BIPHOREP research proposal

Work content

  1. Leaf level studies
    1. Field and laboratory measurements
    2. Emission algorithm development
  2. Canopy scale studies
    1. Intensive field campaigns
    2. Canopy modelling and forest characterization
  3. Regional scale studies
    1. Ambient air measurements
    2. Regional photochemical and emission modelling
Table 1: Organisation and structure of the BIPHOREP proposal by work packages

1 Leaf level studies

1.1 Field and laboratory measurements

These measurements will provide the baseline of the emission estimates to be derived within this project. Enclosure techniques (cuvette and teflon bag) are the basic means to estimate individual species emission. The dependence of emission densities on temperature, relative humidity and photosynthetically active radiation (PAR) will be assessed via concomitant monitoring of these environmental factors. Such data are urgently needed for the most common European higher plants such as trees, whereas for other plants like lichens and mosses, such information is missing completely. The measurements will be mainly carried out in the field during the three intensive summer campaigns. Lichen emissions will be studied in the laboratory from samples collected regularly over the whole vegetative period. In order to cover a wide range of species, and the variation of the emissions over the vegetative period, individual plant emissions will be studied at various field locations in Sweden and Finland.

The leaf level measurements will cover the dominating plants of the European boreal zone. Special emphasis will be given to tree species classified by Geron et al., (1994) as isoprene emitters: spruce, aspen, willow, and oak. As pointed out by Simpson et al., (1995) many of these trees have different subspecies with distinct emission profiles. For example, in the southern parts of Europe oak belongs to the very low emitting class but the north European oaks are high-isoprene emitters. More importantly, isoprene has recently been identified in the VOC emission pattern of the Norway Spruce which is one of the most widespread species in the boreal forest (Steinbrecher et al., 1994). Since many of the plant species to be studied in this project have not been measured before, this project may bring surprises especially for species such as birch or willow, which have large genetic variations inside the genera.

The partition of the work between the participants will be the following:

- FMI measures branch emissions both during the intensive campaigns and during off- campaign periods, using a chamber system identical to that of MISU (Janson, 1993). An automated GC is used for terpene analysis and light hydrocarbons are analysed from canister samples. A GC/MS is used for the compound identification. The main topic will be the dependence of emissions on phenology, in addition to the dependence on light and temperature.

- During the campaigns, IFU measures the biogenic emissions (C5 to C10 hydrocarbons) using leaf and branch enclosure techniques (Steinbrecher, 1994). The time resolution of the VOC concentration measurements by the automated GC is 30 min. Transpiration, net CO2 assimilation, and leaf conductance are determined with a time resolution of 10 s. The focus of these studies will be the characterization of the BVOC emissions and the elucidation of the main environmental factors and physiological processes linked to their synthesis.

- MPI measures the emissions from lichen samples provided by UJ. The emissions of short chained carboxylic acids and aldehydes will be measured by Dr. Kesselmeier, and higher VOCs with the help of Dr. Ciccioli (CNR-Roma) for some selected experiments. For the measurements of organic acids, a well established procedure including cryogenic trapping and ion chromatography analysis is used. The dependence of the emissions on the physiological parameters such as photosynthesis and evapotranspiration will be documented.

- MISU studies the ground vegetation emissions from both wetland (mosses and grasses) and forest floor species, using static chambers of Teflon film (Janson, 1993). The investigated tree species will be pine and spruce. Measurements will cover the main Swedish biotopes, and give the desirable geographical extent and variability of species and genera studied (see Fig. 2). The VOCs to be determined include C2-C5 by canister sampling, C6-C10 by TENAX traps, and carbonyls by DNPH traps. Organic acid emissions are measured by impregnated filters and IC.

- UJ provides the phenological information needed for the field emission measurements. UJ also collects and prepares the lichen samples for analysis by MPI in Mainz, and provides relevant information on their periods of activity and their water content.

The time resolution of all sampling and analysis systems is adequate (5 min - 2 hours, depending on the system) to study the relationships between emission intensities and environmental parameters. Intercalibration of the measurement systems will be done during the intensive campaigns. FMI, IFU, and MISU also participate in the IGBP/IGAC VOC intercalibration activity.

