Leaf, branch, stand and landscape scale measurements of volatile organic compound fluxes from U.S. woodlands.

Natural volatile organic compound (VOC) fluxes were measured in three U.S. woodlands in summer 1993. Fluxes from individual leaves and branches were estimated with enclosure techniques and used to initialize and evaluate VOC emission model estimates. Ambient measurements were used to estimate above canopy fluxes for entire stands and landscapes. The branch enclosure experiments revealed 78 VOCs. Hexenol derivatives were the most commonly observed oxygenated compounds. The branch measurements also revealed high rates of isoprene emission from three genera of plants (Albizia, Chusqua and Mahonia) and high rates of monoterpene emission from three genera (Atriplex, Chrysthamnus and Sorbus) for which VOC emission rates have not been reported. Measurements on an additional 34 species confirmed previous results. Leaf enclosure measurements of isoprene emission rates from Quercus were substantially higher than the rates used in existing emission models. Model predictions of diurnal variations in isoprene fluxes were generally within +/- 35% of observed flux variations. Measurements with a fast response analyzer demonstrated that 60 min is a reasonable time resolution for biogenic emission models. Average daytime stand scale (hundreds of m) flux measurements ranged from about 1.3 mg C m(-2) h(-1) for a shrub oak stand to 1.5-2.5 mg C m(-2) h(-1) for a mixed forest stand. Morning, evening and nighttime fluxes were less than 0.1 mg C m(-2) h(-1). Average daytime landscape scale (tens of km) flux measurements ranged from about 3 mg C m(-2) h(-1) for a shrub oak-aspen and rangeland landscape to about 7 mg C m(-2) h(-1) for a deciduous forest landscape. Fluxes predicted by recent versions (BEIS2, BEIS2.1) of a biogenic emission model were within 10 to 50% of observed fluxes and about 300% higher than those predicted by a previous version of the model (BEIS).