LEAF, BRANCH, STAND AND LANDSCAPE SCALE MEASUREMENTS OF VOLATILE ORGANIC-COMPOUND FLUXES FROM US WOODLANDS

Natural volatile organic compound (VOC) fluxes were measured in three U.S. woodlands in summer 1993. Fluxes from individual leaves and branc hes were estimated with enclosure techniques and used to initialize an d evaluate VOC emission model estimates. Ambient measurements were use d to estimate above canopy fluxes for entire stands and landscapes. Th e branch enclosure experiments revealed 78 VOCs. Hexenol derivatives w ere the most commonly observed oxygenated compounds. The branch measur ements also revealed high rates of isoprene emission from three genera of plants (Albizia, Chusqua and Mahonia) and high rates of moneterpen e emission from three genera (Atriplex Chrysthamnus and Sorbus) for wh ich VOC emission rates have not been reported. Measurements on an addi tional 34 species confirmed previous results. Leaf enclosure measureme nts 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 meas urements 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 day time 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 ab out 7 mg C m(-2) h(-1) for a deciduous forest landscape. Fluxes predic ted by recent versions (BEIS2, BEIS2.1) of a biogenic emission model w ere within 10 to 50% of observed fluxes and about 300% higher than tho se predicted by a previous version of the model (BEIS).