Project

ORGANIC AEROSOLS; TROPOSPHERIC CHEMISTRY; THERMODYNAMICS; MISCIBILITY GAPS; EFFLORESCENCE/DELIQUESCENCE; RADIATIVE FORCING; thermodynamic model; activity coefficient; liquid-liquid phase separation; Raman microscopy; spinodal decomposition; deliquescence; efflorescence

Lead
Lay summary
The physical state of the tropospheric aerosol is still largely unknown despite its importance for cloud formation, multiphase and heterogeneous chemistry in and on aerosol particles, and for the aerosol's radiative properties. Especially the high organic fraction (of largely unknown composition) of the atmospheric particulate matter makes it difficult to predict its physical state.We have developed a thermodynamic activity coefficient model called AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients) that is able to calculate vapor-liquid, solid-liquid, and liquid-liquid equilibria in atmospheric aerosols composed of a wide range of inorganic salts and organic compounds. In the present project, we will assess and generalize AIOMFAC. The main focus will be on aqueous organic systems at low temperatures containing the functionalities typically found in tropospheric aerosol particles, namely alkyl, hydroxyl, carboxyl, carbonyl, ether, aromatic, aldehyde, and vinyl groups. A second focus is the generalization of the model to low temperatures by the development of suitable parameterizations for electrolyte solutions containing the important inorganic constituents of aerosols such as ammonium sulfate and nitrate and sodium chloride. In the experimental part of the project, we will study systems with liquid-liquid phase separations that are confined within the supersaturated region of the inorganic salt with a Raman microscope during humidity cycles. The investigation of micrometer sized droplets allows determining liquid-liquid phase boundaries and metastability phase diagrams that are not accessible by bulk methods. Moreover, we will investigate whether the crystallization of the salt is preceded or even triggered by a liquid-liquid phase separation and how gas/particle partitioning is influenced in the presence of a liquid-liquid phase separation.With the further development of AIOMFAC it will be possible to model the phases of the systems that we investigate experimentally and to predict their phase states based on their chemical composition. Thus, the experimental investigations together with the AIOMFAC model allow to estimate the occurrence probability of liquid-liquid phase separations in tropospheric aerosols, and more generally their hygroscopicity, reactivity, and gas/particle partitioning of semi-volatile species.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

The physical state of the tropospheric aerosol is still largely unknown despite its importance for the aerosol’s radiative properties, for cloud formation, and for multiphase and heterogeneous chemistry in and on aerosol particles. Especially the organic fraction (of mostly unresolved composition) of the particulate matter makes it difficult to predict its physical state.We have developed and corroborated the scientific hypothesis that due to the complexity of the organic fraction of the tropospheric aerosol, organics will mostly stay in the liquid state. This organic liquid takes up and releases water continuously when the relative humidity (RH) changes. In the atmosphere, the organic fraction is usually internally mixed with inorganic salts that are dissolved in aqueous solution particles at high RH and may form crystalline solids at low RH. While the dissolution of the salt (deliquescence) occurs at the relative humidity present above the saturated solution of the salt, high supersaturations of the salt are usually required before atmospheric particles crystallize (effloresce). Experiments and modeling studies have shown that deliquesced aerosols can be present as one-phase systems consisting of an aqueous solution of organics, inorganic salts and water or as two-phase systems consisting of a predominantly organic and a predominantly inorganic aqueous phase. In the SNF Project 200020-103651 (Physical states of mixed organic / inorganic aerosols) that will finish by the end of 2009, we investigate liquid-liquid and liquid-solid phase changes of mixed organic-inorganic droplets. To this end we have developed a thermodynamic activity coefficient model called AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients) that is able to calculate vapor-liquid, solid-liquid, and liquid-liquid equili-bria of systems composed of a wide range of inorganic salts and organic compounds with CHn and OH functional groups. Experimentally we investigated the phase changes of droplets consisting of a mixture of poly(ethylene glycol) 400 and ammonium sulfate with a Raman microscope during humidity cycles. This mixed organic-inorganic model system exhibits liquid-liquid phase separation at relative humidities below 90 % in addition to deliquescence/efflo¬rescence of the ammonium sulfate fraction. This combined modeling and experimental approach allows deepening the understanding of multiphase aerosols by investigating the same system experimentally and theoretically.In this continuation proposal we again apply for funding for two PhD theses that build upon the achievements of SNF Project 200020-103651. The objective of PhD student I is the assessment and generalization of AIOMFAC. The main focus will be on aqueous organic systems at low temperatures containing the functionalities typically found in tropospheric aerosol particles, namely alkyl, hydroxyl, carboxyl, carbonyl, ether, aromatic, aldehyde, and vinyl groups. These systems are treated by AIOMFAC’s short-range part. Time permitting, a second focus is the generalization of the model to low temperatures by the development of suitable parameterizations for electrolyte solutions that are treated in the long- and middle-range part of AIOMFAC. This PhD work comprises the collection of activity data at low temperatures from literature, measurements of the temperature dependence of water activities for solutions that are of special interest to atmospheric aerosols, and to determine temperature dependent AIOMFAC interaction parameters by fitting the model to experimental data.PhD student II will study systems with liquid-liquid phase separations that are confined within the supersaturated region of the inorganic salt. The investigation of micrometer sized drop-lets allows determining liquid-liquid phase boundaries and state diagrams that are not accessible by bulk methods. Moreover, PhD II will investigate whether the crystallization of the salt is preceded or even triggered by a liquid-liquid phase separation and how gas/particle partitioning is influenced in the presence of a liquid-liquid phase separation. With the further development of AIOMFAC it will be possible to model the phases of systems investigated in PhD II and to predict their phase states based on their chemical composition. Thus, the experimental investigations together with the AIOMFAC model allow to estimate the occurrence probability of liquid-liquid phase separations in tropospheric aerosols, and more generally their hygroscopicity, reactivity, and gas/particle partitioning of semi-volatile species.