Measurements of natural hydrocarbon emission fluxes are reported for a northeastern U.S. deciduous forest, for a northwestern U.S. coniferous forest, and for dominant tree species in the Atlanta, Georgia, region. The emission data were obtained by using a vegetation enclosure procedure and a micrometeorological gradient technique. Isoprene fluxes determined by the gradient method in the deciduous forest varied from 2500 #g/m2/h at 20øC to 8000 #g/m2/h at 30øC. Corresponding fluxes obtained with the vegetation enclosure method ranged from 890 to 7300 #g/m2/h. Isoprene fluxes determined by both methods increased exponentially with increasing temperature. Emission rates from isoprene-emitting trees in the Atlanta area were similar to those observed in Pennsylvania. Alpha-pinene fluxes measured in a Douglas fir (Pseudotsuga menziesii) forest ranged from 9 to 1320 #g/me/h. Relative humidity appeared to directly influence the alpha-pinene emission rate. The reasonable agreement between hydrocarbon fluxes obtained with two independent techniques is significant evidence of the general validity of current biogenic hydrocarbon emission rates. levels of approximately 130 ttg/m2/h. In a companion paper, Dement et al. [1975] studied these emissions under varying light, humidity, and temperature conditions. They found that the volatilization rate of monoterpenes is dependent on the vapor pressures of the terpenes, humidity, and the amount of oil present on the surface of the leaf. These investigators also reported that the emission rate is not directly dependent on the photosynthetic activity or on the stomatal opening of the plant. This suggested that the mechanism was physical and that the terpenes were volatilized from the surface of the leaf rather than from the inside. Tingey et al. [1979] used a laboratory gas exchange chamber to study the volatile emissions from live oak (Quercus virginiana) and reported an average emission rate of approximately 30 ttg/g leaf biomass/h. Arnts et al. [1978], in a studysimilar to the gradient profile approach reported in this manuscript, used an energy balance/Bowen ratio technique to estimate the alpha-pinene flux from a plantation of loblolly pine (Pinus taeda). An average flux of 3170 ttg/m2/h was reported, and the authors noted an increase in the emission flux with increasing temperature and water stress and a decrease in flux with increasing wind speed. They also observed substantially lower terpene concentrations in the winter months in ambient samples collected 5 m above the canopy. Using this data base, Knoerr [1980] compared fluxes of alpha-pinene determined from the micrometeorological technique to the results obtained via a branch enclosure method and observed good overall agreement. The most extensive study performed to date on the terpene 2380 570, 1981. Turner, J., Nutrient cycling in a Douglas fir ecosystem with respect to age and nutrient status, Ph.D. thesis, Univ. Wash., Seattle, Wash., 1976. , Assembling a vegetative hydrocarbon emission inventory for the California Sou...

Abstract. 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 30øC and above canopy photosynthetically active radiation flux of 1000 gmol m '2 s -i 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] The nocturnal drainage flow of air causes significant uncertainty in ecosystem CO 2 , H 2 O, and energy budgets determined with the eddy covariance measurement approach. In this study, we examined the magnitude, nature, and dynamics of the nocturnal drainage flow in a subalpine forest ecosystem with complex terrain. We used an experimental approach involving four towers, each with vertical profiling of wind speed to measure the magnitude of drainage flows and dynamics in their occurrence. We developed an analytical drainage flow model, constrained with measurements of canopy structure and SF 6 diffusion, to help us interpret the tower profile results. Model predictions were in good agreement with observed profiles of wind speed, leaf area density, and wind drag coefficient. Using theory, we showed that this one-dimensional model is reduced to the widely used exponential wind profile model under conditions where vertical leaf area density and drag coefficient are uniformly distributed. We used the model for stability analysis, which predicted the presence of a very stable layer near the height of maximum leaf area density. This stable layer acts as a flow impediment, minimizing vertical dispersion between the subcanopy air space and the atmosphere above the canopy. The prediction is consistent with the results of SF 6 diffusion observations that showed minimal vertical dispersion of nighttime, subcanopy drainage flows. The stable within-canopy air layer coincided with the height of maximum wake-to-shear production ratio. We concluded that nighttime drainage flows are restricted to a relatively shallow layer of air beneath the canopy, with little vertical mixing across a relatively long horizontal fetch. Insight into the horizontal and vertical structure of the drainage flow is crucial for understanding the magnitude and dynamics of the mean advective CO 2 flux that becomes significant during stable nighttime conditions and are typically missed during measurement of the turbulent CO 2 flux. The model and interpretation provided in this study should lead to research strategies for the measurement of these advective fluxes and their inclusion in the overall mass balance for CO 2 at this site with complex terrain.

Abstract. Emission rates of reduced sulfur gases from vegetation and soils were measured in various regions of the United States during the summer of 1985. The predominant sulfur gases emitted were hydrogen sulfide, carbonyl sulfide and dimethylsulfide. Typically, vegetative (forests, crops, etc.) emission fluxes varied between approximately 10 and 60 ng S m -2 min -1 . Biogenic sulfur fluxes from mollisol and histisol soils averaged 15 and 217 ng S m -x min -I , respectively. Salt water marsh fluxes with a geometric mean of 293 ng S m -2 min -1 were the highest measured. These biogenic sulfur fluxes are somewhat lower than those measured during the SURE study at some of the same sites. The natural sulfur emission fluxes reported herein together with those data included in the two accompanying manuscripts provide the basis for developing a national inventory of reduced sulfur emissions from soils, crops and trees. When combined these data also will provide a foundation for deriving uncertainty limits associated with these flux estimates.

Abstract. Extensive measurements of ozone and its photochemical precursors and coproducts were made in the 1990 Rural Oxidants in the Southern Environment (ROSE) program. Peroxy radical production, loss, and partitioning are described at a rural site in Alabama, showing the important role that biogenic organic compounds play in ozone production. Estimates of the peroxy radical concentration obtained by four methods along with the measured nitric oxide level are used to predict the instantaneous rate of photochemical ozone production at the site. The four methods agree on the diurnal behavior of peroxy radicals and ozone production rates, while consistent discrepancies between the methods generally are within their combined uncertainties. Selected aircraft measurements are used to derive ozone production rates above the ground site, with the highest rates occurring in the boundary layer and in industrial plumes. The dependences of peroxy radical concentration and ozone production rate on the level of nitrogen oxides exhibit good agreement between the various methods and are consistent throughout the lower troposphere. Surface deposition and entrainment are shown to be as important as photochemical production in determining the diurnal evolution of ozone at this site.