The particles causing absorption and scattering in the atmosphere are mostly nonspherical. While the assumption of "equivalent spheres" appears adequate in some applications in remote sensing, it could be woefully inadequate in others, such as lidar backward scattering. Professor Bohren has been experimenting with approximate methods which directly treat particles of arbitrary shape. One such method is the Purcell-Pennypacker or coupled dipole method, in which a particle is approximated by a lattice of dipoles small compared with the wavelength but still large enough to contain many atoms; each dipole is excited by the incident radiation field and the fields of all the other dipoles. Under this award, Professor Bohren will continue his work on reformulating the coupled dipole method so as to decrease limitations on applicable particle size and refractive index and increase computational efficiency, and on experimenting with embedding exact theories in the coupled dipole method thereby adding to the stock of hybrid theories.