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1.2 Emission algorithm development

The biogenic VOC emissions by plants are controlled by both physiological and physical processes. The most important environmental factors for isoprene emissions are the temperature T and the amount of photosynthetically active radiation PAR (Guenther et al., 1993). For monoterpenes, the short-term emission variations are assumed to depend on the vapor pressure of the monoterpene, and thus, to be controlled by the temperature. On the other hand, not much is known about the factors controlling the long-term variation of emissions. At present, there is no emission algorithm available for other light hydrocarbons than isoprene and monoterpenes.

T and PAR are the only variables that have been explicitly included in the biogenic VOC emissions algorithms (Guenther et al., 1993; Lamb et al., 1993). The current algorithms are typically only applicable to the description of seasonal or climatological variation of the emissions, and there is an urgent need for more accurate biogenic emission rate estimates for detailed numerical modeling of regional atmospheric chemistry and transport. Emission estimates are required for specific plants and vegetation types as well as for each hydrocarbon.

The field emission measurements by FMI, IFU and MISU will provide data for selected tree and plant species, which will be used for the development and testing of genera specific emission algorithms (T, PAR, relative humidity (RH) - dependency) starting from the algorithm developed by Guenther et al. (1993). Very importantly, this project will provide data obtained at lower temperatures (5-25 C), typical of boreal regions. This is essential since significant reductions are expected in emissions with decreasing temperatures. Existing emission measurements usually refer only to temperatures around or in excess of 30 C which places severe restrictions on the applicability of emission algorithms based on those data in boreal regions. The measurements of MISU and FMI will also cover early and late summer, to resolve the posssible phenological dependence of emissions.

The laboratory measurements performed by MPI and the field campaigns will provide necessary temperature and light dependency data for the development of emission algorithms for lichens and other lower plants - such algorithms are currently not available for biogenic VOC emission modeling. The MPI will also study the emission dependencies on photosynthesis and evapotranspiration thus allowing the possible inclusion of these physiological parameters in the emission algorithms.

The development and parameterization of the plant specific and species specific emission algorithms will be the responsibility of each partner involved in the leaf level work package, depending on their designated plant species. To ensure the compatibility of the modeling approaches, this work will be coordinated by MPI.

The species/plant specific and compound specific emission factors calculated using the algorithms developed in this study will be combined with the detailed forest information of the sample plots of the Finnish Forest Research Institute, provided by UJ, to generate landuse average biogenic emission factors which can then be integrated into the canopy scale models and/or used in the regional emission and photochemical modeling. Correct emission algorithms are essential when regional scale emissions are calculated using the ambient air measurements coupled with a photochemical model, and for the photochemical modeling study (see 3.2).

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2 Canopy scale studies

2.1 Intensive field campaigns

(July 1996 Pallas, July 1997 Mekrijärvi, Summer 1998 NOPEX-site, vicinity of Uppsala)

The purpose of the micrometeorological campaigns is to measure biogenic emissions in a natural environment including all components of the ecosystem. This will test how well the emissions calculated starting from the leaf level compare to the emissions measured at canopy scale. The micrometeorological emission flux data, together with the measured water vapour and carbon dioxide fluxes, radiation intensity and temperature will be used to develop a canopy scale emission algorithm. The data gathered during these campaigns will be ideal for the testing of chemical transformation because a wide range of chemical species including carbonyls will be measured. These measurements will also be used to test the effects of chemical transformation on the vertical concentration profiles in the surface layer (Johansson and Janson, 1993).

The first campaign will take place in Lapland (Pallas, 68 N, see Fig. 2) in the northern boreal zone. The second campaign will be carried out in Eastern Finland (Mekrijärvi, 63 N) near the Russian border. This area is between the southern and middle boreal zone. The site of the third campaign will be decided based on the results of the two previous campaigns and to fill the major needs of data detected during the project. The most probable area is the southern hemiboreal zone in Sweden (NOPEX-site, 60 N) but a spruce dominated region in Finland (e.g. Pallas) may also be considered. The season of the campaign will be decided based on the measurements done over the entire vegetative season.

