Isoprene (2-methyl-1,3,-butadiene), produced by many woody and a few herbaceous plant species, is... more Isoprene (2-methyl-1,3,-butadiene), produced by many woody and a few herbaceous plant species, is the dominant volatile organic compound released from vegetation. It represents a non-trivial carbon loss to the plant (typically 0.5–2%, but much higher as temperatures exceed 30°C), and plays a major role in tropospheric chemistry of forested regions, contributing to ozone formation. This review summarizes current knowledge concerning the occurrence of isoprene production within the plant kingdom, and discusses other aspects of isoprene biology which may be of interest to the ecological community. The ability to produce significant amounts of isoprene may or may not be shared by members of the same plant family or genus, but emitting species have been found among bryophytes, ferns, conifers and Ephedra and in approximately one-third of the 122 angiosperm families examined. No phylogenetic pattern is obvious among the angiosperms, with the trait widely scattered and present (and absent) in both primitive and derived taxa, although confined largely to woody species. Isoprene is not stored within the leaf, and plays no known ecological role as, for example, an anti-herbivore or allelopathic agent. The primary short-term controls over isoprene production are light and temperature. Growth in high light stimulates isoprene production, and growth in cool conditions apparently inhibits isoprene, production of which may be induced upon transfer to warmer temperatures. The stimulation of isoprene production by high irradiance and warm temperatures suggests a possible role in ameliorating stresses associated with warm, high-light environments, a role consistent with physiological evidence indicating a role in thermal protection.
The transition between wintertime net carbon loss and springtime net carbon assimilation has an i... more The transition between wintertime net carbon loss and springtime net carbon assimilation has an important role in controlling the annual rate of carbon uptake in coniferous forest ecosystems. We studied the contributions of springtime carbon assimilation to the total annual rate of carbon uptake and the processes involved in the winter-to-spring transition across a range of scales from ecosystem CO 2 fluxes to chloroplast photochemistry in a coniferous, subalpine forest. We observed numerous initiations and reversals in the recovery of photosynthetic CO 2 uptake during the initial phase of springtime recovery in response to the passage of alternating warm-and cold-weather systems. Full recovery of ecosystem carbon uptake, whereby the 24-h cumulative sum of NEE (NEE daily ) was consistently negative, did not occur until 3-4 weeks after the first signs of photosynthetic recovery. A key event that preceded full recovery Communicated by
1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indian... more 1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indiana, and Oklahoma. OZIE was designed to investigate the presumed strong isoprene emission rates from the Missouri Ozarks, where there is a high density of oak trees that are efficient isoprene emitters. Ground, balloon, and aircraft measurements were taken over a three-week study period; 0-D and 3-D chemical models were subsequently used to better understand the observed isoprene emissions from the Ozarks and to investigate their potential regional-scale impacts. Leaf-level measurements for two oak tree species yielded normalized average isoprene emission capacities of 66 mgC g À1 h À1 , in good agreement with values used in current biogenic emissions models. However, the emission capacities exhibited a temperature dependence that is not captured by commonly used biogenic emission models. Isoprene mixing ratios measured aloft from tethered balloon systems were used to estimate isoprene fluxes. These measurementderived fluxes agreed with BEIS3 estimates within the relatively large uncertainties in the estimates. Ground-level isoprene mixing ratios exhibited substantial spatial heterogeneity, ranging from <1 to 35 ppbv. The agreement between measured isoprene mixing ratios and regional-scale chemical transport model estimates was improved upon averaging the ground-level isoprene data observed at several sites within a representative area. Groundlevel formaldehyde (HCHO) mixing ratios were very high (up to 20 ppbv) and were consistently higher than mixing ratios predicted by a regional chemical transport model. The spatial distribution and magnitude of the elevated HCHO concentrations showed good agreement with GOME satellite column observations of HCHO. Citation: Wiedinmyer, C., et al. (2005), Ozarks Isoprene Experiment (OZIE): Measurements and modeling of the ''isoprene volcano,''
High rates of emission of 2-methyl-3-buten-2-ol (MBO) were measured from needles of several pine ... more 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 35øC. Above approximately 42øC, 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 rn above sea level. Subsequent field studies in a ponderosa pine plantation in California ...... 1 -1 confirmed high MBO emissions, which averaged 25 pg C g h for 1-year-old needles, corrected to 30øC and photon flux of 1000 pmol m -2 s -r. A total of 34 pine species growing at Eddy Arboretum in Placerville, California, were investigated, of which 11 exhibited high emissions of MBO (>5 [xg C g-• h-•), 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. concentrations did not correlate well with those of benzene, an indicator of anthropogenic sources, they concluded that there was likely to be a large, unidentified local biogenic source of MBO. Results reported here trace the source of MBO to direct emission from needles of lodgepole and ponderosa pine and identify a number of other species of pine as potentially significant sources of MBO to the atmosphere. The effects on MBO emission rates of varying light and temperature are examined, and implications for tropospheric chemistry are discussed.
