Atmospheric particles basic properties

In sintering, the green compact is placed on a wide-mesh belt and slowly moves through a controlled atmosphere furnace (Fig. 3). The parts are heated to below the melting point of the base metal, held at the sintering temperature, and cooled. Basically a solid-state process, sintering transforms mechanical bonds, ie, contact points, between the powder particles in the compact into metallurgical bonds which provide the primary functional properties of the part. 

The catalytic oxidation of sulphurous acid in aqueous medium to sulphuric acid [138, 84] has been suggested as a probe reaction for the ability of a carbon to activate molecular oxygen at ambient conditions. Besides this remarkable property the reaction is of interest in atmospheric chemistry where it provides a sink for all nonphotochemically oxidized sulfur dioxide [215]. Carbon plays a special role in this environmental application as both pure carbon and active particles (iron oxide [84] for example) anchored to carbon can act as efficient catalysts. The detailed analysis of the reactivity of various types of carbon [138] reveal that basic surface oxides (see Fig. 23) are important to fix the educt HSOj ion. It was found, in addition, that the... 

Figure 3. Basic scheme of the possible photocatalytic action of an atmospheric dust particle with semiconductor properties in which the particle is considered to be covered with a layer of adsorbed atmospheric water. A and B are atmospheric components undergoing, respectively, oxidation and reduction by light-generated holes 0 and electrons Aa and Ba are forms of A and B that have been adsorbed on the particle surface (or absorbed by the water layer). shows the direction of energy change for electrons in the semiconductor particle g is the width of the forbidden band. For simplicity, possible interactions of electrons and holes with water are not shown.

To calculate gas solubility in natural geochemical systems, basic thermodynamic properties such as the Henry s law constant and, in the case of weak electrolytes the dissociation constant, must be combined with a thermodynamic model of aqueous solution behavior. An analogous approach has been used to predict mineral solubilities in concentrated brines (1). Such systems are also relevant to the atmosphere where very concentrated solutions occur as micrometer sized aerosol particles and droplets, which contain very small amounts of water relative to the surrounding gas phase. The ambient relative humidity (RH) controls solute concentrations in the droplets, which will be very dilute near 1(X)% RH, but become supersaturated with respect to soluble constituents (such as NaCl) below about 75% RH. The chemistry of the aerosol is complicated by the non-ideality inherent in concentrated electrolyte solutions. 

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