Journal Article

Shapes of internally mixed hygroscopic aerosol particles after deliquescence, and their effect on light scattering

Adachi, Kouji, Freney, Evelyn J., Buseck, Peter R.

2011

Overview

Hygroscopic aerosol particles change the magnitude of light scattering through condensation and evaporation of water vapor. We collected aerosol particles from two megacities and observed the particle shapes at various values of relative humidity (RH) using an environmental cell within a transmission electron microscope. Many Mexico City samples had sulfate particles that were embedded within weakly hygroscopic organic aerosol, whereas the Los Angeles samples mainly consisted of externally mixed sulfate particles. For the Mexico City samples, when the RH was increased in the microscope, only the sulfate parts deliquesced, but the entire particle did not become spherical, i.e., particles containing deliquescent phases do not necessarily become spherical upon deliquescence. This result conflicts with the assumption used in many models, i.e., that deliquesced particles become spherical. Using a discrete‐dipole approximation to calculate light scattering of simulated particles that resemble the observed ones, we show that, for particles >1.0 μm, the spherical‐shape assumption used in Mie theory underestimates the light scattering by ∼50%, with the exact value depending on the sizes and relative volumes of the constituent phases. Key Points Much pollution sulfate is embedded within weakly hygroscopic organic aerosol Not all internally mixed particles containing deliquesced salts are spherical Assuming sphericity for deliquesced particles can underestimate light scattering

• Publisher: Blackwell Publishing Ltd,American Geophysical Union
• Subject: Earth sciences / Earth, ocean, space / Exact sciences and technology / hygroscopicity / light scattering / organic aerosol / sulfate aerosol / transmission electron microscopy