Abstract
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 84×10 12 g 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. The estimated annual natural NO emission of 2.1×10 12 g of nitrogen (Tg N) from North America is primarily due to soils and lightning, while the estimated 10 Tg C of CO arises from biomass burning and vegetation. Field measurements of ambient concentrations and above canopy fluxes have validated emission estimates for a few compounds from some important landscapes. The uncertainty associated with natural emission estimates ranges from less than 50% for midday summer isoprene emission from some locations to about a factor of 10 for some compounds and landscapes.
Highlights
Natural emissions of volatile compounds are an important component of the earth system responsible forOn assignment to: National Exposure Research Laboratory, US Environmental Protection Agency, Research Triangle Park, NC 27711, USA.determining the composition of the atmosphere
Natural emissions of nitric oxide (NO), carbon monoxide (CO), and non-methane volatile organic compounds (NMVOC) equal or exceed anthropogenic emissions, anthropogenic sources usually dominate within urban areas
Where is a landscape average emission capacity, D is the annual peak foliar density, D is the fraction of foliage present at a particular time of year, the emission activity factors ., 2, and account for the in#uence of photosynthetic photon #ux density (PPFD), temperature, and leaf age, respectively, and is an escape e$ciency that represents the fraction emitted by the canopy that is released into the above-canopy atmosphere
Summary
On assignment to: National Exposure Research Laboratory, US Environmental Protection Agency, Research Triangle Park, NC 27711, USA. A. Guenther et al / Atmospheric Environment 34 (2000) 2205}2230 the appropriate regulatory control strategy (i.e. VOC or NOV control) indicated by a regional oxidant model is sensitive to whether or not isoprene emissions are included and, at some locations, depends on which emission model is used to predict isoprene emission. Guenther et al / Atmospheric Environment 34 (2000) 2205}2230 the appropriate regulatory control strategy (i.e. VOC or NOV control) indicated by a regional oxidant model is sensitive to whether or not isoprene emissions are included and, at some locations, depends on which emission model is used to predict isoprene emission This result emphasizes that accurate estimates of isoprene emissions are needed to determine oxidant control strategies. This review focuses on research reported since 1991, and we refer readers elsewhere (NRC, 1991; Singh and Zimmerman, 1992; Fehsenfeld et al, 1992) for details on earlier research
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