The evaporation and condensation mechanisms of water through a butanol film on sulfuric acid solution are elucidated by molecular dynamics simulation. A previous experiment by Nathanson et al. reported the mass accommodation coefficient alpha to be almost unity, whereas MD simulation of water scattering on the butanol film on water predicted a value of alpha significantly smaller than unity. This discrepancy is elucidated by considering the protonated butanol at the sulfuric acid solution surface, which roughens the surface layer, and the low temperature at the supercooled condition.