Why some countries are so much richer than others is a question of central interest in economics. Low aggregate income per worker in poor countries is mostly accounted for by low labor productivity and high employment in agriculture. This thesis attempts to understand cross-country income difference through examining productivity differences at the sector level - in agriculture and in non-agriculture. Between rich and poor countries, there is a 45-fold difference in agricultural output per worker and a 34-fold difference in mean farm size. In the first chapter, I argue farmer's skill as a plausible explanation for these differences. The model features heterogeneity in innate agricultural skill, on-the-job skill accumulation, and span-of-control in agricultural production. I show that low total factor productivity (TFP) in poor countries not only induces more individuals with low innate skill to choose farming, but also reduces the incentive to accumulate skill. Between rich and poor countries, the model generates substantial difference in farmer's skill, which translates into differences in agricultural productivity and farm size distribution. Quantitatively, the calibrated model explains half of the cross-country differences in agricultural output per worker, and successfully replicates the size distribution of farms in both rich and poor countries. Cross-country productivity differences are asymmetric across sectors. The labor productivity gap between rich and poor countries in agriculture is twice as large as that in the aggregate, and ten times larger than that in non-agriculture. The second chapter shows that these sectoral productivity differences can arise solely from difference in aggregate TFP. I extend the framework in the first chapter to allow for different skill in non-agricultural production as well. Low TFP distorts the allocation of skills across sectors and discourages skill accumulation on the job. To discipline the initial skill distribution and skill accumulation, the model is calibrated to match earnings distribution and age-earnings profiles in both agriculture and non-agriculture in the U.S. The model's implications are then examined using a sample of 70 countries that covers a wide range of development. Between rich and poor countries, the model accounts for most of the productivity differences at the sector level - productivity difference in agriculture in the model is 1.8 times larger than those in the aggregate and 6 times larger than those in non-agriculture. As in the data, the share of farmer in the labor force in the model declines from 85 percent in the poorest countries to less than 2 percent in the richest countries. These results suggest that policy aiming at improving overall efficiency should be prioritized.

Bibliography: leaves 182-198. Improvements in the current production system of inclusion bodies and the downstream processing sequence are essential to maintain a competitive advantage in the market place. Optimisation of fermentation is considered to improve production yield; then flotation as a possible inclusion body recovery method.

To achieve predictive capability for complex environmental systems with coagulation and arsenic sorption, a unified improved coagulation model coupled with arsenic sorption was developed. A unified coagulation model coupled with arsenic sorption was achieved by the following steps: (1) an improved discretized population balance equation (PBE) was developed to obtain the exact solution of conventional coagulation, (2) the improved PBE was extended to an adjustable geometric size interval having higher numerical stability, accuracy, and computational efficiency than existing models for fractal aggregate coagulation that includes agglomeration and fragmentation, (3) a surface complexation equilibrium model and a sorption kinetic model was introduced to predict arsenic sorption behavior onto hydrous metal oxide surfaces, and (4) an improved discretized PBE was coupled with arsenic sorption kinetics and equilibrium models by aid of collision efficiency ?? depending on surface charge (potential) on the hydrous metal oxide particles, colliding particle size ratio, and fluid strain-rate in applied flow system. The collision efficiency ?? into the improved (r,r)ij(r,r)ijdiscretized coagulation model for fractal aggregate yielded a unified improved coagulation model coupled with arsenic sorption kinetics and the equilibrium model. Thus, an improved unified coagulation model could provide high statistical accuracy, numerical stability, and computational efficiency to enhance predictive capability for behavior of arsenic sorption and fractal colloid particle aggregation and break-up, simultaneously. From the investigation, it is anticipated that the unified coagulation model coupled with arsenic sorption kinetics and equilibrium will provide a more complete understanding of the arsenic removal mechanism and its application to water/wastewater treatment. Further, this coupled model can be applied to other water and wastewater treatment systems combined with sorption and filtration processes. These combined processes can be optimized by the coupled model that was developed in this study. By simulating the arsenic sorption and particle size distribution as a pretreatment before filtration (sand filtration or membrane filtration), the overall arsenic removal efficiency and operation cost can be estimated.

Marine aggregates are an important means of carbon transfers downwards to the deep ocean as well as an important nutritional source for benthic organism communities that are the ultimate recipients of the flux. During these last 10 years, data on size distribution of particulate matter have been collected in different oceanic provinces using an Underwater Video Profiler. The cruise data include simultaneous analyses of particle size distributions as well as additional physical and biological measurements of water properties through the water column. First, size distributions of large aggregates have been compared to simultaneous measurements of particle flux observed in sediment traps. We related sediment trap compositional data to particle size (d) distributions to estimate their vertical fluxes (F) using simple power relationships (F=Ad^b). The spatial resolution of sedimentation processes allowed by the use of in situ particle sizing instruments lead to a more detailed study of the role of physical processes in vertical flux. Second, evolution of the aggregate size distributions with depth was related to overlying primary production and phytoplankton size-distributions on a global scale. A new clustering technique was developed to partition the profiles of aggregate size distributions. Six clusters were isolated. Profiles with a high proportion of large aggregates were found in high-productivity waters while profiles with a high proportion of small aggregates were located in low-productivity waters. The aggregate size and mass flux in the mesopelagic layer were correlated to the nature of primary producers (micro-, nano-, picophytoplankton fractions) and to the amount of integrated chlorophyll a in the euphotic layer using a multiple regression technique on principal components. Finally, a mesoscale area in the North Atlantic Ocean was studied to emphasize the importance of the physical structure of the water column on the horizontal and vertical distribution of particulate matter. The seasonal change in the abundance of aggregates in the upper 1000 m was consistent with changes in the composition and intensity of the particulate flux recorded in sediment traps. In an area dominated by eddies, surface accumulation of aggregates and export down to 1000 m occured at mesoscale distances (<100 km).

