Phase separation in organic aerosol

Abstract. The partitioning of semivolatile organic compounds between the gas phase and aerosol particles is an important source of secondary organic aerosol (SOA). Gas-particle partitioning of organic and inorganic species is influenced by the physical state and water content of aerosols, and therefore ambient relative humidity (RH), as well as temperature and organic loading levels. We introduce a novel combination of the thermodynamic models AIOMFAC (for liquid mixture non-ideality) and EVAPORATION (for pure compound vapor pressures) with oxidation product information from the Master Chemical Mechanism (MCM) for the computation of gas-particle partitioning of organic compounds and water. The presence and impact of a liquid-liquid phase separation in the condensed phase is calculated as a function of variations in relative humidity, organic loading levels, and associated changes in aerosol composition. We show that a complex system of water, ammonium sulfate, and SOA from the ozonolysis of α-pinene exhibits liquid-liquid phase separation over a wide range of relative humidities (simulated from 30% to 99% RH). Since fully coupled phase separation and gas-particle partitioning calculations are computationally expensive, several simplified model approaches are tested with regard to computational costs and accuracy of predictions compared to the benchmark calculation. It is shown that forcing a liquid one-phase aerosol with or without consideration of non-ideal mixing bears the potential for vastly incorrect partitioning predictions. Assuming an ideal mixture leads to substantial overestimation of the particulate organic mass, by more than 100% at RH values of 80% and by more than 200% at RH values of 95%. Moreover, the simplified one-phase cases stress two key points for accurate gas-particle partitioning calculations: (1) non-ideality in the condensed phase needs to be considered and (2) liquid-liquid phase separation is a consequence of considerable deviations from ideal mixing in solutions containing inorganic ions and organics that cannot be ignored. Computationally much more efficient calculations relying on the assumption of a complete organic/electrolyte phase separation below a certain RH successfully reproduce gas-particle partitioning in systems in which the average oxygen-to-carbon (O:C) ratio is lower than ~0.6, as in the case of α-pinene SOA, and bear the potential for implementation in atmospheric chemical transport models and chemistry-climate models. A full equilibrium calculation is the method of choice for accurate offline (box model) computations, where high computational costs are acceptable. Such a calculation enables the most detailed predictions of phase compositions and provides necessary information on whether assuming a complete organic/electrolyte phase separation is a good approximation for a given aerosol system. Based on the group-contribution concept of AIOMFAC and O:C ratios as a proxy for polarity and hygroscopicity of organic mixtures, the results from the α-pinene system are also discussed from a more general point of view.

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Liquid-liquid phase separation in secondary organic aerosol particles produced from α-pinene ozonolysis and α-pinene photo-oxidation with/without ammonia

10.5194/acp-2019-19 ◽

2019 ◽

Author(s):

Suhan Ham ◽

Zaeem Bin Babar ◽

Jaebong Lee ◽

Hojin Lim ◽

Mijung Song

Keyword(s):

Phase Separation ◽

Liquid Phase ◽

Secondary Organic Aerosol ◽

Cloud Condensation Nuclei ◽

Aerosol Particles ◽

Organic Aerosol ◽

Liquid Phase Separation ◽

Photo Oxidation ◽

Different Types ◽

Liquid Liquid Phase Separation

Abstract. Recently, liquid–liquid phase separation (LLPS) of secondary organic aerosol (SOA) particles free of inorganic salts has been intensively studied because of their importance on cloud condensation nuclei (CCN) properties. Herein, we investigated LLPS in four different types of SOA particles generated from α-pinene ozonolysis and α-pinene photo-oxidation in the absence and presence of NH3. LLPS was observed in SOA particles produced from α-pinene ozonolysis at ~ 95.8 % relative humidity (RH) and α-pinene ozonolysis with NH3 at ~ 95.4 % RH. However, LLPS was not observed in SOA particles produced from α-pinene photo-oxidation and α-pinene photo-oxidation with NH3. With datasets of average oxygen to carbon elemental ratio (O : C) for different types of SOA particles of this study and previous studies, LLPS occurred when the O : C ratio was less than ~ 0.44 and LLPS did not occur when the O : C ratio was greater than ~ 0.40. When LLPS was observed, the two liquid phases were present up to ~ 100 % RH. This result can help to predict more accurate results of CCN properties of organic aerosol particles.

