In order to examine the effect of uncertainty in PBL depth on the downwind contaminant concentration, we examine a realistic but simple convective atmospheric boundary layer during the IHOP campaign that took place May through June 2002. Using a collection of MM5 forecasts/reanalyses that vary the physics parameterizations and include observed data, we explore the relationship between uncertainty in PBL depth and the resulting uncertainty in the concentration field. These MM5 forecasts were originally developed to create the best' forecast for the given day. We use the various forecasts for PBL depth to define a realistic amount of fuzziness' in that variable and therefore impact downwind concentration forecasts to some degree. We seek to relate the fuzziness' in the forecast PBL depth to the resulting uncertainty in the downwind concentration field.