A ONE-DIMENSIONAL SECTIONAL MODEL TO SIMULATE MULTICOMPONENT AEROSOL DYNAMICS IN THE MARINE BOUNDARY-LAYER - 1 - MODEL DESCRIPTION

A one-dimensional, multicomponent sectional model has been developed t o simulate the temporal and vertical variations of the aerosol size di stribution and composition in the marine boundary layer (MBL). An impo rtant aspect of the model is its ability to handle the transport of ae rosols in an atmosphere with humidity gradients with no numerical diff usion caused by the swelling and shrinking of the particles as they mo ve through the humidity gradients. This is achieved by rewriting the a erosol general dynamical equation (GDE) in terms of dry radius thus tr ansferring all variations in radius caused by temporal and spatial hum idity variations to the rate coefficients appearing in the equations. The model then solves the new GDE in fixed dry size sections, with the humidity dependence of the processes now included in variable coeffic ients. This procedure also results in correct gradient transport. A li miting assumption is that the particles equilibrate instantaneously wi th the ambient water vapor. This assumption limits the maximum particl e size which can be treated in the model to ambient (wet) radii less t han about 30 mu m. All processes currently believed to be important in shaping the MBL size distribution are included in the current version of the model. These include generation of sea-salt aerosol at the oce an surface, nucleation of new particles, coagulation, growth due to co ndensation of gas-phase reaction products, growth due to sulfate forma tion during cloud processing, precipitation scavenging, surface deposi tion, turbulent mixing, gravitational settling, and exchange with the free troposphere. Simple gas-phase chemistry which includes the oxidat ion of dimethylsulfide and SO, to sulfate is incorporated in the curre nt version of the model.