Aqueous-phase processing of gases and of the cloud condensation nucleus (CCN) spectrum is addressed using a trajectory ensemble model. We first derive a set of boundary layer parcel trajectories from a large-eddy simulation model (LES) and then use this trajectory set for subsequent cloud microphysical/chemical studies. We explore this approach in the stratocumulus-capped marine boundary layer and show examples of aqueous-phase processing of gases and CCN. The microphysical model is a Lagrangian parcel model that is driven by the thermodynamic and kinematic fields produced by the LES and calculates CCN deliquescence and droplet growth by condensation on a moving mass grid. It is coupled with a sulfate chemistry model that treats oxidation of S(IV) to S(VI) via ozone and hydrogen peroxide for a size-resolved droplet spectrum. The model reproduces the observed (and previously modeled) aqueous processing of particles that leads to growth of CCN and lowering of their critical supersaturations. Results show that important information on parcel in-cloud residence times and boundary layer mixing timescales can be gleaned from this approach. By averaging the results of 500 parcel trajectories, a more representative picture of CCN and gas processing is obtained compared with that derived from a single parcel simulation representing average conditions. Finally, processed CCN spectra are used as input to a coupled microphysical/dynamical cloud-resolving model, and it is suggested that cloud processing may either enhance or suppress drizzle formation, depending on the concentration and sizes of the CCN. ¿ 1998 American Geophysical Union