Densities of internally mixed organic-inorganic particles from mobility diameter measurements of aerodynamically classified aerosols

Elinor T. Vokes, Ernie R. Lewis, Andrew L. Johnson, Michael I. Cotterell

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Accurate knowledge of particle density is essential for many aspects of aerosol science. Yet, density is often characterized poorly and incompletely for internally mixed particles, particularly for dry particles, with previous studies focused primarily on deliquescent (aqueous) droplets. Instead, densities for dry internally mixed particles are often inferred from mass composition measurements in combination with predictive models assuming ideal mixing, with the accuracy of such models not estimated. We determined particle densities from mobility diameter measurements (using a Scanning Mobility Particle Sizer, SMPS) for dried particles classified by their aerodynamic size (using an Aerosol Aerodynamic Classifier, AAC) for a range of two-component organic-inorganic particles containing known proportions of organic and inorganic species. We examined all permutations of mixing between four different organic (water soluble nigrosin dye, citric acid, polyethylene glycol-400, and ascorbic acid) and three different inorganic (sodium chloride, ammonium sulfate, and sodium nitrate) species. The accuracy and precision in our measured particle densities were ∼5% and ∼1%, respectively, for nonvolatile particles. Substantial deviations in particle density from ideal mixing (up to 20%) were observed. We tested descriptions of the non-ideal mixing for our systems by representing the volume change of mixing using Redlich-Kister (RK) polynomials in terms of mass fraction or in terms of mole fraction, with both approaches performing similarly.

Original languageEnglish
Pages (from-to)688-710
Number of pages23
JournalAerosol Science and Technology
Issue number8
Publication statusPublished - 28 Apr 2022

Bibliographical note

Funding Information:
Elinor T. Vokes was supported through a studentship provided by the Engineering and Physical Sciences Research Council (EPSRC) Center for Doctoral training in Aerosol Science (EP/S023593/1). Michael I. Cotterell acknowledges the support of the Natural Environment Research Council through an Independent Research Fellowship (NE/S014314/1). Ernie R. Lewis was supported by the Scientific Focus Area Science Plan program that is supported by the Office of Biological and Environmental Research in the Department of Energy, Office of Science, through the United States Department of Energy Contract No. DE-SC0012704 to Brookhaven National Laboratory. We thank the EPSRC for funding the purchase of the AAC and CPC used in this work through a Core Equipment Grant (EP/V036440/1). We also acknowledge Dr Bryan Bzdek (University of Bristol) for securing a further EPSRC Core Equipment Grant (EP/S018050/1) that funded the purchase of the aerosol mobility classifier used in this work.

Publisher Copyright:
© 2022 The Author(s). Published with license by Taylor and Francis Group, LLC.


  • Jason Olfert

ASJC Scopus subject areas

  • Environmental Chemistry
  • General Materials Science
  • Pollution


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