This grant supports the deployment of a fast-response, airborne instrument for measuring the concentration of dimethyl sulfide (DMS) during the DYCOMS-II project in July 2001 (Dynamics and Chemistry of Marine Stratocumulus). DMS is an atmospheric trace gas which is an effective indicator of mixing between the marine boundary layer and the free atmosphere. Its main source is the ocean surface and its lifetime in the atmosphere is about one day. It therefore has relatively high concentration in the boundary layer and low concentration in the free atmosphere. Fluctuations in the concentration at the top of the boundary are an indication of the strength of mixing and the rate of mass transfer between the boundary layer and the air above. The instrument is a fast-response ionization mass spectrometer capable of yielding data with a time resolution of 50 ms. In combination with other aircraft instruments that measure temperature, vertical air velocity, humidity, cloud droplet concentration and sizes, and aerosol sizes and composition, it will provide information on entrainment processes at cloud top. The data will be used in the testing and evaluation of large eddy simulations of mixing in marine stratocumulus clouds.