A case study of ships forming and not forming tracks in moderately polluted clouds

The effects of anthropogenic particulate emissions from ships on the radiat ive, microphysical, and chemical properties of moderately polluted marine s tratiform clouds are examined. A case study of two ships in the same air ma ss is presented where one of the vessels caused a discernible ship track wh ile the other did not. In situ measurements of cloud droplet size distribut ions, liquid water content, and cloud radiative properties, as well as aero sol size distributions (outside cloud, interstitial, and cloud droplet resi dual particles) and aerosol chemistry, are presented. These are related to measurements of cloud radiative properties. The differences between the aer osol in the two ship plumes are discussed; these indicate that combustion-d erived particles in the size range of about 0.03-0.3-mu m radius were those that caused the microphysical changes in the clouds that were responsible for the ship track. The authors examine the processes behind ship track formation in a moderate ly polluted marine boundary layer as an example of the effects that anthrop ogenic particulate pollution can have in the albedo of marine stratiform cl ouds.