-
D. SERÇA,
A. GUENTHER,
L. KLINGER,
L. VIERLING,
P. HARLEY,
A. DRUILHET, J. GREENBERG,
B. BAKER,
W. BAUGH,
C. BOUKA-BIONA,
J. LOEMBA-NDEMBI
[show abstract]
[hide abstract]
ABSTRACT: As part of the EXPRESSO program (EXPeriment for the REgional Sources and Sinks of Oxidants), biosphere-atmosphere exchanges of trace gases were investigated in a ground-based forest site of the Republic of Congo. Experiments were carried out in March and November-December 1996. A 60-meter walkup tower was erected in an undisturbed mixed tropical forest typical of upland vegetation in the Nouabalé-Ndoki National Park. Eight belt transects radiating from the tower were used to characterize the species composition and structure of the upland mixed forest. As a comparison, and to investigate horizontal heterogeneity of the trace gases exchanges, additional measurements were made in a nearby monospecific forest stand characteristic of lowland Gilbertiodendron dewevrei(Gilbert. dew.) forest. Micrometeorological data, trace gas concentrations and flux measurements were made from the tower. We report daily above-canopy variation in temperature and radiation, energy partitioning into latent and sensible heat flux, volatile organic compound (VOC) mixing ratios, isoprene and CO2 fluxes. Fluxes of isoprene and CO2 were measured above the canopy using relaxed eddy accumulation and eddy covariance methods, respectively. These fluxes show a seasonal variation between the two experiments, as does energy partitioning. However, difference in isoprene emission between the two seasons are difficult to reconcile with meteorological (T, PAR) data only, and more data such as plant water potential are needed to modeled the seasonal isoprene emission cycle. Isoprene emission at the leaf level was also determined for plant species at both upland and lowland sites using environmentally controlled leaf enclosures. Together with the ecological survey, the leaf level work suggests that lowland Gilbert. dew. forests act as hot spots in terms of isoprene emissions. Future climate and land use changes could greatly affect the isoprene regional emission estimate through changes in the respective proportion of the upland and lowland forests, and the extent of dry versus wet season.
Tellus B 03/2003; 53(3):220 - 234. · 4.38 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Natural volatile organic compound (VOC) fluxes were measured in three U.S. woodlands in summer 1993. Fluxes from individual leaves and branches were estimated with enclosure techniques and used to initialize and evaluate VOC emission model estimates. Ambient measurements were used to estimate above canopy fluxes for entire stands and landscapes. The branch enclosure experiments revealed 78 VOCs. Hexenol derivatives were the most commonly observed oxygenated compounds. The branch measurements also revealed high rates of isoprene emission from three genera of plants (Albizia, Chusqua and Mahonia) and high rates of monoterpene emission from three genera (Atriplex, Chrysthamnus and Sorbus) for which VOC emission rates have not been reported. Measurements on an additional 34 species confirmed previous results. Leaf enclosure measurements of isoprene emission rates from Quercus were substantially higher than the rates used in existing emission models. Model predictions of diurnal variations in isoprene fluxes were generally within +/- 35% of observed flux variations. Measurements with a fast response analyzer demonstrated that 60 min is a reasonable time resolution for biogenic emission models. Average daytime stand scale (hundreds of m) flux measurements ranged from about 1.3 mg C m(-2) h(-1) for a shrub oak stand to 1.5-2.5 mg C m(-2) h(-1) for a mixed forest stand. Morning, evening and nighttime fluxes were less than 0.1 mg C m(-2) h(-1). Average daytime landscape scale (tens of km) flux measurements ranged from about 3 mg C m(-2) h(-1) for a shrub oak-aspen and rangeland landscape to about 7 mg C m(-2) h(-1) for a deciduous forest landscape. Fluxes predicted by recent versions (BEIS2, BEIS2.1) of a biogenic emission model were within 10 to 50% of observed fluxes and about 300% higher than those predicted by a previous version of the model (BEIS).
