The proposal is a continuation to collect more data for publication. The generation of chlorine radicals in the previous experiment was initially tricky and difficult. However er have a nice working system now and need to collect more data to enable publication. Chlorine atom chemistry is common in the lower atmosphere above sea and sea ice. 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