Hari, P., Andreae, M. O., Kabat, P. & Kulmala, M. 2009: A comprehensive network of measuring stations to monitor climate change. Boreal Env. Res. 14: 442–446.

The atmospheric CO₂ concentration and temperature have been rather stable at the time scale of millennia, although rather large variations have occurred during longer periods. The extensive use of fossil fuels and destruction of forests have recently increased the atmospheric CO₂ concentrations. Temperature and circulation of water on the globe are reacting to the increase in the atmospheric CO₂ concentration. Mankind urgently needs knowledge on the present climate change and on its effects on living nature. We propose that a network of comprehensive measuring stations should be constructed, utilizing modern technology to provide documentation of the climate change and data for research related to it. To be able to cover spatial and temporal variations, a hierarchy of stations is needed.

Junninen, H., Lauri, A., Keronen, P., Aalto, P., Hiltunen, V., Hari, P. & Kulmala, M. 2009: Smart-SMEAR: on-line data exploration and visualization tool for SMEAR stations. Boreal Env. Res. 14: 447–457.

Smart-SMEAR is an interface to visualize the data measured at the SMEAR II station in Hyytiälä, Finland. It is designed to be an additional tool for data mining and a simple tool for working together with trace gas and aerosol data, meteorological parameters and air mass back-trajectories, including numerous point sources. The web tool acts as an interface to the station database which can be reached with only few mouse clicks. The tool provides an overview of such a complicated dataset. The database contains all the data from the start of continuous SMEAR II measurements in 1996 until today. Here, we present also some examples of how to use Smart-SMEAR. We present examples of one typical atmospheric nucleation event day, one local pollution event with low photochemical activity, and one long-range-transport pollution day coupled with local pollution events with strong solar radiation. Smart-SMEAR, together with its search engine Smart-Search, has proven to be an efficient tool for studying atmospheric chemistry and atmospheric aerosol dynamics.

Birmili, W., Schwirn, K., Nowak, A., Petäjä, T., Joutsensaari, J., Rose, D., Wiedensohler, A., Hämeri, K., Aalto, P., Kulmala, M. & Boy, M. 2009: Measurements of humidified particle number size distributions in a Finnish boreal forest: derivation of hygroscopic particle growth factors. Boreal Env. Res. 14: 458–480.

Dry and humidified size distributions of atmospheric particles were characterised at the atmospheric research station SMEAR II, Finland between May and July 2004. Particles were classified in a size range between 3 and 800 nm at controlled relative humidities up to 90% by two instruments complementary in size range (HDMPS; Nano-HDMPS). Using the summation method, descriptive hygroscopic growth factors (DHGF) were derived for particle diameters between 70 and 300 nm by comparing dry and humidified size distributions. At 90% relative humidity, DHGF showed mean values between 1.25 and 1.45 in the accumulation mode, between 1.20 and 1.25 in the Aitken mode, and between 1.15 and 1.20 in the nucleation mode. Due to the high size resolution of the method, the transition in DHGF between the Aitken and accumulation modes, which reflects differences in the soluble fraction, could be pinpointed efficiently. For the accumulation mode, experimental DHGFs were compared to those calculated from a simplistic growth model initialised by in-situ chemical composition measurements, and yielded maximum deviations around 0.1. The variation in DHGF could only imperfectly be linked to meteorological factors. A pragmatic parameterisation of DHGF as a function of particle diameter and relative humiditity was derived, and subsequently used to study the sensitivity of the condensational sink parameter (CS) as a function of height in a well-mixed boundary layer.

Jaatinen, A., Hamed, A., Joutsensaari, J., Mikkonen, S., Birmili, W., Wehner, B., Spindler, G., Wiedensohler, A., Decesari, S., Mircea, M., Facchini, M. C., Junninen, H., Kulmala, M., Lehtinen, K. E. J. & Laaksonen, A. 2009: A comparison of new particle formation events in the boundary layer at three different sites in Europe. Boreal Env. Res. 14: 481–498.

In this study, we compare new particle formation (NPF) in the boundary layer at different sites in Europe: Melpitz, Germany (central Europe), San Pietro Capofiume, Italy (southern Europe) and Hyytiälä, Finland (northern Europe) for the period of two years (July 2003–June 2005). NPF was studied based on observations of the particle size distribution, meteorological and gas-phase parameters. Nucleation was found to occur frequently at all stations, however seasonal differences were observed for every station. These differences have a clear correlation with the annual variation of estimated formation rate values. The growth rate reached its maximum values during summer at all three stations. In Hyytiälä the formation and growth of the particles was characterized by low pre-existing condensation sink and most likely high biogenic VOC concentrations associated with the growth season, and in Melpitz and San Pietro Capofiume by the high level of pollution arriving from the nearby industrial and agricultural sources.