Particle size distribution (PSD) of gypseous soils is important in the soil science community. When gypsum constitutes a major portion of the soil, its removal prior to PSD analysis distorts the results and may lead to textures that do not relate to conditions in the field. In order to understand the true characterization of the soil and the gypsum particles, the entire soil sample should be analyzed. Four different approaches to the BaCl2 method presented in the literature (Hesse, 1976, Matar and Douleimy, 1978, Viellefon, 1979) were used to evaluate the use of BaCl2 solution to reduce the solubility of gypsum by forming a protective coating of BaSO4 around gypsum particles. Results showed that the BaCl2 method was unsatisfactory, as dispersion of clays was not sufficient to allow particle size analysis using the pipette method. A procedure using a laser diffraction particle size analyzer (LPSA) was also evaluated. As gypsum is insoluble in methanol, methanol was selected as a possible solution, but it caused flocculation of clays and could not be used to analyze samples containing silicate clays. Gypsum saturated water containing Na hexametaphosphate was evaluated as a solution. First, 20 non-gypseous samples were analyzed on a sand-free basis using saturated gypsum water with Na hexametaphosphate. Results were used to establish a relationship comparing LPSA results and pipette results. An equation y = 1.37x + 2.03 was established relating LPSA clay percent by volume (x) to the pipette clay percent by weight (y). The equation had a R2 value of 0.84 and was significant at the 1% level. From this equation a comparison of 21 gypseous samples was made, between clay percentages of the pipette method and the LPSA method. Results indicate that LPSA can be used to give a satisfactory particle size distribution of gypseous soils when coupled with sand analysis by sieving.

The structure of phytoplankton communities was examined seasonally across five rivers with a focus on small cells and their relative importance. Picophytoplankton (0.2-2 μm), previously considered insignificant in rivers, reached densities as high as those observed in lakes and oceans (~ 10e4-10e5 cells/mL). Their relative importance was not a function of trophic state with the highest contribution to algal biomass found in the most eutrophic river. Body size distributions were analyzed from both chlorophyll-a size fractions and taxonomic enumerations; no significant effect of river or season was detected, suggesting that phytoplankton size distribution is not a useful metric of change in rivers. Unlike lake ecosystems, the rivers were uniformly dominated by small cells (< 20 μm). Taxonomic analyses of the seasonal succession did not reveal a common periodicity of particular divisions (e.g. diatoms). However, strong dominance was more typical of eutrophic rivers even though taxa richness was similar.

Particle size distribution (PSD) of gypseous soils is important in the soil science community. When gypsum constitutes a major portion of the soil, its removal prior to PSD analysis distorts the results and may lead to textures that do not relate to conditions in the field. In order to understand the true characterization of the soil and the gypsum particles, the entire soil sample should be analyzed. Four different approaches to the BaCl2 method presented in the literature (Hesse, 1976, Matar and Douleimy, 1978, Viellefon, 1979) were used to evaluate the use of BaCl2 solution to reduce the solubility of gypsum by forming a protective coating of BaSO4 around gypsum particles. Results showed that the BaCl2 method was unsatisfactory, as dispersion of clays was not sufficient to allow particle size analysis using the pipette method. A procedure using a laser diffraction particle size analyzer (LPSA) was also evaluated. As gypsum is insoluble in methanol, methanol was selected as a possible solution, but it caused flocculation of clays and could not be used to analyze samples containing silicate clays. Gypsum saturated water containing Na hexametaphosphate was evaluated as a solution. First, 20 non-gypseous samples were analyzed on a sand-free basis using saturated gypsum water with Na hexametaphosphate. Results were used to establish a relationship comparing LPSA results and pipette results. An equation y = 1.37x + 2.03 was established relating LPSA clay percent by volume (x) to the pipette clay percent by weight (y). The equation had a R2 value of 0.84 and was significant at the 1% level. From this equation a comparison of 21 gypseous samples was made, between clay percentages of the pipette method and the LPSA method. Results indicate that LPSA can be used to give a satisfactory particle size distribution of gypseous soils when coupled with sand analysis by sieving.

