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ESTIMATES OF REGIONAL NATURAL VOLATILE ORGANIC COMPOUND FLUXES FROM ENCLOSURE AND AMBIENT MEASUREMENTS
Citation:
Guenther, A., P. Zimmerman, L. Klinger, J. Greenberg, C. Ennis, K. Davis, W. Pollock, H. Westberg, G. Allwine, AND C D. Geron*. ESTIMATES OF REGIONAL NATURAL VOLATILE ORGANIC COMPOUND FLUXES FROM ENCLOSURE AND AMBIENT MEASUREMENTS. JOURNAL OF GEOPHYSICAL RESEARCH 101(D1):1345-1359, (1996).
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Description:
The paper discusses results of an investigation at two forested sites in the Southeastern United States. A variety of VOC compounds including methanol, 2-methyl-3-buten-2-ol, 6-methyl-5-hepten-
2-one, isoprene, and 15 monoterpenes were emitted from vegetation at these sites. Diurnal variations in VOC emissions were observed and related to light and temperature. Variations in isoprene emission from individual branches are well correlated with light intensity and leaf temperature, while variations in monoterpene emissions can be explained by variations in leaf temperature alone. Isoprene emission rates for individual leaves can be 75% higher than branch average due to shading on the lower leaves of a branch. Average daytime 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 ground level at the two sites. Isoprene and monoterpenes account for 30 to 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 with OH. Ambient mixing ratios were used to estimate isoprene and monoterpene fluxes by applying box model and mixed-layer gradient techniques. Although the two techniques estimate fluxes averaged over different spatial scales, the average fluxes calculated by the two techniques agree within a factor of 2. The ambient mixing ratios were used to evaluate a biogenic VOC emission model that uses field measurements of plant species composition, remotely sensed vegetation distributions, leaf-level emission potentials determined from vegetation enclosures, and light and temperature dependent emission activity factors. Emissions estimated for a temperature of 30 C and above canopy photosynthetically active radiation flux of 1000 micro mol per s m2 are around 4 mg C per h m2 of isoprene and 0.7 mg C per h m2 of monoterpenes at the ROSE site in western Alabama and 3 mg C per h m2 of isoprene and 0.5 mg C per h m2 of monoterpenes at the SOS-M site in eastern Georgia. Isoprene and monoterpene emissions based on land characteristics data and emission enclosure measurements are within a factor of 2 of estimates based on ambient measurements in most cases. This represents reasonable agreement due to large uncertainties associated with these models and because the observed differences are at least partially due to differences in the size and location of the source region associated with each flux estimate.
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ESTIMATES OF REGIONAL NATURAL VOLATILE ORGANIC COMPOUND FLUXES FROM ENCLOSURE AND AMBIENT MEASUREMENTS