ISSN 1239-6095
© Boreal Environment Research 2000

Contents of Volume 5 Number 4

Kulmala, M., Hari, H. & Vesala, T. 2000: Preface: Aerosol research at SMEAR stations. Boreal Env. Res. 5: 279.
Abstract

Kulmala, M., Hämeri, K., Mäkelä, J. M., Aalto, P.P., Pirjola, L., Väkevä, M., Nilsson, E.D., Koponen, I. K., Buzorius, G., Keronen, P., Rannik, U., Laakso, L., Vesala, T., Bigg, K., Seidl, W., Forkel, R., Hoffmann, T., Spanke, J., Janson, R., Shimmo, M., Hansson, H.-C., O'Dowd, C., Becker, E., Paatero, J., Teinilä, K., Hillamo, R., Viisanen, Y., Laaksonen, A., Swietlicki, E., Salm, J., Hari, P., Altimir, N. & Weber, R. 2000. Biogenic aerosol formation in the boreal forest. Boreal Env. Res. 5: 281–297.
Abstract
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Mäkelä, J.M., Dal Maso, M., Pirjola, L., Keronen, P., Laakso, L., Kulmala, M. & Laaksonen, A. 2000. Characteristics of the atmospheric particle formation events observed at a borel forest site in southern Finland. Boreal Env. Res. 5: 299–313.
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Kulmala, M., Rannik, Ü., Pirjola, L., Dal Maso, M., Karimäki, J., Asmi, A., Jäppinen, A., Karhu, V., Korhonen, H., Malvikko, S.-P., Puustinen, A., Raittila, J., Romakkaniemi, S., Suni, T., Yli-Koivisto, S., Paatero, J., Hari, P. & Vesala, T. 2000. Characterization of atmospheric trace gas and aerosol concentrations at forest sites in southern and northern Finland using back trajectories. Boreal Env. Res. 5: 315–336.
Abstract
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O’Dowd, C.D., Becker, E., Mäkelä. J.M., Kulmala, M. 2000. Aerosol physico-chemical characteristics over a boreal forest determined by volatility analysis. Boreal Env. Res. 5: 337–348.
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Aalto, P. & Kulmala, M. 2000. Using a cloud condensation nuclei counter to study CCN properties and concentrations. Boreal Env. Res. 5: 349–359.
Abstract
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Pirjola, L. & Kulmala, M. 2000. Aerosol dynamical model MULTIMONO. Boreal Env. Res. 5: 361–374.
Abstract
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Kulmala, M., Hari, H. & Vesala, T. 2000: Preface: Aerosol research at SMEAR stations. Boreal Env. Res. 5: 279.

This issue is the first of the two special issues related to the research activities at SMEAR (Station for Measuring Forest Ecosystem-Atmosphere Relations) stations. The SMEAR I station, located in Värriö, has been in operation since 1991 and the SMEAR II station, located in Hyytiälä, since 1995. The second special issue will be published in next year with focus on the forest-atmosphere relationship and forest ecology. Both the issues are based on the presentations given in the APFE (Aerosol Physics & Forest Ecology) seminar during 7–9 February 2000. The SMEAR stations are operated by the APFE group. The APFE group consists of ca. 50 scientists and is one of the centres of Excellence in the University of Helsinki. The aerosol research in the SMEAR stations has focussed on the formation and growth of atmospheric aerosols, particularly the nucleation-mode aerosols. Annually around 50 new-particle formation events have been observed with subsequent growth to the Aitken and even to the accumulation mode. These observations have motivated us to study mechanisms behind these formation events. These phenomena have been investigated from physical, chemical, meteorological, and biological points of view. Clear connections have been found between the formation events and e.g. meteorological conditions, aerosol physical and atmospheric chemical conditions, and biological activities. This issue with six papers is a compact demonstration of the aerosol research conducted within the APFE group. The main features of aerosol formation events are reported in the first four papers. The fifth paper describes a development and construction of a Cloud Condensation Nuclei Counter, and in the last paper a new model development is presented. We would like to thank all authors for their contribution and acknowledge the Academy of Finland and University of Helsinki for their financial support.
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Kulmala, M., Hämeri, K., Mäkelä, J. M., Aalto, P.P., Pirjola, L., Väkevä, M., Nilsson, E.D., Koponen, I. K., Buzorius, G., Keronen, P., Rannik, U., Laakso, L., Vesala, T., Bigg, K., Seidl, W., Forkel, R., Hoffmann, T., Spanke, J., Janson, R., Shimmo, M., Hansson, H.-C., O'Dowd, C., Becker, E., Paatero, J., Teinilä, K., Hillamo, R., Viisanen, Y., Laaksonen, A., Swietlicki, E., Salm, J., Hari, P., Altimir, N. & Weber, R. 2000. Biogenic aerosol formation in the boreal forest. Boreal Env. Res. 5: 281–297.

