A11T-07: FORCAsTing the Influence of a Forest Canopy on the Bi-Directional Exchange of Gases and Aerosols Biosphere-atmosphere interactions play a critical role in governing atmospheric composition, mediating the concentration of key species such as ozone and aerosol, thereby influencing air quality and climate. The FORCAsT (FORest Canopy Atmosphere Transfer) one-dimensional model has been developed to study the emission, deposition, chemistry and transport of volatile organic compounds (VOCs) and their oxidation products in the atmosphere within and above the forest canopy. Recent field measurements at a variety of forest ecosystems have suggested that many oxygenated VOCs exhibit periods of strongly negative (downward) fluxes, for which current understanding cannot account. We apply FORCAsT to simulate within-canopy processes and elucidate their relative contributions to fluxes of key oxygenated species at the top of the canopy under a range of meteorological and chemical (NOx) conditions in an isoprene-dominated forest ecosystem (Harvard Forest). Modelled concentrations and fluxes of methanol, acetone and acetaldehyde cannot be reconciled with observations made during the growing season in 2012 without the inclusion of primary emissions. We also find clear evidence of light-dependent methanol and acetone emissions, in addition to temperature-dependent releases from storage pools. Furthermore, the deposition of oxygenated VOCs appears to be strongly influenced by relative humidity suggesting enhanced deposition to wet surfaces. Our work highlights the need for better understanding and further targeted measurements to constrain the mechanisms controlling these processes.