The presented procedure allows one to recover the film properties present originally at the raindrop surface from the Langmuir trough data supplemented with simultaneously taken rain event characteristics (rain rate and rain drop diameter distribution). It requires the partitioning effect of the surfactant molecules between the surface and bulk phases to be estimated where the entering quantities are: the partitioning coefficient K_p=Γ/c and a degree of the rain water interfacial system area development A_r/V_r evaluated here using fatty acid concentrations as model input data. The latter parameter depends on particular rain event signatures. The concept is quantified using the Langmuir trough film and disdrometer data on a set of rain water events observed at Gdańsk, Poland in May 2002.