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Supplementary material to "Liquid-liquid phase separation in secondary organic aerosol particles produced from α-pinene ozonolysis and α-pinene photo-oxidation with/without ammonia"

10.5194/acp-2019-19-supplement ◽

2019 ◽

Author(s):

Suhan Ham ◽

Zaeem Bin Babar ◽

Jaebong Lee ◽

Hojin Lim ◽

Mijung Song

Keyword(s):

Phase Separation ◽

Liquid Phase ◽

Secondary Organic Aerosol ◽

Aerosol Particles ◽

Organic Aerosol ◽

Liquid Phase Separation ◽

Photo Oxidation ◽

Supplementary Material ◽

Liquid Liquid Phase Separation

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Modeling the gas-particle partitioning of secondary organic aerosol: the importance of liquid-liquid phase separation

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-2199-2012 ◽

2012 ◽

Vol 12 (1) ◽

pp. 2199-2258 ◽

Cited By ~ 1

Author(s):

A. Zuend ◽

J. H. Seinfeld

Keyword(s):

Phase Separation ◽

Liquid Phase ◽

Relative Humidity ◽

Organic Compounds ◽

Organic Aerosol ◽

Liquid Phase Separation ◽

Organic Loading ◽

Computational Costs ◽

Ideal Mixing ◽

Liquid Liquid Phase Separation

Abstract. The partitioning of semivolatile organic compounds between the gas phase and aerosol particles is an important source of secondary organic aerosol (SOA). Gas-particle partitioning of organic and inorganic species is influenced by the physical state and water content of aerosols, and therefore ambient relative humidity (RH), as well as temperature and organic loading levels. We introduce a novel combination of the thermodynamic models AIOMFAC (for liquid mixture non-ideality) and EVAPORATION (for pure compound vapor pressures) with oxidation product information from the Master Chemical Mechanism (MCM) for the computation of gas-particle partitioning of organic compounds and water. The presence and impact of a liquid-liquid phase separation in the condensed phase is calculated as a function of variations in relative humidity, organic loading levels, and associated changes in aerosol composition. We show that a complex system of water, ammonium sulfate, and SOA from the ozonolysis of α-pinene exhibits liquid-liquid phase separation over a wide range of relative humidities (simulated from 30% to 99% RH). Since fully coupled phase separation and gas-particle partitioning calculations are computationally expensive, different simplified model approaches are tested with regards to computational costs and accuracy of predictions compared to the benchmark calculation. Both forcing a liquid one-phase aerosol considering non-ideal mixing or assuming an ideal mixture bear the potential for vastly incorrect partitioning predictions. Assuming an ideal mixture leads to substantial overestimation of the particulate organic mass, at high RH by more than 200%. Moreover, the simplified one-phase cases stress two key points for accurate gas-particle partitioning calculations: (1) non-ideality in the condensed phase needs to be considered and (2) liquid-liquid phase separation is a consequence of considerable deviations from ideal mixing in solutions containing inorganic ions and organics that cannot be ignored. Computationally much more efficient calculations relying on the assumption of a complete organic/electrolyte phase separation below a certain RH successfully reproduce gas-particle partitioning in systems in which the average oxygen-to-carbon (O:C) ratio is lower than ~0.6, as in the case of α-pinene SOA, and bear the potential for implementation in atmospheric chemical transport models and chemistry-climate models. A full equilibrium calculation is the method of choice for accurate offline (box model) computations, where high computational costs are acceptable. Such a calculation enables the most detailed predictions of phase compositions and provides necessary information on whether assuming a complete organic/electrolyte phase separation is a good approximation for a given aerosol system. Based on the group-contribution concept of AIOMFAC and O:C ratios as a proxy for polarity and hygroscopicity of organic mixtures, the results from the α-pinene system are also discussed from a more general point of view.