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) the determination of formation mechanisms of aerosol particles in the boreal forest site, and b) the verification of emissions of secondary organic aerosols from the boreal forest site, including the quantification of the amount of condensable vapours produced in photochemical reactions of biogenic volatile organic compounds (BVOC) leading to aerosol formation. Although the exact formation route for 3 nm particles is still unclear, the project results can be summarised as follows: (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, we do not have a direct proof of these phenomena, since it is impossible to determine the composition of 1 to 5-nm-size particles using the present state-of-art instrumentation; (ii) If 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 showed that the amount of the condensable vapour needed for observed growth of aerosol particles is in the range 1–5 x 107 cm–3. The estimations for the vapour source rate are in the range 3–8 x 104 cm–3s–1.
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Mäkelä, J. M., Dal Maso, M., Pirjola, L., Keronen, P., Laakso, L., Kulmala, M. & Laaksonen, A. 2000. Characteristics of the atmospheric particle formation events observed at a borel forest site in southern Finland. Boreal Env. Res. 5: 299–313.

We analysed 184 formation events of new atmospheric aerosol particles, observed at a boreal forest site in Hyytiälä, southern Finland. Recognition, selection and classification of the formation events was based on continuous experimental size distribution data for submicron particles from a period 31 January 1996–18 September 1999 (1327 days). The formation events were classified, and their characteristic features such as the starting time and duration of the particle formation, the number of new particles produced, the particle growth rate at the beginning of the formation burst, and the final particle size after the observed 8-hour growth subsequent to formation, were quantified. The formation rate of 3 nm particles, J3, varied in the range 0.001–1 particles cm–3 s–1. The ultrafine particle growth rates varied in the range 1–17 nm h–1. The possible coupling between the apparent formation rate of new particles and their growth rate subsequent to formation was discussed.
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Kulmala, M., Rannik, Ü., Pirjola, L., Dal Maso, M., Karimäki, J., Asmi, A., Jäppinen, A., Karhu, V., Korhonen, H., Malvikko, S.-P., Puustinen, A., Raittila, J., Romakkaniemi, S., Suni, T., Yli-Koivisto, S., Paatero, J., Hari, P. & Vesala, T. 2000. Characterization of atmospheric trace gas and aerosol concentrations at forest sites in southern and northern Finland using back trajectories. Boreal Env. Res. 5: 315–336.

The trace gas and aerosol concentrations as well as meteorological data (radiation, temperature, humidity) measured in Hyytiälä and Värriö, southern and northern Finland, respectively, were investigated with air mass analyses. The back trajectories of air masses arriving to the sites on the 925-hPa pressure level were calculated 96 hours backwards in time. Two trajectories per day, arriving at 00 UTC and 12 UTC, were computed. The studied time period covered December 1997 to August 1998, November 1998 to July 1999, and September 1999. The arriving air masses were divided into five sectors according to their origin: I = North–West (Arctic Ocean), II = North–East (Northern Russia, Kola Peninsula), III = South–East (Southern Russia, St. Petersburg), IV = South–West (Central Europe, Great Britain), and V Local, circulating air masses. The climatology of various properties of air masses originating from different sectors was studied. The analysis showed differences between typically clean (sectors I and II) and polluted (sectors III and IV) air masses. In air masses from sectors III and IV, the NOx concentrations were high during all seasons, but the O3 concentrations were high during the spring and summer seasons and low in winter. The highest SO2 concentrations arrived from sector III. In Värriö also the air from sector II was accompanied with higher SO2 concentrations and some very high peaks were observed in winter when air masses passed through the industrial area of Kola Peninsula. Nucleation events typically occurred in clean air masses and accumulation mode concentrations were higher in polluted air masses. Although there were some differences between Hyytiälä and Värriö, the overall behaviour was similar at both sites.
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O’Dowd, C.D., Becker, E., Mäkelä. J.M., Kulmala, M. 2000. Aerosol physico-chemical characteristics over a boreal forest determined by volatility analysis. Boreal Env. Res. 5: 337–348.

