Arctic permafrost soils contain a vast reservoir of soil organic carbon (SOC) vulnerable to increasing mobilization and decomposition from polar warming and permafrost thaw. How these SOC stocks are responding to global warming is uncertain, partly due to a lack of information on the distribution and status of SOC over vast Arctic landscapes. Soil moisture and organic matter vary substantially over the short vertical distance of the permafrost active layer. The hydrological properties of this seasonally thawed soil layer provide insights for understanding the dielectric behavior of water inside the soil matrix, which is key for developing more effective physics-based radar remote sensing retrieval algorithms for large-scale mapping of SOC. This study provides a coupled hydrologic-electromagnetic framework to model the frequency-dependent dielectric behavior of active layer organic soil. For the first time, we present joint measurement and modeling of the water matric potential, dielectric permittivity, and basic physical properties of 66 soil samples collected across the Alaskan Arctic tundra. The matric potential measurement allows for estimating the soil water retention curve, which helps determine the relaxation time through the Eyring equation. The estimated relaxation time of water molecules in soil is then used in the Debye model to predict the water dielectric behavior in soil. A multi-phase dielectric mixing model is applied to incorporate the contribution of various soil components. The resulting organic soil dielectric model accepts saturation water fraction, organic matter content, mineral texture, temperature, and microwave frequency as inputs to calculate the effective soil dielectric characteristic. The developed dielectric model was validated against lab-measured dielectric data for all soil samples and exhibited robust accuracy. We further validated the dielectric model against field-measured dielectric profiles acquired from five sites on the Alaskan North Slope. Model behavior was also compared against other existing dielectric models, and an indepth discussion on their validity and limitations in permafrost soils is given. The resulting organic soil dielectric model was then integrated with a multi-layer electromagnetic scattering forward model to simulate radar backscatter under a range of soil profile conditions and model parameters. The results indicate that low frequency (P-,L-band) polarimetric synthetic aperture radars (SARs) have the potential to map water and carbon characteristics in permafrost active layer soils using physics-based radar retrieval algorithms.

In cold regions, the frozen soil-rock mixture (FSRM) is subjected to cyclic loading coupled with freeze-thaw cycles due to seismic loading and ambient temperature changes. In this study, in order to investigate the dynamic mechanical response of FSRM, a series of cyclic cryo-triaxial tests were performed at a temperature of -10 degrees C on FRSM with different coarse-grained contents under different loading conditions after freeze-thaw cycles. The experimental results show that the coarse-grained contents and freeze-thaw cycles have a significant influence on the deformation properties of FSRM under cyclic loading. Correspondingly, a novel binary-medium-based multiscale constitutive model is firstly proposed to describe the dynamic elastoplastic deformation of FSRM based on the coupling theoretical framework of breakage mechanics for geomaterials and homogenization theory. Considering the multiscale heterogeneities, ice-cementation differences, and the breakage process of FSRM under external loading, the relationship between the microscale compositions, the mesoscale deformation mechanism (including cementation breakage and frictional sliding), and the macroscopic mechanical response of the frozen soil is first established by two steps of homogenization on the proposed model. Meanwhile, a mixed hardening rule that combines the isotropic hardening rule and kinematic hardening is employed to properly evaluate the cyclic plastic behavior of FSRM. Finally, comparisons between the predicted results and experimental results show that the proposed multiscale model can simultaneously capture the main feature of stress-strain (nonlinearity, hysteresis, and plastic strain accumulation) and volumetric strain (contraction and dilatancy) of the studied material under cyclic loading.

