Effect of simulated solar radiation on mixed outdoor atmospheric bacterial populations
Slit and Andersen cascade impact samplers were used to collect ambient atmospheric bacteria under clear conditions at noon and night onto Luria-Bertani agar surfaces. The bacteria were then directly exposed to simulated solar radiation (SSR) for increasing periods to evaluate the lethal effect of SSR. The results showed that SSR had marked differential lethal effects depending on the accumulated total solar irradiance, solar spectral property, bacterial species composition, and bacterial laden particle size. The simulated extraterrestrial solar spectrum had a greater lethality than the simulated terrestrial solar spectrum on the atmospheric bacteria collected at night (P<0.05); no such difference was found for the bacteria collected at noon. Statistical differences found only for the bacteria collected at night indicates that daytime solar radiation exposure could eliminate some sensitive bacterial populations from the ambient atmosphere. Sunlight exposure survival of the bacteria collected at noon linearly increased with respect to particle size. For each micrometer increase in particle aerodynamic diameter, there is an almost 11% increase in survival up to at least 7 μm aerodynamic diameter particles. The higher sunlight sensitivity of airborne bacteria in small particles could provide a partial explanation for the preponderance of large-sized culturable bacterial particles in the atmosphere during clear days.