Nanoporous polymer hydrogels offer a desirable combination of mechanical, optical, and transport characteristics that have placed them at the core of a variety of biomedical technologies including engineered tissue scaffolds, substrates for controlled release of pharmaceutical compounds, and sieving matrices for electrophoretic separation of DNA and proteins. Ultimately, we would like to obtain a detailed picture of the nanoscale pore morphology and understand how it can be manipulated so that we can rationally identify gel formulations best suited for a specific application. But this goal has proven elusive because the most fundamental descriptors of the pore network architecture (e.g., the average pore size and its polydispersity) are particularly difficult to measure in polymer hydrogels. Here we introduce an approach that enables both the mean pore size and the pore size distribution to be quantitatively determined without prior knowledge of any physical material parameters A novel technique to prepare TEM samples was developed so that the nanoscale hydrogel pore size, pore shape and distribution are clearly visualized and quantitatively studied for the first time. The pore sizes of the hydrogel are also estimated with rheology. A new fixture is used in the rheometer and the whole polymerization process can be directly studied using an in-situ rheology experiment. A series of thermoporometry experiments are also conducted, and suitable methods and equations to study hydrogel pore size and distribution are chosen. The pore size derived from TEM, rheology, DSC is compared and their values are self-consistent. These techniques help us understand how the nanoporous morphology of crosslinked polyacrylamide hydrogels is influenced by their chemical composition and polymerization conditions. It is interesting to find hydrogels with similar pore size but different distribution. For two hydrogels with similar pore size, the broader the distribution, the faster the release rate and the higher the accumulated release percentage. So we can control the release of trapped molecules by simply varying the hydrogel pore size distribution. This discovery would have a very promising potential in the application of pharmaceuticals.

To achieve predictive capability for complex environmental systems with coagulation and arsenic sorption, a unified improved coagulation model coupled with arsenic sorption was developed. A unified coagulation model coupled with arsenic sorption was achieved by the following steps: (1) an improved discretized population balance equation (PBE) was developed to obtain the exact solution of conventional coagulation, (2) the improved PBE was extended to an adjustable geometric size interval having higher numerical stability, accuracy, and computational efficiency than existing models for fractal aggregate coagulation that includes agglomeration and fragmentation, (3) a surface complexation equilibrium model and a sorption kinetic model was introduced to predict arsenic sorption behavior onto hydrous metal oxide surfaces, and (4) an improved discretized PBE was coupled with arsenic sorption kinetics and equilibrium models by aid of collision efficiency ?? depending on surface charge (potential) on the hydrous metal oxide particles, colliding particle size ratio, and fluid strain-rate in applied flow system. The collision efficiency ?? into the improved (r,r)ij(r,r)ijdiscretized coagulation model for fractal aggregate yielded a unified improved coagulation model coupled with arsenic sorption kinetics and the equilibrium model. Thus, an improved unified coagulation model could provide high statistical accuracy, numerical stability, and computational efficiency to enhance predictive capability for behavior of arsenic sorption and fractal colloid particle aggregation and break-up, simultaneously. From the investigation, it is anticipated that the unified coagulation model coupled with arsenic sorption kinetics and equilibrium will provide a more complete understanding of the arsenic removal mechanism and its application to water/wastewater treatment. Further, this coupled model can be applied to other water and wastewater treatment systems combined with sorption and filtration processes. These combined processes can be optimized by the coupled model that was developed in this study. By simulating the arsenic sorption and particle size distribution as a pretreatment before filtration (sand filtration or membrane filtration), the overall arsenic removal efficiency and operation cost can be estimated.

Marine aggregates are an important means of carbon transfers downwards to the deep ocean as well as an important nutritional source for benthic organism communities that are the ultimate recipients of the flux. During these last 10 years, data on size distribution of particulate matter have been collected in different oceanic provinces using an Underwater Video Profiler. The cruise data include simultaneous analyses of particle size distributions as well as additional physical and biological measurements of water properties through the water column. First, size distributions of large aggregates have been compared to simultaneous measurements of particle flux observed in sediment traps. We related sediment trap compositional data to particle size (d) distributions to estimate their vertical fluxes (F) using simple power relationships (F=Ad^b). The spatial resolution of sedimentation processes allowed by the use of in situ particle sizing instruments lead to a more detailed study of the role of physical processes in vertical flux. Second, evolution of the aggregate size distributions with depth was related to overlying primary production and phytoplankton size-distributions on a global scale. A new clustering technique was developed to partition the profiles of aggregate size distributions. Six clusters were isolated. Profiles with a high proportion of large aggregates were found in high-productivity waters while profiles with a high proportion of small aggregates were located in low-productivity waters. The aggregate size and mass flux in the mesopelagic layer were correlated to the nature of primary producers (micro-, nano-, picophytoplankton fractions) and to the amount of integrated chlorophyll a in the euphotic layer using a multiple regression technique on principal components. Finally, a mesoscale area in the North Atlantic Ocean was studied to emphasize the importance of the physical structure of the water column on the horizontal and vertical distribution of particulate matter. The seasonal change in the abundance of aggregates in the upper 1000 m was consistent with changes in the composition and intensity of the particulate flux recorded in sediment traps. In an area dominated by eddies, surface accumulation of aggregates and export down to 1000 m occured at mesoscale distances (<100 km).