4.5 Article

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

Journal

AEROSOL SCIENCE AND TECHNOLOGY
Volume 56, Issue 8, Pages 688-710

Publisher

TAYLOR & FRANCIS INC
DOI: 10.1080/02786826.2022.2062293

Keywords

Jason Olfert

Funding

  1. Engineering and Physical Sciences Research Council (EPSRC) Center for Doctoral training in Aerosol Science [EP/S023593/1]
  2. Natural Environment Research Council through an Independent Research Fellowship [NE/S014314/1]
  3. Scientific Focus Area Science Plan program - Office of Biological and Environmental Research in the Department of Energy, Office of Science, through the United States Department of Energy Contract [DESC0012704]
  4. EPSRC [EP/V036440/1]
  5. EPSRC Core Equipment Grant [EP/S018050/1]

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This study accurately measured the density of dry internally mixed particles and found significant deviations from ideal mixing, with accuracies and precisions of approximately 5% and 1% respectively. The use of RK polynomials to describe volume changes in mixing was effective in explaining non-ideal mixing.
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 similar to 5% and similar to 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.

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