Numerical assessments of global air quality and potential changes in atmospheric chemical constit... more Numerical assessments of global air quality and potential changes in atmospheric chemical constituents require estimates of the surface fluxes of a variety of trace gas species. We have developed a global model to estimate emissions of volatile organic compounds from natural sources (NVOC). Methane is not considered here and has been reviewed in detail elsewhere. The model has a highly resolved spatial grid (0.5øx 0.5 ølatitude/longitude) and generates hourly average emission estimates. Chemical species are grouped into four categories: isoprene, monoterpenes, other reactive VOC (ORVOC), and other VOC (OVOC). NVOC emissions from oceans are estimated as a function of geophysical variables from a general circulation model and ocean color satellite data. Emissions from plant foliage are estimated from ecosystem specific biomass and emission factors and algorithms describing light and temperature dependence of NVOC emissions. Foliar density estimates are based on climatic variables and satellite data. Temporal variations in the model are driven by monthly estimates of biomass and temperature and hourly light estimates. The annual global VOC flux is estimated to be 1150 Tg C, composed of 44% isoprene, 11% monoterpenes, 22.5% other reactive VOC, and 22.5% other VOC. Large uncertainties exist for each of these estimates and particularly for compounds other than isoprene and monoterpenes. Tropical woodlands (rain forest, seasonal, drought-deciduous, and savanna) contribute about half of all global natural VOC emissions. Croplands, shrublands and other woodlands contribute 10-20% apiece. Isoprene emissions calculated for temperate regions are as much as a factor of 5 higher than previous estimates. 1. Introduction Volatile organic compounds (VOC) are emitted into the atmosphere from natural sources in marine and terrestrial environments. Surface fluxes of these compounds are of interest because of their role in tropospheric chemistry and the global carbon cycle [see Fehsenfeld et al., 1992]. The development of three-dimensional (3-D) computer models of global climate, chemistry, surface emissions, and ecology is progressing to the point where these models can be coupled and used to investigate interactions between the processes simulated by each model. Three-dimensional climate models respond to forcings such as sea surface temperatures, planetary orientation with the sun, and the concentration of chemical species such as CO 2, C H 4, and sulfur compounds. Since VOC emission rates are critical in controlling the OH concentration of much of the troposphere, VOC emissions may play a major role in determining the growth rate of atmospheric CH 4 and CO concentrations. Since 1National Center for Atmospheric Research, Boulder, Colorado. compounds such as CH 4 influence the radiative balance of the 2Institute of Environmental and Biological Sciences, Lancaster, atmosphere, which in turn is related to global temperature and
Biogenic volatile organic compound (BVOC) emissions were studied using vegetation enclosure exper... more Biogenic volatile organic compound (BVOC) emissions were studied using vegetation enclosure experiments. Particular emphasis was given to sesquiterpene compounds (SQT), although monoterpenes (MT) were also characterized. SQT were detected in emissions from seven (out of eight) pine species that were examined. Thirteen SQT compounds were identified; the most abundant ones were beta-caryophyllene, alpha-bergamotene, beta-farnesene, and alpha-farnesene, with emission rates increasing exponentially with temperature. Regression analysis yielded exponential dependencies of both MT and SQT emissions on temperature of the form E = E0 x exp(beta(T - T0)). This resulted in SQT basal emission rates (E0 defined at T0 = 30 degrees C) ranging between &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;4 and 620 ng (carbon) gdw(-1) h(-1) (gdw = gram dry weight). The average value of the exponential temperature response factor beta for SQT emissions, taken from all experiments, was 0.17 degree C(-1), whereas the value for monoterpenes was 0.11 degrees C(-1). The average, total SQT emissions from pines were estimated to be 9, 16, and 29% of the MT emissions at 20, 30, and 40 degrees C respectively. The emission factors and beta-factors determined from these measurements were used to estimate pine tree MT and SQT emission distributions for the contiguous United States using MEGAN (model of emissions of gases and aerosols from nature, Guenther et al., 2006). SQT fluxes reaching 10-40 mg m(-2) for the month of July were estimated for extensive areas of most western and southern U.S. states.
