Volatile organic compound (VOC) emission rate factors are estimated for 49 tree genera based on a review of foliar emission rate measurements. Foliar VOC emissions are grouped into three categories: isoprene, monoterpenes and other VOCs. Typical emission rates at a leaf temperature of 30°C and a light intensity of 1000 μmol m-2 s-1 range from <0.1 to 70 μg C g-1 h-1 for isoprene, <0.1 to 3 μg C g-1 h-1 for monoterpenes, and < 0.5 to 5 μg C g-1 h-1 for other VOCs. Isoprene emission factors are given for biogenic emission models that incorporate canopy shading effects and thus require leaf-level emission rates and for emission models that do not include a canopy model and therefore require branch-level isoprene emission factors which already account for some shading. Landscape-level emission rates are estimated by combining emission rate factors determined for tree genera with species composition and foliar mass data. Landscape emission rate factors are determined for each of the 91 woodland landscapes in the high resolution (1.1 km) gridded land-cover database compiled by the EROS Data Center (EDC) from satellite and ancillary data. This database covers the entire contiguous United States of America. Landscape emission rates are also be determined using gridded tree distribution data, based on aerial photographs and ground measurements, such as that available in the U.S. Forest Service (USES) Easlwide Forest Inventory Database (EFID). Emission rates are reported for 41 of the 65 tree genera in the EFID including all of the most common genera. Total VOC emission rate factors for the 91 EDC woodland-cover types range from 0.8 to 11 mg C m-2h-1 at a standard condition of 30°C and 1000 μmol m-2s-1. These landscape factors are based on branch-level emission factors and thus already incorporate canopy shading effects. The estimated fluxes of isoprene and monoterpenes are in relatively good agreement with field measurements of area-averaged fluxes if accurate species composition data (e.g. from the EFID) are available. Total VOC emission rate estimates range from 0.8 to 4.3 mg C m-2h-1 for scrub woodlands and 2.2 to 11 mg C m-2h-1 for mixed deciduous/coniferous woodlands. The chemical composition of the VOC flux ranges from 8 to 91 isoprene, 1 to 56% for monoterpenes and 8 to 73% for other VOC. On an area-weighted basis, the U.S. average total VOC emission rate factor of 5.1 mg m-2h-1 for all woodlands is comprised of 58% isoprene, 18% monoterpenes and 24% other VOC. In comparison to previous estimates, these emission rates are generally higher for isoprene and lower for monoterpenes. © 1994.

Biogenic nonmethane hydrocarbon (NMHC) emissions were investigated at two field sites in the Republic of South Africa that include five important southern African savanna landscapes. Tropical savannas are a globally important biome with a high potential for biogenic emissions but no NMHC emission measurements in these regions or in any part of Africa have been reported. Landscape average hydrocarbon emissions were estimated by characterizing plant species composition and foliar density at each site, identifying and characterizing NMHC emissions of the most abundant plant species, and identifying and characterizing NMHC emissions of plant species with the highest NMHC emission rates. A hand‐held portable analyzer proved to be a useful tool for identifying plants with high emission rates. A branch enclosure system, with gas chromatography and flame ionization detector, was used to quantify isoprene and monoterpene emission rates. Emission rates were species‐specific and several genera had both high and low emitters. At least some species with high emission rates were identified in most savanna types. High and low emitters were found on both nutrient‐rich and nutrient‐poor soils. Landscape average emission capacities for the five savanna types range from 0.6 to 9 mg C m−2 h−1 for isoprene and about 0.05 to 3 mg C m−2 h−1 for monoterpenes. The savanna emission rates predicted by existing global models are within the range estimated for these five savanna types.

