Low-order ignition models are important tools in the design of aviation gas turbines. In this paper, a stochastic model that predicts the ignition probability in a combustor based on a timeaveraged cold- ow solution is extended to include local fuel concentration uctuations due to the polydisperse nature of the spray. For this, a stochastic approach to modelling such uctuations is considered, and the e ects of the ow and mixture parameters on the resulting equivalence ratio pdfs are investigated. The concentration of fuel in large droplets results in a high variation of the local equivalence ratio, hence a ecting the local ammability factor at the model's cell scale. The extinction criterion of the ignition model based on a critical Karlovitz number is calibrated based on ignition probability data from canonical experiments using jet fuel, suggesting critical Karlovitz values of spray ames between 0.2-0.6, which is to be contrasted with values of 1.5 for gaseous fuels.