Airship measurements of ship's exhaust plumes and their effect on marine boundary layer clouds

High-resolution aerosol, trace gas, and cloud microphysical measurements we re made from an airship during transects across ships exhaust plumes advect ing downwind of ships in the marine boundary layer (MBL). This study was pa rt of the Office of Naval Research Monterey Area Ship Track experiment desi gned to understand the mechanisms by which ships produce cloud tracks visib le in satellite imagery. Measurements made below clouds and close to the sh ips are used to define the concentrations and source strength of effluents, and the size distribution of ship-generated aerosols, Measurements made du ring crossings inside the cloud indicate that ship-generated aerosol increa ses the number and decreases the radii of cloud droplets. Case studies of f our ships are presented, two of which produced cloud tracks and two that di d not. Of the two that did not produce cloud tracks, one did not produce a cloud track because of unfavorable background aerosol loading; the other, b ecause the ship-produced particles were too small relative to the backgroun d aerosol. A simple cloud microphysical model that assumes the MBL dynamics remains the same inside and outside the plume is used to study differences in background and plume cloud formation and reveals the intricate relation ship among the size and number of background aerosols; MBL dynamics (as it effects cloud supersaturation); and the concentration, size, and compositio n of ship-generated particles.