Friday, February 24, 2017 -
12:00pm to 1:00pm
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100F Pierce Hall, 29 Oxford St., Cambridge

Atmospheric & Environmental Chemistry Seminar

 Andreas Zuend, McGill, will discuss "Phase Separation Effects on the Partitioning of Ions and Organics in Multicomponent Aerosols."

The stable equilibrium state of multicomponent mixtures of organic and inorganic aerosol components may consist of one or more condensed phases. The miscibility of all the chemical species is typically limited, which can lead to the separation of a particle into two or more liquid phases. In fact, when the organic aerosol components are only moderately hygroscopic and when inorganic ions are present as dissolved species in an aqueous solution, as is typically the case in tropospheric aerosols, a liquid-liquid phase separation is often the stable thermodynamic equilibrium state at moderate to high relative humidity (in the absence of solid salt phases that may effloresce at low relative humidity). Under these conditions, the different organic components, water, and dissolved ions will partition according to their distinct physicochemical affinities for the two liquid phases. Aside from the partitioning of organics and water to the two phases, an interesting question is how the individual ionic species will differ in their phase preference and degree of partitioning to a certain phase. A question of particular interest on the molecular level is: how will a phase separation affect the molar abundance and thermodynamic activity coefficients of H+ ions? The answer to this question will determine the acidity (pH value) of the different liquid phases, with potential consequences for acid-catalyzed reactions in distinct reaction media. Reliable and efficient aerosol equilibrium calculations are challenging for a number of reasons that may lead to poor performance of standard numerical methods. I will present a new, reliable and thermodynamically consistent method to compute the liquid-liquid equilibrium partitioning of all organic and inorganic species in complex mixtures, including the partitioning of individual ions under the constraint of electrically neutral phases. In addition, this approach can be coupled to an improved equilibrium gas-particle partitioning method for calculations of the partitioning of water, semivolatile organic compounds, ammonia and inorganic acids of atmospheric relevance. We will explore model results on detailed predictions of aerosol particle acidity in single-phase and liquid-liquid phase-separated states as well as particle hygroscopicity under conditions of changing relative humidity and aerosol chemical composition. In addition, recent work on a link between phase separation in submicron aerosols and potential consequences for their cloud condensation nuclei (CCN) activity will be presented.

Speaker Bio:

Contact Name: 

Adam Birdsall

Research Areas: 

Harvard University
Center for the Environment

Address: 26 Oxford Street, 4th Floor, Cambridge
Phone: (617) 495-0368

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