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Scaling BVOC Emissions from Leaf to Canopy and Landscape: How Different Are Predictions Based on Contrasting Emission Algorithms?
Abstract
The vertical distribution of ambient biogenic volatile organic compounds (BVOC) concentrations within a hemiboreal forest canopy was investigated over a period of one year. Variability in temporal and spatial iso-prene concentrations, ranging from 0.1 to 7.5 µg m −3 , can be mainly explained by biogenic emissions from deciduous trees. Monoterpene concentrations exceeded isoprene largely and ranged from 0.01 to 140 µg m −3 and during winter time anthropogenic contributions are likely. Variation in monoter-pene concentrations were found to be largest right above the ground and the vertical profiles suggest a weak mixing leading to terpene accumulation in the lower canopy. Exceptionally high values were recorded during a heat wave in July 2010 with very high midday temperatures above 30 • C for several weeks. During summer months, monoter-pene exceeded isoprene concentrations 6-fold and during winter 12-fold. During summer months, dominance of α-pinene in the lower and of limonene in the upper part of the canopy was observed, both accounting for up to 70 % of the total monoterpene concentration. During wintertime, 3-carene was the dominant species, accounting for 60 % of total monoterpene concentration in January. Possible biogenic monoterpene sources beside the foliage are the leaf litter, the soil and also resins exuding from stems. In comparison, the hemiboreal mixed forest canopy showed similar isoprene but higher monoterpene concentrations than the boreal forest and lower isoprene but substantially higher monoterpene concentrations than the temperate mixed forest canopies. These results have major implications for simulating air chemistry and secondary organic aerosol formation within and above hemiboreal forest canopies. Possible effects of in-cartridge oxidation reactions are discussed as our measurement technique did not include oxidant scavenging. A comparison between measurements with and without scavenging oxidants is presented.
... Simulation of changes in isoprene emissions due to age can be made using algorithms which can be found embedded in e.g. the Model of Emissions of Gases and Aerosols from Nature (MEGAN) for global isoprene flux emissions (Guenther et al. 2006;Guenther et al. 2012;)(see the chapter of Guenther 2013). Leaf age is also indirectly embedded in some of the seasonality functions used to characterize seasonal variations in emissions (Grote et al. 2013;Niinemets et al. 2013). These approaches (species specific measurements and simulation algorithms) are likely to be feasible for monoculture agroforestry plantations, but they can be complex to apply for a highly fragmented urban green-space canopy. ...
... The emission model is typically driven by incident light and air temperature, and can consider the canopy as a big leaf or a two-big-leaf with sunlit and shaded leaf area fractions (Dai et al. 2004;Guenther et al. 2006;Keenan et al. 2009a). Models with layered canopy have also been used simulating the vertical variations in temperature, light and Epsi (Guenther et al. 2006;Guenther 2013;Niinemets et al. 2013) However, the current key limitation of these models is the lack of information of species-specific variations in emission potentials within the canopy ). ...
... MODIS satellite data were used to obtain the fraction of light absorbed by the canopy at 1 km 2 resolution. Nevertheless, simple big-leaf models tend to overestimate the flux (Dai et al. 2004) and for correct integration of fluxes, either layered models (Guenther et al. 2006;Guenther et al. 2012;Niinemets et al. 2013) or two-big-leaf models (de Pury and Farquhar 1997;Dai et al. 2004) should be used. ...
Agroforestry plantations and urban habitats contribute importantly to atmospheric volatile compound fluxes in densely populated areas. Simulation of emissions from such habitats is associated with several key challenges, including high spatial heterogeneity due to habitat fragmentation and high diversity of planted tree species. On the other hand, plants in urban habitats and in agroforestry plantations commonly receive more nutrients and water than species in natural communities, resulting in higher production and potentially greater capacity for volatile production per unit of land area. This chapter reviews the strategies for simulation of biogenic volatile organic compound (BVOC) fluxes from urban habitats and agroforestry plantations and provides an outline for parameterization of volatile emission models for densely populated areas with high vegetation fragmentation and large number of gardened, often exotic, tree species.
... As a further simplification, Niinemets et al. (2013 in this volume) directly linked isoprene emission to photosynthesis. In the so-called C-ratio model, isoprenoid emission was calculated as the product of the gross assimilation rate and monoterpene emission to assimilation rate ratio, r C ( Niinemets et al. 2013 in this volume). While being the simplest model, r C was shown to depend on light and temperature, and thus, required somewhat greater parameterization effort than linking emissions to electron transport rate (Eq. ...
... Nevertheless, comparison of different model approaches (Eqs. 12.11 and 12.14 and C-ratio model) at canopy level indicated that once correctly parameterized, all models performed similarly ( Niinemets et al. 2013 in this volume). ...
... The capacity to emit isoprenoids generally develops gradually during leaf development and reaches a maximum only after full leaf expansion; following maximum leaf expansion, and the emission potentials further gradually decrease with increasing leaf age ( Fischbach et al. 2002). With respect to evergreen species, it is thus important that functional activity continuously decreases with increasing leaf age ( Niinemets et al. 2006Niinemets et al. , 2013Grote 2007). Finally, canopy structure determines microclimatic conditions that affect short-and long-term impacts on emission processes throughout the canopy ( Keenan et al. 2011). ...
This chapter provides a review of past and state-of-the-art leaf-level emission algorithms that are currently in use for modeling the emissions of biological volatile organic compounds (BVOCs) from plants. The chapter starts with a brief overview about historical efforts and elaborates on processes that describe the direct emission responses to environmental factors such as temperature and light. These phenomenological descriptions have been widely and successfully used in emission models at scales ranging from the leaf to the globe. However, while the models provide tractable mathematical functions that link environmental drivers and emission rates, and as such can be easily incorporated in higher scale predictive models, they do not provide the mechanistic context required to describe interactions among drivers and indirect influences on interactions such as those due to acclimation, accumulated stress and ontogeny. Following a discussion of these issues and the limitations they impose on the current state of model prognosis, we describe in some detail the knowledge gaps that need to be filled in order to move BVOC emissions models into forms that are more directly coupled to physiological processes.
... Aboveground BVOC sinks include reactions with oxidative species within the plant canopy (Carter 1994;Fuentes et al. 2007), dry deposition (Guenther 2015) and, critically, uptake by plant stomata such that stomatal BVOC exchange should be treated as bi-directional (Niinemets et al. 2014) (Fig. 1). Despite our recognition of complex interactions among BVOC emissions drivers, ecosystem BVOC flux models tend to focus on emissions from just the plant canopy, rather than including soils and leaf litter (Baldocchi et al. 1999;Niinemets et al. 2013;Guenther 2015), and they are seldom challenged with observations from a broad set of ecosystem components. This study was aimed at enhancing our understanding of ecosystem BVOC emission modeling with regard to soil and litter processes and model inputs. ...
Biogenic volatile organic compounds (BVOCs) play critical roles in ecological and earth-system processes. Ecosystem BVOC models rarely include soil and litter fluxes and their accuracy is often challenged by BVOC dynamics during periods of rapid ecosystem change like spring leaf out. We measured BVOC concentrations within the air space of a mixed deciduous forest and used a hybrid Lagrangian/Eulerian canopy transport model to estimate BVOC flux from the forest floor, canopy, and whole ecosystem during spring. Canopy flux measurements were dominated by a large methanol source and small isoprene source during the leaf-out period, consistent with past measurements of leaf ontogeny and theory, and indicative of a BVOC flux situation rarely used in emissions model testing. The contribution of the forest floor to whole-ecosystem BVOC flux is conditional on the compound of interest and is often non-trivial. We created linear models of forest floor, canopy, and whole-ecosystem flux for each study compound and used information criteria-based model selection to find the simplest model with the best fit. Most published BVOC flux models do not include vapor pressure deficit (VPD), but it entered the best canopy, forest floor, and whole-ecosystem BVOC flux model more than any other study variable in the present study. Since VPD is predicted to increase in the future, future studies should investigate how it contributes to BVOC flux through biophysical mechanisms like evaporative demand, leaf temperature and stomatal function.
... (Niinemets et al. 2013;Noe et al. 2016a). Starting 2015, measurements of soil respiration, tree growth, aerosols and air ions are completing the trace gas measurements. ...
... Analogous simulations were conducted to compare the average C 3 in vitro temperature response functions developed here and three different temperature functions widely used in the literature in simulating photosynthesis, i.e. in vivo Rubisco temperature responses for Nicotiana tabacum (Bernacchi et al., 2001;Walker et al., 2013) and in vitro Rubisco temperature responses for Spinacia oleracea (Jordan and Ogren, 1984). To quantitatively compare different simulated temperature response curves, warm vs. cool C 3 plants and temperature response curves currently in use in the modeling community, mean absolute (σ A ) and root mean squared (σ S ) differences between different model estimates (Willmott and Matsuura, 2005;Niinemets et al., 2013) were calculated through the modeled temperature range of 5-50 ºC. The mean absolute difference was calculated as: ...