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Liquid–liquid phase separation in secondary organic aerosol particles produced from <i>α</i>-pinene ozonolysis and <i>α</i>-pinene photooxidation with/without ammonia

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-9321-2019 ◽

2019 ◽

Vol 19 (14) ◽

pp. 9321-9331 ◽

Cited By ~ 7

Author(s):

Suhan Ham ◽

Zaeem Bin Babar ◽

Jae Bong Lee ◽

Ho-Jin Lim ◽

Mijung Song

Keyword(s):

Phase Separation ◽

Liquid Phase ◽

Secondary Organic Aerosol ◽

Cloud Condensation Nuclei ◽

Aerosol Particles ◽

Organic Aerosol ◽

Liquid Phase Separation ◽

Liquid Phases ◽

Different Types ◽

Liquid Liquid Phase Separation

Abstract. Recently, liquid–liquid phase separation (LLPS) of secondary organic aerosol (SOA) particles free of inorganic salts has been intensively studied due to the importance of cloud condensation nuclei (CCN) properties. In this study, we investigated LLPS in four different types of SOA particles generated from α-pinene ozonolysis and α-pinene photooxidation in the absence and presence of ammonia (NH3). LLPS was observed in SOA particles produced from α-pinene ozonolysis at ∼95.8 % relative humidity (RH) and α-pinene ozonolysis with NH3 at ∼95.4 % RH. However, LLPS was not observed in SOA particles produced from α-pinene photooxidation and α-pinene photooxidation with NH3. Based on datasets of the average oxygen to carbon elemental ratio (O:C) for different types of SOA particles from this study and from previous studies, there appears to be a relationship between the occurrence of LLPS and the O:C of the SOA particles. When LLPS was observed, the two liquid phases were present up to ∼100 % RH. This result can help more accurately predict the CCN properties of organic aerosol particles.

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Liquid–liquid phase separation in organic particles consisting of α-pinene and β-caryophyllene ozonolysis products and mixtures with commercially-available organic compounds

10.5194/acp-2020-318 ◽

2020 ◽

Author(s):

Young-Chul Song ◽

Ariana G. Bé ◽

Scot T. Martin ◽

Franz M. Geiger ◽

Allan K. Bertram ◽

...

Keyword(s):

Phase Separation ◽

Liquid Phase ◽

Organic Compounds ◽

Spinodal Decomposition ◽

Aerosol Particles ◽

Organic Aerosol ◽

Liquid Phases ◽

Two Component ◽

Organic Particles ◽

Liquid Liquid Phase Separation

Abstract. Liquid–liquid phase separation (LLPS) in organic aerosol particles can impact several properties of atmospheric particulate matter, such as cloud condensation nuclei (CCN) properties, optical properties, and gas-to-particle partitioning. Yet, our understanding of LLPS in organic aerosols is far from complete. Here, we report on LLPS of one-component and two-component organic particles consisting of α-pinene- and β-caryophyllene-derived ozonolysis products and commercially-available organic compounds of relevance to atmospheric organic particles. In the experiments involving single-component organic particles, LLPS was observed in 8 out of 11 particle types studied. LLPS almost always occurred when the oxygen-to-carbon elemental ratio (O : C) was ≤ 0.44, but did not occur when O : C was > 0.44. The phase separation occurred by spinodal decomposition, and when LLPS occurred, two liquid phases co-existed up to ~ 100 % relative humidity (RH). In the experiments involving two-component organic particles, LLPS was observed in 23 out of 25 particles types studied. LLPS almost always occurred when the average was O : C ≤ 0.67, but never occurred when the average O : C was > 0.67. The phase separation occurred by spinodal decomposition or growth of a second phase at the surface of the particles. When LLPS occurred, two liquid phases co-existed up to ~ 100 %. These results provide further evidence that LLPS is likely a frequent occurrence in organic aerosol particles in the troposphere, even in the absence of inorganic salts.

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Liquid-liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors

10.5194/acp-2019-367 ◽

2019 ◽

Author(s):

Mijung Song ◽

Adrian M. Maclean ◽

Yuanzhou Huang ◽

Natalie R. Smith ◽

Sandra L. Blair ◽

...

