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^-2 s^-1. 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 NO ₃ and HO levels approaching 1 part per trillion on a volume basis (pptv) and 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^-2 s ^-1) 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.