Volatile organic compounds (VOCs) emitted from vegetation play a central role in photochemical ozone formation together with NOx from anthropogenic sources (e.g. Trainer et al, 1991). Consequently, they may largely offsett the benefits of VOC emission control strategies and thus will lead to an inadequate and uneconomical environmental policy. The ozone control strategies in the U.S.A. have largely failed due to the disregard of biogenic VOCs, and there is a strong need to re-evaluate the anthropogenic VOC emission abatement strategies (National Research Council, 1991). In Europe, the assessment of cost-effective control measures for VOCs and nitrogen oxides in order to cut down ozone levels is an ongoing process in the framework of the Convention on Long-range Transboundary Air Pollution under the auspice of the United Nations Economic Commission for Europe (UNECE).
VOCs are made up of many different compounds with widely varying stability, reactivity (and ozone formation potential) and concentrations. The multitude of VOC sources, the shortcomings of their measuring techniques, and the gaps in our knowledge of factors controlling the VOC source strengths make the present day emission inventories of biogenic VOCs very uncertain. Simpson et al. (1995) estimated that uncertainties in the emissions of the best known species, isoprene, are easily 500%. Globally it has been estimated that natural VOC emissions from land surfaces with about 1000 TgC per annum are one order of magnitude larger than anthropogenic emissions (IPCC, 1990). Thus, it is of a paramount importance to estimate more accurately the VOC emissions, to identify their main components, how they depend on biological activity and environmental factors, and how much they contribute to ozone formation (CEC,1993).
The boreal forest is one of the world's major vegetation regions, forming a continuous belt around the whole northern hemisphere and occupying 12 M km2, of which more than half are situated in Scandinavia and the Russian Federation. Today the average forest cover of Europe is one third of all land surfaces but varying considerably between countries - from 6% in Ireland to 66% in Finland. Boreal coniferous woodlands are characterized by trees such as Norway Spruce (Picea abies), Scots Pine (Pinus sylvestris) and Downy Birch (Betula pubescens). Boreal forests are rich in mosses and lichens but poor in vascular plants (about 250 species). The boreal regions of Europe are sparsely populated and according to existing emission inventories (Simpson, 1994) biogenic VOC emissions surpass largely anthropogenic ones. Both local regional emissions and long-range transport of nitrogen oxides into the region from the densely populated parts of Europe provide the fuel needed for photochemical production of ozone. The prevailing southwesterly winds ensure that is an ubiquous phenomenon in northern Europe (eg. Hakola et al, 1991). Consequently, for instance, elevated ozone concentrations exceeding the vegetation and health protection threshold set by the European Community (directive 92/72/EEC) have been widely observed in Finland (Laurila and Lättilä, 1994), and elsewhere in northwestern Europe (Beck and Grennfelt, 1993).
At the global scale, the extent of the boreal regions make them a very important source of reactive organic trace species to the atmosphere. However, a better knowledge of BVOC emissions is needed for global scale photochemical modelling of trace species, to close the carbon balance and to estimate the source of organic acids which contribute to acid deposition (Fehsenfeld et al., 1992). Moreover, it has been estimated that ecosystems at high northern latitudes will be severely influenced by global warming causing changes in the distribution, functioning and composition of ecosystem and thus in VOC emissions. Thus, a better knowledge on the relationship between BVOC emission and ecosytem functioning is vital for predicting future trends.
In order to have harmonized and intercomparable emission models and emission inventories, it is vital that the various measurements are performed in parallel and the different techniques are intercomparable. This work will be done on different scales (leaf level, canopy level, regional level) throughout the BIPHOREP project. It is thus one of the major objectives of this project to investigate how measurements and estimates carried out at various scales compare to each other and can be aggregated together for the benefit of models and of European or global inventories.
This project will measure emissions using enclosure techniques (IFU, MISU, FMI, and MPI) and micrometeorological methods (IFU, MISU and FMI), and determine their dependence on the environmental factors. Combined with modeling, these measurements will yield plant specific, species specific, and canopy scale emission factors, which can be further integrated with detailed landuse information (UJ) into landscape average emission factors.
In this project, biogenic VOC emissions will also be assessed using inverse emission and photochemical modeling (FMI), together with ambient air measurements (MISU, FMI). This inverse emission assessment will draw on the results of the smaller scale emission studies, and simultanously serve as an independent validation of the leaf level and canopy scale emission models and their upscaling.
Existing emission profiles based on North American studies cannot be representative of European or North European conditions. European studies of biogenic emissions have been carried out within the BIATEX subproject of EUROTRAC (partly CEC funded) and the CEC funded project BEMA. The former has mainly addressed subalpine vegetation species while the latter concentrated on mediterranean species. In those regions, however, the anthropogenic emissions are relatively more significant than they are in the sparsely populated areas of northern and northeastern Eurasia. Also the STEP project Measurement of Biogenic Hydrocarbon Emission from Vegetation Representative for European Ecosystems has briefly discussed the boreal region.