Järvi, L, Mammarella, I., Eugster, W., Ibrom, A., Siivola, E., Dellwik, E., Keronen, P., Burba, G. & Vesala, T. 2009: Comparison of net CO₂ fluxes measured with open- and closed-path infrared gas analyzers in an urban complex environment. Boreal Env. Res. 14: 499–514.

Simultaneous eddy covariance (EC) measurements of CO₂ fluxes made with open-path and closed-path analyzers were done in urban area of Helsinki, Finland, in July 2007–June 2008. Our purpose was to study the differences between the two analyzers, the necessary correction procedures and their suitability to accurately measure CO₂ exchange in such non-ideal landscape. In addition, this study examined the effect of open-path sensor heating on measured fluxes in urban terrain, and these results were compared with similar measurements made above a temperate beech forest in Denmark. The correlation between the two fluxes was good (R² = 0.93) at the urban site, but during the measurement period the open-path net surface exchange (NSE) was 17% smaller than the closed-path NSE, indicating apparent additional uptake of CO₂ by open-path measurements. At both sites, sensor heating corrections evidently improved the performance of the open-path analyzer by reducing discrepancies in NSE at the urban site to 2% and decreasing the difference in NSE from 67% to 7% at the forest site. Overall, the site-specific approach gave the best results at both sites and, if possible, it should be preferred in the sensor heating correction.

Kivekäs, N., Kerminen, V.-M., Raatikainen, T., Vaattovaara, P., Laaksonen, A. & Lihavainen, H. 2009: Physical and chemical characteristics of aerosol particles and cloud-droplet activation during the Second Pallas Cloud Experiment (Second PaCE). Boreal Env. Res. 14: 515–526.

The Second Pallas Cloud Experiment (Second PaCE) was conducted at the Pallas-Sodankylä Global Atmosphere Watch (GAW) station in northern Finland from 16 September to 6 October 2005. Measured parameters included aerosol number size distribution, aerosol chemical composition, aerosol hygroscopic growth factor, cloud droplet number size distribution and meteorological parameters. Air mass back trajectories were also calculated. The particulate volume and the inorganic fraction (IO) of particulate mass depended strongly on the air-mass history: central European air masses contain much more particulate matter and have higher IO than marine air masses. The hygroscopic growth factor of particles was positively correlated with the IO. Aerosol activation into cloud droplets was studied for accumulation mode particles (d_p > 100 nm). The activation of these particles did not show clear dependency on the number concentration of accumulation mode particles or on IO. These two parameters were positively correlated and their effects on the particle activation could not be separated.

Kyrö, E.-M., Grönholm, T., Vuollekoski, H., Virkkula, A., Kulmala, M. & Laakso, L. 2009: Snow scavenging of ultrafine particles: field measurements and parameterization. Boreal Env. Res. 14: 527–538.

Snow scavenging of sub-micrometre aerosol particles is poorly known, even though it is an important aerosol removal mechanism in the polar and mountainous regions as well as in the upper troposphere and in mid-latitudes in winter. In our study, snow scavenging coefficients were calculated using four years of particle number concentration spectra measurements together with meteorological measurements. The obtained experimental median scavenging coefficients varied between 8.7 x 10^–6 s^–1 and 5.2 x 10^–5 s^–1 in the 10 nm to 1 um size range. A parameterization of the results is presented and its functionality is tested using University of Helsinki Multicomponent Aerosol model (UHMA). The parameterization applies to snowfall types of slight continuous snowfall and snow grains with intensities of the order of 0.1 mm h^–1.

Laitinen, T., Hartonen, K., Kulmala, M. & Riekkola, M.-L. 2009: Aerosol time-of-flight mass spectrometer for measuring ultrafine aerosol particles. Boreal Env. Res. 14: 539–549.

An aerosol mass spectrometer for ultrafine aerosol particles was constructed. The operation of the apparatus is based on the aerosol particle collection onto a metal surface over a short period. The near real-time aerosol particle collection makes possible an analysis of the particle phase compounds with short lifetimes. The collected aerosol particles are charged with a bipolar charger, size-separated by a differential mobility analyzer according to their electrical mobility and collected via electrostatic deposition on the electrically charged metal surface of a sampling valve. The collected particles are subjected to a two-step laser desorption ionization before the mass spectrometric analysis. The ionized molecules are analyzed according to their m/z ratios and detected with a micro-channel plate detector. The aerosol mass spectrometer was calibrated with different chemical compounds, and its applicability for the determination of selected organic compounds was studied. The calibration of the mass axis showed a good correlation (R² = 0.9996). Mass spectral data for ultrafine ambient aerosol particles was obtained at sub-picogram concentrations.

