As anthropogenic emissions of ozone (O{sub}3) precursors, fine particulate matter (PM{sub}2.5), and PM{sub}2.5 precursors continue to decrease in the United States, the fraction of O{sub}3 and PM{sub}2.5 attributable to natural sources may become significant in some locations, reducing the efficacy that can be expected from future controls of anthropogenic sources. Modeling studies were conducted to estimate the contribution of biogenic emissions to the formation of O{sub}3 and PM{sub}2.5 in Nashville/TN and the northeastern United States. Two approaches were used to bound the estimates. In an anthropogenic simulation, biogenic emissions and their influence at the domain boundaries were eliminated. Contributions of biogenic compounds to the simulated concentrations of O{sub}3 and PM{sub}2.5 were determined by the deviation of the concentrations-in the anthropogenic case from those in the base case. A biogenic simulation was used to assess the amounts of O{sub}3 and PM{sub}2.5 produced in an environment free from anthropogenic influences in emissions and boundary conditions. In both locations, the contribution of biogenic emissions to O{sub}3 was small (＜23) on a domain-wide basis, despite significant biogenic volatile organic compounds (VOC) emissions (65-89 of total VOC emissions). However, the production of O{sub}3 was much more sensitive to biogenic emissions in urban areas (22-34). Therefore, the effects of biogenic emissions on O{sub}3 manifested mostly via their interaction with anthropogenic emissions of NO{sub}x. In the anthropogenic simulations, the average contribution of biogenic and natural sources to PM{sub}2.5 was estimated at 9 in Nashville/TN and 12 in the northeast domain. Because of the long atmospheric lifetimes of PM{sub}2.5, the contribution of biogenic/natural PM{sub}2.5 from the boundary conditions was higher than the contribution of biogenic aerosols produced within the domain. The elimination of biogenic emissions also affected the chemistry of other secondary PM{sub}2.5 components. Very little PM{sub}2.5 was formed in the biogenic simulations.
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