A thermal volatility technique was used in the boreal forest environment to examine accumulation mode (0.05–0.35 µm radius) physico-chemical properties as a function of air mass origin. Three primary aerosol species were identified in all air masses: (1) a semi-volatile organic component, (2) ammonium sulphate, and (3) a non-volatile component thought to comprise mostly of soot carbon. Under some conditions, sulphuric acid was also identified, as was sea salt. Following nucleation and growth of new particles into accumulation mode sizes, the organic fraction of accumulation mode aerosol, by mass, was observed to increase from 30%, prior to and during the nucleation event, up to 75% by the end of the growth period, indicating a substantial fraction of organic mass condensing onto newly formed particles.
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Aalto, P. & Kulmala, M. 2000. Using a cloud condensation nuclei counter to study CCN properties and concentrations. Boreal Env. Res. 5: 349–359.

A Cloud Condensation Nuclei Counter (CCNC) has been constructed, calibrated and used in field conditions. The CCNC was used in a rural/boreal forest site (Hyytiälä), in marine/coastal sites (Tenerife and Mace Head), and in an urban site (Helsinki). Expressions for determining the soluble volume fraction of particles were derived. The soluble fraction of Aitken mode particles was determined and the CCNC number concentrations were observed. In the marine air masses the soluble fraction was seen to be around 0.8. In the rural site the soluble fraction was around 0.3 and some diurnal variation was seen. In Helsinki the soluble fraction was somewhat higher than in the boreal forest site.
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Pirjola, L. & Kulmala, M. 2000. Aerosol dynamical model MULTIMONO. Boreal Env. Res. 5: 361–374.

We have developed two effective aerosol dynamical models MULTIMONO and MONO32. The models take into account gas-phase chemistry and aerosol dynamics and includes the following processes: (1) emissions of gases and particles; (2) chemical reactions in the gas phase; (3) dry deposition of gases and particles; (4) homogeneous binary H2SO4-H2O or ternary H2SO4-H2O-NH3 nucleation; (5) multicomponent condensation of H2SO4, H2O, HNO3, NH3 and some organic vapour X onto particles; and (6) inter- and intramode coagulation of particles. The particles can be classified into four different size modes which are monodisperse (all particles in a mode possess the same size and composition). In these models the different aerosol properties, such as the particle number concentration, the particle diameter, the mass and composition of the whole distribution, and the mass of particulate matter smaller than 2.5 µm (PM2.5) and smaller than 10 µm (PM10) can be studied. Particles can include soluble material such as sulphate, nitrate, ammonium, and sodium chloride, as well as insoluble material such as organic carbon, elemental carbon, and mineral dust. We have chosen five different particle classes for each size mode (MULTIMONO), or assumed internally-mixed particles (MONO32). The developed models have been compared with a more detailed sectional model AEROFOR2. The comparison shows that the developed models are physically sound. The performed model runs show that the composition of aerosol particles depends mainly on emissions and condensation. Coagulation seems to be of minor importance. The state of mixing can be studied effectively using MULTIMONO or MONO32. E.g. the degree of internal mixing depends on the condensation rate and condensation time. The developed models can be used as sub-models in one-dimensional boundary layer models and three-dimensional Eulerian models.
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