The oceans cover ~70% of the Earth's surface. The lowest part of the atmosphere over the oceans, the marine boundary layer (MBL), is subject to fluxes of sea spray aerosol and gases from the ocean and is linked to the free troposphere (FT) by vertical mixing in convective events but also by large scale mixing events such as frontal passages or tropical easterly waves. Most greenhouse gases (such as carbon dioxide, CO2, nitrous oxide, N2O) have atmospheric lifetimes of hundreds or thousands of years but others, notably methane, CH4, and tropospheric ozone, O3, have much shorter lifetimes (~9 years and weeks, respectively), largely because they have chemical sinks in the troposphere. Together they account for about 30% of the global radiative forcing of all greenhouse gases. Under most conditions, the MBL acts as a sink for O3 due to the low concentrations of NOx; furthermore about 25% of the tropospheric CH4 destruction occurs in the tropical MBL and a further 35% in the tropical marine free troposphere, hence the atmosphere above the (tropical) oceans is a very important region for atmospheric chemistry. The atmospheric oxidation capacity ("self-cleansing" capacity) is to a large extent determined by the hydroxyl radical (OH), O3 and their budgets and cycling; globally most tropospheric OH is found in the tropics. Therefore a quantitative understanding of the composition and chemistry of the marine atmosphere is crucial to examine the atmospheric oxidative capacity and climate forcing. This project will use a regional three-dimensional model (WRF-Chem) to study meteorological and chemical processes in the marine atmosphere. The model results will be compared to data from recent field campaigns in the East Pacific (TORERO, EqPOS) and from the North Atlantic (Cape Verde Atmospheric Observatory and Bermuda). By comparing model results with field data we will be able to see if the processes, sources and chemical reactions that are included in the model are sufficient to explain the data or if modifications and improvements to the reaction mechanism, emissions inventories or details of physical parameterisations will have to be made. Once these improvements have been made and the model is capable of reproducing the data, we will then be able to quantify lifetimes and budgets of important compounds such as ozone and the toxic mercury and to transfer our results to other ocean regions and to inform global models including Earth System models. This project aims to quantitatively describe the underlying processes and not to simply optimise the fit to observational data by unphysical adjustments in the model.