5. AEROSOL AND ENVIRONMENTAL PHYSICS LABORATORY

(www.atm.helsinki.fi/)

 

5.1. PREFACE

 

In 1999 the research activity of the Laboratory of Aerosol and Environmental Physics has focused on basic and applied aerosol science, cloud microphysics and forest-atmosphere interactions. Studies on heat and mass transfer, nucleation, condensation, aerosol dynamics, aerosol measurement technique, atmospheric aerosols, deposition and fluxes of atmospheric gases, and formation and growth of cloud droplets were performed. The main aim of the studies is to develop practical applications, based on mastering fundamental physical and chemical phenomena, to solve different aerosol and environment-related problems.

Formation and growth of atmospheric aerosol particles and cloud droplets have been studied performing field measurements and using computer models developed in the laboratory. Other topics of theoretical and numerical investigations were heat and mass transfer as well as nucleation processes. Different atmospheric nucleation routes have been proposed.

The field station SMEAR II (in Hyytiälä) was constructed during 1995. Continuous measurement activity started in 1996. The main finding so far has been numerous observations of atmospheric nucleation bursts. Also aerosol and gas fluxes have been investigated (see highlights of research). At our field station two EU-founded projects BIOFOR (aerosol formation) and EUROFLUX (CO2 fluxes) were performed during 1999. Participation in other field campaigns during EU-founded project PARFORCE (aerosol formation at coastal site, Mace Head, Ireland) has enhanced our experience on atmospheric particle formation, growth and hygroscopic properties.

The main output of our experimental laboratory work has been the development of tools for investigating nucleation phenomena and cloud condensation nucleus activation. We have also carried out measurements of aerosol particle size distributions in a variety of laboratory systems as well as in atmospheric conditions. Our special interest has been targeted on the nanometer size range using recently developed aerosol instrumentation such as electrical mobility spectrometry and the diffusion battery technique, whereas for micron-sized particles optical counting of particles is typically used.

International co-operation has had a significant role in both the theoretical and the experimental activities of the group. During 1999 various projects (including four EU projects) continued in co-operation with research groups from Austria, Canada, The Czech Republic, Estonia, Germany, Ireland, Italy, Japan, The Netherlands, Russia, Sweden, The United Kingdom and The United States. On the national level we have had close collaboration especially with the department of Forest Ecology in the University of Helsinki, the department of Chemistry in the University of Helsinki, and with the Air Quality Department of the Finnish Meteorological Institute.

The international postgraduate training programme for aerosol and environmental physics (started at the beginning of the autumn semester 1994) was continued during 1999.

Financial support from the Academy of Finland, The Environment and Climate Research Programme (The European Commission), the Nessling Foundation and TEKES is gratefully acknowledged.

Markku Kulmala

 

5.2. ATMOSPHERIC AEROSOLS

 

MODELLING OF FORMATION AND GROWTH OF ATMOSPHERIC AEROSOL PARTICLES

Liisa Pirjola, Markku Kulmala, Ari Laaksonen* and Lauri Laakso

Sectional models (AEROFOR and AEROFOR2) for the formation of sulphuric acid - water particles has been developed. The model includes also ternary nucleation mechanism water-sulphuric acid - ammonia.The model includes gas-phase chemistry and aerosol dynamics. The model has been applied to evaluate aerosol formation at different environmental conditions such as polluted plumes near Kola Peninsula, upper tropospheric aerosols, coastal environment and boreal forest site. Besides that we have studied the effect of terpene chemistry on aerosol formation, and the effect of ions on aerosol formation.

E.g. we have studied how an increased UV-B irradiation due to stratospheric ozone depletion causes via the SO2 oxidation route an enhanced nucleation potential for new H2SO4-H2O particles as well as the growth of particles to CCN size. Using AEROFOR we show that after a nucleation event the nucleated particle concen-tration is linearly dependent on increased UV-B irradiation with a positive slope. On the other hand, due to increased CO2 concentration photosynthetic rates of plants will increase, and it is likely that enhanced photosynthesis in forests will increase emissions of biogenic volatile organic compounds (BVOC) such as isoprene and monoterpenes. We show that the nucleated particle concentration decreases with increasing BVOC emission, but this dependence is not linear.

