10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Understanding the Partitioning of Water and Secondary Organic Matter Using Optically Trapped Single Particles
STEPHEN INGRAM, Young-Chul Song, David Topping, Simon O'Meara, Jonathan P. Reid, University of Bristol
Abstract Number: 384 Working Group: Aerosol Physics
Abstract The physicochemical changes experienced by organic aerosol particles undergoing dehydration into the surrounding gas phase can be drastic, with persuasive evidence supporting the existence of a moisture driven glass transition in secondary organic aerosol (SOA) particles.1
However, at present, the influence of aerosol in a glassy and ultraviscous state on the partitioning of semi-volatile organic compounds (SVOCs) between the particle and gaseous phases is not clear. Indeed, it may not be governed by equilibrium thermodynamics at all, but instead kinetic factors.
We will begin by presenting recent developments2 in assessing the evaporation kinetics of water and an SVOC from benchmark ternary aerosol, where both chemical species volatilise in response to a step change in relative humidity (RH).
We find that the contributions to observed size behaviour from each process can be decoupled and treated separately.
Particles are levitated in a standard optical tweezer instrument3 and sized continuously by analysis of characteristic stimulated peaks in the Raman spectrum. Employing Fickian diffusion modelling4, we extract the compositional dependence of the diffusion constant of water and compare the results to recently published parametrisations in binary aerosol particles.
With reference to a numerical framework developed by Mai et. al.5, we claim that particle phase diffusivity is also crucial to the timescale and reversibility of SVOC evaporation. Such kinetically limited mass transport manifests as a suppression in the observed vapour pressure above the droplet surface, and could have ramifications for mass loadings and size distributions of organic aerosol, were it to be a common phenomenon under atmospheric conditions.6
Moving up in complexity, we also present more recent experiments involving the gas phase ozonolysis of α-pinene in the atmosphere surrounding (dry) saccharide or inorganic particles. Oxidation products of varying volatility are produced which condense onto the surface of the seed particle. The volatility these compounds are then estimated from the size response of the organic shell once the gas phase chemistry is stopped.
It is observed that, under dry conditions, particle phase diffusion of organics is completely arrested: only the outermost layers of SVOC partition back into the gas phase, consistent with a recently proposed non-equilibrium adsorption model of particle growth7. Conversely, under humid conditions, equilibrium is approached and a distribution of organics are observed to volatilise without impedance.
By cycling the RH within the optical trap in this way, it is also possible to homogenise and re-vitrify the resultant droplet, encasing the organic mass in a nonvolatile, glassy matrix at a range of water activities. This allows the effect of different particle morphologies (core-shell vs homogeneous) on the kinetics of gas-particle partitioning to be studied.
These data provide insights that can improve the representation of SOA phase state, hygroscopic response and organic partitioning, potentially refining our ability to predict the evolution of organic mass in, for example, regional transport models.
1. L. Renbaum-Wolff et. al, Proc. Nat. Acad. Sci., 2013, 110(20), 8014. 2. S. Ingram et. al., Phys. Chem. Chem. Phys., 2017, 19, 31634. 3. Wills, J. B., Knox, K. J. and Reid, J. P., Chem. Phys. Lett., 2009, 481, 153 4. S. O’Meara et. al, Atmos. Chem. Phys. Disc., 2016, 16, 5299. 5. Mai, H., M. Shiraiwa, M., Flagan R. C., and Seinfeld, J. H., Environ. Sci. Technol., 2015, 49, 11485. 6. D. O. Topping, personal communication, 2017. 7. V. Perraud et. al., Proc. Nat. Acad. Sci., 109(8), 2012, 2836.