Tree Physiology 02/1996; 16(1_2):17-24. · 2.88 Impact Factor
-
Journal of the American Chemical Society 01/1994; · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Biogenic volatile organic carbon (BVOC) emissions are an important component of the global BVOC budget, contributing more than 90%. Emissions vary with species and vegetation type; therefore to produce accurate global budgets data is required from different vegetation types. This study investigates BVOC emissions from savannas, Kalahari woodlands, Kalahari open shrublands and Mopane woodlands in southern Africa. BVOC emission samples from individual species were collected using leaf cuvettes and the BVOC concentrations were determined by GC-FID/MS. Ten of the 14 woodland species measured were high isoprene emitters, while two showed high monoterpene emission capacities. Landscape average isoprene emission capacities were estimated to be as high as 9, 8 and 1 mg C m−2 h−1 for savannas, woodlands and shrub lands, respectively. The monoterpene emission capacity for Mopane woodlands were estimated at almost 3 mg C m−2 h−1, while for other landscapes it varied between 0.04 and 3 mg C m−2 h−1. Isoprene and monoterpene emissions at a savanna site in South Africa showed a seasonal variation, which is more pronounced for isoprene. During the winter (June–September) estimated emissions were <10 mg C m−2 d−1, with peak emissions (ranging between 20 and 80 mg C m−2 d−1) occurring during the summer months (December–March) when foliar density peaked. The total BVOC emission from southern African (south of the Equator) savannas was estimated to be in the range of 18–74 Tg C yr−1.
Atmospheric Environment.
-
[show abstract]
[hide abstract]
ABSTRACT: Methanol is found throughout the troposphere, with average concentrations second only to methane among atmospheric hydrocarbons. Proposed global methanol budgets are highly uncertain, but all agree that at least 60% of the total source arises from the terrestrial biosphere and primary emissions from plants. However, the magnitude of these emissions is also highly uncertain, and the environmental factors which control them require further elucidation.
Using a temperature-controlled leaf enclosure, we measured methanol emissions from leaves of six plant species by proton transfer reaction mass spectrometry, with simultaneous measurements of leaf evapotranspiration and stomatal conductance. Rates of emission at 30°C varied from 0.3 to 38 ?g g (dry mass)<sup>?1</sup> h<sup>?1</sup>, with higher rates measured on young leaves, consistent with the production of methanol via pectin demethylation in expanding foliage. On average, emissions increased by a factor of 2.4 for each 10°C increase in leaf temperature. At constant temperature, emissions were also correlated with co-varying incident photosynthetic photon flux density and rates of stomatal conductance. The data were analyzed using the emission model developed by Niinemets and Reichstein (2003a, b), with the incorporation of a methanol production term that increased exponentially with temperature. It was concluded that control of emissions, during daytime, was shared by leaf temperature and stomatal conductance, although rates of production may also vary diurnally in response to variations in leaf growth rate in expanding leaves. The model, which generally provided reasonable simulations of the measured data during the day, significantly overestimated emissions on two sets of measurements made through the night, suggesting that production rates of methanol were reduced at night, perhaps because leaf growth was reduced or possibly through a direct effect of light on production. Although the short-term dynamics of methanol emissions can be successfully modeled only if stomatal conductance and compound solubility are taken into account, emissions on longer time scales will be determined by rates of methanol production, controls over which remain to be investigated.
-
G. Brasseur,
E. Atlas,
D. Erickson,
A. Fried, J. Greenberg,
A. Guenther,
P. Harley,
E. Holland,
L. Klinger,
B. Ridley,
G. Tyndall
-
-
[show abstract]
[hide abstract]
ABSTRACT: Isoprene (2-methyl-1,3-butadiene) emission from plants is highly temperature dependent. The influence of long-term variations in growth temperature on isoprene emission rates from bur oak (Quercus macrocarpa) leaves was investigated under controlled environmental conditions. Trees were installed in a growth chamber and exposed to a series of daytime temperatures that were varied after a period of 3-6 weeks. Emission capacity (measured at leaf temperature of 25 degreesC and photosynthetic photon flux density of 900 mu mol m(-2) s(-1)) doubled when growth temperature was increased from 25 to 30 degreesC. Ten days after the growth temperature was decreased to 20 degreesC, isoprene emission capacity fell to 25 to 50% of its peak value. When growth temperature was returned to 30 degreesC, emission capacity doubled within 5 hours and continued to increase over several days. The observed behavior can be described by modifying existing algorithms.
