Juan G. Navea

Carbon dioxide (C16O2 and C18O2) adsorption in zeolite Y materials: effect of cation, adsorbed water and particle size

In this study, CO2 adsorption in the presence and absence of co-adsorbed H2O was investigated in zeolite Y. Several different zeolite Y materials were investigated including commercial NaY, commercial NaY ion-exchanged with Ba2+ and nanocrystalline NaY; herein referred to as NaY, BaY and nano-NaY. Following heating of these zeolites to 573 K and cooling to room temperature, CO2 was adsorbed as a function of pressure. FTIR spectra show that a majority of CO2 adsorbs in the pores of these three zeolites (NaY, BaY and nano-NaY) in a linear complex with the exchangeable cation, as indicated by the intense absorption band near 2350 cm−1, assigned to the ν3 asymmetric stretch of adsorbed CO2. Most interestingly is the formation of carbonate and bicarbonate on the external surface of nano-NaY zeolites as indicated by the presence of several broad absorption bands in the 1200–1800 cm−1 region, suggesting unique sites for CO2 adsorption on the surface of the nanomaterial. For the other two zeolite materials investigated, bicarbonate formation is only evident in BaY zeolite in the presence of co-adsorbed water. Adsorption of 18O-labeled carbon dioxide and theoretical quantum chemical calculations confirm these assignments and conclusions.

Heterogeneous photochemistry of trace atmospheric gases with components of mineral dust aerosol.

Mineral dust aerosol is known to provide a reactive surface in the troposphere for heterogeneous chemistry to occur. Certain components of mineral dust aerosol, such as semiconductor metal oxides, can act as chromophores that initiate chemical reactions, while adsorbed organic and inorganic species may also be photoactive. However, relatively little is known about the impact of heterogeneous photochemistry of mineral dust aerosol in the atmosphere. In this study, we investigate the heterogeneous photochemistry of trace atmospheric gases including HNO(3) and O(3) with components of mineral dust aerosol using an environmental aerosol chamber that incorporates a solar simulator. For reaction of HNO(3) with aluminum oxide, broadband irradiation initiates photoreactions to form gaseous NO and NO(2). A complex dynamic balance between surface adsorbed nitrate and gaseous nitrogen oxide products including NO and NO(2) is observed. For heterogeneous photoreactions of O(3), iron oxide shows catalytic decompositions toward O(3) while aluminum oxide is deactivated by ozone exposure. Furthermore, the role of relative humidity, and, thus, adsorbed water, on heterogeneous photochemistry has been explored. The atmospheric implications of these results are discussed.

A comparative evaluation of water uptake on several mineral dust sources

Environmental context. Dust particles produced from wind blown soils are of global significance as these dust particles not only impact visibility, as evident in the recent 2009 Australian dust storm, but also atmospheric chemistry, climate and biogeochemical cycles. The amount of water vapour in the atmosphere (relative humidity) can play a role in these global processes yet there are few studies and little quantitative data on water-dust particle interactions. The focus of this research is on quantifying water-dust particle interactions for several dust sources including Asia and Africa where dust storms are most prevalent. Abstract. Mineral dust aerosol provides a reactive surface in the troposphere. The reactivity of mineral dust depends on the source region as chemical composition and mineralogy of the aerosol affects its interaction with atmospheric gases. Furthermore, the impact of mineral dust aerosol in atmospheric processes and climate is a function of relative humidity. In this study, we have investigated water uptake of complex dust samples. In particular, water uptake as a function of relative humidity has been measured on three different dust sources that have been characterised using a variety of bulk and surface techniques. For these well-characterised dust samples, it is shown that although there are variations in chemical composition and mineralogy, on a per mass basis, water uptake capacities for the three dusts are very similar and are comparable to single component clay samples. These results suggest that the measured uptake of water of these bulk samples is dominated by the clay component.

Effect of ozone and relative humidity on the heterogeneous uptake of octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane on model mineral dust aerosol components.