* University of Kuopio

 

BIOGENIC AEROSOL FORMATION IN THE BOREAL FOREST (BIOFOR)

Markku Kulmala, Kaarle Hämeri, Jyrki M. Mäkelä, Pasi P. Aalto, Liisa Pirjola, Minna Väkevä, E. Douglas Nilsson, Ismo K. Koponen, Gintautas Buzorius, Petri Keronen, Üllar Rannik, Lauri Laakso, Timo Vesala, Keith Bigg, W. Seidl 1, R. Forkel 1, T. Hoffmann 2, J. Spanke 2, R. Jansson 3, M. Shimmo 3, H.-C. Hansson 3, C. O'Dowd 4,*, E. Becker 4, J. Paatero 5, K. Teinilä 5, R. Hillamo 5, Y. Viisanen 5, A. Laaksonen 6, E. Swietlicki 7, J. Salm 8, P. Hari 9, N. Altimir 9 and R. Weber 10

Aerosol formation and subsequent particle growth in the ambient air have been frequently observed at the boreal forest site (SMEAR II station), Southern Finland. The EU funded project BIOFOR (Biogenic aerosol formation in the boreal forest) has focused on a) to determine formation mechanisms of aerosol particles in the boreal forest site, and b) to verify emissions of secondary organic aerosols from the boreal forest site, and to quantify the amount of condensable vapours produced in photochemical reactions of biogenic volatile organic compounds (BVOC) leading to aerosol formation.

The BIOFOR project consists of the following scientific tasks:

Task 1: Continuous measurements of ultrafine aerosol particle concentrations, their vertical net flux and relevant background data (local meteorology, micrometeorology, vertical profiles of inorganic gases) in the SMEAR II station. Task 1 has produced the basic long-term data on aerosol size distributions and their vertical fluxes accompanied by required meteorological data like temperature, relative humidity, solar radiation and the strength of turbulence. In addition, the state of vegetation (level of photosynthesis) and soil (temperature, water content and bacterial and mycorrhizal activity) was determined.

Task 2:

Three campaigns on aerosol characteristics (aerosol chemistry and hygroscopic properties) and concentrations of organic gases have been performed (14.4.-22.5.1998, 27.7.-21.8.1998 and 15.3.-30.4.1999). The vertical profiles have been determined and the meteorological conditions have been investigated using radiosoundings, sound detection and ranging (SODAR), surface weather observations and back trajectories. Also the vertical profiles of aerosol size distributions and nucleation mode concentrations have been measured. Task 2 has also included the preparation of measurements, calibration of instruments, data evaluation and data delivery.

Task 3:

The data evaluation, model development and comparison of evaluated data and models have been performed. The modeling work has been divided into four work packages: a) atmospheric chemistry models, b) nucleation models (ternary nucleation, ion induced nucleation) c) Lagrangian model along trajectories with aerosol dynamics and gas phase chemistry, and d) boundary layer dynamics model with aerosol dynamics and gas phase chemistry. Also the linkage to biological activity as a level of organic emissions is formed.

The main results of the project can be summarised: i) The most probable formation mechanism is ternary nucleation (water - sulphuric acid - ammonia) and the growth to observable sizes is mainly due to condensation of organic vapours. However, there is not a direct proof of this phenomena since it is impossible using the present state-of-art instrumentation to determine the composition of 1-5 nm size particles. ii) If the nucleation takes place it always occurs in cold air advection in polar and Arctic air masses at low cloudiness and the nucleation is closely connected to the onset of strong turbulence, convection and entrainment in the morning-noon transition from a stable to an unstable stratified boundary layer. iii) The emissions rates for several gaseous compounds have been verified. The model calculations show that the amount of the condensable vapour needed for observed growth of aerosol particles is 2-10 x 107 cm-3. The estimations for source rate gives 7.5-11 x 104 cm-3s-1.

