctic ice with concentration ranges between 4 and 30 pg mL(-1). To our knowledge these are the first levoglucosan concentrations reported for Antarctic ice.     

Albedo and flux extinction coefficients of impure snow for diffuse short-wave radiation

Impurities enter a snowpack as a result of fallout or scavenging by falling snow crystals. Albedos and flux extinction coefficients of soot-contaminated snowcovers are studied using a two-stream approximation of the radiative transfer equation. The effect of soot is calculated by two methods: independent scattering by ice grains and impurities, and the average refractive index for ice grains. Both methods predict a qualitatively similar effect of soot; the albedo is decreased and the extinction coefficient is increased relative to that for pure snow in the visible region, while the infrared properties are largely unaffected. Quantitatively, however, the effect of soot is more pronounced in the average refractive index method. We find that soot contamination provides qualitative explanation for several snow observations.     

Scattering optics of snow

Permanent snow and ice cover great portions of the Arctic and the Antarctic. It appears in winter months in northern parts of America, Asia, and Europe. Therefore snow is an important component of the hydrological cycle. Also, it is a main regulator of the seasonal variation of the planetary albedo. This seasonal change in albedo is determined largely by the snow cover. However, the presence of pollutants and the microstructure of snow (e.g., the size and shape of grains, which depend also on temperature and on the age of the snow) are also of importance in the variation of the snow's spectral albedo. The snow's spectral albedo and its bidirectional reflectance are studied theoretically. The albedo also determines the spectral absorptance of snow, which is of importance, e.g., in studies of the heating regime in snow. We investigate the influence of the nonspherical shape of grains and of close-packed effects on snow's reflectance in the visible and the near-infrared regions of the electromagnetic spectrum. The rate of the spectral transition from highly reflective snow in the visible to almost totally absorbing black snow in the infrared is governed largely by the snow's grain sizes and by the load of pollutants. Therefore both the characteristics of snow and its concentration of impurities can be monitored on a global scale by use of spectrometers and radiometers placed on orbiting satellites.     

Specific absorption coefficient of chromium in (TeO<Subscript>2</Subscript>)<Subscript>0.80</Subscript>(MoO<Subscript>3</Subscript>)<Subscript>0.20</Subscript> glass

We have prepared (TeO₂)_0.80(MoO₃)_0.20 glass samples containing 0.01 to 0.11 wt % chromium and determined their optical transmission in the range from 450 to 2800 nm. The glasses have been shown to have a strong absorption band centered at 660 nm. From the attenuation coefficient as a function of Cr³⁺ concentration in the glasses, we have evaluated their specific absorption coefficient, which has been shown to be 190 ± 2 cm^–1/wt % at the maximum of the absorption band.