http://www.energy.ca.gov/sitingcases/hydrogen_energy/documents/08-AFC-8/applicant/revised_afc/Volume_II/Appendix%20C.pdf 4.7 FUMIGATION MODELING Fumigation can occur when a stable layer of air lies a short distance above the release point of a plume and unstable air lies below. Especially on sunny mornings with light winds, the heating of the earth’s surface causes a layer of turbulence, which grows in depth over time and may intersect an elevated exhaust plume. The transition from stable to unstable surroundings can rapidly draw a plume down toground level and create relatively high pollutant concentrations for a short period. Typically, a fumigation analysis is conducted using the USEPA model SCREEN3 when the project site is rural and the stack height is greater than 10 m. A fumigation analysis will be performed using SCREEN3 to calculate concentrations from inversion breakup fumigation; no shoreline fumigation modeling will be performed for the HECA location. A unit emission rate will be used 1 gram per second) in the fumigation modeling simulations to represent the plant emissions, and the model results will be scaled to reflect expected plant emissions for each pollutant. Inversion breakup fumigation concentrations will be calculated for 1- and 3-hour averaging times using USEPA-approved conversion factors. These multiple-hour model predictions are conservative, since inversion breakup fumigation is a transitory condition that would most likely affect a given receptor location for only a few minutes at a time.