The research activity of the Laboratory of Aerosol and Environmental Physics has focused on basic and applied aerosol science and cloud microphysics during 1997. Studies on heat and mass transfer, nucleation, condensation, aerosol dynamics, aerosol measurement technique, atmospheric aerosols, deposition 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 using computer models developed in the laboratory. Other topics of theoretical and numerical investigations were heat and mass transfer as well as nucleation processes. Based on our theoretical investigations we have shown the existence of clouds without supersaturation (see highlights of research).
The field station SMEAR II (in Hyytiälä) has been constructed during 1995. Continuous measurement activity started in 1996. The main finding so far has been numerous observations of atmospheric nucleation bursts. The field measurements were also performed during different field campaigns. Participation in ACE-2, the North Atlantic Regional Aerosol Characterization Experiments, has been one of the main activities, and took place in June-July, 1997.
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 1997 various projects (including four EU projects) continued in co-operation with research groups from Austria, Canada, The Czech Republic, 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, 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 during1997.
Financial support from the Academy of Finland and the Nessling Foundation is gratefully acknowledged.
5.2. ATMOSPHERIC AEROSOLS
MODELLING OF FORMATION AND GROWTH OF ATMOSPHERIC AEROSOL PARTICLES
Liisa Pirjola, Markku Kulmala, Ari Laaksonen and Anne Toivonen
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 chemistryon 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.
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 concen-tration 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 differen-tial 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 nd 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.
Markku Kulmala, Kaarle Hämeri, Jyrki Mäkelä, Pasi Aalto, Tuula Aalto and Minna Väkevä
ACE-2, The North Atlantic Regional Aerosol Characterization Experiment, is the third experiment co-ordinated by the international Global Atmospheric Chemistry Project (IGAC) that addresses the properties of the atmospheric aerosol relevant to radiative forcing and climate. Two hundred scientists from Europe and the US joined forces to investigate how aerosol particles climate and the extent to which they may offset the greenhouse warming.
Between 16 June and 25 July 1997, scientists equipped coastal and mountain top sites on the Canary Islands, Madeira and Portugal, with the most advanced observational equipment to study man made aerosols from Europe, North Atlantic back-ground aerosol and dust aerosols from the Sahara. Additionally, six research aircrafts performed flights to make measurements within these aerosol plumes and within the clouds. The area was also monitored with four satellites.
The aerosol group of the University of Helsinki took part in two sub-projects of ACE-2, namely FREETROPE and HILLCLOUD, which both were funded by the EU.
The main platform of the FREETROPE project was the Izaña observatory (Tenerife, 2367m asl). This station is the only free tropospheric station in the North Atlantic. Only during night time, free tropospheric (FT) air is subsiding over Tenerife, whereas during daytime, upslope winds develop and Izaña is exposed to marine boundary layer (MBL) air perturbed by local anthropogenic and biogenic emissions. Izaña is exposed to essentially three types of air masses: from the open North Atlantic, from Europe and from the Sahara. Transport in the FT is usually decoupled from that in the MBL.
The FT aerosol was characterised by performing mostly local closure experiments. The various air masses offered a variety of aerosols, hence the local closure experiments tested present understanding over a wide range of aerosol characteristics. Particular attention was given to the state of mixing of marine and desert aerosols when present together with European pollution. Some of the closure experiments of Izaña were duplicated at the MBL station of Hidalgo, and allowed for a systematic comparison between FT and MBL aerosol characteristics (during nighttime).
HILLCLOUD experiment took place in the Taganana area. The hill cap cloud which forms on the ridge at the end of the Taganana valley was used as a natural flow through reactor to study aerosol processing by a MBL cloud. With extensive instrumentation on both the upslope and downslope areas, and an in-cloud station on the ridge, it was possible to make detailed continuous measurements of the atmospheric trace gases entering and leaving the cloud. Measurements were made also of the aerosol size distribution, hygroscopic and chemical properties before entering and after leaving the cloud. Detailed simultaneous measurements of the cloud microphysics, cloud chemistry and properties of the interstitial aerosol were made on the hill top. The passage of the airstream over the hill will sometimes result in considerable entrainment of air from the free troposphere. Detailed measurements of the gases and aerosol in the free tropospheric air were made in FREETROPE and provide these data.
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, Hans-Christen Hansson**, Paul E. Wagner***, Robert 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 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  (Kulmala et al., 1996).
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 multi-component-multiphase Köhler theory, which re-veals 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 Laak-sonen, 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 of 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
Ari Laaksonen, Hanna Arstila, Markku Kulmala, Ismo Napari, Kari Laasonen* and Robert McGraw**
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 compositionsand nucleation rates in various one-component and binary nucleating systems. Furthermore, 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. 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 1997 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 density functional, from which the relevant thermo-dynamic 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
5.5. ULTRAFINE NANOPARTICLE MEASUREMENT AND GENERATION
Jyrki M. Mäkelä, Vilho Jokinen*, Jarkko Augustin, 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 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.
* Finnish Meteorological Institute
** Institut für Experimentalphysik, Univ. Vienna
*** Tampere University of Technology
1. V. Jokinen and J.M. Mäkelä, J.M., J. Aerosol Sci. 28 (1997) 643-648
2. G.P. Reischl, J.M. Mäkelä and J. Necid, Aerosol Sci. Technol. 27 (1997) 651-672
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 Rossý 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, 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 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.
ý Institute of Astrophysics and Atmospheric Physics, Estonian Academy of Sciences
¶ University of Tarto, Estonia
See SMEAR homepage
LONG TERM CARBON DIOXIDE AND WATER VAPOUR FLUXES
Timo Vesala, Üllar Rannik, Petri Keronen, Toivo Pohja* and Erkki Siivola**
The EU-project EUROFLUX (Long term carbon dioxide and water vapour fluxes of European forests and intercations with the climate system; co-ordinator Dr. Riccardo Valentini, University of Tuscia, Italy) has been running from the beginning of February 1996. Long-term measurements of the fluxes of CO2, water vapour and sensible heat are carried out at 15 represantative European forest sites (one of them being SMEAR II in Hyytiälä) encompassing the entire range in climate, species distribution and site conditions. 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 height of 23 m (canopy height is 13 m). The system consists of an ultrasonic fast-response (10 Hz) anemometer (Gill Solent modified to operate with optical fibre) and a fast-response CO2 and H2O gas analyzer (Li-Cor 6262). Both raw data and real-time calculated 1/2 h flux averages are saved.
For different 12 month periods from May 1996 (starting of the measurements) until October1997 the cumulatice water flux (annual flux) at the site varied from 220 to 260 mm, and annual uptake of carbon dioxide from 550 to 810 g/m2, that is 1500 kg - 2200 kg of carbon per hectare. This is the net flux, the difference between photosynthetic uptake for the growth of the biomass and soil respiration due to decomposition.
* Department of Forest Ecology, Univ. Helsinki
** Laboratory of Applied Electronics, Helsinki University of Technology
See EUROFLUX homepage
5.7. 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 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 Univ.
*** Faculty of Agriculture and Forestry, Instrument Centre, Helsinki University