Organic aerosol (OA) is an important, sometimes dominant, fraction of atmospheric aerosol, yet its formation, fate, and global atmospheric budget are poorly understood. The aim of this project is, by means of a joint measurement-modeling effort, to constrain the reactive organic carbon (and hence OA) budget of the atmosphere by focusing on wet deposition processes. Monthly samples from the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) from sites across the continental U.S. will be analyzed for the total amount of deposited organics and also for their carbon oxidation state, providing information about both the wet-deposition flux and the competition between atmospheric deposition and oxidation. Analysis will be carried out using high-resolution electron impact mass spectrometry; samples will be introduced into the mass spectrometer using two different interfaces, allowing for the approximate distinction of organics from the particle phase with those from the gas phase. Model simulations of atmospheric ROC budgets (with a focus on deposition) will be carried out in parallel with measurements, enabling immediate measurement-model comparisons and model refinement. In particular, the GEOS-Chem global chemical transport model, utilizing a newly developed flexible chemistry scheme and expanded tracking of ROC, will be used to quantify and characterize ROC burdens, oxidation states, and fluxes.