Kulmala, M., Hari, P., Laaksonen, A., Vesala, T. & Viisanen, Y. 2005: Research Unit of Physics, Chemistry and Biology of Atmospheric Composition and Climate Change: overview of recent results. Boreal Env. Res. 10: 459–477.

In this paper we present research methods and recent results obtained within activities of the Research Unit of Physics Chemistry and Biology of Atmospheric Composition and Climate Change, which is one of the centres of excellence of the Academy of Finland. The centre forms an integrated attempt to understand various, but interlinked, biosphere–atmosphere interactions applying inter- and multidisciplinary approaches in a coherent manner. The main disciplines used cover aerosol and environmental physics, atmospheric chemistry and physics, micrometeorology, forest ecology and ecophysiology. The main objective of the centre is to study the importance of aerosol particles on climate change. Our scientific approach that starts from basic nucleation theories, is followed by detailed aerosol dynamic/atmospheric chemistry models and well-defined laboratory experiments, and ends with wide continuous field measurements in our research stations and 3D modelling. During the last years the joint efforts within our centre of excellence and the Nordic centres of excellence BACCI (devoted to atmospheric physics and chemistry) and NECC (devoted to carbon balance of northern ecosystems) have increased strongly. A thorough understanding of physical, meteorological, chemical and ecophysiological processes obtained by individual research groups lays the foundation of a unique possibility to study biosphere–aerosol–cloud–climate interactions, or the interplay between carbon exchange, BVOC emissions and formation of new aerosol particles. The necessary requirement is jointly working, real inter-, multi- and cross disciplinary teams. The core of activities is in continuous measurements and database of atmospheric and ecological mass fluxes and aerosol precursors and CO₂-aerosol-trace gas interactions in SMEAR field stations. These are supported by models of particle thermodynamics, transport and dynamics, atmospheric chemistry, boundary layer meteorology and forest growth.

Sogacheva, L., Dal Maso, M., Kerminen, V.-M. & Kulmala, M. 2005: Probability of nucleation events and aerosol particle concentration in different air mass types arriving at Hyytiälä, southern Finland, based on back trajectories analysis. Boreal Env. Res. 10: 479–491.

The probability of a new ultrafine particle formation as well as the spatial sources of the nucleation, Aitken and accumulation modes of particles measured in Hyytiälä, southern Finland, were investigated using air mass history analysis. The studied period covered the years 1997–2003. We estimated the probability of a new ultrafine particle formation for nine sub-areas inside a domain limited by 50°N–80°N in latitude and 30°W–60°E in longitude. For all the seasons North Atlantic and arctic regions dominated as an origin of air masses that accompany nucleation events. The probability of nucleation events in Hyytiälä reached its maximum in spring and summer with north and north-west air masses transfer directions. The highest concentration of nucleation mode particles was observed in arctic and polar air masses, whereas the highest concentration of accumulation mode particles was observed during intrusions of continental air masses.

Lyubovtseva, Y. S., Sogacheva, L., Dal Maso, M., Bonn, B., Keronen, P. & Kulmala, M. 2005: Seasonal variations of trace gases, meteorological parameters, and formation of aerosols in boreal forests. Boreal Env. Res. 10: 493–510.

The data from observations made in the Hyytiälä background station over the period 1997–2003 were analyzed in order to investigate the possible links between physical, chemical and meteorological parameters for event and nonevent days during different seasons. The seasonal patterns of average trace gas (O₃, H₂O, NO_x = NO + NO₂, SO₂) concentrations, UV-A irradiation intensity, temperature, relative humidity and condensation sink were established and used for evaluating their influence on the seasonal dependence of the formation and growth rates of particles in the nucleation mode. For the forest region at Hyytiälä, an estimate was made on the contribution of the oxidation products of SO₂ and volatile organic compounds (VOCs) by O₃ and OH to the formation and growth of the nucleation mode particles. The condensation sink was shown to play a crucial role in the chemical dynamics of gaseous precursors and aerosol particles. The variability of the sink was attributed mainly to the humidification of the pre-existing aerosol particle population, depending thus strongly on the relative humidity. Our results indicate that the observed growth rate of new particles is connected with condensation of secondary organic compounds originating from the oxidation of terpenes.

