Determining activity coefficients of SOA from isothermal evaporation in a laboratory chamber
Submitter
Jimenez, Jose-Luis — University of Colorado
Ziemann, Paul J — University of Colorado
Area of research
Aerosol Properties
Journal Reference
Science
Activity coefficients (γ), a measure of non-ideal molecular interactions, were quantified using gas- and particle-phase observations of chamber SOA formation and isothermal evaporation for a wide range of seed particles. Increasing γ were observed as the polarity of the generated SOA molecules and pre-existing seed diverged.
Impact
Non-ideal molecular interactions in aerosol particles influence the partitioning of semi-volatile organic compounds (SVOCs). However, few direct measurements exist that determine activity coefficients (γ), which quantify nonideality, for individual oxidized organic compounds in different atmospheric mixtures. Our results demonstrate a method to quantify non-ideal behavior, and show that it can occur in multicomponent mixtures and therefore influence OA formation, evolution, and lifetimes. The inclusion of γ in estimating G/P partitioning is crucial for accurately predicting aerosol mass concentration and composition.
Summary
Effective saturation concentrations (c*) and activity coefficients (γ) of SVOCs in different seed particles were inferred using a combination of novel laboratory methods using environmental Teflon chambers and a model/fitting framework. Measurements were conducted under dry and humid conditions using seeds of different properties that are atmospherically relevant. The study uses a new “fast burst” method for quickly producing atmospherically relevant oxidized compounds that allows for a decoupling of the timescales of reaction (10 s), condensation (100s of s), vapor wall loss (1000 s), and net particle evaporation (several 1000s of s), together with state-of-the-art gas (NO3- CIMS) techniques capable of making fast (seconds) molecular-level measurements and standard detection particle methods (scanning mobility particle sizer) for tracking slower particle evaporation. Investigations of the isothermal evaporation of the bulk SOA, treating particles as a single liquid phase, showed that c* and γ were smaller when the SVOC and the seed particle had similar polarity (similar lipophilicity or hydrophilicity), otherwise c* and γ increased when the vapor and the particle were dissimilar, reflecting less favorable interactions. γ tends to increase from ~1 to ~5 as the organic seeds and the SOA have more differing polarities. High computed γ =74 for a wet ammonium sulfate/SOA system indicates phase separation. Peak SOA formation generally decreases as γ increases, which is consistent with more favorable partitioning of SVOC compounds when the seed or absorbing phase has more similar polarity to the partitioning compounds. γ for some individual species was also quantified based on gas-phase CIMS measurements of c*. However, the bulk SOA γ cannot be explained by the simplified speciated SVOC-seed interactions, suggesting incomplete compositional understanding of the bulk SOA.
Figure 1. Experimental SOA formation followed by evaporation (red) compared to kinetic models with different activity coefficients (γ). From journal.
Figure 2. Activity coefficients (γ) versus seed n-octanol/water partitioning coefficient, KOW, a metric for polarity (lipophilicity/hydrophilicity). From journal.