1 Fraunhofer-Institute for Atmospheric Environmental Research, Deutschland

2 Institute of Spectrochemistry and Applied Spectroscopy (ISAS), Dortmund

3 Air Pollution Laboratory, Institute of Applied Environmental Research, Stockholm Univ.

4 Univ. Sunderland, Centre for Marine and Atmospheric Sciences, School of the Environment

* Present address, Department of Physics, Univ. Helsinki

5 Finnish Meteorological Institute (FMI), Air Quality Research

6 Department of Applied Physics, Univ. Kuopio

7 Division of Nuclear Physics, Lund Univ.

8 Institute of Environmental Physics, Univ. Tartu

9 Department of Forest Ecology, Univ. Helsinki

10 School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA

 

NEW PARTICLE FORMATION AND FATE IN THE COASTAL ENVIRONMENT (PARFORCE)

C.D. O'Dowd 1,2, E. Becker 2, C. Hoell 2, J.M. Mäkelä 1, M. Kulmala 1, K. Hämeri 1, M. Väkevä 1, L. Pirjola 1, P. Aalto 1, H.-C. Hansson 3, J. Strom 3, S.G. Jennings 4, M. Geever 4, G. de Leeuw 5, G. Kunz 5, H. Berresheim 6, C.N. Hewitt 7, J. Sartin 7, R.M Harrison 8, A.G. Allen 8, Y. Viisanen 9, P. Korhonen 9, S. Rapsomanikis 10 and T. Hoffman 11

Elevated aerosol particle concentrations have been observed for some time in the coastal environment and have been associated with the formation of new particles from the gas phase. It has been illustrated that these events occurred regularly in the coastal atmosphere and that they were associated with the occurrence of low tide and solar irradiation. Additionally, they highlighted a possible link with the OH radical, suggesting the photochemical production of one or more of the aerosol precursor species. Following these initial reports, the PARFORCE programme, funded by the EU, was enabled. The objectives of this investigation were to elucidate and understand the processes and conditions which promote and control homogeneous heteromolecular nucleation in the coastal boundary layer. Primary objectives were:

(1) Determine the conditions and the rates under which homogeneous heteromolecular nucleation occurs in the coastal boundary layer (i.e. pre-existing aerosol surface area; precursor gas concentration; micro- and macro-scale meteorology). (2) Examine whether these nucleation events can be explained by binary or ternary heteromolecular nucleation of the following schemes: H2SO4-H2O or NH3-H2SO4-H2O or NH3-H2O-MSA/HCl/HNO3, or whether alternative nucleation schemes are likely to explain the observed events i.e. organic embryo formation followed by organic and/or sulphate growth. (3) Examine the influence of anthropogenic/continental air parcel mixing on coastal nucleation. (4) Explore the growth rates of newly-formed particles. (5) Examine the long-term frequency of occurrence of coastal nucleation bursts and their duration. Two intensive field campaigns were conducted at Mace Head during 1998 and 1999 supported by continuous measurements of ultra-fine particle concentration for a two year period.

