10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
UV Intensity Calculated in Approximations of Clusters of Bacteria and Bacterial Spores for Predicting Viability
STEVEN HILL, Dan Mackowski, Frank Handler, Jana Kesavan, Adam Driks, David Doughty, US Army Research Lab
Abstract Number: 1011 Working Group: Infectious Bioaerosol
Abstract Understanding the photo-inactivation of bacteria by ultraviolet (UV) light is important in predicting and warning of biological hazards and in using germicidal UV to sanitize the air or surfaces. Bacterial spores tend to remain viable in the environment far longer than do vegetative cells. Clusters of bacteria are more difficult to inactivate with UV than are individual bacteria. Here we mathematically model Bacillus subtilis spores as homogeneous spheres and study clusters of these spheres. Each cluster of spores may be encased in an encompassing sphere of the same or different composition, or may be clean, with only air in the spaces between the spores. We use these models to better understand and estimate the distributions of UV intensities within particles containing bacteria, the shielding of some bacteria from UV, and variations of the UV intensities with wavelength. Two methods are used to calculate the UV intensities. The primary method is the multi-sphere T-matrix (MSTM) method, which provides exact solutions for the UV intensities in the spheres of the cluster. It is applicable to clusters of bacteria, with or without additional material in the spaces between the bacteria, where the individual spores or bacteria are reasonably approximated as homogeneous spheres. Because the MSTM is computationally intensive, especially for larger clusters, e.g., greater than 5-micrometer diameter, we also used a homogeneous sphere model. This model is applicable to the special case of particles formed by drying liquid droplets containing bacteria where the droplets include sufficient soluble material to cover all the spores in a cluster, and where the optical properties of the non-spore material is not too different from that of the spores. The MTSM (or a similar code capable of representing the spheres and the material in between the spores) is essential for cases where the refractive index in the spaces between the spores in a cluster is significantly different from the refractive index of the spores. Such differences are especially important when the cluster is made of clean spores, i.e., with air in the spaces between spores. Calculated intensities, using clusters containing as many as 100 spores, show that the UV exposure of individual spores within the cluster can be orders of magnitude smaller than for isolated spores under the same ambient light conditions. These effects, which are due to both absorptive and refractive shielding, are far more significant for particles in a fixed orientation, as when held on a surface. Such effects are less significant for particles rotating in air. We suggest that modeling capabilities such as these can help in predicting the time-dependence of the inactivation of bacteria exposed to UV of various intensities and spectral content. A limitation of the model is the uncertainty in the estimations of the complex refractive index of the bacteria (and of some of the other materials which may be in a cluster) at the wavelengths studied. This uncertainty is greater at wavelengths larger than approximately 290 nm, where absorption of light by nucleic acids, proteins and calcium dipicolinate no longer dominate the absorption. Other limitations at present are the assumptions that each spore is a homogenous sphere, and that reflections of UV from surfaces touching the cluster do not affect the UV inside the spores of the cluster.