VOC fluxes will be measured using the gradient method which relates the fluxes to the concentration differences (Fowler and Duyzer, 1989). Other required parameters (e.g. friction velocity, roughness length) will be obtained from the accompanying eddy correlation measurements. The basic experimental set up designed for the campaigns is very mobile which gives the possibility to select a site as suitable as possible. The system is not dependent on getting electricity from mains. The core of the sampling system are two automated GC:s, one (IFU) sampling air just above the canopy, and the other (FMI) lifted to a height of 40-50 m by a crane lifter. The sampling heights will be optimised according to Garratt (1992) and Schuepp et al. (1990) to have optimum gradient and fetch. The two GC:s will be intercalibrated at the start of the campaign, and at regular intervals thereafter. At both levels, FMI will measure momentum, heat, water vapour, carbon dioxide, and ozone fluxes directly by the eddy correlation method.

In order to widen the range of chemical species covered, FMI will take canister samples and MISU will collect TENAX-tube, and DNPH-trap samples and samples for organic acid analysis. This will take place at both levels to get concentration gradients. The sampling resolution of these measurements will always be high enough (usually 30 min) to resolve the biological and environmental functioning of the ecosystem. These data will also be very valuable to the study of chemical transformations and the comparison of analytical accuracies, and essential to cover the whole range of compounds. Intercomparison of the analytical methods will be done following the procedure of Steinbrecher et al. (1994).

The gradient method requires a high accuracy and sensitivity for measuring the concentration gradients (Businger, 1986). Based on previous measurements carried out by FMI in the study area, the VOC concentration levels are in the sub-ppb range. Therefore, the campaigns will be conducted during the warmest months of summer, when the concentrations are expected to be the highest. Analytical comparisons and cross-checks between the instruments will be done during the campaigns. Since the momentum, heat, water vapour, carbon dioxide, and ozone fluxes will be measured directly by eddy correlation at both levels, we will have continuous and detailed information on the biological and microclimatological functioning of the canopy. This will allow us to trace the biogenic VOC species and to calculate their fluxes in relation with other biological and physical processes.

Additionally, IFU and FMI will endeavour VOC flux measurements by the relaxed eddy accumulation method, which is a method "half-way" to direct eddy-correlation measurements. In this method, utilizing a combination of a fast response sonic anemometer and sampling valves, up and down drafts of air are collected separately so that the chemical analysis can be done with low time resolution (Majevski et al., 1993). The fluxes are calculated from the concentration differences in the up and down drafts and turbulent data. This method is not yet a standard one and needs technical testing.

UJ will provide assistance in conducting the field campaigns. UJ is also responsible for the gathering and analysis of the forest data of the experimental sites.

FMI has good experience using crane lifters as flux measurement platforms for the eddy- correlation measurements. The set-up is very mobile and there are no safety threats for the personnel. All the necessary instruments can be lifted to the desired measurement height. Supporting measurements by FMI will include also nitrogen oxides concentrations, ozone concentration, and the photolysis rate of nitrogen dioxide. Concentrations of nitrogen oxides are the basic information needed for the evaluation of the photochemistry.

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2.2 Canopy modeling and forest characterization

Accurate emission estimates require correct information on forest and forest foliar biomass characterisation as well as estimates of the physical environmental conditions inside the forest. For the forest characterization, UJ will utilize the forest and biomass data from the permanent sample plot network of over 1000 sites of the Finnish Forest Research Institute. This database includes the distribution of individual species and their sizes.

Our emission calculations will be performed using this network of sites having detailed information of the canopy. More importantly, the generalization of these data will be carried out in order to get typical boreal canopy structures of the subzones of the European boreal region. These data will be then compiled so that the standard boreal forest canopies can be used as default canopies if only very rough information is available, as might be the case, for Bexample, with the Russian boreal territory. In addition, these forest data will be compared to the gridded landuse data from the LANDSAT/GIS which is available at FMI. Over all, our purpose is to produce forest data base as comprehensive and detailed as possible based on the currently available information but for a wider use. Thus, the forest data will be expanded to cover larger areas (including Russia) using either existing maps (with forest cover, type etc.) or digitized materials when practical. The forest characterization task will be the responsibility of UJ.

UJ will provide the partners with the information of the most common lichens and especially the lichen information needed by MPI for characterizing lichen emissions. For characterizing the lichen emissions, lichen data is available from the National Forest Inventories of the Finnish Forest Research Institute.