1] In February 2001, as part of the Southern African Regional Science Initiative (SAFARI 2000), i... more 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 À2 h À1 and a maximum measured flux of approximately 1.0 mg m À2 h À1 . 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 À2 h À1 , 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. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere-composition and chemistry; 1615 Global Change: Biogeochemical processes (4805) Citation: Harley, P., L. Otter, A. Guenther, and J. Greenberg, Micrometeorological and leaf-level measurements of isoprene emissions from a southern African savanna,
1] In February 2001, as part of the Southern African Regional Science Initiative (SAFARI 2000), i... more 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 À2 h À1 and a maximum measured flux of approximately 1.0 mg m À2 h À1 . 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 À2 h À1 , 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. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere-composition and chemistry; 1615 Global Change: Biogeochemical processes (4805) Citation: Harley, P., L. Otter, A. Guenther, and J. Greenberg, Micrometeorological and leaf-level measurements of isoprene emissions from a southern African savanna,
1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indian... more 1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indiana, and Oklahoma. OZIE was designed to investigate the presumed strong isoprene emission rates from the Missouri Ozarks, where there is a high density of oak trees that are efficient isoprene emitters. Ground, balloon, and aircraft measurements were taken over a three-week study period; 0-D and 3-D chemical models were subsequently used to better understand the observed isoprene emissions from the Ozarks and to investigate their potential regional-scale impacts. Leaf-level measurements for two oak tree species yielded normalized average isoprene emission capacities of 66 mgC g À1 h À1 , in good agreement with values used in current biogenic emissions models. However, the emission capacities exhibited a temperature dependence that is not captured by commonly used biogenic emission models. Isoprene mixing ratios measured aloft from tethered balloon systems were used to estimate isoprene fluxes. These measurementderived fluxes agreed with BEIS3 estimates within the relatively large uncertainties in the estimates. Ground-level isoprene mixing ratios exhibited substantial spatial heterogeneity, ranging from <1 to 35 ppbv. The agreement between measured isoprene mixing ratios and regional-scale chemical transport model estimates was improved upon averaging the ground-level isoprene data observed at several sites within a representative area. Groundlevel formaldehyde (HCHO) mixing ratios were very high (up to 20 ppbv) and were consistently higher than mixing ratios predicted by a regional chemical transport model. The spatial distribution and magnitude of the elevated HCHO concentrations showed good agreement with GOME satellite column observations of HCHO. Citation: Wiedinmyer, C., et al. (2005), Ozarks Isoprene Experiment (OZIE): Measurements and modeling of the ''isoprene volcano,''
As part of the Large Scale Biosphere-Atmosphere Experiment in Amazô nia (LBA), we have developed ... more As part of the Large Scale Biosphere-Atmosphere Experiment in Amazô nia (LBA), we have developed a bottom-up approach for estimating canopy-scale fluxes of isoprene. Estimating isoprene fluxes for a given forest ecosystem requires knowledge of foliar biomass, segregated by species, and the isoprene emission characteristics of the individual tree species comprising the forest. In this study, approximately 38% of 125 tree species examined at six sites in the Brazilian Amazon emitted isoprene. Given logistical difficulties and extremely high species diversity, it was possible to screen only a small percentage of tree species, and we propose a protocol for estimating the emission capacity of unmeasured taxa using a taxonomic approach, in which we assign to an unmeasured genus a value based on the percentage of genera within its plant family which have been shown to emit isoprene.
Diurnal branch-level emission rates of biogenic volatile organic compounds (BVOC) including isopr... more Diurnal branch-level emission rates of biogenic volatile organic compounds (BVOC) including isoprene, monoterpenes (MT), and sesquiterpenes (SQT) were determined at the University of Michigan Biological Station for the tree species red maple (Acer rubrum), red oak (Quercus rubra), paper birch (Betula papyrifera), white pine (Pinus strobus), and big tooth aspen (Populus grandidentata). These emission rates were combined with detailed biomass distribution and meteorological data and incorporated into the canopy model, model of emissions of gasses and aerosols from nature (MEGAN), for estimating whole-canopy fluxes of isoprene. The modeled half-hour fluxes ðmg C m À2 h À1 Þ and cumulative seasonal fluxes ðmg C m À2 Þ compared favorably with results from direct, canopy-level eddy covariance (EC) isoprene measurements; modeled cumulative seasonal flux deviated less than 30% from the EC results. Significant MT emissions were found from four of the five tree species. MT emissions from three of these were both light-and temperature-dependent and were approximately one order of magnitude greater than light-independent MT emissions. SQT emissions were identified from three of the five tree species. The model was modified to incorporate SQT and both light-dependent and light-independent MT emissions for determining fluxes. Isoprene comprised 495% of the total terpenoid flux with MT and SQT comprising 4% and 0.3%, respectively. The average cumulative fluxes (in mg C m À2 ) from June through September were 2490 for isoprene, 105 for MT, and 7 for SQT. A simple box model analysis was used to estimate the contribution of the isoprene, MT, and SQT emissions to the total OH reactivity. These results confirm that isoprene dominates OH reactions especially during daytime hours. Emissions of reactive MT and SQT increase the BVOC+OH reactivity, but are still lower than estimates of BVOC fluxes at this site necessary for affecting OH reactivity to the significant degree suggested by recent reports. r (D. Helmig). a Standard deviations ðN ¼ 4) of the specific leaf masses are in parentheses. b Mean basal emission rate (BER) of branch enclosure experiments performed during 2003 and 2005. Standard deviations of isoprene emission rates are listed in parentheses.