Anttila, T., Vehkamäki, H., Napari, I. & Kulmala, M. 2005: Effect of ammonium bisulphate formation on atmospheric water-sulphuric acid-ammonia nucleation. Boreal Env. Res. 10: 511–523.

The effect of formation of stable ammonium bisulphate clusters on the ternary water-sulphuric acid-ammonia nucleation was investigated by means of the classical thermodynamics. The performed calculations show that most of the sulphuric acid in the atmosphere is likely to be bound to stable ammonium bisulphate clusters. Consequently the nucleation rates calculated with the new model are orders, sometimes even tens of orders, of magnitude lower than the rates produced by the previous ternary model which neglected the formation of these clusters. The new nucleation rates compare favourably with the available experimental results, unlike the old ones, which are far too high. In contrast to the old ternary model, the revised model does not predict significant nucleation rates that would explain new particle formation events every time they are observed, and we have to look for other nucleation mechanisms like multi-component nucleation involving organics or ion induced nucleation to understand atmospheric new particle formation.

Grini, A., Korhonen, H., Lehtinen, K. E. J., Isaksen, I. S. A. & Kulmala, M. 2005: A combined photochemistry/aerosol dynamics model: model development and a study of new particle formation. Boreal Env. Res. 10: 525–541.

A new coupled photochemistry/aerosol model has been developed for studies of multicomponent aerosol dynamics. The photochemistry module, based on the reaction mechanisms of OSLO-CTM2 global model, has been extended with a simple oxidation scheme of volatile organic gases to produce condensable products. The aerosol module is a further development of UHMA (University of Helsinki Multicomponent Aerosol model) which incorporates all major aerosol dynamic processes with a special focus on new particle formation and growth. The new model explicitly incorporates production and loss terms from photochemistry into the calculation of condensation fluxes and nucleation rates, making the two models completely interactive. In this paper, we apply the new model to investigate the role of photochemistry and biogenic emissions in new aerosol formation and growth. Although using a simplified photochemistry scheme, the model allows for analysis of the evolution of chemical composition of an air mass simultaneously with the aerosol size distribution. The simulation results suggest a classification of air masses which goes beyond describing the air as "clean" or "polluted".

Mordas, G., Kulmala, M., Petäjä, T., Aalto, P. P., Matulevicius, V., Grigoraitis, V., Ulevicius, V., Grauslys, V., Ukkonen, A. & Hämeri, K. 2005: Design and performance characteristics of a condensation particle counter UF-02proto. Boreal Env. Res. 10: 543–552.

In this paper we present a design of a new swirling flow condensation particle counter UF-02proto, which has a continuous sampling flow. The performance and characteristics of the instrument were investigated. We studied experimentally the concentration range of the instrument and the detection efficiency as a function of particle diameter for silver particles. The lower cut-off size of the CPC, i.e. the limiting size when 50% of the particles are successfully accounted for, was determined to be 4.35–4.46 nm, depending slightly on a form of a fitted exponential step-function. The counting efficiency of the CPC at high particle concentrations was experimentally investigated using 20 nm silver particles. The maximum observable number concentration with a single particle counting method was approximately 100000 cm^–3 with an accuracy of 20%. The operation of the CPC UF-02proto was also compared with that of a commercially available CPC when operating both instruments in parallel. The CPC proved to be suitable for a variety of applications with a wide range of particle concentrations and particle sizes.

Ruuskanen, T. M., Kolari, P., Bäck, J., Kulmala, M., Rinne, J., Hakola, H., Taipale, R., Raivonen, M., Altimir, N. & Hari, P. 2005: On-line field measurements of monoterpene emissions from Scots pine by proton-transfer-reaction mass spectrometry. Boreal Env. Res. 10: 553–567.