The present study provides a synthesis of the in vitro and in vivo temperature responses of Rubisco Michaelis–Menten constants for CO2 (Kc) and O2 (Ko), specificity factor (Sc,o) and maximum carboxylase turnover rate ( k c a t c kcatc) for 49 species from all the main photosynthetic kingdoms of life. Novel correction routines were developed for in vitro data to remove the effects of study-to-study differences in Rubisco assays. The compilation revealed differences in the energy of activation (∆Ha) of Rubisco kinetics between higher plants and other photosynthetic groups, although photosynthetic bacteria and algae were under-represented and very few species have been investigated so far. Within plants, the variation in Rubisco temperature responses was related to species’ climate and photosynthetic mechanism, with differences in ∆Ha for k c a t c kcatc among C3 plants from cool and warm environments, and in ∆Ha for k c a t c kcatc and Kc among C3 and C4 plants. A negative correlation was observed among ∆Ha for Sc/o and species’ growth temperature for all data pooled, supporting the convergent adjustment of the temperature sensitivity of Rubisco kinetics to species’ thermal history. Simulations of the influence of varying temperature dependences of Rubisco kinetics on Rubisco-limited photosynthesis suggested improved photosynthetic performance of C3 plants from cool habitats at lower temperatures, and C3 plants from warm habitats at higher temperatures, especially at higher CO2 concentration. Thus, variation in Rubisco kinetics for different groups of photosynthetic organisms might need consideration to improve prediction of photosynthesis in future climates. Comparisons between in vitro and in vivo data revealed common trends, but also highlighted a large variability among both types of Rubisco kinetics currently used to simulate photosynthesis, emphasizing the need for more experimental work to fill in the gaps in Rubisco datasets and improve scaling from enzyme kinetics to realized photosynthesis.
... The work towards the SMEAR site establishment started in 2008 when an intensive and integrated measurement campaign combining fl ux, trace gas concentration, aerosol particles and plant and soil measurements were carried out in Järvselja; this campaign provided the fi rst estimates of forest productivity by eddy covariance measurements in Estonia (Noe et al., 2011). Until the year 2013, these measurements were performed during intensive campaigns that led to several publications based on the collected data (Noe et al., 2012;Niinemets et al., 2013;Bourtsoukidis et al., 2014;Smolander et al., 2014). The campaigns were organized such that each provided new data eventually to cover the whole annual cycle. ...
Establishment of the SMEAR Estonia at a hemiboreal mixed deciduous broad-leaved-evergreen needle-leaved forest at Järvselja, SouthEastern Estonia, has strongly enhanced the possibilities for national and international cooperation in the fi elds of forest ecosystem – atmosphere research and impacts of climatic changes on forest ecosystems, atmospheric trace gases, aerosols and air ions. The station provides a multitude of comprehensive continuously measured data covering key climatic and atmospheric characteristics (state and dynamics of solar radiation, trace gases, aerosols and air ions, meteorological parameters) and forest ecosystem traits (net primary productivity, individual tree growth, gas-exchange characteristics, soil variables). The station follows a multidisciplinary and multiscale approach covering processes in spatial dimensions ranging from nanometres to several hundred square kilometres, being thus able to signifi cantly contribute to worldwide measurement networks and the SMEAR network. Here we present an overview of the station, its data produced and we envision future developments towards sustainable research and development of the large-scale scientifi c infrastructure SMEAR Estonia.
... Integrated and inter-disciplinary measurements of atmosphere-biosphere relations began in southeast Estonia in August 2008 at Järvselja. The first measurement campaign, which took place in close co-operation among the University of Helsinki, University of Tartu, University of Innsbruck and Estonian University of Life Sciences, produced a short but comprehensive data set (Noe et al. 2011) that initiated further common research (Noe et al. 2012, Bourtsoukidis et al. 2014, Niinemets et al. 2013, Laan et al. 2014, Smolander et al. 2014) utilizing the location in Järvselja, also in the following years. ...
Atmospheric air ions, clusters and aerosol particles participate in a variety of atmospheric processes and considerably affect e.g. global climate and human health. When measured, air ions as well as atmospheric clusters and particles have been observed to be present practically always and everywhere. In this overview, we present a brief summary of the main achievements and legacy of the series of workshops organized mainly by the University of Helsinki and the University of Tartu. The legacy covers the development and standardization of new instruments, such as ion spectrometers, mass spectrometers and aerosol particle counters, as well as work toward theoretical understanding of new-particle formation and evolution of atmospheric clusters. One important legacy is the establishment of the SMEAR-Estonia station at Järvselja.
... Different pathways with contrasting regulatory mechanisms are responsible for the production of different classes of volatiles (for recent reviews on volatile production see Li & Sharkey 2013;Monson 2013;Rajabi Memari, Pazouki & Niinemets 2013). The rate of volatile isoprenoid synthesis in constitutively emitting species is often tightly linked to photosynthetic metabolism, and therefore the emissions primarily respond to light and temperature, similarly to photosynthesis (for reviews see Niinemets et al. 2010c;Grote et al. 2013;Li & Sharkey 2013;Niinemets et al. 2013a). The emissions also respond to CO 2 concentration, although often differently from photosynthesis (Niinemets et al. 2010c;Sun et al. 2012b;Grote et al. 2013;Li & Sharkey 2013). ...
Biogenic volatile organic compound (BVOC) emissions are widely modeled as inputs to atmospheric chemistry simulations. However, BVOC may interact with cellular structures and neighboring leaves in a complex manner during volatile diffusion from the sites of release to leaf boundary layer and during turbulent transport to the atmospheric boundary layer. Furthermore, recent observations demonstrate that the BVOC emissions are bidirectional, and uptake and deposition of BVOC and their oxidation products are the rule rather than the exception. This review summarizes current knowledge of within-leaf reactions of synthesized volatiles with reactive oxygen species (ROS), uptake, deposition and storage of volatiles and their oxidation products as driven by adsorption on leaf surface and solubilization and enzymatic detoxification inside leaves. The available evidence indicates that due to reactions with ROS and enzymatic metabolism, the BVOC gross production rates are much larger than previously thought. The degree to which volatiles react within leaves and can be potentially taken up by vegetation depends on compound reactivity, physicochemical characteristics, as well as their participation in leaf metabolism. We argue that future models should be based on the concept of bidirectional BVOC exchange and consider modification of BVOC sink/source strengths by within-leaf metabolism and storage.
Bioenergy plant production is expected to rapidly expand in Europe in the near future. This might not only affect resource availability but will also influence the environment. Since many bioenergy plants do emit different amounts and different compositions of biogenic volatile organic compounds (BVOCs) compared to conventional agricultural crops, the new blend of highly reactive compounds might change the chemical composition of the atmosphere. BVOCs have a strong potential to enhance the photochemical O3 production, increase the formation of secondary organic aerosols (SOA), and prolong CH4 lifetime due to fast reactions with OH. These environmental impacts of bioenergy plants on air quality and the regional climate, however, are difficult to evaluate since accurate field observations of relevant crops are not available.
Therefore, I studied a large range of BVOC fluxes from the most prominent bioenergy plants in Germany, which are maize, ryegrass, and oilseed rape, by applying field measurements and biogeochemical modeling. The plants were cultivated in Dedelow, Brandenburg, Germany and observed throughout the vegetative and reproductive development stages. Combining automatically moving large chambers and a proton transfer reaction–mass spectrometer (PTR-MS), I quantified the emission of numerous highly reactive terpenoids, together with several other BVOCs, including alcohols, aldehydes, ketones, benzenoids, and fatty acid derivatives. The characteristic seasonal BVOC flux pattern of each species, could be divided into groups and was associated to the different plant growth stages. The observations from the field campaigns were used to parameterize a biogeochemical ecosystem model coupled to a process-based BVOC emission model. The parameters for the BVOC model were fitted for each compound individually and comprise the dtandardized emission factor, an emission function curvature coefficient, and the fractionation into a light dependent (de novo emission) and light independent (pool emission) function. Therefore, I merged a mechanistic process-based de novo model with a pool emission approach into a joint BVOC emission model which was embedded in the biogeochemical framework LandscapeDNDC. Finally, total annual emissions were calculated in dependence on simulated plant growth and photosynthesis.
Simulated BVOC emissions show that considerable differences between the investigated bioenergy plants exist with oilseed rape having 37-fold higher total annual emissions than maize (oilseed rape: 91.3 ± 8.0 mmol m-2 a-1 ; maize: 2.5 ± 0.1; and ryegrass: 15.7 ± 0.6). The differences in potential annual impacts on air chemistry are less pronounced between the plants, due to the large fraction of highly reactive terpenoids in the maize BVOC emissions. In particular, the difference is reduced to the 6-fold when the potential impact on OH-reactivity (a measure for O3 and SOA forming potential as well as indirect radiative
forcing) is considered and to the 4.5-fold when the theoretically produced electric-
ity yield is additionally taken as a reference.