The optical properties of light absorbing soot aerosols generally change through interactions with weakly absorbing particles, resulting in complex mixing states, and have been highlighted as a major uncertainty in assessing their radiative forcing and climatic impact. The single scattering properties of soot aggregates partially embedded in the host sulfate particle (semi-embedded soot-containing mixtures) are investigated for two kinds of morphologies with intersecting and non-intersecting surfaces. The surfaces cannot be overlapped in the non-intersecting surface morphology, while the intersecting surface morphology is unconstrained. Based on the modified diffusion limited aggregation (DLA) algorithm, the models with non-intersecting surfaces are simulated and applied for the single scattering calculations of semi-embedded soot-containing mixtures using the superposition T-matrix (STM) method. For comparison, the models with intersecting surfaces are simulated with the same morphological parameters, but some soot monomers are intersected by the host sphere. Due to the limitation of current STM method, the optical properties of these models with intersecting surfaces are calculated using the discrete dipole approximation (DDA) method. The soot volume fractions outside sulfate host (F-s,F-out) are introduced and applied to characterize the mixing states of the soot-containing aerosols. These simulations show that the absorption cross-sections of those internally, deeply, half and slightly embedded mixed soot particles (F-s,F-out = 0.0, 02, 0.5, 0.8) are similar to 105%, similar to 65%, similar to 43% and similar to 14% larger than the semi-external mixtures (F-s,F-out = 1.0), respectively. The results also indicate that the differences of extinction cross-sections, single scattering albedo (SSA) and asymmetry parameter (ASY) between simulations with intersecting and non-intersecting surfaces are small ( < 1%) for semi-embedded soot-containing mixtures with the same morphological parameters. Within the range of visible and near-infrared wavelengths, the relative deviations of absorption cross-sections between these different morphologies are also small ( < similar to 5%). Therefore, based on these simulations, the single scattering properties of semi-embedded soot-containing mixtures are rarely influenced by the morphological differences between the absorbing spheres intersecting and non-intersecting the non-absorbing host, which can nearly be ignored in the single scattering (C) 2015 Elsevier Ltd. All rights reserved.

The effects of coating on black carbon (BC) optical properties and global climate forcing are revisited with more realistic approaches. We use the Generalized Multiparticle Mie method along with a realistic size range of monomers and clusters to compute the optical properties of uncoated BC clusters. Mie scattering is used to compute the optical properties of BC coated by scattering material. When integrated over the size distribution, we find the coating to increase BC absorption by up to a factor of 1.9 (1.8-2.1). We also find the coating can significantly increase or decrease BC backscattering depending on shell size and how shell material would be distributed if BC is uncoated. The effect of coating on BC forcing is computed by the Monte-Carlo Aerosol Cloud Radiation model with observed clouds and realistic BC spatial distributions. If we assume all the BC particles to be coated, the coating increases global BC forcing by a factor of 1.4 from the 1.9 x absorption increase alone. Conversely, the coating can decrease the forcing by up to 60% or increase it by up to 40% by only the BC backscattering changes. Thus, the combined effects generally, but not necessarily, amplify BC forcing.

[1] The optical properties and hence the radiative forcing of atmospheric aerosols are determined, in part, by the way in which the various constituents are externally or internally mixed. The mixing state must be known to compute the effective refractive index, water activity, and size distribution of the aerosols. In this study we found that the percentage difference in the optical properties, including extinction, single scattering albedo, and asymmetry parameter, between an internal mixture and external mixture of black carbon and ammonium sulfate can be over 25% for the dry case and over 50% for the wet case for typical mass mixing ratios. The differences are a result of a complicated combination of nonlinear Mie theory on the refractive index, assumptions about the coagulated particle sizes for internal mixtures, and the role of water uptake and deliquescence as a function of relative humidity. The computed optical properties are used to estimate the globally average clear-sky direct radiative forcing for different mixing assumptions. The results are displayed as a function of relative humidity to conveniently see the mixing effects for dry aerosols at less than the crystallization point, for dry internal and wet external mixtures between the crystallization and deliquescence points, and for fully wet mixtures above the deliquescence point. For a 9: 1 ammonium sulfate to black carbon mass ratio, nearly all the cooling effect predicted for an external mixture is lost for the internally mixed assumption, especially for relative humidities less than the deliquescence point.