This presentation is the second of three to summarize results obtained jointly by ENSCO Inc. and Battelle Memorial Institute to determine a method for modeling the rate of inactivation of B. anthracis spores under arbitrary environmental conditions using observed weather data. For part II, we describe a method for estimating the rate of inactivation of B. anthracis by natural sunlight using observed weather data.

The modeling technique employs 1) observed inactivation rates of the microorganism under laboratory conditions, 2) a previously published weighting function for the dependence on ultraviolet wavelength of inactivation of a similar microorganism, and 3) a radiative transfer model. First, a radiative transfer model (RTM) simulates downward irradiance for a given set of environmental conditions (for a particular location, time of day, altitude, cloudiness, ozone column concentration, etc.). Then the UV irradiance spectrum output by the RTM is scaled by the weighting function and integrated to calculate the biologically effective irradiance. Next the effective irradiance is applied as a scale factor to a laboratory determined rate of decay to estimate the decay constant appropriate for that particular set of environmental conditions. The dependence of the rate of inactivation on environmental variables that affect UVB radiation, such as solar zenith angle, ozone column concentration, cloud cover, aerosol index, etc. will be explored graphically.

This method for predicting the inactivation of B. anthracis spores would be a critical component of any emergency response model, including downwind hazard models used by agencies supporting domestic preparedness or the military in their support of national defense.