The primary results from PARFORCE are briefly summarised here: coastal nucleation occurs on ~90% of days of the year in all air mass types. The events are related to biogenic emissions during low-tide exposure of marine biota, and are photo-chemically driven. These nucleation events typically last from 2-8 hours depending on tide amplitude and concentrations can often exceed 1,000,000 cm-3, even in the cleanest marine air. Formation rates of detectable sized particles (3 nm) are in the range 104-105 particles cm-3s-1 and nucleation rates are of the order of 107 cm-3s-1. Coastal new particles are encountered for sulphuric acid concentrations of the order of ~2 x 106 molecules cm-3 or higher, although there is no correlation between peak sulphuric acid concentration and peak particle concentration or production rate. Modelling studies suggest that ternary nucleation of sulphuric acid-water-ammonia can cause nucleation for the given environmental conditions and vapour concentrations, however, the sulphuric acid concentrations cannot grow these new particles to detectable sizes. In order to grow new particles to >3 nm, an additional source of condensable species is required. This additional species is thought to be a halocarbon derivative, many of which are observed to be released from the shore biota. Airborne measurements indicate that these coastal nucleation events are not unique to Mace Head and occur all along the coastline. Given off shore flow, these coastal plumes can extend out over the ocean for several 100s of km. Modelling predictions indicated that after 2-3 days, these coastal plumes can lead to enhancements of radiatively-active aerosol and CCN (d>100 nm) by 2-10 times above that of the pre-existing population.

1 University of Helsinki, Department of Physics

2 University of Sunderland

3 Stockholm University

4 NUI, Galway

5 TNO, The Netherlands

6 DWD

7 Lancaster University

8 University of Birmingham

9 FMI

10 Demokritos University of Thrace

11 ISAS Dortmund

 

 

5.3.FORMATION AND GROWTH OF CLOUD DROPLETS &

 

Markku Kulmala, Pekka Korhonen*, Ari Laaksonen, Jukka Hienola, Pasi Aalto, Jyrki M. Mäkelä, Timo Vesala, Timo Mattila, Kaarle Hämeri, Antti Siivola, H.-C. Hansson**, P.E. Wagner***, R.J. Charlson+ and Frank Arnold++

The main goal of the project is to investigate the formation and growth of cloud droplets. During the study we focus on the realistic - multicomponent - systems. Our contribution to the worldwide "Climate Change problem" is to establish reliable, experimentally well tested theoretical basis for the description of different physical and physicochemical processes which can affect the radiative properties of aerosols and clouds.

Atmospheric aerosol particles influence the Earth's radiation balance both directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei (CCN). Increased aerosol and CCN concentrations lead not only to increased scattering of light back to space, but also to higher cloud albedos. Enhanced CCN concentrations can also lead to increased cloud lifetimes.

In our recent studies we have explored how condensable trace gases (HNO3, HCl and NH3 in our examples) influence the cloud droplet number concentrations due to the increased amount of hygroscopic matter in developing CCN [1-4] (Kulmala et al. 1993, 1995, b; Korhonen et al. 1996a, b). For the description of the cloud envronment, an adiabatic air parcel model has been used. We have also applied an entraining air parcel model to examine the influence of entrainment of drier air during the activation process. Cloud dynamics (e.g. different air updraft velocities) has been identified to be an important factor [1,3].

In more recent studies [5] (e.g. Kulmala et al. 1996), we have developed the microphysical model further and concentrated especially in the description of the initial aerosol particle distribution. In this context a realistic four-mode particle distribution has been applied: two modes in both size and hygroscopicity, i.e. more and less hygroscopic particles in both Aitken and accumulation modes [5] (Kulmala et al., 1996).

Recently we have developed [6] (Mattila et al., 1996; Kulmala and Mattila, 1996) the model further. In the latest version we have investigated the droplet growth using HNO3, HCl, NH3 and water condensing simultaneously.

We have also applied our model on supercooled cirrus clouds [7]. In these conditions the nitric acid will also enhance droplet formation significantly.

We have also presented the concept of multicomponent-multiphase Köhler theory, which reveals that stable cloud droplets of size 1-10 µm could exist in air with a relative humidity of less than 100 % [8].

All our simulations show, that if the concentration of trace gases changes from background values to the values found from the polluted areas it effects also on the formation and growth of cloud droplets. This change will affect on radiative properties of a single cloud by increasing the optical thickness of a cloud.

