Spectral data from satellite and ground-based remote sensing instruments are used to monitor the effects of atmospheric aerosols on climate. Determinations of how aerosols interact with sunlight or infrared radiation from Earth’s surface rely upon an aerosol model that approximates the sizes, shapes, and compositions of the particles. Particle composition and shape are typically modeled as a single material within a simplified geometric form such as a sphere or spheroid. In this research, we use scanning electron microscopy and 3-dimensional spatial modeling of individual particles to show how the optical behavior of actual particles in the atmosphere compares with aerosol models that utilize homogeneous geometric shapes for particles.
Important technologies used in this research are focused ion-beam (FIB) tomography for generating particle 3-D spatial models and the discrete dipole approximation method [B.T. Draine and P.J. Flatau, 1994, J. Opt. Soc. Am., 11, 1491-1499] for calculating particle optical properties. FIB tomography is performed with a scanning electron microscope equipped with an ion-beam column. The ion beam slices through the particle incrementally as the electron beam images each slice. Element maps of the particle may be acquired with energy-dispersive x-ray spectroscopy. The images and maps are used to create the 3-D spatial model, from which the discrete dipole approximation software [Draine and Flatau, User Guide for the Discrete Dipole Approximation Code DDSCAT 7.3, 2013] is used to calculate optical extinction, single scattering albedo (SSA, ratio of scattering to extinction), asymmetry parameter, and the phase function.
Aerosol remote sensing techniques for monitoring climate typically separate particles into two size classes: <1 μm and >1 μm. Examples of particles <1 μm are tailpipe soot and secondary organic aerosol formed by oxidation of gas-phase organic carbon within the atmosphere. Examples of particles >1 μm are mineral dust from arid regions and urban road dust.
We investigate dust particles selected from samples collected at urban sites (e.g., Los Angeles and Seattle) and at the Mauna Loa Observatory (MLO) in Hawaii. Particles from the MLO samples are identified as Asian dust. For the urban and Asian dust samples, we compare the optical properties for the actual particles with the particles as simple geometric shapes, which include spheres, ellipsoids, cubes, and tetrahedra. The geometric particles are volume- and compositionally-equivalent to the original particles.
This research is intended to help atmospheric scientists who utilize aerosol models to derive aerosol optical properties from remote-sensing spectra. We show how remote-sensing aerosol models may be improved by incorporating additional geometric particle-shape information that accounts for composition and morphological features of different types of atmospheric dust. Improved particle composition and shape information may then be used to improve atmosphere-ocean circulation models for creating accurate climate change scenarios.
J. M. Conny and D.L. Ortiz-Montalvo, “Effect of Heterogeneity and Shape on Optical Properties of Urban Dust Based on 3-Dimensional Modeling of Individual Particles,” J. Geophys. Res.- Atmospheres, in review.
J. M. Conny, S.M. Collins, A.A. Herzing, “Qualitative Multiplatform Microanalysis of Individual Heterogeneous Atmospheric Particles from High-Volume Air Samples,” Anal. Chem. 2014, 86, 9709-9716.
J. M. Conny, “Internal Composition of Atmospheric Dust Particles from Focused Ion-Beam Scanning Electron Microscopy,” Environ. Sci. Technol. 2013, 47, 8575-8581.
J. M. Conny and G.A. Norris, “Scanning Electron Microanalysis and Analytical Challenges of Mapping Elements in Urban Atmospheric Particles,” Environ. Sci. Technol. 2011, 45, 7380-7386.