We have carried out kinetic and reaction yield studies to determine the effect of O(3) on the heterogeneous reaction of two cyclic volatile methylsiloxanes (cVMS), octamethylcyclotetrasiloxane (D(4)) and decamethylcyclopentasiloxane (D(5)), with model mineral dust aerosol in order to obtain a better understanding of the atmospheric fate of cVMS. The heterogeneous chemistry was studied in an environmental reaction chamber using FT-IR spectroscopy to monitor the reaction progress. The uptake kinetics and the reaction extent for D(4) and D(5) in the presence of O(3) were quantified for two components of mineral dust aerosol, hematite and kaolinite. Some experiments with a carbonaceous particulate, carbon black, were also performed for D(5). The relative humidity (RH) inside the chamber was varied to investigate the influence of surface adsorbed water on the heterogeneous chemistry of the dust samples. With the dust samples, but not carbon black, the coadsorption of O(3) introduced a new reaction pathway, characterized by a linear, zero-order, decay of both gas phase cVMS and ozone. The new pathway does not saturate on the time scale of our experiments. Elevated RH was observed to decrease the total uptake of cVMS and ozone by the end of the experiment, but the characteristic linear decay was still present. The atmospheric loss of cVMS due to heterogeneous uptake is enhanced due to O(3), even at higher RH values, but the overall loss rate is reduced at RH values typical of the troposphere.

Low temperature cell for cavity ring down absorption studies

Phase shift cavity ring down is a technique that due to its long optical path length is an ideal method to detect weak absorptions. Coupling the method to a custom fitted cryostat allows gas phase molecules to be studied at cryogenic temperatures in a thermally isolated vacuum chamber. A novel design is described to construct the complete instrument. With optical cavities of length 10⩽l⩽43cm, optical path lengths between 200m and 6km have been achieved. High vibrational overtones C–H (Δυ=5) are measured at 130K (methane), 150K (ethylene), and 155K (ethane). Oscillator strengths of each molecule calculated at different temperatures are in excellent agreement. The experimental setup can be used to study kinetics and spectroscopy of atmospheric molecules, planetary atmospheres, and molecular complexes in the gas phase. Low temperatures can be obtained using liquid He or liquid N2 as cryogens.

Comparative evaluation of iron leach from different sources of fly ash under atmospherically relevant conditions

Environmental context Iron, a limiting nutrient of plankton in the ocean, is deposited to the sea from atmospheric aerosols. In particular, atmospheric acidic conditions promote dissolution of iron from fly ash, a by-product of coal-fired power plants. Here, we report that the iron leached from fly ash depends on its source region, and that the type of combustion process may influence the iron species mobilized. Abstract Fly ash, an iron-containing by-product of coal-fired power plants, has been observed in atmospheric aerosol plumes. Under the acidic atmospheric conditions resulting from the uptake of atmospheric gases, iron leached from fly ash can impact global biogeochemical cycles. However, the fly ash source region, as well as its generating power plant, plays an important role in the amount, speciation and lability of iron. Yet no comparative studies have been made on iron leached from fly ash from different sources. This study reports the iron mobilisation by proton-promoted dissolution from well-characterised fly ash samples from three distinctive locations: the USA Midwest, north-east India and Europe. In addition, pH dependency was also investigated. Proton-promoted dissolution showed a variability between source regions with a relative iron leach in the order USA Midwestern>north-east Indian>European ash. In addition, the initial rate of iron leach suggests that source region is indeed a determining factor in the iron leaching capacity of fly ash, because dissolution from Midwestern fly ash is also faster than both European and Indian ash. Finally, the combustion process of fly ash proved to be significant for the iron speciation, given that well-combusted fly ash samples leached mostly Fe3+ rather than bioavailable Fe2+. The role of fly ash should therefore be taken into account in order to better understand the effects of combustion particles in atmospheric iron deposition.

