Relative humidity deliquescence

The predicted solids based on thermodynamic measurements and models derived from them depend strongly on chemical composition. However, small aqueous aerosols remain meta-stable with decreasing RH until reaching a crystallization relative humidity(CRH). In contrast, solid aerosols t e up water at the thermodynamically favored deliquescence relative humidity(DRH). This hysteresis causes a dependence of aerosol phase on RH history. 

Upon drying, sea salt particles remain in a metastable highly concentrated solution state below their deliquescence relative humidity of —75%. Only when they reach their crystallization (or effluescence) point, which is —45% relative humidity for NaCl, will they assume the crystalline form. This hysteresis effect is well documented by laboratory experiments (e.g., Shaw and Rood, 1990 Tang, 1997 Pmppacher and Klett, 1997 Lee and Hsu, 2000) and implies that, in the MBL, sea salt aerosol will usually be present in an aqueous form. Only in very dry marine regions and in the free troposphere, where the relative humidity is less than 45%, these particles can be expected to be dry. Even then a semiliquid layer can be present on the surface which makes sruface reactions easier. 

TABLE 10.1 Deliquescence Relative Humidities of Electrolyte Solutions at 298 K... 

Water equilibrium between the gas and aerosol phases at the point of deliquescence requires that the deliquescence relative humidity of a salt will then satisfy... 

The behavior of the same system at a higher relative humidity, 75%, is illustrated in Figure 10.18. Recall that the deliquescence relative humidities for the NH4HSO4 and... 

The life persistency of a smoke cloud is deterrnined chiefly by wind and convection currents in the air. Ambient temperature also plays a part in the continuance or disappearance of fog oil smokes. Water vapor in the air has an important role in the formation of most chemically generated smokes, and high relative humidity improves the performance of these smokes. The water vapor not only exerts effects through hydrolysis, but it also assists the growth of hygroscopic (deliquescent) smoke particles to an effective size by a process of hydration. Smoke may be generated by mechanical, thermal, or chemical means, or by a combination of these processes (7). 

The presence of moisture on steel above the critical humidity but below the saturation point may be caused by an adsorption mechanism or by the presence of particles of deliquescent salts on the surface. Once rusting has begun, the composition of the rust already formed will influence the relative humidity at which further rusting will occur, because rusts formed in polluted atmospheres contain hygroscopic salts. The method by which moisture reaches the surface is probably less important, however, than the length... 

With many electrolytes, AP is so large that the solid, when exposed to moist air, picks up water (deliquesces). This occurs with calcium chloride, whose saturated solution has a vapor pressure only 30% that of pure water. If dry CaCl2 is exposed to air in which the relative humidity is greater than 30%, it absorbs water and forms a saturated solution. Deliquescence continues until the vapor pressure of the solution becomes equal to that of the water in the air. 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

A deliquescent material takes up moisture freely in an atmosphere with a relative humidity above a specific, well-defined critical point. That point for a given substance is defined as the critical relative humidity (RH0). Relative humidity (RH) is defined as the ratio of water vapor pressure in the atmosphere divided by water vapor pressure over pure water times 100% [RH = (PJP0) X 100%]. Once moisture is taken up by the material, a concentrated aqueous solution of the deliquescent solute is formed. The mathematical models used to describe the rate of moisture uptake involve both heat and mass transport. 

The effect of relative humidity and temperature on the physical and structural properties of the 1 1 isopropanol solvatomorph of warfarin has been studied [58], Below the critical relative humidity of 60-68% the solid is not hygroscopic, but becomes deliquescent at higher values of relative humidity without exchange of water for isopropanol. Storage of the solvate-morph at elevated temperatures causes formation of an amorphous solid owing to loss of isopropanol, which may proceed through an intermediate crystalline phase. 

The relative humidity at which a solid begins to deliquesce, RH0, can be determined in two ways directly, by measuring the relative humidity above a saturated solution of the substance or indirectly, by measuring the steady state moisture uptake rate at relative humidities above RH0 and then extrapolating to the relative humidity at which the moisture uptake rate is zero [1,30,31],... 

Fig. 1 Water vapor adsorption and deliquescence of a water-soluble solid (a) Atmospheric relative humidity, RHj < RHq (b) RH = RH0 and (c) RHj > RH0.

If the relative humidity is above the deliquescence point of NaCl (75% at 25°C), the sea salt particles are... 

It should be noted that as with all analytical techniques that involve subjecting the sample to vacuum conditions before and/or during the analysis, separation of components via selective crystallization is expected (e.g., Ge et al., 1998a). Hence these particles may not have actually existed in these crystalline forms at relative humidities above their deliquescence points in the atmosphere, although the various constituents observed were clearly present. 

Humidity can affect explosives and propellants both adversely and beneficiently. If the humidity is very high, an explosive can pick up enough moisture to cause it to deteriorate or malfunction. A prime example of this is Ammonium Nitrate which must be stored and handled in special dehumidified areas to prevent caking. The humidity above which a substance deliquesces, or below which it ceases to be hygroscopic is called the critical relative humidity (CRH). 

C. Sulfamates prepared from weak bases form acidic solutions, whereas those prepared from strong bases produce neutral solutions. The pH of 5 wt % solution of ammonium sulfamate is 5.2. Crystals of ammonium sulfamate deliquesce at relative humidity of 70% and higher. Both ammonium sulfamate [7773-06-0] and potassium sulfamate [13823-30-2] hbemte ammonia at elevated temperatures and form the corresponding imidodisulfonate (12). Inorganic sulfamates are quite water-soluble, except for the basic mercury salt. Some relative solubilities of sulfamates at 25°C in 100 g of water are ammonium, 103 g sodium, 106 g magnesium, 119 g calcium, 67 g barium, 34.2 g zinc, 115 g and lead, 218 g. The properties of a number of sulfamates may be found in the literature (see Table 5). 

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