Keyword(s):

Phase Separation ◽

Liquid Phase ◽

Diesel Fuel ◽

Diffusion Coefficients ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Liquid Phase Separation ◽

Small Diffusion ◽

Wide Range ◽

Liquid Liquid Phase Separation

Abstract. Information on liquid-liquid phase separation (LLPS) and viscosity (or diffusion) within secondary organic aerosol (SOA) is needed to improve predictions of particle size, mass, reactivity, and cloud nucleating properties in the atmosphere. Here we report on LLPS and viscosities within SOA generated by the photooxidation of diesel fuel vapors. Diesel fuel contains a wide range of volatile organic compounds, and SOA generated by the photooxidation of diesel fuel vapors may be a good proxy for SOA from anthropogenic emissions. In our experiments, LLPS occurred over the relative humidity (RH) range of ~ 70 % to ~ 100 %, resulting in an organic-rich outer phase and a water-rich inner phase. These results may have implications for predicting the cloud nucleating properties of anthropogenic SOA since the organic-rich outer phase can lower the kinetic barrier for activation to a cloud droplet. At &leq; 10 % RH, the viscosity was in the range of &geq; 1 × 108 Pa s, which corresponds to roughly the viscosity of tar pitch. At 38–50 % RH the viscosity was in the range of 1 × 108–3 × 105 Pa s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (O : C) and molar mass as well as predictions based on the number of carbon, hydrogen, and oxygen atoms. Based on the measured viscosities and the Stokes–Einstein relation, at &leq; 10 % RH diffusion coefficients of organics within diesel fuel SOA is &leq; 5.4 × 10−17cm2 s−1 and the mixing time of organics within 200 nm diesel fuel SOA particles (τmixing) is &gtrsim; 50 h. These small diffusion coefficients and large mixing times may be important in laboratory experiments, where SOA is often generated and studied using low RH conditions and on time scales of minutes to hours. At 38–50 % RH, the calculated organic diffusion coefficients are in the range of 5.4 × 10−17 to 1.8 × 10−13 cm2 s−1 and calculated τmixing values are in the range of ~ 0.01 h to ~ 50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.

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Liquid–liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-12515-2019 ◽

2019 ◽

Vol 19 (19) ◽

pp. 12515-12529 ◽

Cited By ~ 5

Author(s):

Mijung Song ◽

Adrian M. Maclean ◽

Yuanzhou Huang ◽

Natalie R. Smith ◽

Sandra L. Blair ◽

...

Keyword(s):

Phase Separation ◽

Liquid Phase ◽

Diesel Fuel ◽

Diffusion Coefficients ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Liquid Phase Separation ◽

Small Diffusion ◽

Wide Range ◽

Liquid Liquid Phase Separation

Abstract. Information on liquid–liquid phase separation (LLPS) and viscosity (or diffusion) within secondary organic aerosol (SOA) is needed to improve predictions of particle size, mass, reactivity, and cloud nucleating properties in the atmosphere. Here we report on LLPS and viscosities within SOA generated by the photooxidation of diesel fuel vapors. Diesel fuel contains a wide range of volatile organic compounds, and SOA generated by the photooxidation of diesel fuel vapors may be a good proxy for SOA from anthropogenic emissions. In our experiments, LLPS occurred over the relative humidity (RH) range of ∼70 % to ∼100 %, resulting in an organic-rich outer phase and a water-rich inner phase. These results may have implications for predicting the cloud nucleating properties of anthropogenic SOA since the presence of an organic-rich outer phase at high-RH values can lower the supersaturation with respect to water required for cloud droplet formation. At ≤10 % RH, the viscosity was ≥1×108 Pa s, which corresponds to roughly the viscosity of tar pitch. At 38 %–50 % RH, the viscosity was in the range of 1×108 to 3×105 Pa s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (O:C) and molar mass as well as predictions based on the number of carbon, hydrogen, and oxygen atoms. Based on the measured viscosities and the Stokes–Einstein relation, at ≤10 % RH diffusion coefficients of organics within diesel fuel SOA is ≤5.4×10-17 cm2 s−1 and the mixing time of organics within 200 nm diesel fuel SOA particles (τmixing) is 50 h. These small diffusion coefficients and large mixing times may be important in laboratory experiments, where SOA is often generated and studied using low-RH conditions and on timescales of minutes to hours. At 38 %–50 % RH, the calculated organic diffusion coefficients are in the range of 5.4×10-17 to 1.8×10-13 cm2 s−1 and calculated τmixing values are in the range of ∼0.01 h to ∼50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.

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