Water Adsorption Isotherms on Fly Ash from Several Sources

In this study, horizontal attenuated total reflection (HATR) Fourier-transform infrared (FT-IR) spectroscopy was combined with quartz crystal microbalance (QCM) gravimetry to investigate the adsorption isotherms of water on fly ash, a byproduct of coal combustion in power plants. Because of composition variability with the source region, water uptake was studied at room temperature as a function of relative humidity (RH) on fly ash from several regions: United States, India, The Netherlands, and Germany. The FT-IR spectra show water features growth as a function of RH, with water absorbing on the particle surface in both an ordered (ice-like) and a disordered (liquid-like) structure. The QCM data was modeled using the Brunauer, Emmett, and Teller (BET) adsorption isotherm model. The BET model was found to describe the data well over the entire range of RH, showing that water uptake on fly ash takes place mostly on the surface of the particle, even for poorly combusted samples. In addition, the source region and power-plant efficiency play important roles in the water uptake and ice nucleation (IN) ability of fly ash. The difference in the observed water uptake and IN behavior between the four samples and mullite (3Al2O3·2SiO2), the aluminosilicate main component of fly ash, is attributed to differences in composition and the density of OH binding sites on the surface of each sample. A discussion is presented on the RH required to reach monolayer coverage on each sample as well as a comparison between surface sites of fly ash samples and enthalpies of adsorption of water between the samples and mullite.

Differential expression of zinc transporters accompanies the differentiation of C2C12 myoblasts

Zinc transporters facilitate metal mobilization and compartmentalization, playing a key role in cellular development. Little is known about the mechanisms and pathways of Zn movement between Zn transporters and metalloproteins during myoblast differentiation. We analyzed the differential expression of ZIP and ZnT transporters during C2C12 myoblast differentiation. Zn transporters account for a transient decrease of intracellular Zn upon myogenesis induction followed by a gradual increase of Zn in myotubes. Considering the subcellular localization and function of each of the Zn transporters, our findings indicate that a fine regulation is necessary to maintain correct metal concentrations in the cytosol and subcellular compartments to avoid toxicity, maintain homeostasis, and for loading metalloproteins needed during myogenesis. This study advances our basic understanding of the complex Zn transport network during muscle differentiation.

Effects of Coadsorbed Water on the Heterogeneous Photochemistry of Nitrates Adsorbed on TiO2

Nitric acid, a well-known sink of NO x gases in the atmosphere, has been found to be photoactive while adsorbed on tropospheric particles. When adsorbed onto semiconductive metal oxides, nitrates photochemical degradation can be interpreted as a photocatalytic process. Yet, the photolysis of nitrate ions on the surface of aerosols can also be initiated by changes in the symmetry of the ion upon adsorption. In this study, we use quantum chemistry to model the vibrational spectra of adsorbed nitrate on TiO2, a semiconductor component of atmospheric aerosols, and determine the kinetics of the heterogeneous photochemical degradation of nitrate under simulated solar light. Frequencies and geometry calculations suggest that the symmetry of chemisorbed nitrate ion depends strongly on coadsorbed water, with water changing the reactive surface of TiO2. Upon irradiation, surface nitrate undergoes photolysis to yield nitrogen-containing gaseous products including NO2, NO, HONO, and N2O, in proportions that depend on relative humidity (RH). In addition, the heterogeneous photochemistry rate constant decreases an order of magnitude, from (5.7 ± 0.1) × 10-4 s-1 on a dry surface to (7.1 ± 0.8) × 10-5 s-1 when nitrate is coadsorbed with water above monolayer coverage. Little is known about the roles of coadsorbed water on the heterogeneous photochemistry of nitrates on TiO2, along with its impact on the chemical balance of the atmosphere. This work discusses the roles of water in the photolysis of surface nitrates on TiO2 and the concomitant renoxification of the atmosphere.

Photochemistry of Atmospheric Particles

Abstract Given the considerable interest in aerosol particles in the atmosphere as these particles play a key role in health, climate, and chemistry, there is an outstanding need for fundamental molecular studies of a wide range of aerosol processes. In the past, there has been a tremendous amount of interest in heterogeneous atmospheric chemistry, that is, reactions that are mediated by liquid or solid aerosol particles. Given the demonstrated importance of heterogeneous thermal chemistry in the atmosphere, it is surprising how little attention has been directed toward the photochemistry of atmospheric particles, that is, reactions initiated or significantly enhanced with light. This section focuses on the photochemistry of atmospheric particles. An important aspect of this is to develop a molecular scale level understanding of photochemistry in the aerosol environment and on surfaces, including details of the modifications to ground and excited electronic states of surface bound species and the ramifications for the product branching ratio.