Thus, the results indicate that BVOC fluxes from large-scale bioenergy fields should be better differentiated, especially with regard to BVOC composition and reactivity. Additionally, the large impact of plant phenology on emission factors demands for elaborated models that should be based on measurements that cover the whole plant growth period.
The book deals with a highly relevant interdisciplinary topic: tree-atmosphere interactions. Plant-driven volatile organic compound (BVOC) emissions play a major role in atmospheric chemistry, including ozone and photochemical smog formation in the troposphere, and they extend the atmospheric lifetime of the key greenhouse gas, methane. Furthermore, condensation of photo-oxidation products of BVOCs leads to formation of secondary organic aerosols with profound implications for the earth's solar radiation budget and climate. Trees represent the plant life form that most contributes to BVOC emissions, which gives global forests a unique role in regulating atmospheric chemistry.
This book, written by leading experts in the field, focuses on recent advancements in understanding the controls on plant-driven BVOC emissions, including efforts to quantitatively predict emissions using computer models. Particular emphasis is on elicitation of emissions under biotic and abiotic stresses, molecular mechanisms of volatile synthesis and emission and the role of emissions in plant stress tolerance. Potentials and limitations of genetic engineering of volatile emissions are also covered. This book addresses all biological scales of organization from molecules to globe and makes a major leap in summarizing and synthesizing the existing information.
The main goal of the book is to provide state-of-the-art summary of the exciting field of tree volatile emissions and offer a perspective for future investigations. The book is intended to serve as an invaluable resource for graduate students starting a thesis project on tree volatile emissions as well as serves as a contemporary source of reference for teachers, scientists and professional within and outside the exciting field of plant-driven volatile emissions.
Biogenic volatile organic compounds (BVOCs) play a central role in atmospheric chemistry via their high reactivity in the gas phase and via their participation in atmospheric new particle formation and secondary organic aerosol formation. The emissions of BVOC to the atmosphere depend on several climate-related variables, making these compounds part of complex, yet potentially very important, climate feedback mechanisms. Here we illustrated the role of BVOCs in enhancing gross primary production (GPP) and cloud droplet number concentrations. The first of these phenomena forms a positive feedback loop for the terrestrial carbon sink (GPP feedback), whereas the second one forms a negative feedback loop for the ambient temperature increase (temperature feedback).
The implementation of biogenic emissions in regional air quality and global climate models requires numerical code and input datasets that are compatible with these regulatory and scientific tools. Canopy- and landscape-level emission models can be developed using a scaling up approach where emissions are first calculated on a leaf scale and then scaled up to higher scale using a canopy model that describes the environmental conditions in different canopy locations. Alternatively, big-leaf models can be used that simulate canopy emissions based on the physiological potentials of uppermost leaves in the canopy. Finally, with development of flux technology for measurement of whole canopy emission fluxes, canopy-level emission models have been derived that simulate the emissions on the basis of whole canopy environmental responses. Here the potentials and limitations of different model frameworks are compared and perspectives for future model developments are offered.
This book summarizes recent advancements in the resolution and quantification of the controls on tree BVOC emissions, including efforts toward synthetic projections using computer models. Major progress has been achieved in understanding the molecular mechanisms of volatile synthesis and emission, the role of emissions in plant stress tolerance and elicitation of emissions under biotic and abiotic stresses. Use of this rich source of insight not only allows for improvement of regional air quality estimations under current climate and atmospheric conditions, but it also allows for improvements to the models and observations needed to predict BVOC emissions under future climate and atmospheric conditions. As our understanding of physiological mechanisms, taxonomic distribution and multi-trophic interactions in forest ecosystems increases further, we will be able to tackle some of the large-scale feedback loops between BVOC emissions, plant stress, and climate that have eluded us for so long.
We present a unifying model for isoprene emission by photosynthesizing leaves based on the hypothesis that isoprene biosynthesis depends on a balance between the supply of photosynthetic reducing power and the demands of carbon fixation.
We compared the predictions from our model, as well as from two other widely used models, with measurements of isoprene emission from leaves of P opulus nigra and hybrid aspen ( P opulus tremula × P . tremuloides ) in response to changes in leaf internal CO 2 concentration ( C i ) and photosynthetic photon flux density ( PPFD ) under diverse ambient CO 2 concentrations ( C a ).
Our model reproduces the observed changes in isoprene emissions with C i and PPFD , and also reproduces the tendency for the fraction of fixed carbon allocated to isoprene to increase with increasing PPFD . It also provides a simple mechanism for the previously unexplained decrease in the quantum efficiency of isoprene emission with increasing C a .
Experimental and modelled results support our hypothesis. Our model can reproduce the key features of the observations and has the potential to improve process‐based modelling of isoprene emissions by land vegetation at the ecosystem and global scales.
The majority of volatile organic compounds emitted from the terrestrial biosphere (BVOCs) are highly reactive hydrocarbons that have been shown to affect atmospheric composition across the full range of temporal scales from fractions of seconds to centuries and spatial scales from μm to global. Furthermore, biogenic emissions are thought to account for around 90 % of the total quantity of non-methane hydrocarbons released into the atmosphere each year. As a result, BVOCs have substantial air quality and climate impacts, and there is an urgent need to quantify and map their emissions as precisely as possible. In this chapter we outline the use of computer models to estimate annual global emissions of BVOCs and the on-going efforts to validate and constrain the output from such models. The current generation of BVOC emission models generally includes only the constitutive emissions of a handful of compounds: chiefly isoprene, monoterpenes and methanol, which are thought to account for about 80 % of the total flux from the biosphere. At present, it is estimated by global models that total annual emission of isoprene amounts to around 500 Tg of carbon, with the emissions dominated by tropical ecosystems and by tree species. The emissions of monoterpenes are similarly distributed, although high levels of monoterpene emissions are also seen from the boreal forests. There is currently no consensus on the annual estimate of monoterpene emission, with estimates ranging from 30 to 150 Tg of carbon. Apart from these main compounds, the biosphere emits many hundreds of different compounds, some of which are produced as a short-lived, transient response to stress rather than as constitutive emissions. We discuss the role that biogenic emissions of reactive trace gases play in the Earth system as a whole, and consider the potential feedbacks that exist between BVOC emissions, atmospheric composition, air quality and climate, and the terrestrial biosphere, and how these can be studied with Earth system models. We finally suggest ways of improving and further developing the global models.
The ability of secondary organic aerosol (SOA) produced from the ozonolysis of α-pinene and monoterpene mixtures (α-pinene, β-pinene, limonene and 3-carene) to become cloud droplets was investigated. Monoterpene SOA is quite active and would likely be a good source of cloud condensation nuclei (CCN) in the atmosphere. A static CCN counter and a Scanning Mobility CCN Analyser (a Scanning Mobility Particle Sizer coupled with a Continuous Flow counter) were used for the CCN measurements. A decrease in CCN activation diameter for α-pinene SOA of approximately 3 nm h−1 was observed as the aerosol continued to react with oxidants. Hydroxyl radicals further oxidize the SOA particles thereby enhancing the particle CCN activity with time. The initial concentrations of ozone and monoterpene precursor (for concentrations lower than 40 ppb) do not appear to affect the activity of the resulting SOA. Köhler Theory Analysis (KTA) is used to infer the molar mass of the SOA sampled online and offline from atomized filter samples. KTA suggests that the aged aerosol (both from α-pinene and the mixed monoterpene oxidation) is primarily water-soluble (around 70–80%), with an estimated average molar mass of 180±55 g mol−1 (consistent with existing SOA speciation studies). CCN activity measurements of the SOA mixed with (NH4)2SO4 suggest that the organic can depress surface tension by as much as 10 nM m−1 (with respect to pure water). The droplet growth kinetics of SOA samples are similar to (NH4)2SO4, except at low supersaturation, where SOA tends to grow more slowly. The CCN activity of α-pinene and mixed monoterpene SOA can be modelled by a very simple implementation of Köhler theory, assuming complete dissolution of the particles, no dissociation into ions, molecular weight of 180 g mol−1, density of 1.5 g cm−3, and surface tension to within 10–15% of water.
