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The magnitudes, distributions, controlling processes and uncertainties associated with North American natural emissions of oxidant precursors are reviewed. Natural emissions are responsible for a major portion of the compounds, including non-methane volatile organic compounds (NMVOC), carbon monoxide (CO) and nitric oxide (NO), that determine tropospheric oxidant concentrations. Natural sources include soil microbes, vegetation, biomass burning, and lightning. These sources are strongly influenced by human activities that have led to significant changes in the magnitude and distribution of natural emissions in the past two centuries. The total NMVOC flux of about 84×10¹²g of carbon (Tg C) is comprised primarily of isoprene (35%), 19 other terpenoid compounds (25%) and 17 non-terpenoid compounds (40%). Vegetation is predicted to contribute about 98% of the total annual natural NMVOC emission …