ESTIMATES OF REGIONAL NATURAL VOLATILE ORGANIC-COMPOUND FLUXES FROM ENCLOSURE AND AMBIENT MEASUREMENTS

Natural volatile organic compound (VOC) emissions were investigated at two forested sites in the southeastern United States. A variety of VO C compounds including methanol, 2-methyl-3-buten-2-ol, 6-methyl-5-hept en-2-one, isoprene and 15 monoterpenes were emitted from vegetation at these sites. Diurnal variations in VOC emissions were observed and re lated to light and temperature. Variations in isoprene emission from i ndividual branches are well correlated with light intensity and leaf t emperature while variations in monoterpene emissions can be explained by variations in leaf temperature alone. Isoprene emission rates for i ndividual leaves tend to be about 75% higher than branch average emiss ion rates due to shading on the lower leaves of a branch. Average dayt ime mixing ratios of 13.8 and 6.6 ppbv C isoprene and 5.0 and 4.5 ppbv C monoterpenes were observed at heights between 40 m and 1 km above g round level the two sites. Isoprene and monoterpenes account for 30% t o 40% of the total carbon in the ambient non-methane VOC quantified in the mixed layer at these sites and over 90% of the VOC reactivity wit h OH. Ambient mixing ratios were used to estimate isoprene and monoter pene fluxes by applying box model and mixed-layer gradient techniques, Although the two techniques estimate fluxes averaged over different s patial scales, the average fluxes calculated by the two techniques agr ee within a factor of two. The ambient mixing ratios were used to eval uate a biogenic VOC emission model that uses field measurements of pla nt species composition, remotely sensed vegetation distributions, leaf level emission potentials determined from vegetation enclosures, and light and temperature dependent emission activity factors. Emissions e stimated for a temperature of 30 degrees C and above canopy photosynth etically active radiation flux of 1000 mu mol m(-2) s(-1) are around 4 mg C m(-2) h(-1) of isoprene and 0.7 mg C m(-2) h(-1) of monoterpenes at the ROSE site in western Alabama and 3 mg C m(-2) h(-1) of isopren e and 0.5 mg C m(-2) h(-1) of monoterpenes at the SOS-M site in easter n Georgia. Isoprene and monoterpene emissions based on land characteri stics data and emission enclosure measurements are within a factor of two of estimates based on ambient measurements in most cases. This rep resents reasonable agreement due to the large uncertainties associated with these models and because the observed differences are at least p artially due to differences in the size and location of the source reg ion (''flux footprint'') associated with each flux estimate.