Geophysical Research Letters. 28(9):1707-1710.
-
[show abstract]
[hide abstract]
ABSTRACT: Terpenoid emissions were measured at a grassland site in Inner Mongolia, China during four campaigns over a 2-year period. Emissions were strongly correlated with light and temperature and the variations could be simulated using a canopy emission model. Substantial seasonal and interannual variations in isoprene emissions were also observed. Area averaged isoprene emissions normalized to standard above-canopy temperature and light conditions (30 degrees C and 1500 mu mol m(-2) s(-1)) ranged from about 50 to 500 mu g m(-2) h(-1). These rates are more than an order of magnitude higher than those previously reported for temperate grasslands but are lower than emission rates observed from ground cover vegetation at higher latitudes. Isoprene emissions from this Inner Mongolia grassland may be dominated by emissions from sedges, e.g. Carex appendiculata. Total monoterpene emissions normalized to a standard temperature of 30 degrees C were only about 3 mu g m(-2) h(-1) and consisted primarily of carene and limonene with smaller contributions of a-pinene and beta-pinene. A model sensitivity study showed that grass and other herbaceous ground cover can contribute > 10% of the total isoprene emission from certain regions, such as Inner Mongolia, but are < 4% on a global annual scale. (c) 2006 Elsevier Ltd. All rights reserved.
Atmospheric Environment. 40(30):5753-5758.
-
B. Baker,
J. H. Bai,
C. Johnson,
Z. T. Cai,
Q. J. Li,
Y. F. Wang,
A. Guenther, J. Greenberg,
L. Klinger,
C. Geron,
R. Rasmussen
[show abstract]
[hide abstract]
ABSTRACT: Canopy scale fluxes of isoprene and monoterpenes were investigated in both wet and dry seasons above a rubber tree (Hevea brasiliensis)/secondary tropical forest in the Yunnan province of southwestern China. Drought conditions were unusually high during the dry season experiment. The eddy covariance measurement technique was used to measure isoprene fluxes, while monoterpene fluxes were modeled based on leaf level emission measurements. Maximum observed isoprene fluxes occurred during the wet season and daytime average fluxes were about 1 mg Cm-2 h(-1). Dry season fluxes were much lower with a daytime average of 0.15 mg Cm-2 h(-1). Wet season isoprene fluxes compare quite well with isoprene fluxes observed from other tropical forests. Monoterpene fluxes came, almost entirely, from Hevea brasiliensis, which is a light-dependent monoterpene emitter. Modeled wet season total monoterpene fluxes were about 2 mg Cm-2 h(-1) (average for the daytime), and in the dry season were undetectable. Extreme drought conditions, and the drought deciduous nature of Hevea brasiliensis may be the cause of the low dry season fluxes. (C) 2004 Elsevier Ltd. All rights reserved.
Atmospheric Environment. 39(2):381-390.
-
[show abstract]
[hide abstract]
ABSTRACT: Thirteen common plant species ill the Mojave and Sonoran Desert regions of the western US were tested for emissions of biogenic non-methanc volatile organic compounds (BVOCs). Only two of the species examined emitted isoprene at rates of 10 mu gCg(-1) h(-1) or greater. These species accounted for < 10% of the estimated vegetative biomass in these arid regions of low biomass density, indicating that these ecosystems are not likely a strong source of isoprene. However, isoprene emissions from these species continued to increase at much higher leaf temperatures than is observed from species in other ecosystems. Five species, including members of the Ambrosia genus, emitted monoterpenes at rates exceeding 2 mu g Cg(-1)h(-1). Emissions of oxygenated compounds, such as methanol, ethanol, acetone/propanal, and hexanol, from Cut branches of several species exceeded 10 mu g Cg(-1) h(-1), warranting further investigation ill these ecosystems. Model extrapolation of isoprene emission measurements verifies recently published observations that desert vegetation is a small source of isoprene relative to forests. Annual and daily total model isoprene emission estimates from an eastern US mixed forest landscape were 10-30 times greater than isoprene emissions estimated from the Mojave site. Monoterpene (and possibly oxygenated terpene and sesquiterpene) emissions may be more comparable, as annual forest terpene emission model estimates were 3-8 times greater than those from the Mojave Desert, and were within a factor of 2 for peak summertime fluxes. Primary productivity and leaf biomass of desert ecosystems are very dependent oil annual precipitation, and our model results indicate that there call be at least a three-fold difference in total annual BVOC emissions between dry and wet years. We recommend additional studies of desert plant BVOC emissions, especially those that focus on sesquiterpenes, oxygenated compounds, and the effects of soil moisture, temperature, humidity, and seasonality. Landscape flux Studies are needed to test BVOC model estimates and to verify BVOC influences on regional atmospheric chemistry. Published by Elsevier Ltd.