As biogenic emissions are driven by the temperature and sunlight conditions affecting each emitting leaf, it is essential to know how these parameters vary within the forest canopy. In canopy models, the photosynthetically active radiation within the canopy (PAR) is described as a function of the measured above-canopy PAR and the leaf area index (LAI) at each vertical level of the canopy layer (e.g. Guenther et al., 1993, Lamb et al., 1993, Geron et al., 1994). An accurate representation of the reduction of the PAR at the lower levels of the canopy is especially important for isoprene, since isoprene emissions are strongly controlled by PAR. The leaf area index (the fraction of the projected leaf area of the ground area) must be estimated specifically for each species. It also depends on plant phenology, which may affect the seasonal emission patterns. In addition to the environmental parameters, the plant biomass distribution within the canopy must be known for the emission calculations.

UJ will develop its present canopy models (Kellomäki, 1995; Kellomäki and Kolström, 1992) into a practical model which is easy to use but accurate enough to calculate physical parameters (e.g. temperature, PAR, relative humidity) inside the canopy. These parameters are needed in the model to properly calculate the biogenic VOC emissions since they are strongly dependent on the environmental conditions. The model will accept various levels of initial forest information:

  1. no data, boreal forest default parameters,
  2. county or country average stem volume and species distribution known,
  3. Landsat/GIS-type information available,
  4. detailed information (Finnish Forest Research Institute-plot) available.
The canopy model developed in this work package can be run using either continuous time series or monthly averages as meteorological input data.

The canopy modeling task will be coordinated by UJ but IFU and FMI will be in close cooperation with UJ in developing the landscape average emission factors. The emission factors will be constructed using the detailed canopy model and the forest characterization data, and utilizing the plant specific and species specific emission factors obtained from the leaf level work package. The landscape average emission factors are an essential link when upscaling the smaller scale emission results for the regional scale emission modeling and photochemical modeling tasks of the regional scale work package.

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3 Regional scale studies

3.1 Ambient air measurements

Boreal regions are characterised by a strong seasonal cycle of the biological activity. In the southern parts of the boreal zone, the growing season starts at the end of April, and in the northern parts in June. Photochemistry is mainly active from March to October. Ozone concentrations are the highest during spring in the northern boreal zone but during summer in the southern boreal zone. To properly understand the influence of biogenic sources on ozone formation, we need VOC measurements covering the April-September period. From ambient air concentration measurements alone we can qualitatively estimate the seasonal cycle of emissions.

For example, isoprene has been observed in ambient air at the Pallas WMO/GAW station in Northern Finland from the beginning of June until October. The growing season starts there in the beginning of June. Isoprene is typically emitted by higher plants as a by-product of photosynthesis. The emission mechanisms of many other chemical compounds, such as terpenes and other alkenes than isoprene, are not so directly and instantaneously related to photosynthesis. For example, 1-butene is observed at Pallas from April to October and the temperature dependence of the 1-butene concentrations is quite different of that of isoprene. To our knowledge, there are no ambient air measurements of terpenes covering the whole April-October period to qualitatively estimate the seasonal cycle of emissions.

The seasonal cycle of concentrations in the boreal region will be measured by sampling air at two elevated sites representing average boundary layer air. Samples (canister, TENAX, DNPH) will be taken twice a week at the Pallas station in northern Finland and at a site in eastern Finland and analysed in the laboratory. FMI will arrange for the sampling at the Finnish Stations. MISU will analyse terpenes from the TENAX-samples and carbonyls from the DNPH cartridges. FMI will analyse C2-C6 hydrocarbons from the canister samples.

The measured concentrations will be studied with respect to the growing season, temperature, and solar radiation. VOC concentrations of anthropogenic and potentially biogenic species and their photochemical ozone forming potential will be compared as in Laurila and Hakola (1995) using the method presented by Chameides et al. (1992). Thus, we can estimate the contribution of individual species to the local photochemistry. Preliminary results have shown that in summer most of the local VOC reactivity is due to biogenic species. Furthermore, it will be very interesting to compare the concentrations of carbonyls, which are oxidation products of the measured hydrocarbons, to their parent species (Montzka et al., 1993, Hakola et al., 1994). This will give much new information of the roles of the biogenic versus anthropogenic species with respect to the photochemical cycle.