Natural volatile organic compound (VOC) fluxes were measured in three U.S. woodlands in summer 19... more 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).
Isoprene emission capacity measurements are presented from 18 North American oak (Quercus) specie... more Isoprene emission capacity measurements are presented from 18 North American oak (Quercus) species and species from six other genera previously found to emit signi"cant quantities of isoprene. Sampling was conducted at physiographically diverse locations in North Carolina, Central California, and Northern Oregon. Emissions from several sun leaves of each species were measured at or near standard conditions (leaf temperature of 303C and photosynthetically active radiation of 1000 mol m\ s\) using environmentally controlled cuvette systems and gas chromatography with reduction gas detectors. Species mean emission capacity ranged from 39 to 158 g C g\ h\ (mean of 86), or 22 to 79 nmol m\ s\ (mean of 44). These rates are 2}28 times higher than those previously reported from the same species, which were summarized in a recent study where isoprene emission rates were assigned based on published data and taxonomy. These discrepancies were attributed to di!erences in leaf environment during development, measurement technique (branch or plant enclosure versus leaf enclosure), and lack of environmental measurements associated with some of the earlier branch enclosure measurements. Mass-based emission capacities for 15 of 18 oak species, sweetgum (Liquidambar styraciyua), and poplars (Populus trichocarpa and P. deltoides) were within ranges used in current biogenic volatile organic compound (BVOC) emission models, while measured rates for the remaining three oak species, Nyssa sylvatica, Platanus occidentalis, Robinia pseudoacacia, Salix nigra, and Populus hybrids (Populus trichocarpa ; P. deltoides) were considerably higher. In addition, mean speci"c leaf mass of the oak species was 30% higher than assumed in current emission models. Emission rates reported here and in other recent studies support recent conclusions that isoprene emission capacities for sun leaves of high emitting species may be better represented by a value of 100$50 g C g\ h\ during hot summer conditions. We also "nd that intermediate isoprene emission rates previously suggested for some tree species may not represent their true emission capacities, and that broadleaf plant species may have either low ( (1.0 g C g\ h\) or very high ( &100 g C g\ h\) genetic capacity to emit isoprene when mature foliage is exposed to a high ambient temperature and light environment. Published by Elsevier Science Ltd.
The magnitudes, distributions, controlling processes and uncertainties associated with North Amer... more The magnitudes, distributions, controlling processes and uncertainties associated with North American natural emissions of oxidant precursors are reviewed. Natural emissions are responsible for a major portion of the compounds, including non-methane volatile organic compounds (NMVOC), carbon monoxide (CO) and nitric oxide (NO), that determine tropospheric oxidant concentrations. Natural sources include soil microbes, vegetation, biomass burning, and lightning. These sources are strongly in#uenced by human activities that have led to signi"cant changes in the magnitude and distribution of natural emissions in the past two centuries. The total NMVOC #ux of about 84;10 g of carbon (Tg C) is comprised primarily of isoprene (35%), 19 other terpenoid compounds (25%) and 17 non-terpenoid compounds (40%). Vegetation is predicted to contribute about 98% of the total annual natural NMVOC emission. The estimated annual natural NO emission of 2.1;10 g of nitrogen (Tg N) from North America is primarily due to soils and lightning, while the estimated 10 Tg C of CO arises from biomass burning and vegetation. Field measurements of ambient concentrations and above canopy #uxes have validated emission estimates for a few compounds from some important landscapes. The uncertainty associated with natural emission estimates ranges from less than 50% for midday summer isoprene emission from some locations to about a factor of 10 for some compounds and landscapes.