The rate of constitutive isoprenoid emissions from plants is driven by plant emission capacity under specified environmental conditions ( E S, the emission factor) and by responsiveness of the emissions to instantaneous variations in environment. In models of isoprenoid emission, E S has been often considered as intrinsic species-specific constant invariable in time and space. Here we analyze the variations in species-specific values of E S under field conditions focusing on abiotic stresses, past environmental conditions and developmental processes. The reviewed studies highlight strong stress-driven, adaptive (previous temperature and light environment and growth CO2 concentration) and developmental (leaf age) variations in E S values operating at medium to long time scales. These biological factors can alter species-specific E S values by more than an order of magnitude. While the majority of models based on early concepts still ignore these important sources of variation, recent models are including some of the medium- to long-term controls. However, conceptually different strategies are being used for incorporation of these longer-term controls with important practical implications for parameterization and application of these models. This analysis emphasizes the need to include more biological realism in the isoprenoid emission models and also highlights the gaps in knowledge that require further experimental work to reduce the model uncertainties associated with biological sources of variation.
The Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1) is a modeling framework for estimating fluxes of 147 biogenic compounds between terrestrial ecosystems and the atmosphere using simple mechanistic algorithms to account for the major known processes controlling biogenic emissions. It is available as an offline code and has also been coupled into land surface models and atmospheric chemistry models. MEGAN2.1 is an update from the previous versions including MEGAN2.0 for isoprene emissions and MEGAN2.04, which estimates emissions of 138 compounds. Isoprene comprises about half of the estimated total global biogenic volatile organic compound (BVOC) emission of 1 Pg (1000 Tg or 1015 g). Another 10 compounds including methanol, ethanol, acetaldehyde, acetone, α-pinene, β-pinene, t −β-ocimene, limonene, ethene, and propene together contribute another 30% of the estimated emission. An additional 20 compounds (mostly terpenoids) are associated with another 17% of the total emission with the remaining 3% distributed among 125 compounds. Emissions of 41 monoterpenes and 32 sesquiterpenes together comprise about 15% and 3%, respectively, of the total global BVOC emission. Tropical trees cover about 18% of the global land surface and are estimated to be responsible for 60% of terpenoid emissions and 48% of other VOC emissions. Other trees cover about the same area but are estimated to contribute only about 10% of total emissions. The magnitude of the emissions estimated with MEGAN2.1 are within the range of estimates reported using other approaches and much of the differences between reported values can be attributed to landcover and meteorological driving variables. The offline version of MEGAN2.1 source code and driving variables is available from http://acd.ucar.edu/~guenther/MEGAN/MEGAN.htm and the version integrated into the Community Land Model version 4 (CLM4) can be downloaded from http://www.cesm.ucar.edu/ .
The vertical distribution of ambient biogenic volatile organic compounds (BVOC) concentrations within a hemiboreal forest canopy was investigated over a period of one year. Variability in temporal and spatial isoprene concentrations can be mainly explained by biogenic emissions from deciduous trees, ranging from 0.1 to 7.5 μg m−3. Monoterpene concentrations exceeded isoprene largely and ranged from 0.01 to 140 μg m−3 and during winter time anthropogenic contributions are likely. Variation in monoterpene concentrations found to be largest right above the ground and the vertical profile suggest a weak mixing leading to terpene accumulation in the lower canopy. Exceptionally high values were recorded during a heat wave in July 2010 with very high midday temperatures above 30 °C for several weeks. During summer months, monoterpene exceeded isoprene concentrations 6-fold and during winter 12-fold. The relative contribution of diverse monoterpene species to the ambient concentrations revealed a dominance of α-pinene in the lower and of limonene in the upper part of the canopy, both accounting for up to 70 % of the total monoterpene concentration during summer months. The main contributing monoterpene during wintertime was Δ3-carene accounting for 60 % of total monoterpene concentration in January. Possible biogenic monoterpene sources beside the foliage are the leaf litter, the soil and also resins exuding from stems. In comparison, the hemiboreal mixed forest canopy showed similar isoprene but higher monoterpene concentrations than the boreal forest and lower isoprene but substantially higher monoterpene concentrations than the temperate mixed forest canopies. These results have major implications for simulating air chemistry and secondary organic aerosol formation within and above hemiboreal forest canopies.
Various bacterial and plastidic plant terpenoids are synthesized via the non-mevalonate 1-deoxy-D-xylulose-5-phosphate (DOXP) pathway. The antibiotic and herbicidal compound fosmidomycin is known to inhibit growth of several bacteria and plants, but so far its mode of action was unknown. Here we present data which demonstrate that the DOXP pathway of isoprenoid biosynthesis is efficiently blocked by fosmidomycin. The results point to the DOXP reductoisomerase as the probable target enzyme of fosmidomycin.
Biogenic emission of hydrocarbons plays an important role in the interactions between plants, especially trees, and the atmosphere. Among these volatile organic compounds isoprene (2-methyl-1,3-butadiene) is the predominant component emitted by many photosynthesizing leaves. Its rapid atmospheric breakdown substantially affects the oxidation potential of the atmosphere. An enzyme, isoprene synthase, extracted from leaves of European oak (Quercus robur L.) was previously found to catalyse the Mg2+-dependent elimination of diphosphate from dimethylallyldiphosphate to form isoprene. The present paper describes the seasonal variation of this enzyme activity in Quercus robur (L) leaves in 1995. The enzymatic data obtained were used to create an additional term for the isoprene emission algorithm (ISOG93). The addition of this correction term for the seasonality of isoprene synthase to the emission model improved considerably the simulation of seasonal isoprene emission rates in oaks, avoiding over- and underestimations in the current modeling approach.
Model calibration is a critical phase in the modelling process, and the need for a well-established calibration strategy is obvious. Therefore a systematic approach for model calibration is proposed which is guided by the intended model use, and which is supported by adequate techniques, prior knowledge and expert judgement. The success of calibration will be primarily limited by the nature, amount and quality of the available data, in relation to the complexity of the model; additional limitations are the effectiveness of the applied techniques and the availability of time, man- and computer power, adequate expertise and financial resources. These limitations will often preclude a unique calibrated model. As a consequence, calibration studies should provide information on the non-uniqueness and/or uncertainty which will be left in the model (parameters) after calibration, and this uncertainty should be adequately accounted for in subsequent model applications.
The Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1) is a modeling framework for estimating fluxes of biogenic compounds between terrestrial ecosystems and the atmosphere using simple mechanistic algorithms to account for the major known processes controlling biogenic emissions. It is available as an offline code and has also been coupled into land surface and atmospheric chemistry models. MEGAN2.1 is an update from the previous versions including MEGAN2.0, which was described for isoprene emissions by Guenther et al. (2006) and MEGAN2.02, which was described for monoterpene and sesquiterpene emissions by Sakulyanontvittaya et al. (2008). Isoprene comprises about half of the total global biogenic volatile organic compound (BVOC) emission of 1 Pg (1000 Tg or 1015 g) estimated using MEGAN2.1. Methanol, ethanol, acetaldehyde, acetone, α-pinene, β-pinene, t-β-ocimene, limonene, ethene, and propene together contribute another 30% of the MEGAN2.1 estimated emission. An additional 20 compounds (mostly terpenoids) are associated with the MEGAN2.1 estimates of another 17% of the total emission with the remaining 3% distributed among >100 compounds. Emissions of 41 monoterpenes and 32 sesquiterpenes together comprise about 15% and 3%, respectively, of the estimated total global BVOC emission. Tropical trees cover about 18% of the global land surface and are estimated to be responsible for ~80% of terpenoid emissions and ~50% of other VOC emissions. Other trees cover about the same area but are estimated to contribute only about 10% of total emissions. The magnitude of the emissions estimated with MEGAN2.1 are within the range of estimates reported using other approaches and much of the differences between reported values can be attributed to land cover and meteorological driving variables. The offline version of MEGAN2.1 source code and driving variables is available from
Various aspects of the biochemistry of photosynthetic carbon assimilation in C3 plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO2(p(CO2)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O2(p(O2)), the dependence of net CO2 assimilation rate on p(CO2) and irradiance, and the influence of p(CO2) and irradiance on the temperature dependence of assimilation rate.
On-line measurements of atmospheric VOC concentrations in the European boreal zone with a proton transfer reaction mass spectrometer were conducted at SMEAR II station in Hyytiälä, south-western Finland on 2-22 July 2004. The measurements showed a strong diurnal variation of several compounds. A factor analysis performed for the concentration data was used to classify the measured VOC masses into three classes based on the behavior of their concentrations. The masses in the first class had a high diurnal variation with maximum values in the afternoon. Compounds contributing to masses in this category were e.g. methanol, acetone, methyl-vinyl-ketone and hexanal. The concentrations of masses in the second class had also a high diurnal variation, but with maxima during the night when the mixing of the atmospheric surface layer was weak. Monoterpenes and phenol are compounds contributing to the masses in this category. The masses in the third class did not have a marked diurnal cycle and were not dependent on the local meteorological parameters. The masses having a strong positive loading on this factor were those associated with anthropogenic compounds with relatively long atmospheric life-times, such as benzene. Considering the difference in the measurement height, the total monoterpene concentration measured by the PTR-MS was consistent with the concentration measured by gas chromatography-mass spectrometer with adsorbent sampling.
