Relationship between solubility and solubility product: The roles of crystal sizes and crystallographic directions

Grain size- and crystallographic direction-dependence are among the fundamental characteristics of crystal solubility. However, such important material properties are routinely ignored and solubility is often conveniently approximated by a solubility product. In this study, we attempt to outline the relationship between solubility and solubility product using thermodynamic arguments, and to provide observations that demonstrate the occurrence of circumstances where the solubility product cannot properly approximate crystal solubility. Theoretical analysis shows that solubility is always greater than solubility product, but the difference is inversely related to the grain size. Furthermore, the difference can be crystallographic direction specific if the total surface energy change upon the attachment of an individual growth unit is nonequivalent for each symmetrically unrelated crystal faces. In situ AFM experiments conducted on the cleavage face of calcite demonstrate that the steps exhibit direction- and length-dependent behavior. Specifically, the measured critical step lengths are consistent with the predicted inverse relationship to saturation states. Moreover, step retreat at and advance at are observed simultaneously in a narrow range of saturation at near equilibrium conditions, indicating the existence of direction specific solubility. Whereas these findings justify the rationale for approximating solubility by solubility product in cases where large crystals are concerned, the results imply that the size and direction effect should not be ignored if nanocrystal growth/dissolution is the subject of interest.     

Influence of road network and population demand assumptions in evacuation modeling for distant tsunamis

Tsunami evacuation planning in coastal communities is typically focused on local events where at-risk individuals must move on foot in a matter of minutes to safety. Less attention has been placed on distant tsunamis, where evacuations unfold over several hours, are often dominated by vehicle use and are managed by public safety officials. Traditional traffic simulation models focus on estimating clearance times but often overlook the influence of varying population demand, alternative modes, background traffic, shadow evacuation, and traffic management alternatives. These factors are especially important for island communities with limited egress options to safety. We use the coastal community of Balboa Island, California (USA), as a case study to explore the range of potential clearance times prior to wave arrival for a distant tsunami scenario. We use a first-in–first-out queuing simulation environment to estimate variations in clearance times, given varying assumptions of the evacuating population (demand) and the road network over which they evacuate (supply). Results suggest clearance times are less than wave arrival times for a distant tsunami, except when we assume maximum vehicle usage for residents, employees, and tourists for a weekend scenario. A two-lane bridge to the mainland was the primary traffic bottleneck, thereby minimizing the effect of departure times, shadow evacuations, background traffic, boat-based evacuations, and traffic light timing on overall community clearance time. Reducing vehicular demand generally reduced clearance time, whereas improvements to road capacity had mixed results. Finally, failure to recognize non-residential employee and tourist populations in the vehicle demand substantially underestimated clearance time.     

Hydro-mechanical analysis of rainfall-induced fines migration process within unsaturated soils

Seepage-induced fines migration under rainfall infiltration is a main cause leading to shallow failures in loose colluvial slopes. To describe the full process of fines migration within unsaturated soils during rainfall infiltration and the associated hydro-mechanical behaviors, a seepage-erosion-deformation coupled formulation is proposed in this paper. The governing equations proposed are implemented into a finite element code and used to investigate the influences of skeleton deformation on the rainfall infiltration process through unsaturated soil columns. The numerical results were presented in detail for a better understanding of the rainfall-induced fines migration process within unsaturated soils. Further, the obtained results are integrated into an infinite slope model for slope stability analysis. The results show that, the skeleton deformation will affect the rainfall infiltration rate and hence the timing of slope failures; meanwhile their influences are more evident if the fines deposition process is taken into account. Moreover, the slope stability could be reduced gradually due to the soil strength loss along with loss of fine particles. Therefore, particular attentions should be paid to analyzing the stability of soil slopes susceptible to internal erosion.     

Analysis of climate policy targets under uncertainty

Although policymaking in response to the climate change threat is essentially a challenge of risk management, most studies of the relation of emissions targets to desired climate outcomes are either deterministic or subject to a limited representation of the underlying uncertainties. Monte Carlo simulation, applied to the MIT Integrated Global System Model (an integrated economic and earth system model of intermediate complexity), is used to analyze the uncertain outcomes that flow from a set of century-scale emissions paths developed originally for a study by the U.S. Climate Change Science Program. The resulting uncertainty in temperature change and other impacts under these targets is used to illustrate three insights not obtainable from deterministic analyses: that the reduction of extreme temperature changes under emissions constraints is greater than the reduction in the median reduction; that the incremental gain from tighter constraints is not linear and depends on the target to be avoided; and that comparing median results across models can greatly understate the uncertainty in any single model.     

Carbon sequestration potential of parkland agroforestry in the Sahel

Establishing parkland agroforestry on currently treeless cropland in the West African Sahel may help mitigate climate change. To evaluate its potential, we used climatically suitable ranges for parklands for 19 climate scenarios, derived by ecological niche modeling, for estimating potential carbon stocks in parkland and treeless cropland. A biocarbon business model was used to evaluate profitability of hypothetical Terrestrial Carbon Projects (TCPs), across a range of farm sizes, farm numbers, carbon prices and benefit sharing mechanisms. Using climate analogues, we explored potential climate change trajectories for selected locations. If mature parklands covered their maximum range, carbon stocks in Sahelian productive land would be about 1,284?Tg, compared to 725?Tg in a treeless scenario. Due to slow increase rates of total system carbon by 0.4?Mg?C?ha^?1 a^?1, most TCPs at carbon prices that seem realistic today were not feasible, or required the participation of large numbers of farmers. For small farms, few TCP scenarios were feasible, and low Net Present Values for farmers made it unlikely that carbon payments would motivate many to participate in TCPs, unless additional benefits were provided. Climate analogue locations indicated an uncertain climate trajectory for the Sahel, but most scenarios projected increasing aridity and reduced suitability for parklands. The potentially severe impacts of climate change on Sahelian ecosystems and the uncertain profitability of TCPs make the Sahel highly risky for carbon investments. Given the likelihood of degrading environmental conditions, the search for appropriate adaptation strategies should take precedence over promoting mitigation activities.