Isoprene (2-methyl-1,3,-butadiene), produced by many woody and a few herbaceous plant species, is... more Isoprene (2-methyl-1,3,-butadiene), produced by many woody and a few herbaceous plant species, is the dominant volatile organic compound released from vegetation. It represents a non-trivial carbon loss to the plant (typically 0.5–2%, but much higher as temperatures exceed 30°C), and plays a major role in tropospheric chemistry of forested regions, contributing to ozone formation. This review summarizes current knowledge concerning the occurrence of isoprene production within the plant kingdom, and discusses other aspects of isoprene biology which may be of interest to the ecological community. The ability to produce significant amounts of isoprene may or may not be shared by members of the same plant family or genus, but emitting species have been found among bryophytes, ferns, conifers and Ephedra and in approximately one-third of the 122 angiosperm families examined. No phylogenetic pattern is obvious among the angiosperms, with the trait widely scattered and present (and absent) in both primitive and derived taxa, although confined largely to woody species. Isoprene is not stored within the leaf, and plays no known ecological role as, for example, an anti-herbivore or allelopathic agent. The primary short-term controls over isoprene production are light and temperature. Growth in high light stimulates isoprene production, and growth in cool conditions apparently inhibits isoprene, production of which may be induced upon transfer to warmer temperatures. The stimulation of isoprene production by high irradiance and warm temperatures suggests a possible role in ameliorating stresses associated with warm, high-light environments, a role consistent with physiological evidence indicating a role in thermal protection.
The transition between wintertime net carbon loss and springtime net carbon assimilation has an i... more The transition between wintertime net carbon loss and springtime net carbon assimilation has an important role in controlling the annual rate of carbon uptake in coniferous forest ecosystems. We studied the contributions of springtime carbon assimilation to the total annual rate of carbon uptake and the processes involved in the winter-to-spring transition across a range of scales from ecosystem CO 2 fluxes to chloroplast photochemistry in a coniferous, subalpine forest. We observed numerous initiations and reversals in the recovery of photosynthetic CO 2 uptake during the initial phase of springtime recovery in response to the passage of alternating warm-and cold-weather systems. Full recovery of ecosystem carbon uptake, whereby the 24-h cumulative sum of NEE (NEE daily ) was consistently negative, did not occur until 3-4 weeks after the first signs of photosynthetic recovery. A key event that preceded full recovery Communicated by
1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indian... more 1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indiana, and Oklahoma. OZIE was designed to investigate the presumed strong isoprene emission rates from the Missouri Ozarks, where there is a high density of oak trees that are efficient isoprene emitters. Ground, balloon, and aircraft measurements were taken over a three-week study period; 0-D and 3-D chemical models were subsequently used to better understand the observed isoprene emissions from the Ozarks and to investigate their potential regional-scale impacts. Leaf-level measurements for two oak tree species yielded normalized average isoprene emission capacities of 66 mgC g À1 h À1 , in good agreement with values used in current biogenic emissions models. However, the emission capacities exhibited a temperature dependence that is not captured by commonly used biogenic emission models. Isoprene mixing ratios measured aloft from tethered balloon systems were used to estimate isoprene fluxes. These measurementderived fluxes agreed with BEIS3 estimates within the relatively large uncertainties in the estimates. Ground-level isoprene mixing ratios exhibited substantial spatial heterogeneity, ranging from <1 to 35 ppbv. The agreement between measured isoprene mixing ratios and regional-scale chemical transport model estimates was improved upon averaging the ground-level isoprene data observed at several sites within a representative area. Groundlevel formaldehyde (HCHO) mixing ratios were very high (up to 20 ppbv) and were consistently higher than mixing ratios predicted by a regional chemical transport model. The spatial distribution and magnitude of the elevated HCHO concentrations showed good agreement with GOME satellite column observations of HCHO. Citation: Wiedinmyer, C., et al. (2005), Ozarks Isoprene Experiment (OZIE): Measurements and modeling of the ''isoprene volcano,''
High rates of emission of 2-methyl-3-buten-2-ol (MBO) were measured from needles of several pine ... more 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 35øC. Above approximately 42øC, 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 rn above sea level. Subsequent field studies in a ponderosa pine plantation in California ...... 1 -1 confirmed high MBO emissions, which averaged 25 pg C g h for 1-year-old needles, corrected to 30øC and photon flux of 1000 pmol m -2 s -r. A total of 34 pine species growing at Eddy Arboretum in Placerville, California, were investigated, of which 11 exhibited high emissions of MBO (>5 [xg C g-• h-•), 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. concentrations did not correlate well with those of benzene, an indicator of anthropogenic sources, they concluded that there was likely to be a large, unidentified local biogenic source of MBO. Results reported here trace the source of MBO to direct emission from needles of lodgepole and ponderosa pine and identify a number of other species of pine as potentially significant sources of MBO to the atmosphere. The effects on MBO emission rates of varying light and temperature are examined, and implications for tropospheric chemistry are discussed.