In recent years evidence has emerged that the amount of isoprene emitted from a leaf is affected by the CO2 growth environment. Many - though not all - laboratory experiments indicate that emissions increase significantly at below-ambient CO2 concentrations and decrease when concentrations are raised to above-ambient. A small number of process-based leaf isoprene emission models can reproduce this CO2 stimulation and inhibition. These models are briefly reviewed, and their performance in standard conditions compared with each other and to an empirical algorithm. One of the models was judged particularly useful for incorporation into a dynamic vegetation model framework, LPJ-GUESS, yielding a tool that allows the interactive effects of climate and increasing CO2 concentration on vegetation distribution, productivity, and leaf and ecosystem isoprene emissions to be explored. The coupled vegetation dynamics-isoprene model is described and used here in a mode particularly suited for the ecosystem scale, but it can be employed at the global level as well. Annual and/or daily isoprene emissions simulated by the model were evaluated against flux measurements (or model estimates that had previously been evaluated with flux data) from a wide range of environments, and agreement between modelled and simulated values was generally good. By using a dynamic vegetation model, effects of canopy composition, disturbance history, or trends in CO2 concentration can be assessed. We show here for five model test sites that the suggested CO2-inhibition of leaf-isoprene metabolism can be large enough to offset increases in emissions due to CO2-stimulation of vegetation productivity and leaf area growth. When effects of climate change are considered atop the effects of atmospheric composition the interactions between the relevant processes will become even more complex. The CO2-isoprene inhibition may have the potential to significantly dampen the expected steep increase of ecosystem isoprene emission in a future, warmer atmosphere with higher CO2 levels; this effect raises important questions for projections of future atmospheric chemistry, and its connection to the terrestrial vegetation and carbon cycle.
Isoprene plays an important role in regulating the atmospheric trace gas composition, in particular the tropospheric ozone concentrations. Therefore realistic estimates of the seasonal variation of isoprene emission source strengths of strong isoprene-emitting deciduous trees such as pedunculate oak (Quercus robur L.) are required in temperate regions of Europe. In 1995 to 1997 a study was conducted to survey the annual fluctuations of oak isoprene synthase activity and photosynthetic pigment contents, the latter as a parameter for the development of the photosynthetic apparatus of oak leaves. Depending on annual temperature and light profiles (photosynthetic photon flux densities (PPFD)), different seasonal patterns of isoprene synthase activity were observed with maximum activities of 18.4+/-10.6nmolm-2s-1, 14.1+/-5.8nmolm-2s-1, and 19.9+/-7.9nmolm-s-1 in 1995, 1996, and 1997, respectively. On the basis of isoprene synthase activity, chlorophyll a measurements, and phenological data collected from pedunculate oaks of 89 ecological regions covering all of Germany a model was developed for the calculation of the seasonal variation of oak isoprene synthase activity in relation to annual fluctuation of temperature and PPFD. By coupling this model to a numeric process-based isoprene emission model it was possible to predict isoprene emission rates of individual pedunculate oak trees with a deviation of 55%.
Isoprenoid biosynthesis in plants proceeds via two independent pathways: 1) the cytosolic classical acetate/mevalonate pathway (biosynthesis of sterols, sesquiterpenes, triterpenoids) and 2) via the non-mevalonate 1-deoxy-D-xylulose-5-phosphate (DOX-P) pathway for the biosynthesis of plastidic isoprenoids such as carotenoids, phytol (side-chain of chlorophylls), plastoquinone-9, isoprene, mono- and diterpenes. Both pathways form isopentenyl-diphosphate (IPP) as precursors, from which all other isoprenoids are formed via head-to-tail addition. The present knowledge of the novel 1-deoxy-D-xylulose-5-phosphate (DOX-P) pathways for isopentenyl-diphosphate biosynthesis, which is apparently located in plastids, is reviewed in this contribution. It provides a new insight into chloroplast metabolism.
Der pflanzliche 1-Desoxy-D-xylulose-5-phosphat-Weg in der Biosynthese von Isoprenoiden
Die Isoprenoidbiosynthese der Pflanzen verläuft über zwei voneinander unabhängige Biosynthesewege: 1. über den cytosolischen Acetat/Mevalonat-Weg (Biosynthese von Sterolen, Sesquiterpenen und Triterpenoiden) und 2. über den Mevalonatunabhängigen 1-Desoxy-D-xylulose-5-phosphat (DOX-P)-Weg für die Bildung der plastidären Isoprenoide, z.B. Carotinoide, Phytol (Seitenkette von Chlorophyll), Plastochinon-9, Isopren, Mono- und Diterpene. Beide Biosynthesewege bilden Isopentenyldiphosphat (IPP) als aktive Isoprenoidstufe, aus der sich durch Kopf-Schwanz-Addition alle Isoprenoide ableiten. Der gegenwärtige Stand unserer Kenntnis des 1-Desoxy-D-xylulose-5-phosphat-Biosynthesewegs für die Bildung von IPP, der offensichtlich in Plastiden lokalisiert ist, wird in diesem Beitrag zusammengefaßt. Der IPP-Biosyntheseweg gibt neue Einblicke in den Chloroplasten-Stoffwechsel.
Physiological responses to seasonal drought were explored for Psychotria limonensis (Rubiaceae), an abundant understory shrub in a seasonally dry tropical forest in Panama. Control and irrigated plants were compared at the beginning and again at the end of the 4-mo dry season. Stomatal conductance remained high throughout for irrigated plants, but fell to very low levels for control plants late in the dry season. Net assimilation rates under both saturating and ambient light were unaffected by irrigation. As a consequence, instantaneous water-use efficiency (assimilation ÷ evapotranspiration), derived from gas exchange measures, and long-term water-use efficiency, estimated from stable carbon isotope ratios of leaf tissue, were similar for both treatments. The maintenance of high assimilation rates despite drought may be related to osmotic adjustment. Control plants had more negative osmotic potentials at full turgor and higher moduli of elasticity in the late dry season.
We investigated growth, leaf monoterpene emission, gas exchange, leaf structure and leaf chemical composition of 1-year-old Quercus ilex L. seedlings grown in ambient (350 mul l(-1)) and elevated (700 mul l(-1)) CO2 concentrations ([CO2]). Monoterpene emission and gas exchange were determined at constant temperature and irradiance (25 degreesC and 1000 mu mol m(-2) s(-1) of photosynthetically active radiation) at an assay [CO2] of 350 or 700 mul l(-1). Measurements were made on intact shoots after the end of the growing season between mid-October and mid-February. On average, plants grown in elevated [CO2] had significantly increased foliage biomass (about 50%). Leaves in the elevated [CO2] treatment were significantly thicker and had significantly higher concentrations of cellulose and lignin and significantly lower concentrations of nitrogen and minerals than leaves in the ambient [CO2] treatment. Leaf dry matter density and leaf concentrations of starch, soluble sugars, lipids and hemi-cellulose were not significantly affected by growth in elevated [CO2]. Monoterpene emissions of seedlings were significantly increased by elevated [CO2] but were insensitive to short-term changes in assay [CO2]. On average, plants grown in elevated [CO2] had 1.8-fold higher monoterpene emissions irrespective of the assay [CO2]. Conversely, assay [CO2] rapidly affected photosynthetic rate, but there was no apparent long-term acclimation of photosynthesis to growth in elevated [CO2]. Regardless of growth [CO2]. photosynthetic rates of all plants almost doubled when the assay [CO2] was switched from 350 to 700 mul l(-1). At the same assay [CO2], mean photosynthetic rates of seedlings in the two growth CO2 treatments were similar. The percentage of assimilated carbon lost as monoterpenes was not significantly altered by CO2 enrichment. Leaf emission rates were correlated with leaf thickness, leaf concentrations of cellulose, lignin and nitrogen, and total plant leaf area. In all plants, monoterpene emissions strongly declined during the winter independently of CO2 treatment. The results are discussed in the context of the acquisition and allocation of resources by e. ilex seedlings and evaluated in terms of emission predictions.
Biogenic volatile organic compounds (BVOCs) play a central role in atmospheric chemistry via their high reactivity in the gas phase and via their participation in atmospheric new particle formation and secondary organic aerosol formation. The emissions of BVOC to the atmosphere depend on several climate-related variables, making these compounds part of complex, yet potentially very important, climate feedback mechanisms. Here we illustrated the role of BVOCs in enhancing gross primary production (GPP) and cloud droplet number concentrations. The first of these phenomena forms a positive feedback loop for the terrestrial carbon sink (GPP feedback), whereas the second one forms a negative feedback loop for the ambient temperature increase (temperature feedback).
