Relative humidity efflorescence

Martin S. T., Han J. H., and Hung H. M. (2001) The size effect of hematite and alumina inclusions on the efflorescence relative humidities of aqueous ammonium sulfate particles. Geophys. Res. Lett. 28, 2601-2604. 

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]. 

This process is known as efflorescence. It would appear that the relative humidity of the air was rather low for this to occur. When rehydrated and gently dried, the hydrate reappeared indicating a reversible reaction ... 

Since the 7>(H20) of 7.92 is less than the vapor pressure of water at the same temperature (23.8 torr), CUSO4 5H20 will not always effloresce. It will effloresce only on a dry day when the partial pressure of water in the air is less than 7.92 torr. This will occur when the relative humidity is less than... 

Citric acid monohydrate melts at 100°C. It loses water at 75° C, becomes anhydrous at 135°C, and fuses at 153°C. At relative humidities (RH) lower than approximately 65%, it effloresces at 25° C the anhydrous acid is formed at humidities below approximately 40%. At RH between approximately 65 and 75%, it sorbs insignificant amoimts of moisture, but above this, substantial amoimts are absorbed (Fig. 1). " ... 

Heat of solution —16.3kJ/mol (—3.9 kcal/mol) at 25°C Hygroscopicity at relative humidities less than about 65%, citric acid monohydrate effloresces at 25°C, the anhydrous acid being formed at relative humidities less than about 40%. At relative humidities between about 65% and 75%, citric acid monohydrate absorbs insignificant amounts of moisture, but under more humid conditions substantial amounts of water are absorbed. 

Loss of water usually causes a breakdown in the structure of the crystal this is commonly seen with sodium sulfate, whose vapor pressure is sufficiently large that it can exceed the partial pressure of water vapor in the air when the relative humidity is low. What one sees is that the well-formed crystals of the decahydrate undergo deterioration into a powdery form, a phenomenon known as efflorescence. When a solid is able to take up moisture from the air, it is described as hygroscopic. A small number of anhydrous solids that have low vapor pressures not only take up atmospheric moisture on even the driest of days, but will become wet as water molecules are adsorbed onto their surfaces this is most commonly observed with sodium hydroxide and calcium chloride. With these solids, the concentrated solution that results continues to draw in water from the air so that the entire crystal eventually dissolves into a puddle of its own making solids exhibiting this behavior are said to be deliquescent. 

Under a low relative humidity condition, due to the high evaporation rate, the jxrsition of the wet-dry interface will move to the inner part of the element and closer to the bottom of the element, where a satmation p>eak will build up with a very small width, forming supersaturation, and resulting in sub-efflorescence and more severe deterioration. The breadth of efflorescence zone decreases and the average concentration of the pore solution will remain constant at nearly the initial concentration of exposme solution (as shown in Fig. 4 (c)). 

Bertram AK, Martin ST, Hanna SJ, Smith ML, Bodsworth A, Chen Q, Kuwata M, Liu A, You Y, Zorn SR (2011) Predicting the relative humidities of liquid-liquid phase separation, efflorescence, and deliquescence of mixed particles of ammonium sulfate, organic material, and water using the organic-to-sulfate mass ratio of the particle and the oxygen-to-carbon elemental ratio of the organic component. Atmos Chem Phys 11 10995-11006... 

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