Mixing of the Organic Aerosol Fractions:  Liquids as the Thermodynamically Stable Phases

Abstract

An increasing number of single-particle measurements show that organic and inorganic constituents of the atmospheric aerosol are internally mixed within the particles. Therefore, the phases of the aerosol will be influenced by both mixing of the organic substances with each other and mixing between organic and inorganic constituents. In this work, the mixing properties of the organic aerosol fractions have been investigated theoretically and experimentally with respect to melting and deliquescence. We show that a liquid (or an amorphous solid) is the thermodynamically stable phaseseven in the absence of water as a solventsprovided that a sufficiently high number of miscible components are present. Furthermore, we show experimentally that the deliquescence relative humidity of aqueous solutions of dicarboxylic acids decreases with an increasing number of components present in the solution. A five-component mixture consisting of malic, malonic, maleic, glutaric, and methylsuccinic acids deliquesces at a relative humidity (RH) as low as 45.4% RH, while the pure dicarboxylic acids exhibit deliquescence points between 72 and 96% RH. A further reduction of the deliquescence relative humidity is observed when an inorganic salt is added to the dicarboxylic acid fivecomponent mixture. For NaCl, deliquescence of the eutonic composition occurred at 41.3%, for ammonium sulfate at 36.4%, and for ammonium nitrate even at 27.1% RH. Interactions between the solutes lead to either higher or lower solubilities in the multicomponent mixture as compared to the respective single-component aqueous solutions. In the mixed dicarboxylic acids/inorganic salt solutions, the solubilities of ammonium nitrate and sulfate are increased by ∼40%, the one of sodium chloride is decreased by a similar amount. In summary, these mixing properties suggest that small fractions of organic species prevent tropospheric aerosols from becoming fully solid, and the organic fraction may even stay fully liquid irrespective of tropospheric humidity.

Internal mixing of the organic aerosol by gas phase diffusion of semivolatile organic compounds

“…Experimental and theoretical thermodynamic considerations indicate that the organic particulate matter remains liquid down to very low relative humidity, if it consists of a high enough number of miscible components. It therefore absorbs/desorbs molecules from/to the gas phase (Marcolli et al, 2004). This assumption is in accordance with smog chamber measurements, which are best described by a gas/particle absorptive partitioning model (Odum et al, 1996;Liang et al, 1997).…”

“…A high degree of internal mixing of the organic fraction with inorganic constituents is experimentally confirmed by an increasing number of single-particle measurements (Murphy and Thomson, 1997;Lee et al, 2002Lee et al, , 2003. Moreover, the internal mixing leads to a systematic depression of melting and deliquescence points of organic and mixed organic/inorganic aerosols, thus leading to an aerosol population in the lower troposphere which is predominantly liquid (Marcolli et al, 2004).…”

“…The internal mixing of the organic aerosol constituents leads to a systematic depression of melting and deliquescence points of organic and mixed organic/inorganic aerosols (Marcolli et al, 2004). Figure 3 shows the abundance of organic substances in the water-soluble organic fraction of samples from Tokyo, 1992, in terms of weight percent (Sempéré and Kawamura, 1994).…”

“…Figure 3 shows the abundance of organic substances in the water-soluble organic fraction of samples from Tokyo, 1992, in terms of weight percent (Sempéré and Kawamura, 1994). The grey bars give the calculated solubilities assuming ideal behavior (Marcolli et al, 2004). The listed substances will be internally mixed within a second (e.g.…”

Ice nucleation in sulfuric acid/organic aerosols: implications for cirrus cloud formation

“…Therefore, acidcatalyzed reactions on sulfuric acid aerosols could play a role in transforming organic compounds from those that likely depress or have no effect on ice nucleation temperatures to those that actually elevate the ice nucleation temperatures through heterogeneous nucleation. However, if no reaction transforms the shorter carbon chain organic compounds to larger organic compounds, the solubility limit will likely not be reached for the shorter carbon chain, highly soluble organic species in the atmosphere (Marcolli et al, 2004).…”