Aerosol dynamics in ship tracks

摘要

Ship tracks are a natural laboratory to isolate the effect of anthropogenicaerosol emissions on cloud properties. The Monterey Area Ship Tracks (MAST)experiment in the Pacific Ocean west of Monterey, California, in June 1994, providesan unprecedented data set for evaluating our understanding of the formation andpersistence of the anomalous cloud features that characterize ship tracks. Thedata set includes conditions in which the marine boundary layer is both cleanand continentally influenced. Two case studies during the MAST experiment areexamined with a detailed aerosol microphysical model that considers an externalmixture of independent particle populations. The model allows tracking individualparticles through condensational and coagulational growth to identify the sourceof cloud condensation nuclei (CCN). In addition, a cloud microphysics model wasemployed to study specific effects of precipitation. Predictions and observationsreveal important differences between clean (particle concentrations below 150 cm -3)and continentally influenced (particle concentrations above 400 cm-3 ) backgroundconditions: in the continentally influenced conditions there is a smaller change inthe cloud effective radius, drop number and liquid water content in the ship trackrelative to the background than in the clean marine case. Predictions of changesin cloud droplet number concentrations and effective radii are consistent withobservations although there is significant uncertainty in the absolute concentrationsdue to a lack of measurements of the plume dilution. Gas-to-particle conversion ofsulfur species produced by the combustion of ship fuel is predicted to be importantin supplying soluble aerosol mass to combustion-generated particles, so as to renderthem available as CCN. Studies of the impact of these changes on the cloud'spotential to precipitate concluded that more complex dynamical processes must berepresented to allow sufficiently long drop activations for drizzle droplets to form.