Ambient particles are ubiquitous in the atmosphere and exert a strong influence on the climate by altering the Earth's radiative balance. Aerosol interactions with sunlight arise from their optical properties (e.g., scattering and absorption); however these properties depend highly on the diverse chemical composition, structures, sizes, and shapes of the particles. Furthermore, hidden imbedded phases and voids are commonly present in heterogeneously-mixed particles and can affect their optical properties as well as their projected radiative forcing. The objective of this project is to characterize the morphology and composition of ambient particles and study their optical properties using a newly developed method that combines advanced microscopy techniques and optical property modeling. Focused ion-beam scanning electron microscopy (FIB-SEM) was used to reconstruct the three-dimensional (3-D) configuration of select ambient particles. 3-D reconstructions included particle's inherent voids and inclusions whether they were light-absorbing or light-scattering phases. Absorption and scattering behavior of select 3-D reconstructions was resolved using a Discrete Dipole Approximation (DDA) optical property model and compared to Mie scattering theory. DDA captured the complexities in particle morphology that other models like the Mie theory do not. Efficiencies in scattering, extinction, absorption and backscattering were evaluated in this study. Preliminary results indicate that the shape of the particle was the dominating factor affecting the variation of optical properties and that backscattering efficiency was the most sensitive to differences in shape. These results suggest that assigning ambient particles as spheres in climate models could result in large errors in predicting radiative forcing.
Proceedings of the 108th Annual Conference and Exhibition of the Air & Waste Management Association (A&WMA)
June 22-25, 2015
Connecting the Dots: Environmental Quality to Climate
aerosol optical properties, ambient particles, focused ion-beam, microscopy techniques, optical property modeling, scanning electron microscopy, 3-D particle reconstructions, 3-D models