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 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 tend to be about 75% higher than branch average emission rates 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 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 two. 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 μmol 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 isoprene and 0.5 mg C m−2 h−1 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 two of estimates based on ambient measurements in most cases. This represents reasonable agreement due to the 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 (“flux footprint”) associated with each flux estimate.

[1]  Hal Westberg,et al.  A biogenic hydrocarbon emission inventory for the U.S.A. using a simple forest canopy model , 1993 .

[2]  S. Montzka,et al.  The observation of a C5 alcohol emission in a North American pine forest , 1993 .

[3]  John C. Wyngaard,et al.  Evaluation of turbulent transport and dissipation closures in second-order modeling , 1989 .

[4]  R. Olson,et al.  Geoecology: a county-level environmental data base for the conterminous United States , 1980 .

[5]  R. Atkinson Gas-phase tropospheric chemistry of organic compounds: a review , 1990 .

[6]  Patrick R. Zimmerman,et al.  Natural volatile organic compound emission rate estimates for U.S. woodland landscapes , 1994 .

[7]  C. N. Hewitt,et al.  A global model of natural volatile organic compound emissions , 1995 .

[8]  Donald H. Lenschow,et al.  Biogenic nonmethane hydrocarbon emissions estimated from tethered balloon observations , 1994 .

[9]  D. Lilly Models of cloud-topped mixed layers under a strong inversion , 1968 .

[10]  S. Montzka,et al.  Isoprene and its oxidation products, methyl vinyl ketone and methacrolein, in the rural troposphere , 1993 .

[11]  Hal Westberg,et al.  A national inventory of biogenic hydrocarbon emissions , 1987 .

[12]  Russell K. Monson,et al.  Isoprene and monoterpene emission rate variability: Observations with Eucalyptus and emission rate algorithm development , 1991 .

[13]  Thomas E. Pierce,et al.  An improved model for estimating emissions of volatile organic compounds from forests in the eastern United States , 1994 .

[14]  Alex Guenther,et al.  SEASONAL AND SPATIAL VARIATIONS IN NATURAL VOLATILE ORGANIC COMPOUND EMISSIONS , 1997 .

[15]  R. Stull An Introduction to Boundary Layer Meteorology , 1988 .

[16]  B. Lanne,et al.  Biosynthesis of 2-methyl-3-buten-2-ol, a pheromone component of Ips typographus (Coleoptera: Scolytidae) , 1989 .

[17]  M. Khalil,et al.  Tropospheric OH: model calculations of spatial, temporal, and secular variations , 1991 .

[18]  P. Zimmerman,et al.  Nonmethane hydrocarbons in remote tropical, continental, and marine atmospheres , 1984 .

[19]  Jesslyn F. Brown,et al.  Development of a land-cover characteristics database for the conterminous U.S. , 1991 .

[20]  Michael O. Rodgers,et al.  Ozone precursor relationships in the ambient atmosphere , 1992 .

[21]  Patrick R. Zimmerman,et al.  Measurements of atmospheric hydrocarbons and biogenic emission fluxes in the Amazon Boundary layer , 1988 .

[22]  D. Jacob,et al.  Simulation of summertime ozone over North America , 1993 .

[23]  R. Fall,et al.  Sub-parts per billion detection of isoprene using a reduction gas detector with a portable gas chromatograph , 1993 .

[24]  Mark H. Hansen,et al.  The Eastwide forest inventory data base: users manual. , 1992 .

[25]  R. Monson,et al.  Isoprene and monoterpene emission rate variability: Model evaluations and sensitivity analyses , 1993 .