The implementation of biogenic emissions in regional air quality and global climate models requires numerical code and input datasets that are compatible with these regulatory and scientific tools. Canopy- and landscape-level emission models can be developed using a scaling up approach where emissions are first calculated on a leaf scale and then scaled up to higher scale using a canopy model that describes the environmental conditions in different canopy locations. Alternatively, big-leaf models can be used that simulate canopy emissions based on the physiological potentials of uppermost leaves in the canopy. Finally, with development of flux technology for measurement of whole canopy emission fluxes, canopy-level emission models have been derived that simulate the emissions on the basis of whole canopy environmental responses. Here the potentials and limitations of different model frameworks are compared and perspectives for future model developments are offered.
The emission of biogenic volatile organic compounds (BVOCs), including isoprenoid compounds, methanol and oxygenated organic compounds, is controlled by both the existing metabolic potential of a leaf and gene expression responses that modulate the existing metabolic potential to increase or decrease compound biosynthesis and emission rate. This capability to respond both instantaneously and in the long term to environmental variation provides plants with flexibility in their adaptions to biotic and abiotic stresses, which are also encountered in short and long-term time frames. This chapter reviews the mechanistic basis of the immediate controls of volatile BVOC emissions by light, temperature, and ambient CO2 and O2 concentrations, as well as the genetic responses that involve changes in gene expression patterns. Photosynthesis ultimately provides the carbon for BVOC production, though under non-stressed conditions the photosynthetic rate itself is rarely so low that it limits BVOC emissions. However, various metabolic pathways compete for substrates that are produced from photosynthate, including cytosolic pathways, such as the mevalonic acid (MVA) pathway and chloroplastic pathways such as the 2-C-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate pathway (MEP/DOXP). Controls over the use of substrate are regulated among these pathways through feedback mechanisms, specificity in the transport of metabolites across organelle membranes, and the channeling of NADPH reductant and ATP to specific steps in the pathways. This chapter emphasizes that these interactive controls provide the major explanation for longer-term physiological controls of emissions. Emissions of several types of compounds are considered, including isoprenoids, methanol, and green leaf volatiles such as various aldehydes and ketones.
Plants emit more than 30,000 different volatile organic compounds and the extent to which stomata exert control over the emissions of these compounds varies widely. Each of these compounds has unique physico-chemical characteristics, including volatility that characterizes the partitioning between water/air and water/lipid phases. For different volatile compounds, volatility differs by over six orders of magnitude. The volatility of each compound, and to a lesser extent the anatomical characteristics of the leaf, determine for each compound the extent to which the aqueous and lipid phases within the leaf comprise temporary non-specific storage pools between the site of synthesis and the substomatal cavities. This chapter emphasizes that the pool size of each volatile in leaf lipid and water phases is the chief determinant of the strength of stomatal control as well as the responsiveness of emissions to rapid changes in light and temperature.
Recently, attention among scientists has been focused on potential global climate change as well as on the deposition of pollutants and their impacts. These perspectives emphasize the role of ecosystems as exchange surfaces between atmosphere and vegetation and between vegetation and groundwater (Dickenson 1988; Bolin 1988; Ulrich 1987). Particularly with respect to northern taiga and tundra regions, it is important to determine whether climate change may have already altered or may in the future alter rates (positive or negative) of ecosystem carbon storage (Oberbauer et al. 1992; Oechel and Billings 1992). Furthermore, it is important to understand environmental controls on carbon fluxes and carbon storage, because the gradients in soil temperature, water availability, and available light energy in the Arctic are large and these will strongly affect the integrated values of net carbon dioxide (Tenhunen et al. 1992) and methane exchange (Whalen and Reeburgh 1988, 1990) in polar regions. Even when viewed simplistically and at the regional scale, temporal and spatial variation in ecosystem material exchange characteristics must be considered when estimating carbon balances (Miller et al. 1983). At smaller scales such as the watershed, temporal and spatial variation in ecosystem structure, species composition, physiology, and environmental conditions determine momentary net gas exchange rates, but also provide clues concerning the manner in which ecosystem properties may be shifted regionally in a future climate (Chapin et al. 1992).
The classically described stresses to which plants are subjected in mediterranean type climates are water stress and heat stress during the summer and cold stress during the winter. The general behavioral tendencies in gas exchange of leaves of shrubs subjected to these climate conditions were summarized nicely by Larcher (1961).
Evaluation and comparison of the performance of soil organic matter models is often based upon visual/graphical comparison of the simulated values produced by the model with actual values from field experiments. Such methods provide an immediate qualitative description of the differences, highlighting trends, different types of errors and distribution patterns of simulated and measured values. However, model evaluations or comparisons should ideally incorporate both a qualitative visual/graphical assessment and a quantitative statistical appraisal. Statistical methods have been selected that are suitable for quantitative evaluation and comparison of soil organic matter models. The methods included each provide information on some distinct aspect of the accuracy of the simulation. Quantifying association and coincidence provides two different measures of the overall similarity between the simulated and measured values. A high coincidence indicates that the simulated values closely correspond to the measured values, whereas a high association indicates that the shape of the simulated curve is similar to that measured. The total difference between the simulated values and measurements may be expressed in terms of consistent and inconsistent errors. Consistent errors quantify the extent of model bias towards either over- or under-prediction of observations. Inconsistent errors correspond to errors which cancel out because the model shows no inclination towards either over- or under-prediction. If the experiments have been replicated, the total difference between simulated values and measurements can be more usefully expressed in terms of systematic and random errors. Systematic errors represent the failure of the model to simulate differences between the experiments. Random errors represent experimental error. Finally, the maximum difference between any pair of simulated and measured values can be used to indicate where a small number of very large errors are contributing to a large proportion of the overall error calculated, so that other statistical values may become unreliable. If used in conjunction with visual/graphical methods, the statistical techniques described in this paper provide a rigorous method for the quantitatively evaluating the performance of soil organic matter models, or indeed other models.
Various woody species belonging to the mediterranean macchia ecosystem near Montalto di Castro, Italy were studied in respect of their strategies of water resources acquisition and water-use efficiencies, using stable isotopes analysis. Evergreen species seem to depend more on rain-water utilization that the deciduous ones, which utilize ground-water almost exclusively. this pattern is paralleled by the distribution of δ13C values which show a greater water-use efficiency for the evergreen species vs the deciduous ones. -Authors
This book covers the proceedings of the Second European Symposium on the
Physico-Chemical Behavior of Atmospheric Pollutants, held September 29
to October 1, 1981, in Varese, Italy. These symposia are organized about
every second year to coordinate research within the European Community
and in Austria, Switzerland, Sweden, and Yugoslavia under a joint
agreement called COST (Cooperation Scientifique et Technique).The book
consists of 13 articles on the identification and analysis of
pollutants, 14 on chemical and photochemical reactions, 12 on aerosols,
12 on pollutant cycles, and 21 on transport and modeling and field
experiments and ends with five useful summaries written by the session
chairman.
Isoprene emission is related to photosynthesis but the nature of the relationship is not yet known. To explore this relationship we have examined the rate of isoprene emission, photosynthesis, and the contents of photosynthetic metabolites in leaves of velvet bean (Mucuna deeringeniana L.) and red oak (Quercus rubra L.) in response to a light-to-dark transition and to changes in air composition. Isoprene emission fell when darkness was imposed and the drop was associated with reduced amounts of ribulose-1,5-bisphosphate and ATP. The rate of isoprene emission and ATP content were reduced to the same extent by exposure to low O2 or high CO2 partial pressures. Only when O2 and CO2 were simultaneously removed from the air did the rate of isoprene emission drop without a corresponding change in ATP. The results demonstrate that when carbon is not limiting, isoprene emission is highly correlated with ATP content. When synthesis of phosphoglyceric acid is inhibited, however, carbon availability may control isoprene production.
This chapter provides a review of past and state-of-the-art leaf-level emission algorithms that are currently in use for modeling the emissions of biological volatile organic compounds (BVOCs) from plants. The chapter starts with a brief overview about historical efforts and elaborates on processes that describe the direct emission responses to environmental factors such as temperature and light. These phenomenological descriptions have been widely and successfully used in emission models at scales ranging from the leaf to the globe. However, while the models provide tractable mathematical functions that link environmental drivers and emission rates, and as such can be easily incorporated in higher scale predictive models, they do not provide the mechanistic context required to describe interactions among drivers and indirect influences on interactions such as those due to acclimation, accumulated stress and ontogeny. Following a discussion of these issues and the limitations they impose on the current state of model prognosis, we describe in some detail the knowledge gaps that need to be filled in order to move BVOC emissions models into forms that are more directly coupled to physiological processes.
