Marine aerosols represent the most important aerosol fraction in the Earth atmosphere. Field studies have revealed that fatty acids form an organic film at the surface of sea salt particles, altering the properties of the aerosol. By means of classical molecular dynamics simulation, the surface organization of palmitic acid (PA) on a salt surface, NaCl, has been investigated at two different temperatures, 235 and 300 K, and with relative humidity varying from 0 to 40%. Calculations show that water promotes the formation of well-ordered close-packed PA islands. As a result, some area of the salt may be covered by water only or by PA molecules supported by water. Depending on the relative humidity, the hydrophilic/hydrophobic character of the sea salt surface varies. This heterogeneous coating gives rise locally to very different surface properties and hence may affect the transfer of gas phase species to the salt and their reactivity.