Forests not only assimilate large amounts of airborne pollutants and carbon dioxide but also release substantial amounts of reactive hydrocarbons. In regions such as the eastern United States, for example, forests emit numerous reactive hydrocarbons whose fluxes often exceed the anthropogenic emissions. In this presentation, the degree of chemical processing within and just above canopies is reviewed for selected forest ecosystems under the influence of different regimes of nitrogen oxides. Theoretical and field studies have been completed to determine the amounts of pollutants and aerosol yields resulting from biogenic hydrocarbon reactions. In addition, studies have been undertaken to determine the influences of the resulting phytogenic aerosols on surface energy balance and cloud formation processes. Conclusions for such investigations reveal that phytogenic aerosols can alter regional climate through two related processes. First, phytogenic aerosols can effectively act as cloud condensation nuclei, thereby influencing cloud formation processes. Increased cloud formation will result in reduced sunlight reaching the surface. Because of aerosol-cloud interactions, this light attenuation may produce regional surface energy balance changes comparable to the thermal energy generated by greenhouse gases. Additionally, clouds contribute to transform direct to diffuse sunlight. Due to the fact that forests increase their carbon uptake in response to large levels of diffuse light, there are novel and "positive" feedback loops operating over forest ecosystems. These loops may give rise to reduced levels of sunlight reaching the Earth's surface and result in lowering the thermal energy forcing due to increased removal of greenhouse gases such as carbon dioxide. These regional processes will be presented and discussed during the seminar.