5. AEROSOL AND ENVIRONMENTAL PHYSICS

5.1. PREFACE


The research activity of the Laboratory of Aerosol and Environmental Physics has focused on basic and applied aerosol science and cloud microphysics during 1996. 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.


The effect of condensable trace gases on cloud droplet formation was studied using computer models developed in the laboratory. Other topics of theoretical and numerical investigations were heat and mass transfer, formation and growth of aerosol particles, and nucleation processes.


The field station SMEAR II (in Hyytiälä) has been constructed during 1995. Continuous measurement activity started in 1996. The main output so far was numerous observations of atmospheric nucleation bursts. The field measurements were also performed during different field campaigns. The preparations for ACE-2, the North Atlantic Regional Aerosol Characterization Experiments, have been one of the main activities.The laboratory has also participated in the ARCTIC OCEAN-96 expedition.


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 nanometer size range using recent aerosol instrumentation such as electrical mobility spectrometry and 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 theoretical and experimental activities of the group. During 1996 various projects (including three EU projects) continued in co-operation with research groups from Austria, Canada, Czech Republic, Italy, Japan, Nethelands, Russia, Sweden, United Kingdom and 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 in the beginning of the fall semester 1994) was continued during1996. The Laboratory organized The Fourteenth International Conference on Nucleation and Atmospheric Aerosols with more than 300 participants in Helsinki 26-30 August.


In the autumn 1996 the APFE group (Aerosol Physics and Forest Ecology) was chosen as a group of excellence of the University of Helsinki. The other important milestone was the starting of new professorship in Environmental Physics and Chemistry on Aug. 1.


Financial support from the Academy of Finland and the Nessling Foundation is gratefully acknowledged.


Markku Kulmala

5.2. CHEMICAL AND PHYSICAL CONVERSION IN COLD ATMOSPHERE AND THE EFFECT OF RADIATION &

Markku Kulmala, Pekka Korhonen*, Ari Laaksonen, Pasi Aalto, Jyrki M. Mäkelä, Timo Vesala, Timo Mattila, Petri Keronen, Kaarle Hämeri, Antti Siivola, Hans-Christen Hansson**, Paul E. Wagner***

The main goal of the project is to investigate the formation and growth of atmospheric aerosol particles and cloud droplets. During the study we are focusing on the realistic - multicomponent - systems. Our contribution to 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 (HNO and HCl in our examples) influence the cloud droplet number concentrations due to the increased amount of hygroscopic matter in developing CCN [1-4]. 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 entrainm ent of drier air during the activation process. Cloud dynamics (e.g. different air updraft velocities) has been identified to be an important factor [3,1].


In more recent studies [5], 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 acc umulation modes [5].


In our latest studies we have developed [7,6] the model further. In the latest version we have investigated the droplet growth using HNO<sub>3</sub>, HCl, NH<sub>3</sub> and water condensing simultaneously.


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.


In our recent studies we have also investigated the formation and growth of atmospheric aerosol particles using aerosol models, and by conducting field experiments in Finnish Arctic. Several routes for atmospheric particle production via homogeneous nucleation are proposed. Sulphuric acid - water nucleation is studied in more detail, and occasions of new particle production in Finnish Arctic are identified.

* Finnish Meteorological Institute

** Univ. of Stockholm, Dept .of Meteorology

***University of Vienna 

References

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

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 (1996) 249-266

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 proper-ties due to NO<sub>x</sub> emissions. Geophysical Research Letters, Vol. 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 B, Vol. 48B (1996) 347-360

6. Markku Kulmala, Timo Mattila: The effect of Ammonia and acids on cloud droplet forma-tion. In Proceedings of The NOSA/ NORSAC Symposium 1996, Ed. by Poul Hummelshøj, Helsingør, Denmark, 15-17 Nov. 1996. Risø -R-934(EN), pp. 16-17

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

& Supported by the Academy of Finland (SILMU-project)

5.3. NUCLEATION STUDIES

Ari Laaksonen, Hanna Arstila, Markku Kulmala, Robert McGraw *, Kari Laasonen **, Oleg Vassiliev ***

Theoretical studies on gas-liquid nucleation studies were continued in 1996.

Statistical foundations of the scaling relations developed earlier for the number of molecules in the critical nucleus, g*, and the work of nucleus formation, W*, were explored. It was shown that the function D(T) (dependent only on temperature), which gives the difference between classical and density functional W*, can be related to the rigidity coefficient. Furthermore, making use of the fact that the classical theory predicts the critical nucleus size correctly down to about g* = 50, a new equation was derived for the size-dependent surface tension that differs from the Tolman relation. It was shown that density functional calculations support the new formula.


