Particles deliquescence

Multicomponent aerosol particles exhibit behavior similar to that of single-component salts. As the ambient RH increases the salt mixture is solid, until the ambient RH reaches the deliquescence point of the mixture, at which the aerosol absorbs atmospheric moisture and produces a saturated solution. A typical set of data of multicomponent particle deliquescence, growth, evaporation, and then crystallization is shown in Figure 10.7 fora KCl-NaCl particle. Note that the DRH for the mixed-salt particle occurs at 72.7% RH, which is lower than the DRH of either NaCI (75.3%) or KC1 (84.2%). 

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... 

Here our interest is in the application of homogeneous nucleation theory to produce the comprehensive plots of meta-stable crystallization. Fig. 1 illustrates the meta-stable efflorescence paths(solid lines) of (NH4)2S04 and (NH4)3H(S04)2 particles as a function of RH with the decreasing rate of ARH = 0.005 min with the deliquescence paths(O). Fig. 2 shows the expectation time of the aqueous particle composed of (NH4)2S04 and H2SO4... 

Wang Jia, Zhang Jibiao. The effect of micro-droplets formation caused by the deliquescence of the deposited salt particle on atmospheric corrosion of metals. Proceedings 16th International Corrosion Congress, Beijing, China, September 19-24, 2005. 

In addition, the high concentrations of ions in solutions of high ionic strength such as sea salt particles (especially near their deliquescence point) can alter gas solubility. In this case, the Henry s law constants must be modified using Setchenow coefficients to take this effect into account (e.g., Kolb et al., 1997). 

Figure 7.14 shows the calculated ratio of S(IV) oxidation with the uptake and reaction of N03 to that without the NO, contribution as a function of the chloride concentration in particles (Rudich et al., 1998). For reference, the saturation concentration of Cl- in sea salt particles (i.e., at the deliquescence point) is 6 M at room temperature. Under the assumptions of these particular calculations, the rate of aqueous-phase oxidation of S(IV) is estimated to increase by as much as 25% when N03 chemistry is taken into account. This uptake and reaction of NO, also decrease its gas-phase concentrations. 

The ammonium nitrate formed in reaction (54) can exist either as a solid particle or in solution, and since this reaction is an equilibrium, it can redissociate to form the reactants. The deliquescence point for NH4N03 at 25°C is 62% RH i.e., at a water vapor concentration corresponding to 62% RH, the solid particle dissolves to form a concentrated liquid solution. 

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

Atmospheric aerosols are hygroscopic, taking up and releasing water as the RH changes (see also Section C.l) because some of the chemical components are themselves deliquescent in pure form. For example, sodium chloride, the major component of sea salt, deliquesces at 298 K at an RH of 75%, whereas ammonium sulfate, (NH4)2S04, and ammonium nitrate, NH4N03, deliquesce at 80 and 62% RH, respectively. (See Table 9.16 for the deliquescence points of some common constituents of atmospheric particles.) De-... 

It should be noted that hysteresis occurs as the particles are dried out i.e., the liquid solution does not form a solid particle as the water evaporates at the same RH as it went through the solid - liquid transition. Typically one must reach RHs 20-30% or more below the deliquescence point in order to dry the particle. Figure 9.41, for example, shows the uptake of water by solid (NH4)2S04 and its subsequent dehydration (Tang et al., 1995). At 80% RH the solid deliquesces but does not solidify (effluoresce) on drying until an RH of 37% is reached. (The presence of other species has been shown to increase this effluorescence... 

Tang, I. N., Deliquescence Properties and Particle Size Change of Hygroscopic Aerosols, in Generation of Aerosols and Facilities for Exposure Experiments (K. Willeke, Ed.), Chap. 7, pp. 153-167, Ann Arbor Science Publishers, Ann Arbor, MI, 1980. 

Wagner, J., E. Andrews, and S. M. Larson, Sorption of Vapor Phase Octanoic Acid onto Deliquescent Salt Particles, J. Geophys. Res., 101, 19533-19540 (1996). 