The flux of volatile organic chemicals from natural vegetation influences various atmospheric properties including oxidation state of the troposphere via the hydroxyl radical (OH), photochemical haze production and the concentration of greenhouse gases (CH4, H2O, CO). Because the Volatile Organic Compound (VOC) flux in the present-day world varies markedly with both vegetation cover and with climate, changes in the emission of VOCs may have damped or amplified past climate changes. Here we conduct a preliminary study on possible changes in VOC emission resulting from broad scale vegetation and climate change since the Last Glacial Maximum (LGM). During the general period of the LGM (~25-17,000 years before present {BP}), global forest cover was considerably less than in the present potential situation. The change in vegetation would have resulted in a ~30% reduction in VOC emission at 643 Tg y-1 relative to the present potential vegetation (912.9 Tg y-1). Uncertainty in global vegetation cover during the LGM bounds the VOC estimate by ±15%. In contrast, during the warmer early-to-mid Holocene (8000 and 5000 BP), with greater forest extent and less desert than during the late Holocene (0 BP), emission rates of VOCs seem likely to have been higher than at present. Further modifications in VOC emission may have been mediated by a reduction in mean tropical lowland temperatures (by around 5-6°C) decreasing the LGM VOC emission estimate by 38% relative to the warmer LGM scenario. Increased VOC emissions due to forest expansion and increased tropical temperatures since the LGM may have served as a significant driver of climate change over the last 15 ka y through the influence of VOC oxidation; this can impact tropospheric radiative balance through reductions in the concentration of OH, increasing the concentration of CH4. The error limits on past VOC emission estimates are large, given the uncertainties of present-day VOC emission rates, paleoecosystem distribution, tropical paleoclimatic conditions, and physiological assumptions regarding controls over VOC emission. Nevertheless, the potential significance of changes in natural VOC emission over the last 20 ka and their influence on climate are an important unknown that should at least be borne in mind as a limit on the understanding of past atmospheric conditions. Elucidation of the role of VOCs in climate change through paleoclimatic general circulation model simulations may improve understanding of past and future changes in climate. (C) 2000 Elsevier Science Ltd.

Vegetation composition and biomass were surveyed for three specific sites in Atlanta, GA; near Rhinelander, WI; and near Hayden, CO. At each research site emissions of biogenic volatile organic compounds (BVOCs) from the dominant vegetation species were sampled by enclosing branches in bag enclosure systems and sampling the equilibrium head space onto multi-stage solid adsorbent cartridges. Analysis was performed using a thermal desorption technique with gas chromatography (GC) separation and mass spectrometry (MS) detection. Identification of BVOCs covering the GC retention index range (stationary phase DB-1) from approximately 400 to 1400 was achieved (volatilities C4-C14).

Landscape flux potentials for biogenic volatile organic compounds (BVOCs) were derived for three ecosystems in the continental U.S. (Fernbank Forest, Atlanta, GA; Willow Creek, Rhinelander, WI; Temple Ridge, CO). Analytical data from branch enclosure measurements were combined with ecological survey data for plant species composition and biomass. Other quantitative flux measurements at the leaf and landscape level were incorporated to scale the results from the enclosure measurements to the landscape level. Flux estimates were derived by using a one week ambient temperature and light record (30 min time resolution) and adjusting all emission rates to these conditions with temperature and light correction algorithms.

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.

Isoprene fluxes were estimated using eight different measurement techniques at a forested site near Oak Ridge, Tennessee, during July and August 1992. Fluxes from individual leaves and entire branches were estimated with four enclosure systems, including one system that controls leaf temperature and light. Variations in isoprene emission with changes in light, temperature, and canopy depth were investigated with leaf enclosure measurements. Representative emission rates for the dominant vegetation in the region were determined with branch enclosure measurements. Species from six tree genera had negligible isoprene emissions, while significant emissions were observed for Quercus, Liquidambar, and Nyssa species. Above‐canopy isoprene fluxes were estimated with surface layer gradients and relaxed eddy accumulation measurements from a 44‐m tower. Midday net emission fluxes from the canopy were typically 3 to 5 mg C m−2 h−1, although net isoprene deposition fluxes of −0.2 to −2 mg C m−2 h−1 were occasionally observed in early morning and late afternoon. Above‐canopy CO2 fluxes estimated by eddy correlation using either an open path sensor or a closed path sensor agreed within ±5%. Relaxed eddy accumulation estimates of CO2 fluxes were within 15% of the eddy correlation estimates. Daytime isoprene mixing ratios in the mixed layer were investigated with a tethered balloon sampling system and ranged from 0.2 to 5 ppbv, averaging 0.8 ppbv. The isoprene mixing ratios in the mixed layer above the forested landscape were used to estimate isoprene fluxes of 2 to 8 mg C m−2 h−1 with mixed layer gradient and mixed layer mass balance techniques. Total foliar density and dominant tree species composition for an approximately 8100 km2 region were estimated using high‐resolution (30 m) satellite data with classifications supervised by ground measurements. A biogenic isoprene emission model used to compare flux measurements, ranging from leaf scale (10 cm2) to landscape scale (102 km2), indicated agreement to within ±25%, the uncertainty associated with these measurement techniques. Existing biogenic emission models use isoprene emission rate capacities that range from 14.7 to 70 μg C g−1 h−1 (leaf temperature of 30°C and photosynthetically active radiation of 1000 μmol m−2 s−1) for oak foliage. An isoprene emission rate capacity of 100 μg C g−1 h−1 for oaks in this region is more realistic and is recommended, based on these measurements.