Numerical assessments of global air quality and potential changes in atmospheric chemical constit... more Numerical assessments of global air quality and potential changes in atmospheric chemical constituents require estimates of the surface fluxes of a variety of trace gas species. We have developed a global model to estimate emissions of volatile organic compounds from natural sources (NVOC). Methane is not considered here and has been reviewed in detail elsewhere. The model has a highly resolved spatial grid (0.5øx 0.5 ølatitude/longitude) and generates hourly average emission estimates. Chemical species are grouped into four categories: isoprene, monoterpenes, other reactive VOC (ORVOC), and other VOC (OVOC). NVOC emissions from oceans are estimated as a function of geophysical variables from a general circulation model and ocean color satellite data. Emissions from plant foliage are estimated from ecosystem specific biomass and emission factors and algorithms describing light and temperature dependence of NVOC emissions. Foliar density estimates are based on climatic variables and satellite data. Temporal variations in the model are driven by monthly estimates of biomass and temperature and hourly light estimates. The annual global VOC flux is estimated to be 1150 Tg C, composed of 44% isoprene, 11% monoterpenes, 22.5% other reactive VOC, and 22.5% other VOC. Large uncertainties exist for each of these estimates and particularly for compounds other than isoprene and monoterpenes. Tropical woodlands (rain forest, seasonal, drought-deciduous, and savanna) contribute about half of all global natural VOC emissions. Croplands, shrublands and other woodlands contribute 10-20% apiece. Isoprene emissions calculated for temperate regions are as much as a factor of 5 higher than previous estimates. 1. Introduction Volatile organic compounds (VOC) are emitted into the atmosphere from natural sources in marine and terrestrial environments. Surface fluxes of these compounds are of interest because of their role in tropospheric chemistry and the global carbon cycle [see Fehsenfeld et al., 1992]. The development of three-dimensional (3-D) computer models of global climate, chemistry, surface emissions, and ecology is progressing to the point where these models can be coupled and used to investigate interactions between the processes simulated by each model. Three-dimensional climate models respond to forcings such as sea surface temperatures, planetary orientation with the sun, and the concentration of chemical species such as CO 2, C H 4, and sulfur compounds. Since VOC emission rates are critical in controlling the OH concentration of much of the troposphere, VOC emissions may play a major role in determining the growth rate of atmospheric CH 4 and CO concentrations. Since 1National Center for Atmospheric Research, Boulder, Colorado. compounds such as CH 4 influence the radiative balance of the 2Institute of Environmental and Biological Sciences, Lancaster, atmosphere, which in turn is related to global temperature and
Biogenic volatile organic compound (BVOC) emissions were studied using vegetation enclosure exper... more Biogenic volatile organic compound (BVOC) emissions were studied using vegetation enclosure experiments. Particular emphasis was given to sesquiterpene compounds (SQT), although monoterpenes (MT) were also characterized. SQT were detected in emissions from seven (out of eight) pine species that were examined. Thirteen SQT compounds were identified; the most abundant ones were beta-caryophyllene, alpha-bergamotene, beta-farnesene, and alpha-farnesene, with emission rates increasing exponentially with temperature. Regression analysis yielded exponential dependencies of both MT and SQT emissions on temperature of the form E = E0 x exp(beta(T - T0)). This resulted in SQT basal emission rates (E0 defined at T0 = 30 degrees C) ranging between &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;4 and 620 ng (carbon) gdw(-1) h(-1) (gdw = gram dry weight). The average value of the exponential temperature response factor beta for SQT emissions, taken from all experiments, was 0.17 degree C(-1), whereas the value for monoterpenes was 0.11 degrees C(-1). The average, total SQT emissions from pines were estimated to be 9, 16, and 29% of the MT emissions at 20, 30, and 40 degrees C respectively. The emission factors and beta-factors determined from these measurements were used to estimate pine tree MT and SQT emission distributions for the contiguous United States using MEGAN (model of emissions of gases and aerosols from nature, Guenther et al., 2006). SQT fluxes reaching 10-40 mg m(-2) for the month of July were estimated for extensive areas of most western and southern U.S. states.
