The Earth¿s climate is strongly influenced by clouds. The oxidative processing of pollutants in clouds affects cloud droplet size and optical properties, important climatic effects. Common cloud pollutants are naturally occurring organic surfactants forming organic films on the droplet. Chlorine atom chemistry is common in the marine atmosphere. In this work we propose to monitor the air-water interface when monolayers of deuterated stearic acid are oxidised by gas-phase chlorine atoms. The objectives are:1) To determine the rate of film oxidation by chlorine radicals ¿ is it atmospheric relevant?2) To determine if accommodation of oxidant at the surface effects the rate of oxidation3) To determine if oxygen is chain carrier in the radical chain reaction.The work will support a new joint STFC/NERC student with ancillary measurements at laser science facility using laser tweezers