Ideal Mixing of the Complex Refractive Index for Organic Internally Mixed Light Absorbing Aerosol Particles

SIMON XI CHEN, Gwen Rachel Lawson, Justin Langridge, Kate Szpek, James Allan, Michael Cotterell, University of Bristol

     Abstract Number: 50
     Working Group: Aerosol Physics

Abstract
To predict variations in the complex refractive index of aerosol particles, densities for dry internally mixed particles are needed in physically based models and often are predicted by models assuming ideal mixing. Yet, Vokes et al., (2022) have reported substantial deviations in measured effective densities from expectations of an ideal mixing model for two-component organic-inorganic mixtures, which might impact the aerosol particle optical properties.

Here we used an Aerodynamic Aerosol Classifier (AAC) to classify particles by their aerodynamic diameter in the range 100 - 400 nm. Combined with a Scanning Mobility Particle Sizer (SMPS) for quantifying the particle mobility diameter distribution, we determined the particle densities for two-component nonreactive light absorbing organic particles containing nigrosin and sucrose. Concurrently, we used cavity ring-down spectroscopy (CRDS) and photoacoustic spectroscopy (PAS) to measure the ensemble-mean extinction and absorption cross sections for the aerodynamically classified particles, from which we retrieved the complex refractive indices (RI) for the nigrosin-sucrose mixed particles at optical wavelengths of 405 and 658 nm. Both measured densities and the real (n) and imaginary (k) components of nigrosin-sucrose mixed particles were in close agreement with ideal mixing predictions as well as those that have an underlying physical basis, in stark contrast to results for inorganic-organic mixed particles.

In this contribution, we also assess the accuracy of complex RI retrievals from both the extinction and absorption, as well as from the extinction only. Comparisons of n and k from the two retrieval approaches show small differences, contrary to previous assessments that had used electrical mobility selection to control the particle size.