* Finnish Meteorological Institute

** Univ. Stockholm, Dept. of Meteorology

*** University of Vienna

+ University of Washington, Departments of Atmospheric Sciences and Chemistry

++ Max-Planck-Inst. für Kernphysik, Heidelberg

1. P. Korhonen, M. Kulmala, T. Vesala: Model Simulation of the Amount of Soluble Mass During Cloud Droplet Formation. Atmospheric Environment, Vol. 30, 1773-1785, 1996a.

2. P. Korhonen, M. Kulmala, H.-C. Hansson, I.B. Svenningsson, N. Rusko: Hygroscopicity of pre-existing particle distribution and formation of cloud droplets: a model study. Atmospheric Research, Vol. 41, 249-266, 1996b

3. M. Kulmala, A. Laaksonen, P. Korhonen, T. Vesala, T. Ahonen, J.C. Barrett: The effect of atmospheric nitric acid vapour on CCN activation. J. Geophys Res., Vol. 98, No. D12 (1993) 22949-22958

4. Markku Kulmala, Pekka Korhonen, Ari Laaksonen, Timo Vesala: Changes in cloud properties due to NOx emissions. Geophys. Res. Lett. 22 (1995) 239-242

5. Markku Kulmala, Pekka Korhonen, Timo Vesala, Hans-Christen Hansson, Kevin Noone, Birgitta Svenningsson: The effect of hygroscopicity on cloud droplet formation. Tellus 48B (1996) 347-360

6. Markku Kulmala, Timo Mattila, Anne Toivonen: The effect od Ammonia and acids on cloud droplet formation. J. Aerosol Sci. Vol. 28 (1997) S419-420

7. Ari Laaksonen, Jukka Hienola, Markku Kulmala, Frank Arnold: Supercooled cirrus cloud formation modified by nitric acid pollution of the upper troposphere. Geophys. Res. Lett. 24 (1997) 3009-3012

8. Markku Kulmala, Ari Laaksonen, Robert J. Charlson, Pekka Korhonen: Clouds without supersaturation. Nature 388 (1997) 336-337

& Supported by the Academy of Finland

 

5.4. NUCLEATION STUDIES

 

Markku Kulmala, Pekka Korhonen, Ari Laaksonen, Hanna Vehkamäki, Ismo Napari, Kari Laasonen* and Robert McGraw**

The aim of theoretical nucleation studies is to predict nucleation rate, i.e. number of new parti-cles formed per unit time, when the ambient conditions of nucleating vapours are known. We have used classical nucleation theory and den-sity functional theory in studying nucleus com-positionsand nucleation rates in various one-component and binary nucleating systems. Fur-thermore, we have carried out ab initio calcula-tions in order to gain information of the struc-tures and proton transfer barriers of the smallest gas-phase sulfuric acid/water clusters, i.e. hy-drates containing 1-3 water molecules. It was found that 3 water molecules is almost sufficient to cause the first proton transfer reaction to take place.

During 1999 we applied the density functional theory in studying the interfacial curvature free energy of Lennard-Jones clusters in connection with the Kelvin relation. The key idea of the density functional theory is to consider nucleat-ing system as an inhomogenous fluid, in which the particle density varies through space. The most important quantity is the free energy den-sity functional, from which the relevant thermo-dynamic properties the system can be calcu-lated, i.e. the pressure and chemical potential. These can be used to obtain the density profile of the gas-liquid interface, the work of formation of the critical nucleus in the supersaturated va-pour, and finally the nucleation rate. Density functional theory works best for nonpolar va-pours, e.g. for argon-krypton system. In the immediate future our goal is to apply the theory to partially immiscible model systems.

Classical theory for ternary water-sulphuric acid - ammonia nucleation has been developed and applied to atmospheric conditions. The results show that this mechanism is very probable in atmospheric ocnditions.