Atmospheric Environment. 40(9):1645-1660.
-
[show abstract]
[hide abstract]
ABSTRACT: High rates of emission of 2-methyl-3-buten-2-ol (MBO) were measured from needles of several pine species. Emissions of MBO in the light were 1 to 2 orders of magnitude higher than emissions of monoterpenes and, in contrast to monoterpene emissions from pines, were absent in the dark. MBO emissions were strongly dependent on incident light, behaving similarly to net photosynthesis. Emission rates of MBO increased exponentially with temperature up to approximately 35 degrees C. Above approximately 42 degrees C, emission rates declined rapidly. Emissions could be modeled using existing algorithms for isoprene emission. We propose that emissions of MBO from lodgepole and ponderosa pine are the primary source of high concentrations of this compound, averaging 1-3 ppbv, found in ambient air samples collected in Colorado at an isolated mountain site approximately 3050 m above sea level. Subsequent field studies in a ponderosa pine plantation in California confirmed high MBO emissions, which averaged 25 mu g C g(-1) h(-1) for 1-year-old needles, corrected to 30 degrees C and photon flux of 1000 mu mol m(-2) s(-1). A total of 34 pine species growing at Eddy Arboretum in Placerville, California, were investigated, of which 11 exhibited high emissions of MBO (>5 mu g C g(-1) h(-1)), and 6 emitted small but detectable amounts. All the emitting species are of North American origin, and most are restricted to western North America. These results indicate that MBO emissions from pines may constitute a significant source of reactive carbon and a significant source of acetone, to the atmosphere, particularly in the western United States.
Journal of Geophysical Research-Atmospheres. 103(D19):25479-25486.
-
[show abstract]
[hide abstract]
ABSTRACT: In temperate regions the chemistry of the lower troposphere is known to be significantly affected by biogenic volatile organic compounds (VOCs) emitted by plants, The chemistry of the lower troposphere over the tropics, however, is poorly understood, in part because of the considerable uncertainties in VOC emissions from tropical ecosystems. Present global VOC models predict that base emissions of isoprene from tropical rainforests are considerably higher than from savannas. These global models of VOC emissions which rely mainly on species inventories are useful, but significant improvement might be made with more ecologically based models of VOC emissions by plants. Ecosystems along a successional transect from woodland savanna to primary rainforest in central Africa were characterized for species composition and vegetation abundance using ground surveys and remotely sensed data. A total of 336 species (mostly trees) at 13 sites were recorded, and 208 of these were measured for VOC emissions at near-optimal light and temperature conditions using a leaf cuvette and hand-held photoionization detector (PID). A subset of 59 species was also sampled using conventional VOC emission techniques in order to validate the PID technique. Results of ecological and VOC emission surveys indicate both phylogenetic and successional patterns along, the savanna-rainforest transect. Genera and families of trees which tend to emit isoprene include Lophira, Irvingia, Albizia, Artocarpus, Ficus, Pterocarpus, Caesalpiniaceae, Arecaceae, and Moraceae. Other taxa tend to contain stored VOCs (Annonaceae and Asteraceae). Successional patterns suggest that isoprene emissions are highest in the relatively early successional Isoberlinia forest communities and progressively decrease in the later successional secondary and primary rainforest communities. Stored VOCs appear to increase along the savanna-rainforest succession, but these data are more tentative. These findings are consistent with successional patterns of isoprene and terpene fluxes in North American forests and highlight the feasibility of constructing better predictive models of VOC emissions.