1] In February 2001, as part of the Southern African Regional Science Initiative (SAFARI 2000), i... more 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 À2 h À1 and a maximum measured flux of approximately 1.0 mg m À2 h À1 . 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 À2 h À1 , 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. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere-composition and chemistry; 1615 Global Change: Biogeochemical processes (4805) Citation: Harley, P., L. Otter, A. Guenther, and J. Greenberg, Micrometeorological and leaf-level measurements of isoprene emissions from a southern African savanna,
1] In February 2001, as part of the Southern African Regional Science Initiative (SAFARI 2000), i... more 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 À2 h À1 and a maximum measured flux of approximately 1.0 mg m À2 h À1 . 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 À2 h À1 , 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. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere-composition and chemistry; 1615 Global Change: Biogeochemical processes (4805) Citation: Harley, P., L. Otter, A. Guenther, and J. Greenberg, Micrometeorological and leaf-level measurements of isoprene emissions from a southern African savanna,
1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indian... more 1] The Ozarks Isoprene Experiment (OZIE) was conducted in July 1998 in Missouri, Illinois, Indiana, and Oklahoma. OZIE was designed to investigate the presumed strong isoprene emission rates from the Missouri Ozarks, where there is a high density of oak trees that are efficient isoprene emitters. Ground, balloon, and aircraft measurements were taken over a three-week study period; 0-D and 3-D chemical models were subsequently used to better understand the observed isoprene emissions from the Ozarks and to investigate their potential regional-scale impacts. Leaf-level measurements for two oak tree species yielded normalized average isoprene emission capacities of 66 mgC g À1 h À1 , in good agreement with values used in current biogenic emissions models. However, the emission capacities exhibited a temperature dependence that is not captured by commonly used biogenic emission models. Isoprene mixing ratios measured aloft from tethered balloon systems were used to estimate isoprene fluxes. These measurementderived fluxes agreed with BEIS3 estimates within the relatively large uncertainties in the estimates. Ground-level isoprene mixing ratios exhibited substantial spatial heterogeneity, ranging from <1 to 35 ppbv. The agreement between measured isoprene mixing ratios and regional-scale chemical transport model estimates was improved upon averaging the ground-level isoprene data observed at several sites within a representative area. Groundlevel formaldehyde (HCHO) mixing ratios were very high (up to 20 ppbv) and were consistently higher than mixing ratios predicted by a regional chemical transport model. The spatial distribution and magnitude of the elevated HCHO concentrations showed good agreement with GOME satellite column observations of HCHO. Citation: Wiedinmyer, C., et al. (2005), Ozarks Isoprene Experiment (OZIE): Measurements and modeling of the ''isoprene volcano,''
As part of the Large Scale Biosphere-Atmosphere Experiment in Amazô nia (LBA), we have developed ... more As part of the Large Scale Biosphere-Atmosphere Experiment in Amazô nia (LBA), we have developed a bottom-up approach for estimating canopy-scale fluxes of isoprene. Estimating isoprene fluxes for a given forest ecosystem requires knowledge of foliar biomass, segregated by species, and the isoprene emission characteristics of the individual tree species comprising the forest. In this study, approximately 38% of 125 tree species examined at six sites in the Brazilian Amazon emitted isoprene. Given logistical difficulties and extremely high species diversity, it was possible to screen only a small percentage of tree species, and we propose a protocol for estimating the emission capacity of unmeasured taxa using a taxonomic approach, in which we assign to an unmeasured genus a value based on the percentage of genera within its plant family which have been shown to emit isoprene.
Diurnal branch-level emission rates of biogenic volatile organic compounds (BVOC) including isopr... more Diurnal branch-level emission rates of biogenic volatile organic compounds (BVOC) including isoprene, monoterpenes (MT), and sesquiterpenes (SQT) were determined at the University of Michigan Biological Station for the tree species red maple (Acer rubrum), red oak (Quercus rubra), paper birch (Betula papyrifera), white pine (Pinus strobus), and big tooth aspen (Populus grandidentata). These emission rates were combined with detailed biomass distribution and meteorological data and incorporated into the canopy model, model of emissions of gasses and aerosols from nature (MEGAN), for estimating whole-canopy fluxes of isoprene. The modeled half-hour fluxes ðmg C m À2 h À1 Þ and cumulative seasonal fluxes ðmg C m À2 Þ compared favorably with results from direct, canopy-level eddy covariance (EC) isoprene measurements; modeled cumulative seasonal flux deviated less than 30% from the EC results. Significant MT emissions were found from four of the five tree species. MT emissions from three of these were both light-and temperature-dependent and were approximately one order of magnitude greater than light-independent MT emissions. SQT emissions were identified from three of the five tree species. The model was modified to incorporate SQT and both light-dependent and light-independent MT emissions for determining fluxes. Isoprene comprised 495% of the total terpenoid flux with MT and SQT comprising 4% and 0.3%, respectively. The average cumulative fluxes (in mg C m À2 ) from June through September were 2490 for isoprene, 105 for MT, and 7 for SQT. A simple box model analysis was used to estimate the contribution of the isoprene, MT, and SQT emissions to the total OH reactivity. These results confirm that isoprene dominates OH reactions especially during daytime hours. Emissions of reactive MT and SQT increase the BVOC+OH reactivity, but are still lower than estimates of BVOC fluxes at this site necessary for affecting OH reactivity to the significant degree suggested by recent reports. r (D. Helmig). a Standard deviations ðN ¼ 4) of the specific leaf masses are in parentheses. b Mean basal emission rate (BER) of branch enclosure experiments performed during 2003 and 2005. Standard deviations of isoprene emission rates are listed in parentheses.