The first step of extending the scaling relations to binary systems was taken with a study, in which the classical and density functional predictions of the nucleation of a nonideal system were compared in detail. It was shown that, even though the classical theory produces unphysical activity plots (figures showing how the individual vapor densities can be varied keeping the nucleation rate constant) for the system considered, it is able to describe the dependence of nucleation rate on vapor density correctly when the vapor mole fraction is held constant. This translates, according to the so called nucleation theorem, into a correct prediction of the total number of molecules n1+n2 in the critical nucleus, although the individual numbers n1 and n2 are predicted incorrectly.


The effect of associate and hydrate formation on homogeneous nucleation rate was studied. To get reasonable estimates for hydrate concentrations in binary system, stable geometries and binding energies of small sulphuric acid-water clusters were determined using quantum mechanical ab initio calculations.


The kinetics of homogeneous nucleation process was considered. The nucleation rate was found to be unsensitive to changes in collision probabilities, and association correction to monomer-cluster kinetics was found to approximate fairly well the cluster-cluster kinetics also in highly associated cases.

* Brookhaven National Laboratory, USA 

** University of Oulu

*** UCLA, USA

5.4. ULTRAFINE NANOPARTICLE MEASUREMENT AND GENERATION

Jyrki M. Mäkelä, Vilho Jokinen, Jarkko Augustin, Georg P. Reischl*, Jorma Keskinen**, Ari Ukkonen**, Anatoli Baklanov***, Sergei Dubtsov***

Measurement of size and electrical mobility as well as detection of ultrafine nanoparticles and small ions have been carried out in the size range 0.7-40 nm (diameter) corresponding to electrical mobility range 4.0-0.001 cm2/Vs. The electrical mobility distribution is measured by combining classification of particles by Differential Mobility Analyzer (DMA) with detection by Condensation Nucleus Counter and Faraday Cup Electrometer. From the mobility spectra obtained, the size distribution of the particles can be determined. The measurements are typically carried out in atmospheric pressure.


The main aim of the study is to establish the DMA-technique in the nanometer size range. 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 char acterize 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.

The work is closely related to formation processes and dynamics of atmospheric ultrafine and fine aerosol particles as well as related topics in nanotechnology, mainly particle and cluster formation and generation.

* Inst. of Experimental Physics, Univ. Vienna

** Tampere University of Technology

*** Institute of Chemical Kinetics and Combustion SB RAS, Novosibirsk

5.5. FOREST-ATMOSPHERE INTERACTIONS

GAS EXCHANGE, DEPOSITION AND AIR POLLUTION

Timo Vesala, Markku Kulmala, Pasi Aalto, Tuula Ahonen, Kaarle Hämeri, Vilho Jokinen, Petri Keronen, Jyrki M. Mäkelä, Ûllar Rannik, Pertti Hari*, Toivo Pohja*, Tapani Lahti **, Erkki Siivola**, John Grace+, John Moncrieff+, Juhan Ross***, Hannes Tammet ****

A general goal is to understand the behavoiur 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; since1995) 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). For six weeks in 1996, 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 de termined also. The station offers represantative and valuable continuous data of atmosphere-biosphere interactions for northern pine forest. As an example, we have detected particle formation within the forest and their upward transport.


Measurements of irradiance (photoactive, global, reflected, net, diffusive, UV), rain and pressure offer basic meteorological data. The preceding tasks are accomplished by 70 m-high mast. Aerosol measurements aim to the understanding of particle formation, their h groscopic 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 will offer representative and valuable continuous data of atmosphere-biosphere interactions for northern pine forest. Already during the test period, some interesting phenomena have been detected: for example, in unpolluted conditions pines seem to emit significant amounts of nitrogen oxides and winter-time soil respiration is quite large.


* 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
**** Univ. Tartu, Dept. of Environmental Physics

See SMEAR homepage.

LONG TERM CARBON DIOXIDE AND WATER VAPOUR FLUXES

Timo Vesala, Ûllar Rannik, Petri Keronen, Toivo Pohja*, 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 representative 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 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. Footprint (area representing the measured flux) analysis has been done both by means of turbulent diffusion equation and by a Lagrangian Monte Carlo simulations.


In the project the eddy covariance (EC) technique is used and fluxes are obtained from time averages of product of 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. Footprint (area representing the measured flux) analysis has been done both by means of turbulent diffusion equation and by a Lagrangian Monte Carlo simulations.


The preliminary results from 1996 (since 19th April) show that the carbon dioxide sink strength (annual cumulative flux) is 1 kg/m and the cumulative evapo-transpiration is 250 mm in Hyytiälä site.

*Dept. of Forest Ecology, Univ. Helsinki

**Laboratory of Applied Electronics, Helsinki University Technology

See EUROFLUX home page