Journal of Geophysical Research-Atmospheres. 103(D1):1443-1454.
-
[show abstract]
[hide abstract]
ABSTRACT: Improved vegetation distribution and emission data for Africa south of the equator were developed for the Southern African Regional Science Initiative ( SAFARI 2000) and were combined with biogenic volatile organic compound (BVOC) emission measurements to estimate BVOC emissions for the southern African region. The BVOCs are estimated to total 80 Tg C yr(-1) for the region, with isoprene and monoterpenes contributing 56 and 7 Tg C yr(-1), respectively. The large uncertainties, particularly in terms of basal emission capacity assignment, associated with these outputs are discussed. Woodlands are predicted to be the dominant vegetation type, covering 23% of southern Africa, and are the largest annual source of isoprene ( 20 Tg C), monoterpenes (3 Tg C), and other VOCs (4 Tg C). Mopane savannas and woodlands are predicted to contribute over 75% of all monoterpenes, primarily from light-dependent emission processes. Rain forests cover only 3.5% of the total area but have high annual emission rates (9.8 g C m(-2) yr(-1)). In the tropical regions with high rainfall, warm temperatures, and high plant productivity throughout the year, the seasonal variation in VOC emissions was small. In subtropical regions, dominated by highly seasonal savannas and grasslands, large variations were predicted, with emissions declining by up to 85% during dry winter periods (June-August) due to low leaf area index after leaf drop.
Journal of Geophysical Research-Atmospheres. 108(D13):doi 10.1029/2002jd002609.
-
[show abstract]
[hide abstract]
ABSTRACT: Biogenic volatile organic carbon (BVOC) emissions are an important component of the global BVOC budget, contributing more than 90%. Emissions vary with species and vegetation type; therefore to produce accurate global budgets data is required from different vegetation types. This study investigates BVOC emissions from savannas, Kalahari woodlands, Kalahari open shrublands and Mopane woodlands in southern Africa. BVOC emission samples from individual species were collected using leaf cuvettes and the BVOC concentrations were determined by GC-FID/ MS. Ten of the 14 woodland species measured were high isoprene emitters, while two showed high monoterpene emission capacities. Landscape average isoprene emission capacities were estimated to be as high as 9, 8 and 1 mg Cm-2 h(-1) for savannas, woodlands and shrub lands, respectively. The monoterpene emission capacity for Mopane woodlands were estimated at almost 3 mg Cm-2 h(-1), while for other landscapes it varied between 0.04 and 3 mg Cm-2 h(-1). Isoprene and monoterpene emissions at a savanna site in South Africa showed a seasonal variation, which is more pronounced for isoprene. During the winter (June-September) estimated emissions were < 10 mg Cm-2 d(-1), with peak emissions (ranging between 20 and 80mg Cm-2 d(-1)) occurring during the summer months (December-March) when foliar density peaked. The total BVOC emission from southern African (south of the Equator) savannas was estimated to be in the range of 18-74 Tg C yr(-1). (C) 2002 Elsevier Science Ltd. All rights reserved.
Atmospheric environment. 36(26):12.
-
[show abstract]
[hide abstract]
ABSTRACT: Biogenic volatile organic compounds (BVOCs) and their role in atmospheric oxidant formation were investigated at a forest site near Oak Ridge, Tennessee, as part of the Nashville Southern Oxidants Study (SOS) in July 1995. Of 98 VOCs detected, a major fraction were anthropogenic VOCs such as chlorofluorocarbons (CFCs), alkanes, alkenes and aromatic compounds. Isoprene was the dominant BVOC during daytime. Primary products from BVOC oxidation were methylvinylketone, methacrolein and 3-methylfuran Other compounds studied include the BVOCs cr-pinene, camphene, beta-pinene, p-cymene, limonene and cis-3-hexenyl acetate and a series of light alkanes, aromatic hydrocarbons and seven of the CFCs. The correlation of meteorological parameters, with the mixing ratios of these different compounds, reveals information on atmospheric oxidation processes and transport. Long-lived VOCs show very steady mixing ratio time series. Regionally and anthropogenically emitted VOCs display distinct diurnal cycles with a strong mixing ratio decrease in the morning from the breakup of the nocturnal boundary layer. Nighttime mixing ratio increases of CFCs and anthropogenic VOCs are suspected to derive from emissions within the Knoxville urban area into the shallow nocturnal boundary layer. In contrast, the time series of BVOCs and their oxidation products are determined by a combination of emission control, atmospheric oxidation and deposition, and boundary layer dynamics. Mixing ratio time series data for monoterpenes and cis-3-hexenyl acetate suggest a temporarily emission rate increase during and after heavy rain events. The isoprene oxidation products demonstrate differences in the oxidation pathways during night and day and in their dry and wet deposition rates.