The second main result of the seasonal ambient air measurements is to provide air concentrations for the modelling task of this work package. The emissions at regional level, for example, will be inversely estimated using a combination of an emission model and a photochemical model (see 2.3.2). In Europe, there are no datasets available which cover the light hydrocarbons, terpenes and carbonyls over the whole photochemical season (April - September) at representative sites. Thus, our results will provide a much needed database for the evaluation of photochemical models and for studies on the contribution of biogenic species in photochemistry.

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3.2 Regional photochemical and emission modeling

The field and laboratory measurements carried out in this project will create a unique database of biogenic VOC, ozone and NOx concentrations, together with environmental and meteoro- logical parameters, which can be used in the development and testing of local, regional, and larger scale atmospheric chemistry models. The inclusion of several reaction products of e.g. isoprene (such as carbonyls and organic acids) is particularly valuable for the evaluation of hydrocarbon oxidation mechanisms included in photochemical model packages. The intensive campaign measurements, performed within the boundary layer, with simultaneous meteoro- logical observations, will allow the study of hydrocarbon profiles over the forest canopy and their diurnal cycle. This information is needed for the quantification of the sink processes and their relative importance (e.g. how the chemical lifetimes are affected by turbulent mixing the boundary layer).

IFU and FMI will use the field data gathered in this project to develop and test BVOC oxida- tion schemes, and to perform sensitivity and uncertainty studies of chemical modules (i.a. RADM, CIT, the Harwell photochemical model). Special emphasis will be given to the reaction pathways of isoprene and terpenes, which are expected to be the dominating BVOCs emitted in the boreal region. The BVOC emissions also play a role in photochemical ozone formation, and the models will be used to estimate the relative importance of biogenic and anthropogenic VOC emissions for the ozone levels in northern Europe. Especially, situations of NOx rich air carried from the more polluted areas of Central Europe to the boreal forested regions will be investigated with respect to their potential for ozone formation. Under these conditions, it is possible that the full ozone forming potential of the BVOC emissions could be realised, resulting in local or regional peak ozone concentrations. This work will thus add to the knowledge of the environmental effects that European air pollutants may cause in more remote areas. The incremental reactivities of the individual BVOCs will be studied using the photochemical modules and by comparing reactivity scaled biogenic VOC concentrations to those of anthropogenic species. IFU will also model the vertical distribution of the BVOC, ozone and nitrogen compounds and study the effect of atmospheric stability on the chemical sink processes.

The RADM and CIT chemical models, and the Harwell photochemical package are all well documented and widely used in Europe and in other countries (e.g. Stockwell, 1994; Stockwell et al., 1990; Derwent and Jenkin, 1991). The RADM and CIT models are based on the grid method, while the Harwell photochemical package is typically combined with predetermined air mass trajectories over an emission field. The strength of the Eulerian gridded approach is the more accurate representation of the atmospheric advection and diffusion, whereas, with current computing techniques, the Lagrangian trajectory models can include more detailed chemical modules. Thus, the Harwell trajectory model and the CIT model are complementary approaches, and by comparing the results of both models it is possible to get an estimate of the magnitude of the modelling uncertainty. This comparison of the model results will be especially relevant with respect to the photochemical reaction mechanisms, as the RADM model incorporates a lumped organic chemistry scheme, while in the Harwell model the hydrocarbon oxidation pathways and the reaction intermediates are represented in great detail. This work will thus bring new information to the modellers worldwide on the reaction pathways, removal mechanisms and effects of the hitherto less well captured BVOC compounds.

As a separate modelling task, FMI will use the Harwell photochemical model in combination with the emission algorithms and the landscape average emission factors to make an assessment of the regional VOC emissions. For this study, the measurements of the seasonal cycle of VOCs, gathered at regular intervals at elevated sites (above the tree line, but within the boundary layer), will be used. This study is also an independent validation of the emission algorithms and the emission models developed in the leaf level and canopy scale work packages of the BIPHOREP project. It will also utilize the results of the sensitivity studies of the photochemical packages and the terpene and isoprene oxidation schemes developed in the first part of this modeling task.

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