The goal of the project is to quantify, by means of model simulations and analysis of in-situ and satellite observations, the response of the South East Pacific (SEP) cloud layer and of its properties to forcing by natural and anthropogenic gas phase emissions, accounting for both aerosol formation from the gas phase and incloud aqueous oxidation. In the proposed work, the transport of aerosol precursor gases from their sources to locations over the SEP will be characterized by ensembles of trajectories extracted from simulations with the Weather Research and Forecasting (WRF) model. A previously developed Lagrangian model of aerosol processes, which describes aerosol formation from the gas phase and subsequent growth of the particles, will be operated along these trajectories. The trajectory ensembles will be chosen based on the VOCALS-REx campaign area, and on the movement of air masses carrying distinctly different loads of aerosol precursor gases. The aerosol properties obtained from these Lagrangian model runs will be validated with observations from the VOCALS-REx campaign, and correlated with satellite-derived cloud properties, providing a first quantification of the effect of aerosols on clouds. They will then serve to initiate Large Eddy Simulations of the SEP region cloud cover. Aerosol formation from the gas phase will be accounted for in these simulations with a previously developed algorithm. These simulations close the connection between emissions of aerosol precursor gases and cloud properties in the region; their results will be compared with VOCALS-REx measurements and satellite observations, and provide a second quantification of the effect of aerosols on clouds. The proposed work is designed to support the VOCALS science program, which aims at the understanding of aerosol, cloud and climate processes in the SEP region, and of their connection to large scale climate via their impact on the ITCZ and the ENSO.