Journal of Geophysical Research-Atmospheres. 103(D17):22397-22414.
-
[show abstract]
[hide abstract]
ABSTRACT: [1] In February 2001, as part of the Southern African Regional Science Initiative (SAFARI 2000), isoprene fluxes were measured for 8 days using the relaxed eddy accumulation technique from a 21-m tower in a Combretum-Acacia savanna in Kruger National Park, 13 km from Skukuza, RSA. Despite warm and sunny conditions, midday isoprene concentrations were low, averaging 0.39 nL/L. Fluxes of isoprene increased through the morning hours, with midday fluxes averaging 0.34 mg m(-2) h(-1) and a maximum measured flux of approximately 1.0 mg m(-2) h(-1). Consistent with these low fluxes, leaf enclosure measurements of woody species within the tower footprint determined that only one isoprene-emitting species, Acacia nigrescens, was present in significant numbers, comprising less than 10% of the woody biomass. Combining enclosure data with species composition and leaf area index data from the site, we estimated that the isoprene emission capacity of the vegetation within the vicinity of the tower was very low, approximately 0.47 mg m(-2) h(-1), and patchy. Under these circumstances, low and variable fluxes are expected. Additional leaf enclosure measurements, for a total of 121 species, were made at other locations, and approximately 35% of the species was found to emit significant amounts of isoprene. Important isoprene emitting plant families included Caesalpinaceae, Mimosaceae, Papilionaceae, Euphorbiaceae, Moraceae, and Myrtaceae. Twelve members of the important savanna genus Acacia were measured, of which five species, all belonging in Subgenus Aculeiferum, Section Aculeiferum, were found to emit significant amounts of isoprene. In contrast, the plant family, Combretaceae, dominant in many savanna ecosystems, was found to contain no species which emit isoprene.
Journal of Geophysical Research-Atmospheres. 108(D13):-.
-
[show abstract]
[hide abstract]
ABSTRACT: A global three-dimensional (3-D) chemistry and transport model was used to demonstrate that a factor of 2 decrease in isoprene and monoterpene emissions results in significant (10-30%) changes in predicted concentration distributions of compounds such as OH, MPAN, NOx, H2O2, O-3, and CO. Isoprene and monoterpenes were predicted to have a particularly strong impact on tropical regions, including central Africa. The 1996 Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) study included a number of experiments that improved our ability to predict isoprene and monoterpene emissions from central Africa, The results of these experiments have been incorporated into an isoprene emission model that predicts hourly emissions on a spatial scale of about 1 km(2). The model uses procedures that are suitable for estimating global emissions but uses regional measurements to accurately parameterize the model. Comparisons with above-canopy aircraft and tower flux measurements demonstrate that the model can estimate emissions within a factor of 2 for regions where ground measurements of model inputs are available. The annual central African isoprene emission predicted by our revised model (35 Tg C) is only 14% less than that predicted by our earlier model, but there are considerable differences in estimates of individual model variables. The models differ by more than a factor of 5 for specific times and locations, which indicates that there are large uncertainties in emission estimates for at least some locations and seasons. The good agreement obtained for the EXPRESSO study field sites, however, suggests that the model can predict reasonable estimates if representative field measurements are used to parameterize the model.
Journal of Geophysical Research-Atmospheres. 104(D23):30625-30639.
-
[show abstract]
[hide abstract]
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 30 degrees C and above canopy photosynthetically 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 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.
Journal of Geophysical Research-Atmospheres. 101(D1):1345-1359.
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Journal of Geophysical Research-Atmospheres. 101(D20):25859-25865.