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. 

For example, Neubauer et al. (1998) have shown that the spectra can be very sensitive to the amount of water present and whether the particle is aqueous, i.e., above the deliquescence point, or solid (but holding adsorbed water on the surface). Figure 11.72 shows the... 

Figure 12.22 shows the composition in terms of the weight percent HNO, and H2S04 as a function of temperature as solid SAT is cooled from 194 K under conditions corresponding to a pressure of 50 rnbar in an atmosphere containing 5 ppm HzO and an HNO, concentration of 10 ppb (Koop and Carslaw, 1996). Under these particular conditions, as the temperature falls below 192 K, the SAT is in equilibrium with a liquid film on the particle containing both HN03 and H20. The particular temperature at which SAT deliquesces is a function of the water vapor and gaseous nitric acid concentrations as shown in Fig. 12.23. As the temperature falls further and more HNO, and HzO are taken up into the liquid, the solid SAT dissolves completely, forming a ternary solution of the two acids and water. This solution can then act again to nucleate PSCs.

Take as an example, a small dry particle of NaCl of a given mass (mu) that is introduced into air at a water vapor pressure corresponding to SA in Fig. 14.38a. Assuming that the RH is above the deliquescence point of NaCl, 75% at 25°C, the particle will take up water, dissolve, and form a stable droplet of radius rA. Similarly, if the air saturation ratio increases to Su, the particle will, under equilibrium conditions, take up water and grow to radius ru. 

Sodium biphosphate was known as a dimorphous salt .. . of the 4 atoms of water which the crystals contain, they lose, I find, 2 atoms at the temperature of 212° (F.), and not a particle more till heated up to about 875°. After heating to 212° it contains 3 atoms base, namely, one atom soda and 2 atoms water united to a double atom of phosphoric acid. The salt cannot sustain the loss of any portion of this water without assuming a new train of properties. Several other forms were obtained by heating to higher temperatures, and at a low red heat a glass was obtained which was deliquescent, not crystallisable from solution, and which gave the reactions of phosphoric acid ignited per se. In modern symbols—... 

Experimental problems with TGA are usually connected with sample preparation for instance, homogeneous or very disperse particle sizes may yield different results, while the presence of humidity adsorbed on the surface of the particles may mask or alter the response. Deliquescent or highly hygroscopic samples yield poorly reproducible results because it can be difficult to discriminate between removal of wetting solvent and removal of structural solvent. It is useful to accompany DSC experiments with TGA experiments. Heat absorption in a DSC plot may correspond to solvent loss and not to a phase transition (see above). Importantly, as shown below, a desolvation process may sometimes induce the formation of another polymorph or pseudo-polymorph not otherwise attainable. 

Chloride ions from supermicron sea-salt particles are replaced by sulfate, nitrate and minor contributions of oxalate, malonate and succinate. The principal mechanisms causing accumulation of sulfate in sea-salt particles are cloud processing and, to a lesser degree, heterogeneous reactions taking place in deliquescent particles. Mechanisms for the chloride replacement by nitrate are not clear [54]. 

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. 

Hygroscopic behavior has been well characterized in laboratory studies for a variety of materials, for example, ammonium sulfate (Figure 14), an important atmospheric material. When an initially dry particle is exposed to increasing RH it rapidly accretes water at the deliquescence point. If the RH increases further the particle continues to accrete water, consistent with the vapor pressure of water in equilibrium with the solution. The behavior of the solution at RH above the deliquescence point is consistent with the bulk thermodynamic properties of the solution. However, when the RH is lowered below the deliquescence point, rather than crystallize as would a bulk solution, the material in the particle remains as a supersaturated solution to RH well below the deliquescence point. The particle may or may not undergo a phase transition (efflorescence) to give up some or all of the water that has been taken up. For instance, crystalline ammonium sulfate deliquesces at 79.5% RH at 298 K, but it effloresces at a much lower RH, 35% (Tang and Munkelwitz, 1977). This behavior is termed a hysteresis effect, and it can be repeated over many cycles. 

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