The ability of biogenic secondary organic aerosol (SOA) to contribute to the concentration of cloud condensation nuclei (CCN) in the atmosphere is examined. Aerosol is generated by the ozonolysis reaction of monoterpenes (alpha-pinene, beta-pinene, 3-carene, and limonene) and sesquiterpenes (beta-caryophyllene, alpha-humulene, and alpha-cedrene) in a 10 m3 temperature-controlled Teflon smog chamber. In some cases, a self-seeding technique is used, which enables high particle concentrations with the desired diameters without compromising particle composition and purity. The monoterpene SOA is excellent CCN material, and it activates similarly (average activation diameter equals 48 +/- 8 nm at 1% supersaturation for the species used in this work) to highly water-soluble organic species. Its effective solubility in water was estimated to be in the range of 0.07-0.40 g solute/g H2O. CCN measurements for sesquiterpene SOA (average activation diameter equals 120 +/- 20 nm at 1% supersaturation for the species used in this work) show that it is less CCN active than monoterpene SOA. The initial terpene mixing ratio (between 3 and 100 ppb) does not affect the CCN activation for freshly generated SOA.
Monoterpene emission rates from young and adult Scots pines (Pinus sylvestris), a typical central European conifer, were measured under ambient conditions using a dynamic enclosure chamber. We investigated diurnal and seasonal cycles of monoterpene emissions and branch-to-branch and plant-to-plant variabilities of emission rates. The four most abundant monoterpenes usually emitted from Pinus sylvestris were α-pinene, 3-carene, camphene, and β-pinene. Emissions of individual monoterpenes were highly correlated to each other and increased exponentially with temperature. We obtained β coefficients for the temperature dependence of different monoterpenes between 0.08 and 0.13 K−1. The β coefficients varied with season by a factor of 2; the standard emission rates varied by more than 1 order of magnitude. Highest standard emission rates were found in April; lowest standard emission rates were found in July and October. In July and October the standard emission rates from two different branches of the same tree showed no significant differences; in September they differed by a factor of 2. Seasonal variations of a single branch and branch-to-branch variations in the spectrum of emitted monoterpenes were small. On the other hand, different individual Scots pines emitted a completely different spectrum of monoterpenes, indicating that the monoterpene emission spectrum is only typical for an individual plant but not for the whole plant species. The temperature normalized standard emission rates were found to be highly variable. Values for the sum of monoterpenes ranged between 0.06 and 3.7 μg g(dry weight)−1 h−1 (micrograms monoterpenes per gram dry weight (dw) of needles and hour). Temperature-normalized monoterpene emission rates and temperature dependencies of the emissions were used to calculate monthly flux estimates of monoterpenes for the Hartheimer Wald.
Stomatal conductance remained high throughout for irrigated plants, but fell to very low levels for control plants late in the dry season. Net assimilation rates under both saturating and ambient light were unaffected by irrigation. As a consequence, instantaneous water use efficiency were similar for both treatments. Maintenance of high assimilation rates despite drought may be related to osmotic adjustment. Control plants had more negative osmotic potentials at full turgor and higher moduli of elasticity in the late dry season. -from Authors
Many plants synthesize isoprene. Because it is volatile and reacts rapidly with hydroxyl radicals, it is emitted to the atmosphere and plays a critical role in atmospheric chemistry. Determining effective remediation efforts for ozone pollution requires accurate isoprene-emission inventories. Temperature and light effects on isoprene emission from plants over minutes to a few hours are fairly well known, but effects over a few days (i.e., influenced by weather) are also possible. We measured isoprene emission and photosynthesis under constant temperature and light (known as the basal emission rate, which reflects the capacity for isoprene emission) during eight field trips from 1994 to 1996. Measurements were made at the tops of oak trees at two sites between May and September. On six of the trips, the effect of short-term (minutes to hours) temperature changes was also investigated. The basal emission rate of isoprene was highly correlated with the average temperature of the previous two days. Including the average daily dose of photosynthetically active radiation for the previous two days improved the correlation. Using averages from one, four, or seven days before the measurement resulted in lower correlation coefficients. Including a variable basal emission rate will improve the accuracy of isoprene-emission models.
Henry's law constants (HLCs or air‐water partition coefficients) for organic compounds of environmental concern are reviewed. Frequently, the most significant factor influencing HLC values for a particular compound is temperature. Conditions are delineated where other parameters (pH, compound hydration, compound concentration, complex mixtures, dissolved salts, suspended solids, dissolved natural organic material [DOM], surfactants, and natural water sample composition) may also significantly affect HLC values. HLC estimation techniques utilizing (1) thermodynamically based quantitative property‐property relationships (QPPRs), including the vapor pressure/aqueous solubility ratio (VP/AS) method, and (2) various quantitative structure‐property relationships (QSPRs), including use of UNIFAC, are summarized. Major limitations noted were: (1) the VP/AS approach — lack of reliable/accurate vapor pressure and aqueous solubility data, (2) UNIFAC — errors emanating from required extrapolation of vapor‐liquid equilibrium (VLE) data, and (3) other QSPRs — predictions limited to a single temperature (25°C). Following a review of HLC experimental determination techniques, 25 studies establishing directly measured HLC temperature‐dependent relationships (covering 130 compounds) are summarized and discussed. From these data, the average (and typical range) slope of the temperature‐dependent line was found to correspond to a 60% (30 to 100%), 140% (85 to 250%), and 90% (45 to 170%) increase in HLC per 10°C rise in temperature for hydrocarbons (omitting pesticides and polychlorinated biphenyls [PCBs]), pesticides and PCBs, and nonhydrocarbons, respectively. Finally, the directly measured values were compared with QPPR‐and QSPR‐predicted values.
The mechanisms of volatile organic compound (VOC) emissions from Scots pine (Pinus sylvestris L.) were investigated in laboratory experiments. The plants emitted mainly monoterpenes and acetone. Isoprene was emitted only in small amounts, but the mechanisms of its emissions were similar to those of the other compounds. Isoprene, acetone, and monoterpene emissions from Scots pine could be well described by an algorithm that considers emissions caused by evaporation of VOCs out of pools and emissions in parallel with their biosynthetic production. Monoterpene emissions were mainly affected by temperature. In some cases, monoterpene emissions were also influenced by photosynthetic active radiation implying that monoterpene emissions from Pinus sylvestris occur from storage pools as well as from processes that are linked to monoterpene biosynthesis. The coupling of monoterpene emissions with photosynthesis was confirmed by results of experiments with 13CO2. The 13CO2 exposure resulted in emission of 13C labeled monoterpenes during 13CO2 exposure as well as during the night following the exposure. Similar results were also obtained for isoprene emissions. Scots pine emitted isoprene during illumination as well as in darkness. The emitted isoprene was labeled during 13CO2 exposure and in the night following the exposure. The results obtained for monoterpene emissions in the laboratory were compared to those of outdoor measurements with Scots pine. While the temperature dependencies of emission rates were comparable to those obtained from laboratory experiments, a PAR dependence was not detectable. Temperature variations during outdoor measurements prevented a detection of this dependence.
The algorithm developed by Guenther et al. [1991] to describe the organic emission of isoprene-emitting plants has been used for prediction monoterpene emission from Quercus ilex L., an evergreen oak typical of the Mediterranean basin. The dependence of monoterpene emission on photosynthetically active radiation and temperature has been verified through laboratory experiments carried out on single leaves as well as through field measurements at branch level. While the algorithm describes well monoterpene emission under stationary state conditions, it is less accurate when rapid fluctuations of light and temperature take place. Because of this, the isoprene algorithm is capable of predicting the response of Quercus ilex L. with an accuracy better than +/-25% only in 65% of the environmental situations experienced by the plant. Field and laboratory observations consistently indicate that temperature oscillations can be an important source for the discrepancies between predicted and observed values as they can generate bursts of emission with values twice as high as those predicted by the algorithm. Possible causes generating these effects are analyzed and critically discussed. In spite of the observed limitations, the isoprene algorithm can successfully describe the biogenic emission from Quercus ilex L., and its use is advantageous as it greatly simplifies regional and global emission models, especially if the light dependence of monoterpene emission is proven to be a widespread phenomenon.
A process-based model was constructed around the current knowledge of the biochemical pathway of isoprene synthesis, with the objective of producing a new model of high mechanistic content to simulate the effects of environmental change on rates of isoprene emission, and thus enable the prediction of emission rates under future climates. The model was based on the three potentially limiting processes underlying isoprene synthesis: pyruvate supply to provide the substrate of isoprene carbon, supply of adenosine triphosphate (ATP) for phosphorylation to dimethylallyl pyrophosphate (DMAPP), and the rate of isoprene synthesis from DMAPP, which was controlled by the temperature dependency of the enzyme isoprene synthase. Using mechanistic methods wherever possible, model simulations predicted the relative effects of changing photon flux density, carbon dioxide concentrations and temperature on leaf isoprene emission rates. The model was used to predict the interactive effects of elevated concentrations of carbon dioxide and temperature on rates of isoprene emission. Simulations indicated that the effects of carbon dioxide and temperature on isoprene emission rates were complicated by the interactive effects of two of the controlling rate-limiting processes in the synthesis of isoprene, namely phosphorylation rates and isoprene synthase activity. Under present concentrations of carbon dioxide and at photon flux density levels above ca. 500 μmol m−2 s−1 the controlling rate process is the temperature dependency of isoprene synthase.