* University of Oulu

** Brookhaven National Laboratory, USA

 

5.5. ULTRAFINE NANOPARTICLE MEASUREMENT AND GENERATION

 

Jyrki M. Mäkelä, Georg P. Reischl* and Jorma Keskinen**

Measurement on size and electrical mobility as well as detection of ultrafine nanoparticles and small ions have been carried out in the size range of 0.7-40 nm (diameter) corresponding to electrical mobility range of 4.0-0.001 cm2/Vs. The main aim of the study is to establish the DMA-technique in the nanometer size range, in normal atmospheric temperature and pressure. The recent work has been concentrated on flow arrangement of the DMA instrument, on resolu-tion of DMA at 2-10 nm size range and on the physical concept of mobility equivalent diameter of particle to characterize the size of the nanoparticles. Also Transmission Electro Micro-scope (TEM) has been used to characterize the generated nanoclusters down to 3-4 nm (particle diameter). Moreover, work has been carried out to develop a set of ion cluster mobility peaks to serve as calibration and testing standards in the nanometer particle size range.

* Institut für Experimentalphysik, Univ. Vienna

** Tampere University of Technology

 

5.6. FOREST-ATMOSPHERE INTERACTIONS

 

GAS EXCHANGE, DEPOSITION AND AIR POLLUTION

Timo Vesala, Markku Kulmala, Pasi Aalto, Tuula Aalto, Kaarle Hämeri, Petri Keronen, Jyrki M. Mäkelä, Üllar Rannik, Tiina Markkanen, Pertti Hari*, Toivo Pohja*, Tapani Lahti**, Erkki Siivola**, John Grace***, Juhan Ross‡ and Hannes Tammet¶

A general goal is to understand the behaviour of air pollutants in the atmosphere and their deposition to Scots pine forest together with carbon and water exchange. To reach this aim, continuous long-term field measurements have been carried out in Värriö (SMEAR I; since 1991) and Hyytiälä (SMEAR II; since 1995) environmental measurement stations combining the physico-chemical and biological knowledge.

In SMEAR II, the work can be divided into categories of air (meteorology, gas exchange on stand level, aerosols), tree (gas exchange on branch level, sap flow) and soil. Eddy covariance instrumentation detects three wind velocity components, temperature and carbon dioxide and water vapour concentrations with high response (10 Hz). The vertical transport of aerosol particles was also measured by eddy covariance technique. In a longer time scale, CO2, H2O, SO2, O2, NOx, temperature and wind speed and direction will be measured at several vertical levels to detect gradients. Besides these, fluxes of above mentioned gaseous components into a pine branch enclosed by a transparent cuvette will be monitored. Measurements of irradiance (photoactive, direct, global, reflected, net, diffuse, UV), rain and pressure offer basic meteorological data. The preceding tasks are accomplished by 73 m-high mast and 15 m-high tower. Aerosol measurements aim to the understanding of particle formation, their hygroscopic properties and formation of clouds. The sap flow in stems and the water flow and content in the ground will be determined also. The station offers represantative and valuable continuous data of atmosphere-biosphere interactions for northern pine forest. It also helps in understanding of scaling, taking information at one scale (like shoot) and using it to derive processes at another scale (like canopy).

* Dept. of Forest Ecology, Univ. Helsinki

** Laboratory of Applied Electronics, Helsinki University of Technology

+ Institute of Ecology and Resource Management, University of Edinburgh, U.K.

‡ Institute of Astrophysics and Atmospheric Physics, Estonian Academy of Sciences

¶ University of Tarto, Estonia

See SMEAR homepage: http://honeybee.helsinki.fi/HYYTIALA/smear

 

LONG TERM CARBON DIOXIDE AND WATER VAPOUR FLUXES

Timo Vesala, Üllar Rannik, Petri Keronen, Tiina Markkanen, Toivo Pohja* and Erkki Siivola**

Long-term measurements of the fluxes of CO2, water vapour and sensible heat have been carried out at SMEAR II station in Hyytiälä since 1996. The measurements are part of the global FLUXNET project. The aim of organised flux network with standard measurement and data presentation protocols and an active centralised archive,would benefit the wider global change science community.

