Relative humidity effects saturated water vapor pressure

Saturated salt solutions and sulfuric acid solutions establish relative humidity by reducing the vapor pressure above an aqueous solution (a colligative effect). Saturated salt solutions at controlled temperature maintain a constant relative humidity as long as excess salt and bulk solution are present. As water is added or removed from the solution, moisture from the head-space will either condense or evaporate (as appropriate), with subsequent dissolution or precipitation of salt to maintain the equilibrium vapor pressure. Because the degree of vapor pressure depression is dependent on the number of species in solution and, further, since the solubility of most salts is somewhat dependent on temperature, the relative humidity generated is also temperature dependent. Hence, use of the same salt at different temperatures can result in different relative humidities. Refs. can be consulted for specific saturated salt solutions that result in defined relative humidities as a function of... 

Only two possibilities exist for explaining the existence of cloud formation in the atmosphere. If there were no particles to act as cloud condensation nuclei (CCN), water would condense into clouds at relative humidities (RH) of around 300%. That is, air can remain supersaturated below 300% with water vapor for long periods of fime. If this were to occur, condensation would occur on surface objects and the hydrologic cycle would be very different from what is observed. Thus, a second possibility must be the case particles are present in the air and act as CCN at much lower RH. These particles must be small enough to have small settling velocity, stay in the air for long periods of time and be lofted to the top of the troposphere by ordinary updrafts of cm/s velocity. Two further possibilities exist - the particles can either be water soluble or insoluble. In order to understand why it is likely that CCN are soluble, we examine the consequences of the effect of curvature on the saturation water pressure of water. 

This analysis can be applied to a small dry salt particle exposed to increasing relative humidity. The particle remains solid until, if it is hygroscopic, a characteristic relative humidity less than 100% at which it absorbs water and dissolves, forming a saturated solution. The relative humidities at which this occurs for saturated solutions of various salts are shown in Table 9.3. These values will vary with crystal size because of the Kelvin effect. For sodium chloride,. solution takes place at a relative humidity of 75% at which the diameter about doubles. With increasing relative humidity, the equilibrium relationship between drop size and vapor pressure is determined by the interaction of the Kelvin effect and vapor pressure lowering. 

Humidity is defined as the concentration of water molecules in the atmosphere. In practice, parameters important in the measurement of humidity are partial pressure of water, mixing ratio, specific humidity, absolute humidity, mole fraction of water vapor, relative humidity (r. h.), and dew-point temperature. Of these measurements, relative humidity, which is the ratio of the actual water pressure to the saturated pressure, is widely used. The concentration of water molecules in the air is low and, moreover, the effects of water are very complicated not only chemically but also physically. The measurement of humidity is difficult compared to the measurement of temperature. In many industries from the electronic industry to agriculture, there is a demand for humidity control. For example, a dry atmosphere is required for the pro-... 

This initial stage of droplet formation deserves a careful explanation. Over a flat, pure water surface at 100% relative humidity (saturation with respect to water), water vapor is in equilibrium, which means that the number of water molecules leaving the water surface is balanced by the number arriving at the surface. Molecules at water surfaces are subjected to intermolecular attractive forces exerted by the nearby molecules below. If the water surface area is increased by adding curvature, molecules must be moved from the interior to the surface layer, in which case energy is required to oppose the cohesive forces of the liquid. As a consequence, for a pure water droplet to be at equilibrium, the relative humidity has to exceed the relative humidity at equilibrium over a flat, pure water surface, or be supersaturated. The flux of molecules to and from a surface produces what is known as vapor pressure. The equilibrium vapor pressure is less over a salt solution than it is over pure water at the same temperature. This effect balances to some extent the increase in equilibrium vapor pressure caused by the surface curvature of small droplets. Droplets with high concentrations of solute can then be at equilibrium at subsaturation. 

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