A Monte Carlo code is developed for simulating energy spectra of alpha particles from aerosol samples. Geometrical detection efficiency and energy loss of the alpha particles in the source itself and in the material between the source and detector are simulated. Different characteristics of the aerosol particles and medium material are taken into account in the computation. An excellent agreement with earlier results and measurements is found. The code can be applied for example in designing optimal aerosol sampling methods in direct alpha spectrometry.