A ONE-DIMENSIONAL SECTIONAL MODEL TO SIMULATE MULTICOMPONENT AEROSOL DYNAMICS IN THE MARINE BOUNDARY-LAYER - 2 - MODEL APPLICATION

The dynamics of aerosols in the marine boundary layer (MBL) are simula ted with the marine boundary layer aerosol model (MARBLES), a one-dime nsional, multicomponent sectional aerosol model [Fitzgerald et al., th is issue; Gelbard et al., this issue]. First, to illustrate how the va rious aerosol processes influence the particle size distribution, the model was run with one or two processes operating on the same initial size distribution. Because of current interest in the effects of cloud processing of aerosols and exchange of aerosols with the free troposp here (FT) on marine aerosol size distributions, these two processes ar e examined in considerable detail. The simulations show that the effec t of cloud processing (characteristic double-peaked size distribution) in the upper part of the MBL is manifested at the surface on a timesc ale that is much faster than changes due to exchange with the FT, assu ming a typical exchange velocity of 0.6 cm s(-1). The model predicts t hat the FT can be a significant source of particles for the MBL in the size range of the cloud-processing minimum, between the unactivated i nterstitial particles and the cloud condensation nuclei (CCN) which ha ve grown as a result of conversion of dissolved SO, to sulfate in clou d droplets. The model was also used to simulate the evolution of the a erosol size distribution in an air mass advecting from the east coast of the United States out over the ocean for up to 10 days. The modific ation of a continental aerosol size distribution to one that is remote marine in character occurs on a timescale of 6-8 days. Nucleation was not observed in the base case 10-day advection simulation which assum ed rather typical meteorological conditions. However, significant nucl eation was predicted under a more favorable (albeit, atypical) combina tion of conditions which included significant precipitation scavenging (5 mm h(-1) of rain for 12 hours), colder temperatures by 10 degrees C (283 K at the surface decreasing to 278 K at 1000 m) and a high DMS flux (40 mu mol m(-2) d(-1)). In a test of model self initialization, long-term (8-10 days) predictions of marine aerosol size distributions were found to be essentially independent of initial conditions.