In the project the eddy covariance (EC) technique is used and fluxes are obtained from time averages of product of turbulent fluctuations in vertical wind velocity and measured scalar (concentration or temperature). In Hyytiälä, EC measurements are presently carried out at the heights of 23 and 46 m (canopy height is 13 m). The system consists of an ultrasonic fast-response (10 Hz) anemometers (Gill Solent modified to operate with optical fibre) and a fast-response CO2 and H2O gas analyzers (Li-Cor 6262). Both raw data and real-time calculated 1/2 h flux averages are saved.

Based on 44 months measurement series, the annual, gap-filled values of the net ecosystem exchange of carbon dioxide were 230 gcm-2, 260 gcm-2 and 190 gcm-2 in 1997, 1998 and 1999, respectively. Along the spring and summer, the light saturated carbon dioxide uptake rate reaches the maximum value of 12 µmol-2s-1 by the end of June. In the autumn, the uptake does not decline gradually but ceases rapidly around the beginning of November.

* Department of Forest Ecology, Univ. Helsinki

** Laboratory of Applied Electronics, Helsinki University of Technology

See EUROFLUX home page: http://www.unitus.it/eflux/euro.html

 

 

5.7. WATER TRANSPORT OF TREES

 

Timo Vesala, Sanna Sevanto, Teemu Hölttä, Asko Valli, Martti Perämäki*, Eero Nikinmaa*, Pertti Hari*, Raimo Sepponen**, Jussi Timonen+ and Franciska Sundholm++

The mechanisms of the ascent of sap in plants, especially in tall trees, is not known. The commonly accepted explanation is based on the cohesion-tension theory according to which water ascends plants in a metastable state under tension, i.e. with xylem pressure more negative than that of the vapour pressure of water. In the whole project, co-ordinated by T. Vesala, mechanisms of water transport in trees are investigated by means of nuclear magnetic resonance technique and modelling of flow through porous medium. Hypotheses thus obtained for macro and microstructures of xylem are tested by analysing fibres and modelling tree growth. The project combines environmental physcis, flow simulations, forest ecology and polymer chemistry with the development of a novel measurement technique.

The subproject running at Department of Physics and Department of Forest Ecology has developed the model based on the cohesion theory and on the assumption that fluctuating water tension driven by transpiration together with the elasticity of wood material causes variations in the diameter of a tree stem and branches. The model was tested against field measurements of the diurnal xylem diameter change at different heights of Scots pine trees at SMEAR II station (Hyytiälä). Measured shoot-scale transpiration or ecosystem-scale evapo-transpiration and soil water potential were used as input data for the model. The good agreement has been obtained and the stem diameter variations can be used as the indicator of the sap flow.

Huge tension makes water transport intrinsically vulnerable to cavitation, which decreases hydraulic conductivity and may lead to irreversible embolism of conduits and it is thus harmful for plants. There exist controversial ideas on the cavitation processes and the ways plants possibly repair embolism. We have examined theoretically physics of cavitation and the repair mechanisms and these characteristics will be included in the sap flow model.

* Department of Forest Ecology, Univ. Helsinki

** Helsinki University of Technology

+ University of Jyväskylä

++ Department of Chemistry, Univ. Helsinki

 

5.8. PROJECT ON ENVIRONMENTAL ANALYSIS

 

Markku Kulmala, Masahiko Shimmo, Jyrki Viidanoja, Katri Puhto, Marja-Liisa Riekkola*, Franciska Sundholm*, Pertti Hari** and Antti Uusi-Rauva***

One of the major problems in the analysis of the environmental questions is to combine physico-chemical and biological knowledge. In order to be able to solve this problem we have started joint efforts on environmental analysis. Three departments and two field stations from two dif-ferent faculties are participating in the project. We have a comperehensive point of view and we investigate the whole analysis chain.

* Deparment of Chemistry, Univ. Helsinki

** Department of Forest Ecology, Univ. Helsinki

*** Faculty of Agriculture and Forestry, Instrument Centre, Univ. Helsinki