We measured the daily patterns of monoterpene emissions from Scots pine (Pinus sylvestris) trees in a boreal coniferous forest in August and September 2004, using an on-line chamber method combined with a proton-transfer-reaction mass spectrometry (PTR-MS) analyzer. The on-line measurements were made in two chambers with a one-year old shoot inside. Simultaneous measurements were performed for carbon dioxide (CO₂) exchange, transpiration (H₂O), exchange of trace gases (NO_x, O₃), photosynthetic photon flux density (PPFD), air temperature and relative humidity (RH). The composition of the monoterpene emission did not change during the measurement period, with [delta]³-carene and [alpha]-pinene being the dominant species. The total mono-terpene emission rate (per needle area) was on average 0.5 ng m^–2 s^–1, varied from non detectable to 2.1 ng m^–2 s^–1, and showed a typical diurnal pattern with afternoon maximum and nighttime minimum. The emission rates determined with this on-line chamber method were in agreement with results from a simultaneously used established adsorbent sampling technique with offline GC-MS analysis. The monoterpene emissions from the chamber walls were correlated with the chamber temperature and this measurement artifact was dominating at night. Emission rates normalized to 30 °C, using temperature regression coefficient of 0.09 °C^–1, ranged from 2.1 µg g(dw)^–1 h^–1 to 4.4 µg g(dw)^–1 h^–1. Measurements of emission dynamics of biogenic volatile organic compounds (BVOCs) together with plant physiological activity are urgently needed for the development of mechanistic BVOC emission models in order to assess their regional and global influence.

Launiainen, S., Rinne, J., Pumpanen, J., Kulmala, L., Kolari, P., Keronen, P., Siivola, E., Pohja, T., Hari, P. & Vesala, T. 2005: Eddy covariance measurements of CO₂ and sensible and latent heat fluxes during a full year in a boreal pine forest trunk-space. Boreal Env. Res. 10: 569–588.

The eddy covariance method was used to determine turbulent heat fluxes and CO₂ flux below a boreal Scots pine canopy 3 m above the forest floor. Data were filtered using standard deviation of the vertical velocity as a measure of the turbulent mixing in the trunk space along with the non-stationary criterion. The turbulent transfer in the trunk space was dominated by large (15–100 m) intermittent eddies, which were detectable by the eddy covariance technique. Heat fluxes exhibited clear annual and diurnal course and amounted to 20%–30% of the fluxes above the canopy. The forest floor was a source of carbon all-year-round and the CO₂ efflux was mainly controlled by soil temperature. Photosynthesis of the forest floor vegetation decreased daytime fluxes of CO₂ by 1.0–1.5 µmol m^–2 s^–1 compared with nocturnal values (~3.0 µmol m^–2 s^–1). The eddy covariance method provided a similar daily cycle to the chamber method but there was a discrepancy between their mean levels.

Pumpanen, J. & Ilvesniemi, H. 2005: Calibration of time domain reflectometry for forest soil humus layers. Boreal Env. Res. 10: 589–595.

Time domain reflectometry (TDR) has become a widely used method for determining the volumetric water content of soils. However, due to the differences in bulk density and surface area, the relationship between the dielectric constant and water content in organic soils is very different from that in mineral soil. It is therefore impossible to have a universal calibration suitable for all soil types. In this article we describe the relationship between the apparent dielectric constant (K_a) and volumetric water content ([theta]_v) for homogenized and undisturbed humus layers (Of + Oh) from forested soil using three empirical models. There was a clear relationship between the K_a and [theta]_v and this relationship was best described (R² = 0.968) with a logarithmic equation of the form [theta]_v = aln(K_a) – b. Accurate determination of sample volume was the main source of variation in the calibration, having a greater effect on the calibration results than differences in bulk density.