In 1998 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 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 and 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 E.U.-founded projects BIOFOR (aerosol formation) and EUROFLUX (CO2 fluxes) were performed during 1998. Participation in other field campaigns during the E.U.-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 1998 various projects (including four E.U. projects) continued in co-operation with research groups from Austria, Canada, The Czech Republic, Germany, Italy, Japan, The Netherlands, Russia, Sweden, The United Kingdom and The United States. At the national level we have had close collaboration especially with the department of Forest Ecology 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 1998.
MODELLING OF FORMATION AND GROWTH OF ATMOSPHERIC AEROSOL PARTICLES
Liisa Pirjola, Markku Kulmala, Ari Laaksonen* and E. Douglas Nilsson
A sectional model (AEROFOR) for the formation of sulphuric acid - water particles has been developed. 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 and upper tropospheric aerosols. Besides that we have studied the effect of terpene chemistry 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 concentration 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
MEASUREMENTS OF ATMOSPHERIC AEROSOLS
Pasi Aalto, Gintautas Buzorius, Kaarle Hämeri, Petri Keronen, Markku Kulmala, Jyrki M. Mäkelä, Üllar Rannik, Timo Vesala and Minna Väkevä
Measurements have been performed on the concentration and size distribution of submicron aerosol particles in the ambient air. The size distribution of aerosol particles may be generally interpreted as a fingerprint of atmospheric pollution. Moreover, various kinds of quantitative information about atmospheric physico-chemical processes may be obtained by observing the dynamics of ambient particle size distribution. Size classification of submicron aerosol particles is performed by differential mobility analysers, based on electrical mobility of charged aerosol particles. For detection of particles, condensation particle counters are typically used.
In addition to continuous monitoring of the number size distribution
(range 3-500 nm) in Hyytiälä SMEAR II station in Southern Finland
and in the Department of Physics, downtown of Helsinki, we have also participated
in several field campaigns. There, accompanying size distribution measurements
we have also performed experimental studies on hygroscopic growth and volatility
properties of aerosol particles, as well as their potential to act as cloud
condensation nuclei. For ambient studies a novel method for measurement
of vertical particle fluxes have also been developed. In the end
of 1997, a continuous monitoring unit of submiron particle size distribution
was started in Eastern Lapland (SMEAR I, Värriö). Additionally,
studies on laboratory aerosols are made in instrumentation testing and
development purposes. We have worked on e.g. expanding of the lower edge
of measurable particle size range down to 2 nm.
The main goal of the project is to investigate the formation and growth of cloud droplets. During the study we are focusing 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 environment, 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] (Kulmala et al., 1993; Korhonen et al., 1996a).
In more recent studies  (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 .
Recently we have developed  (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 . 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 % .
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, Department of Meteorology
*** University of Vienna
+ University of Washington, Departments of Atmospheric Sciences and Chemistry
++ Max-Planck-Institut 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: Hygrocopicity 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
The aim of theoretical nucleation studies is to predict nucleation rate, i.e. number of new particles formed per unit time, when the ambient conditions of nucleating vapours are known. We have used classical nucleation theory and density functional theory in studying nucleus compositions and nucleation rates in various one-component and binary nucleating systems. Furthermore, we have carried out ab initio calculations in order to gain information of the structures and proton transfer barriers of the smallest gas-phase sulfuric acid/water clusters, i.e. hydrates 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 1998 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 nucleating system as an inhomogenous fluid, in which the particle density varies through space. The most important quantity is the free energy density functional, from which the relevant thermodynamic properties the system can be calculated, 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 vapour, and finally the nucleation rate. Density functional theory works best for nonpolar vapours, e.g. for argon-krypton system. In the immediate future our goal is to apply the theory to partially immiscible model systems.
* University of Oulu
** Brookhaven National Laboratory, USA
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 resolution 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 Microscope (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
A general goal is to understand the behaviour of air pollutants in the atmosphere and their deposi-tion 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 covari-ance 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, O3, 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 meteoro-logical 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 Manage-ment, 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.
The annual uptake of carbon dioxide has been about 800 g/m2, that is about 2000 kg of carbon per hectare. This is the net flux, the difference between photoynthetic uptake for the growth of the biomass and soil respiration due to decomposition. The repiration losses during winter time (November - Februaray) is about 200 g/m2.
* 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
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. However, 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.
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 and soil water potential were used as input data for the model. The good agreement was obtained especially at the lower positions of the tree stem. The results suggest that the uptake of water from soil is the most restrictive part in the soil-tree-atmosphere continuum and that the time lag between transpiration and flow at the base of tree is less than half an hour (34-year-old trees). The next objective is to take into account the cavitation.
* Department of Forest Ecology, University of Helsinki
** Helsinki University of Technology
+ University of Jyväskylä
++ Department of Chemistry, University of Helsinki
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 different faculties are participating in the project. We have a comperehensive point of view and we investigate the whole analysis chain.
* Deparment of Chemistry, Helsinki University
** Department of Forest Ecology, Helsinki University
*** Faculty of Agriculture and Forestry, Instrument Centre, Helsinki University