Periods of soil dryness are typical for the climate of Southern Europe. Since drought induced restriction in carbon acquisition may reduce carbon reemission through terpenoids, drought stress can be a factor influencing the overall emission of terpenoids in the Mediterranean area. In this area orange plantations are common. In order to investigate the relationship between the water status of Citrus sinensis and terpenoid emissions, monoterpene and sesquiterpene emission rates were followed during a drought treatment and subsequent recovery. A dynamic enclosure technique was used to determine emissions of terpenoids from a branch of a young orange tree in a greenhouse. Terpenoid emissions from Citrus sinensis consisted mainly of the sesquiterpene beta-caryophyllene and the monoterpene trans-beta-ocimene. Taken together these compounds accounted for 82% of the total terpenoid carbon emission from orange leaves. Other terpenoids were emitted in traces. Under severe drought stress the emission of both compounds was reduced to less than 6% of the prestress level. Mild drought stress induced a decrease in the trans-beta-ocimene emission rate whereas the beta-caryophyllene emission showed no response to slight drought stress. During the first phase of the experiment, before severe drought stress occured, the beta-caryophyllene emission rate was closely related to temperature. The emission increased by the 5.6 fold with a 10 degrees C increase in temperature. At standard conditions (30 degrees C, PPFD 1069 mu mol photons m(-2) s(-1)) trans-beta-ocimene was emitted from orange leaves at a rate of 0.33 mu g g(-1) h(-1) and beta-caryophyllene at a rate of 0.41 mu g g(-1) h(-1) given on a total dry weight basis, rates corresponding to 10.5 and 13.1 ng m(-2) s(-1), respectively, calculated per total projected leaf area. With flowers the total terpenoid carbon emission was 7.8 fold the emission from the same branch without flowers. The data indicate, that severe drought stress and flowering should be considered in terpenoid emission inventories to avoid error in the estimation of total terpenoid emissions from vegetation in model calculations. (C) 1999 Elsevier Science Ltd. All rights reserved.
Atmospheric hydrocarbons are continuously monitored at the rural site of Taenikon, Switzerland. As expected for a rural area, highest isoprene concentrations are found in summer. However, elevated concentrations are also measured on some occasions in winter, in particular during events with long-lasting surface inversions, temperatures constantly below 0°C and snow covering the vegetation. During such events, concentrations of isoprene are strongly correlated with those of 1,3-butadiene, a substance that is mainly due to human activities. For these periods, a molar ratio between the concentrations of isoprene and those of 1,3-butadiene of 0.42 is observed. This value, together with the concentrations of 1,3-butadiene, is used to estimate the anthropogenic fraction of the atmospheric isoprene for the whole of 1997. It is found that the fraction is close to 100% in January–February and again in November–December. On the other hand, as early as March, a considerable amount of the observed isoprene appears to be of biogenic origin, although isoprene emissions by trees are negligible. The relative anthropogenic contribution is minimal in midsummer, when biogenic emissions are highest. For this time of the year, the anthropogenic contribution is largest during the early morning hours, in agreement with the traffic peak on nearby country roads.
Canopy scale monoterpene emissions were measured using the micrometeorological gradient method at two sites, Huhus and Hyytiälä, in the European boreal zone. At both sites the dominant tree species was Pinus sylvestris. The dominant monoterpene emitted by the forests was α-pinene, followed by Δ3-carene and β-pinene/myrcene. At the more homogenous Huhus site monoterpene emission correlated reasonably well with the IR-temperature of the canopy and the air temperature inside the canopy, and the common exponential equation could be used to describe the emissions. The β-coefficient obtained was 0.15°C−1, which is somewhat higher than the commonly used value of 0.09°C−1. The emission potential calculated using the measured canopy scale emissions and β=0.09°C−1 was 1.2μgg−1dwh−1 which is close to 1.5μgg−1dwh−1 used in recent emission inventories. The measured emissions at the more heterogeneous Hyytiälä site were not well correlated with the meteorological parameters. However, the coefficients obtained using the Huhus measurements predicted the magnitude of the fluxes measured at Hyytiälä reasonably well.
Concentrations of monoterpenes, 1,8-cineol and light hydrocarbons were measured in Pötsönvaara, Ilomantsi, Eastern Finland during two growing seasons in 1997 and 1998. The measuring site was located on the top of a hill, outside a mixed forest. The monthly average summer concentrations of isoprene were 0.3–1.7 ppbC and monoterpenes and 1,8-cineol together 1.6–3.2 ppbC. Isoprene and α-pinene were the most abundant compounds throughout the growing season, but β-pinene, Δ3-carene, camphene, 1,8-cineol, sabinene and limonene were found as well. Isoprene and sabinene concentrations started to increase later than the concentrations of other compounds, and were better correlated with each other than with other compounds. Diurnal variations of monoterpenes show a minimum in the daytime and a maximum at night, except sabinene at midsummer, that has maximum concentrations during the day. The field data support the idea that the effective temperature sum can be used to predict the initiation of emissions of isoprene and also terpene emissions from Betula pendula.
Two of the most typical Mediterranean tree species (Pinus pinea [Pp] and Quercus ilex [Qi]) were screened for emissions of monoterpenes during the period of June 1997–July 1998 in the field at a semi-rural location near Terrassa (Barcelona, Spain) using a bag-enclosure sampling method followed by gas chromatography analysis with mass selective detection (GC/MSD). A mean of about eight samples per day were measured. A periodical sampling throughout 1 yr allowed to examine data for long-term influences. The main compounds emitted from Pp were linalool, limonene, trans-ocimene and 1,8-cineole (80% on average). Eighty percent of total emissions in Qi were β-pinene, α-pinene, myrcene and sabinene, followed by limonene, β-phellandrene, γ-terpinene and trans-ocimene (20%). On average, the standard monoterpene emission rate from Qi was approximately three times higher than from Pp. Diurnal and seasonal emission variations were characterized with regard to temperature and PAR. For both species a statistically significant variation in monoterpene emissions was observed between seasons for 1 yr period. For Pp, the seasonal variability not accounted for by PAR and temperature is also estimated and compared with existing models in the literature.
A dynamic soil enclosure was used to characterise monoterpene emissions from 3 soil depths within a Picea sitchensis (Sitka spruce) forest. In addition, a dynamic branch enclosure was used to provide comparative emissions data from foliage. In all cases, limonene and α-pinene dominated monoterpene soil emissions, whilst camphene, β-pinene and myrcene were also present in significant quantities. α-Phellandrene, 3-carene and α-terpinene were occasionally emitted in quantifiable amounts whilst cymene and cineole, although tentatively identified, were always non-quantifiable. Total daily mean monoterpene emission rates, normalised to 30°C, varied considerably between soil depths from 33.6μgm−2h−1 (range 28.3–38.4) for undisturbed soil, to 13.0μgm−2h−1 (8.97–16.4) with uppermost layer removed, to 199μgm−2h−1 (157–216) with partially decayed layer removed, suggesting that the surface needle litter was the most likely source of soil emissions to the atmosphere. Relative monoterpene ratios did not vary significantly between layers. Foliar monoterpenes exhibited a similar emission profile to soils with the exceptions of camphene and 3-carene whose contributions decreased and increased, respectively. Emission rates from foliage, normalised to 30°C were found to have a daily mean of 625ngg−1 dwh−1 (299–1360). On a land area basis however, total soil emissions were demonstrated to be relatively insignificant to total emissions from the forest ecosystem.
The rate constants for the gas-phase reactions of sabinene and camphene, two monoterpenes emitted from vegetation, with OH and NO3 radicals and O3 have been determined at 296±2 K and one atmosphere total pressure of air. The OH and NO3 radical reaction rate constants were determined using relative rate techniques. Using rate constants of k(OH + isoprene) = 1.01 × 10−10 cm3 molecule−1 s−1, k(NO3 + trans-2-butene) = 3.87 × 10−13 cm3 molecule−1 s−1 and k(NO3 + 2-methyl-2-butene) = 9.33 × 10−12 cm3 molecule−1 s−1, the following OH and NO3 radical reaction rate constants (in cm3 molecule−1 s−1 were obtained: OH radical reaction; sabinene, 1.17 × 10−10 and camphene, 5.33 × 10−11; NO3 radical reaction; sabinene, 1.01 × 10−11, and camphene, 6.54 × 10−13. The absolute O3 reaction rate constants determined were (in cm3 molecule−1 s−1 units): sabinene, 8.07 × 10−17, and camphene, 9.0 × 10−19. These rate constants are compared to literature data for other structural-related alkenes and monoterpenes.