Climate change in Arctic landscapes may increase freeze-thaw frequency within the active layer as well as newly thawed permafrost. Freeze-thaw is a highly disruptive process that can deform soil pores and alter the architecture of the soil pore network with varied impacts to water transport and retention, redox conditions, and microbial activity. Our objective was to investigate how freeze-thaw cycles impacted the pore network of newly thawed permafrost aggregates to improve understanding of what type of transformations can be expected from warming Arctic landscapes. We measured the impact of freeze-thaw on pore morphology, pore throat diameter distribution, and pore connectivity with X-ray computed tomography (XCT) using six permafrost aggregates with sizes of 2.5 cm3 from a mineral soil horizon (Bw; 28-50 cm depths) in Toolik, Alaska. Freeze-thaw cycles were performed using a laboratory incubation consisting of five freeze-thaw cycles (-10 C to 20 C) over five weeks. Our findings indicated decreasing spatial connectivity of the pore network across all aggregates with higher frequencies of singly connected pores following freeze-thaw. Water-filled pores that were connected to the pore network decreased in volume while the overall connected pore volumetric fraction was not affected. Shifts in the pore throat diameter distribution were mostly observed in pore throats ranges of 100 mu m or less with no corresponding changes to the pore shape factor of pore throats. Responses of the pore network to freeze-thaw varied by aggregate, suggesting that initial pore morphology may play a role in driving freeze-thaw response. Our research suggests that freeze-thaw alters the microenvironment of permafrost aggregates during the incipient stage of deformation following permafrost thaw, impacting soil properties and function in Arctic landscapes undergoing transition.

Most climate models predict that the timing, magnitude, and duration of snow cover will change over much of the Northern Hemisphere. Because snow cover effectively buffers soil against changes in air temperature, fluctuations in snowpack could alter freeze-thaw cycling, resulting in shifts in macroaggregate stability and subsequent detachment. Moreover, vegetation type could modify these effects; however, these interactions remain unexplored. In this study, we experimentally manipulated snow cover in an agricultural field and in an adjacent 13-year-old restored prairie to assess changes to soil aggregation and detachment over a three-winter period (November-April 2014-17). Treatments consisted of complete snow removal, natural snow cover, and a sustained snowpack simulated via straw insulation. Averaged over the course of the study, snow removal resulted in a 5% and 15% over-winter reduction in wet-aggregate stability (WAS) and mean weight diameter (MWD), respectively. Conversely, natural snow cover and straw insulation resulted in a 3% and 15% over-winter increase in WAS and MWD, respectively. However, over-winter changes to WAS and MWD did not persist but instead appeared to return to a set point by the end of each growing season regardless of vegetation type. In addition, we found an offset in WAS; it was approximately 11% higher in the prairie than in the agricultural field, likely due to increased root and microbial activity in the prairie. No similar offset was observed in MWD between vegetation types. These responses in soil aggregation did not result in significant springtime changes to soil critical shear stress, measured as a proxy for soil detachment potential. The results of this study suggest that future investigations into over-winter soil processes should consider vegetation type, temporal soil aggregation dynamics, and more detailed quantification of freeze-thaw cycling.

Tar balls are frequently found in slightly aged biomass burning plumes. They are spherical in shape, have diameters between similar to 100 and 300 nm, are amorphous and composed mostly of oxygen and carbon. Tar balls are light absorbing and considered to be a component of brown carbon. Tar balls have been typically reported and analyzed as individual spheres: however, in a recent study, we reported the presence of significant fractions of fractal-like aggregates made of several tar balls in fire plumes from different geographical locations. Aggregation affects the optical properties of particles; therefore, we use T-Matrix and Lorenz-Mie simulations to explore the effects of aggregation on the tar balls' optical properties in the 350 - 1150 nm wavelength range. We also evaluate the effects of different refractive indices available from the literature, different monomer numbers, and monomer sizes, as these are key factors determining the aggregates optical properties. Furthermore, we estimate the simple forcing efficiency for low and high surface albedos. Aggregates have a single scattering albedo (SSA) higher than that of individual tar balls (Delta SSA(550) (nm) up to 0.22). The hemispherical upscatter fraction of individual tar balls is more than 100% larger than for tar ball aggregates in many cases. The top of the atmosphere simple forcing efficiency over dark surfaces shows large variabilities with an increase up to similar to 53% for tar ball aggregates compared to individual tar balls. These results demonstrate that aggregation of tar balls can have a significant impact on their optical properties and radiative forcing. (C) 2019 Elsevier Ltd. All rights reserved.

Throughout most of the northern hemisphere, snow cover decreased in almost every winter month from 1967 to 2012. Because snow is an effective insulator, snow cover loss has likely enhanced soil freezing and the frequency of soil freeze-thaw cycles, which can disrupt soil nitrogen dynamics including the production of nitrous oxide (N2O). We used replicated automated gas flux chambers deployed in an annual cropping system in the upper Midwest US for three winters (December-March, 2011-2013) to examine the effects of snow removal and additions on N2O fluxes. Diminished snow cover resulted in increased N2O emissions each year; over the entire experiment, cumulative emissions in plots with snow removed were 69% higher than in ambient snow control plots and 95% higher than in plots that received additional snow (P < 0.001). Higher emissions coincided with a greater number of freeze-thaw cycles that broke up soil macroaggregates (250-8000 A mu m) and significantly increased soil inorganic nitrogen pools. We conclude that winters with less snow cover can be expected to accelerate N2O fluxes from agricultural soils subject to wintertime freezing.

Incense sticks and mustard oil are the two most popular combustion fuels during rituals and social ceremonies in Asian countries. Given their widespread use in both closed and open burning activities, it is important to quantify the spectral radiative properties of aerosols emitted from the combustion of both fuels. This information is needed by climate models to assess the impact of these aerosols on radiative forcing. In this study, we used a 3-wavelength integrated photoacoustic-nephelometer - operating simultaneously at 405, 532 and 781 nm - to measure the optical coefficients of aerosols emitted from the laboratory combustion of mustard oil lamp and two types of incense sticks. From the measured optical coefficients at three wavelengths, time-varying single scattering albedo (SSA), absorption Angstrom exponent (AAE), and scattering Angstrom exponent (SAE) were calculated. For incense smoke particles, the time-averaged mean AAE values were found to be as high as 8.32 (between 405 and 532 nm) and 6.48 (between 532 and 781 nm). This spectrally-varying characteristic of AAE indicates that brown carbon - a class of organic carbon which strongly absorbs solar radiation in the blue and near ultraviolet - is the primary component of incense smoke aerosols. For aerosols emitted from the burning of mustard oil lamp, the time-averaged mean ME values were similar to 1.3 (between 405 and 781 nm) indicating that black carbon (BC) is the primary constituent. Scanning electron microscopy combined with image processing revealed the morphology of incense smoke aerosols to be non-coalescing and weakly-bound aggregates with a mean two-dimensional (2-d) fractal dimension (D-f)=1.9 +/- 0.07, while the mustard oil smoke aerosols had typical fractal-like BC aggregate morphology with a mean 2-d D-f=1.85 +/- 0.09. (C) 2012 Elsevier Ltd. All rights reserved.