Measurements of VOC emission factors from European boreal regions are nearly absent. Only Janson (1992, 1993) has performed monoterpene emission rate measurements from coniferous trees. High isoprene emission densities from spruce in the Alps, measured recently by Steinbrecher and Rabong (1994), makes it potentially the most important species also in northern Eurasia. However, we are bound to face new unsuspected results since up to now very few species have been measured. The existing methodologies for emission inventories cover isoprene and few monoterpenes. Our goal is to quantify also other light hydrocarbons than isoprene, monoterpenes and carbonyls. Recently, direct emissions of organic acid from plants have also been observed (Kesselmeier et al., 1994). The dependence of emission densities on environmental conditions (light and temperature) is presently known only for isoprene and some monoterpenes and a limited number of plants.
To achieve this objective we have to make forest biomass characterisations and use canopy modelling. Measurements of emission factors of individual species are needed for calculating emission factors and emission algorithms on the basis of the main controlling factors. Emissions must be calculated at canopy and regional levels, and validation of the calculated emissions must be done by measurements of fluxes using micrometeorological methods, and photochemical modelling. All emission factors and estimates, for both deciduous and coniferous species, will be presented as a function of climatological and phenological parameters.
Usually emission inventories cover only trees. Preliminary studies in Canada (Klinger et al., 1994) and in Sweden (Janson, unpublished results) have indicated that ground cover flora and lichens may emit substantial amounts of VOCs. In boreal forests, a relatively large share of the biomass consists of other species than trees. Since algae and lichens do not have stomates and cuticles, their emission mechanism is very different from that of higher plants. Measurements of these components will be conducted in this project at leaf level and they will be included in the canopy and regional scale emission flux measurements. Studies at these scales have not been done in the Eurasian boreal region. Using the detailed characterization of the forest sites provided by partner UJ, it will be possible to validate the canopy model used for upscaling the leaf level emission measurements.
Published European biogenic emission inventories use country average forest characterisation and very rough canopy biomass parameters (Simpson et al., 1995). Forest characterisation will be enhanced substantially by combining extensive satellite landuse data from the LANDSAT dataset with detailed forest information from the ground based forest inventory of the Finnish Forest Research Institute. These data are also needed for the canopy modelling (Kellom„ki, 1995).
By providing a clear relationship between emission factors and phenological and climato- logical parameters it will be possible to extrapolate these emission distributions from Scandinavia to the largest boreal forest area of the globe, namely in Russia and Siberia (see Fig. 1.). Our objectives can thus be considered as a contribution to the goals of the core-project IGAC of the IGBP Programme, especially as to its subproject GEIA (Global Emissions Inventory Activity), and HESS (High-latitude Ecosystems as Sources and Sinks of Trace Gases).
This project will complete existing European studies of biogenic emissions carried out within EUROTRAC/BIATEX, BEMA, and STEP. However, these studies do not cover the northern region of Europe and our project BIPHOREP will thus complement them by the addition of the northern part of a north-south transect as planned in the TERI-initiative.
In Europe, the average ozone concentration at ground level has risen from 10 ppb at the end of the last century to about 25-30 ppb today (eg. Volz and Kley, 1988). Regional episodes of ozone in Europe are superimposed on these higher background levels, with the result that higher episodic levels of ozone occur, aggravating ozone's adverse impacts on human health and vegetation. As explained above, BVOC emissions may have a significant role in this excess ozone formation that leads to changes in the overall oxidizing capacity of the troposphere.
On the other hand, when NOx concentrations are very low, the biogenic hydrocarbons can decrease ozone concentrations. Therefore, model studies are needed in various NOx environ- ments, to find out the role BVOCs have in the ozone budget, when the composition of the BVOCs is known.
Another crucial question is the consequences for the hydroxyl radical OH, the key to the self- cleaning ability of the atmosphere. Since the competition for the OH radical between the different VOCs will depend on the local conditions, it is necessary to study the different reaction pathways with a chemical model including all relevant parameters. Thus, the determination of BVOC emissions for northern Europe together with ambient concentration measurements of the key compounds will constitute an important step towards the understanding of the fate of BVOCs in the atmosphere.
Concomitant measurements of VOCs, ozone, and nitrogen oxides backed with measurements of the photolysis rates J(NO2) and J(O3) allow to investigate the photochemical balance prevailing under different meteorological and chemical (air mass properties) conditions.
The BIPHOREP partners have a selection of models available for the comparison and further development of the BVOC oxidation schemes and for the assessment of the BVOC contribution in photochemical ozone formation in northern Europe. One appropriate and tested model is the Harwell photochemical model of Derwent and Jenkin (1991) with extensive chemistry that the FMI has already implemented, updated with new emission data for Scandinavia and run under different scenarios. This model covers all Europe and utilizes the EMEP emission fields. IFU has the gridded RADM and CIT models, which also include photochemical reaction schemes and are applicable throughout Europe. Through further model developments, based on the findings of the BIPHOREP project (e.g. BVOC emission rates), we will be able to calculate comparatively the ozone formation from both anthropogenic and biogenic emissions, and make an assessment of the environmental effects of pollutants carried from the densely populated areas of Europe into more remote regions.