Biogenic Hydrocarbon Chemistry within and Above a Mixed Deciduous Forest

This manuscript includes findings from field and numerical modeling investigations designed to quantify the degree and rates of biogenic hydrocarbon chemical processing within and above a mixed deciduous forest in the southeastern United States. The study site was under the influences of nitrogen oxide and hydrocarbon emissions from suburban automobile traffic. The most common ambient biogenic hydrocarbons measured within and above the forest included isoprene, α-pinene, and d-limonene. Isoprene was the most abundantly produced biogenic hydrocarbon, with maximum isoprene flux densities reaching 50 nmol m s. Isoprene and its reaction products (methyl vinyl ketone and methacrolein) comprised over 75% of the measured hydrocarbon mass. Substantial nitrate (NO3) and hydroxyl (HO) radical formation occurred within the forest canopy, with maximum NO3 and HO levels approaching 1 part per trillion on a volume basis (pptv) and J Atmos Chem DOI 10.1007/s10874-006-9048-4 J. D. Fuentes (*) Department of Environmental Sciences, University of Virginia, Clark Hall, 291 McCormick Road, Charlottesville, VA 22904, USA e-mail: [email protected] D. Wang Environmental Technology Center, Environment Canada, 335 River Road South, Ottawa, ON K1A 0H3, Canada D. R. Bowling Department of Biology, University of Utah, Salt Lake City, UT 84112, USA M. Potosnak Division of Earth and Environmental Sciences, Desert Research Institute, Reno, NV 89512, USA R. K. Monson Department of Ecology and Evolutionary Biology, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA W. S. Goliff Division of Atmospheric Sciences, Desert Research Institute, Reno, NV 89512, USA W. R. Stockwell Department of Chemistry, Howard University, Washington, DC 20059, USA 0.05 pptv, respectively. These NO3 and HO levels, combined with within-canopy ozone (O3) mixing ratios of 60 parts per billion (ppbv), reacted with biogenic hydrocarbons and produced substantial amounts (0.6 ppbv) of peroxy radicals. The main conclusion from this investigation is that forested ecosystems capable of high rates (>50 nmol m s) of biogenic hydrocarbon emissions, and in the vicinity of modest rates of nitrogen oxide emissions from suburban automobile traffic, can support a unique and active photochemistry within the forest canopy. In these areas it may not be valid to use biogenic emissions estimated from measurements made at the foliage level for regional-scale air quality modeling because the underlying processes are nonlinear. Regional-scale air quality models should include chemical preprocessing of biogenic hydrocarbons before they are emitted to the full regional modeling grid in order to accurately represent the photochemical production of pollutants on the wider scale.