Modeling and laboratory measurements of the development of superfog

Smoke and fog combinations from a nearby prescribed burns traveling over roadways have substantially reduced visibility (“superfog”, < 3 m.) resulting in significant auto accidents such as the 2008 I-4 incident in Florida and I-10 incident in Mississippi. Land managers need to recognize conditions that will result in superfog. Physical and numerical modeling of superfog formation has been accomplished. Superfog forms when water vapor from combustion, live fuel, and soil condenses on smoke particles as ambient air cools. Empirical relations developed for naturally occurring advection fogs suggest relatively large liquid water content (LWC) of 5 g m^-3 is needed for superfog formation. Such high LWC is thermodynamically difficult to achieve and is possible in the atmosphere only under extreme conditions. It has been hypothesized that extremely hygroscopic cloud condensation nuclei (CCN) from the smoldering phase of a fire can produce a large number of droplets smaller in size than in naturally occurring fog making it is feasible to achieve superfog conditions at relatively low LWC (2 g m^-3). Laboratory generated fogs resembling near superfog visibilities have been measured by Phase Doppler Particle Analyzer (PDPA) system to determine particle number density and size distribution. Measurements indicate that mean droplet diameter of 3 μm was larger than expected for superfog while producing similar low visibility conditions. A sensitivity study of droplet size distributions and number densities was carried out to understand the impact on visibility and LWC. It was found that lower LWC (2 g m^-3) is sufficient to form superfog when droplets of mean diameter of 2 μm or less, with log normal size distribution, are present. Results suggest that the required number of droplets has to be larger than 10⁵ droplets per cubic centimeter. Large deviation in droplet size distribution requires much larger amounts of LWC for significant visibility reduction. The presence of CCN from smoke had a significant impact on the droplet size distribution. Superfog formation was modeled for various concentrations of solute CCN. For high concentrations of solute pollutants, water vapor readily condensed to a large number of droplets forming superfog. Modeled and measured effects of pollutant CCN concentration, LWC and droplet size distribution on visibility will be presented.