Natural volatile organic compound (VOC) fluxes were measured in three U.S. woodlands in summer 19... more 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).
Isoprene emission capacity measurements are presented from 18 North American oak (Quercus) specie... more Isoprene emission capacity measurements are presented from 18 North American oak (Quercus) species and species from six other genera previously found to emit signi"cant quantities of isoprene. Sampling was conducted at physiographically diverse locations in North Carolina, Central California, and Northern Oregon. Emissions from several sun leaves of each species were measured at or near standard conditions (leaf temperature of 303C and photosynthetically active radiation of 1000 mol m\ s\) using environmentally controlled cuvette systems and gas chromatography with reduction gas detectors. Species mean emission capacity ranged from 39 to 158 g C g\ h\ (mean of 86), or 22 to 79 nmol m\ s\ (mean of 44). These rates are 2}28 times higher than those previously reported from the same species, which were summarized in a recent study where isoprene emission rates were assigned based on published data and taxonomy. These discrepancies were attributed to di!erences in leaf environment during development, measurement technique (branch or plant enclosure versus leaf enclosure), and lack of environmental measurements associated with some of the earlier branch enclosure measurements. Mass-based emission capacities for 15 of 18 oak species, sweetgum (Liquidambar styraciyua), and poplars (Populus trichocarpa and P. deltoides) were within ranges used in current biogenic volatile organic compound (BVOC) emission models, while measured rates for the remaining three oak species, Nyssa sylvatica, Platanus occidentalis, Robinia pseudoacacia, Salix nigra, and Populus hybrids (Populus trichocarpa ; P. deltoides) were considerably higher. In addition, mean speci"c leaf mass of the oak species was 30% higher than assumed in current emission models. Emission rates reported here and in other recent studies support recent conclusions that isoprene emission capacities for sun leaves of high emitting species may be better represented by a value of 100$50 g C g\ h\ during hot summer conditions. We also "nd that intermediate isoprene emission rates previously suggested for some tree species may not represent their true emission capacities, and that broadleaf plant species may have either low ( (1.0 g C g\ h\) or very high ( &100 g C g\ h\) genetic capacity to emit isoprene when mature foliage is exposed to a high ambient temperature and light environment. Published by Elsevier Science Ltd.
The magnitudes, distributions, controlling processes and uncertainties associated with North Amer... more The magnitudes, distributions, controlling processes and uncertainties associated with North American natural emissions of oxidant precursors are reviewed. Natural emissions are responsible for a major portion of the compounds, including non-methane volatile organic compounds (NMVOC), carbon monoxide (CO) and nitric oxide (NO), that determine tropospheric oxidant concentrations. Natural sources include soil microbes, vegetation, biomass burning, and lightning. These sources are strongly in#uenced by human activities that have led to signi"cant changes in the magnitude and distribution of natural emissions in the past two centuries. The total NMVOC #ux of about 84;10 g of carbon (Tg C) is comprised primarily of isoprene (35%), 19 other terpenoid compounds (25%) and 17 non-terpenoid compounds (40%). Vegetation is predicted to contribute about 98% of the total annual natural NMVOC emission. The estimated annual natural NO emission of 2.1;10 g of nitrogen (Tg N) from North America is primarily due to soils and lightning, while the estimated 10 Tg C of CO arises from biomass burning and vegetation. Field measurements of ambient concentrations and above canopy #uxes have validated emission estimates for a few compounds from some important landscapes. The uncertainty associated with natural emission estimates ranges from less than 50% for midday summer isoprene emission from some locations to about a factor of 10 for some compounds and landscapes.
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