Forests and vegetation emit biogenic volatile organic compounds (BVOCs) into the atmosphere which, once oxidized, can partition into the particle phase, forming secondary organic aerosols (SOAs). This thesis reports on a unique and comprehensive analysis of the impact of BVOC emissions on atmospheric aerosols and climate. A state-of-the-art global aerosol microphysics model is used to make the first detailed assessment of the impact of BVOC emissions on aerosol microphysical properties, improving our understanding of the role of these emissions in affecting the Earth's climate.   The thesis also reports on the implications for the climate impact of forests. Accounting for the climate impacts of SOAs, taken together with the carbon cycle and surface albedo effects that have been studied in previous work, increases the total warming effect of global deforestation by roughly 20%.