Atmospheric organic matter forms core-shell aerosol particles on mineral surfaces

Core-shell particles have been created in an optical trap by placing thin films (∼ 10−40 nm) of atmospheric matter collected and extracted from urban aerosol and wood-burning smoke samples onto mineral silica particles (diameter ∼2 μm). The structures formed by these samples were a symmetrical core-shell and not an engulfing structure or asymmetric structure. Mie scattering of light from the optically trapped particles demonstrated that the experimental and calculated Mie spectra were consistent with the scattering of light by core-shell aerosol, and not with the scattering of homogeneous or partially-engulfed aerosol. The work presented may be the first experimental demonstration of organic materials extracted from the atmosphere favouring a symmetrical core-shell morphology on solid particles. Furthermore oxidation of these core-shell particles with ozone demonstrated a thinning shell of organic film and no change in morphology . Figure 1 demonstrates our initial results with an increasing thicker core-shell film of woodsmoke extract on a silica core particle and Figure 2 demonstrates the ability to follow refractive index and film thickness during oxidation of this thin film. The Mie scattering allows calculation of the refractive indices as a function of wavelength and film thicknesses. The refractive index of ∼40nm thick shells were determined to a precision of ∼0.01 and ∼0.006, and the thickness of the shell was determined to a precision of 0.2nm and 0.1nm for the aged urban and wood-smoke samples respectively.  The core-shell morphology, significantly reduces the complexity of atmospheric modelling of the radiative properties of these aerosol.