Airship Measurements of Ship’s Exhaust Plumes and Their Effect on Marine Boundary Layer Clouds

Abstract

High-resolution aerosol, trace gas, and cloud microphysical measurements were made from an airship during transects across ships exhaust plumes advecting downwind of ships in the marine boundary layer (MBL). This study was part of the Office of Naval Research Monterey Area Ship Track experiment designed to understand the mechanisms by which ships produce cloud tracks visible in satellite imagery. Measurements made below clouds and close to the ships are used to define the concentrations and source strength of effluents, and the size distribution of ship-generated aerosols. Measurements made during crossings inside the cloud indicate that ship-generated aerosol increases the number and decreases the radii of cloud droplets. Case studies of four ships are presented, two of which produced cloud tracks and two that did 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, because the ship-produced particles were too small relative to the background 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 relationship among the size and number of background aerosols; MBL dynamics (as it effects cloud supersaturation); and the concentration, size, and composition of ship-generated particles.