Laurila, T., Tuovinen, J.-P. & Hatakka, J. 2009: Ozone concentration variations observed in northern Finland in relation to photochemical, transport and cloud processes. Boreal Env. Res. 14: 550–558.

Ozone concentrations measured at the Pallas-Sodankylä Global Atmosphere Watch station, located north of the Arctic Circle on a low mountain, were analysed together with data on nitrogen oxides concentrations and cloudiness. Air parcel trajectories were used to characterise the source area of air masses. The monthly mean ozone concentrations at Pallas showed a maximum in April and a broad minimum in late summer/autumn. In summer, the highest concentrations were observed in southern air masses on average, while the Atlantic and Arctic cases dominated in the earlier and latter part of the year, respectively. In autumn/early winter, there were large differences between the source areas. Throughout the year, ozone concentrations were depleted in the presence of clouds. In summer, the reduction was larger in unpolluted than polluted cases. It is suggested that the formation of nitrate radical and the scavenging of dinitrogen pentoxide act as an efficient ozone sink in winter, while in summer the difference between clear and cloudy conditions is due to the direct and indirect effects of aqueous chemistry.

Leppä, J., Kerminen, V.-M., Laakso, L., Korhonen, H., Lehtinen, K. E. J., Gagné, S., Manninen, H. E., Nieminen, T. & Kulmala, M. 2009: Ion-UHMA: a model for simulating the dynamics of neutral and charged aerosol particles. Boreal Env. Res. 14: 559–575.

A new aerosol dynamical box model, Ion-UHMA (University of Helsinki Multicomponent Aerosol model for neutral and charged particles), is introduced in this paper. The model includes basic dynamical processes (condensation, coagulation and deposition) as well as ion–aerosol attachment and ion–ion recombination. The formation of particles is treated as model input or, alternatively, the model can be coupled with an existing nucleation model. Ion-UHMA was found to be able to reproduce qualitatively the measured time evolution of the particle number size distribution, when the particle formation and growth rates as well as concentrations of particles > 20 nm in diameter were taken from measurements. The simulated charging state of freshly formed particles during a new particle formation event evolved towards charge equilibrium in line with previously-derived analytical formulae. We provided a few illustrative examples to demonstrate possible applications, to which the Ion-UHMA model could be used in the near future.

Leskinen, A., Portin, H., Komppula, M., Miettinen, P., Arola, A., Lihavainen, H., Hatakka, J., Laaksonen, A. & Lehtinen, K. E. J. 2009: Overview of the research activities and results at Puijo semi-urban measurement station. Boreal Env. Res. 14: 576–590.

We introduce a new measurement station that we established in 2005 in an observation tower at Puijo in Kuopio. At Puijo we measure several meteorological parameters, aerosol and cloud droplet size distribution, aerosol optical properties and trace gas concentrations. We summarize the research activities at the station during its three-year history and present overall results. We compare the results from Puijo with those measured at the ground level in Kuopio and at the Finnish background stations. We also characterize the measured parameters according to the wind direction and air mass origins, based on trajectory analysis, for the effects of local and remote sources. Our conclusion is that the Puijo tower is a very good place to gather experimental data on cloud formation and aerosol–cloud interaction. In addition to cloud experiments, we would suggest the Puijo measurement station for studies of particle formation, which we also observed frequently.

Manninen, H. E., Petäjä, T., Asmi, E., Riipinen, I., Nieminen, T., Mikkilä, J., Hõrrak, U., Mirme, A., Mirme, S., Laakso, L., Kerminen, V.-M. & Kulmala, M. 2009: Long-term field measurements of charged and neutral clusters using Neutral cluster and Air Ion Spectrometer (NAIS). Boreal Env. Res. 14: 591–605.

To understand the very first steps of atmospheric particle formation and growth, measurements at the size where atmospheric nucleation occurs are crucially needed. We present number size distributions of > 0.8 nm ions and > 1.8 nm particles measured with the Neutral cluster and Air Ion Spectrometer (NAIS) at a boreal forest site in Hyytiälä, Finland, during 2006–2007. Pools of neutral and charged clusters seem to be present all the time below 3 nm. The total concentration of 1.8–3.0 nm clusters was ~1000 cm^–3, ranging from 50 to 6000 cm^–3, whereas the concentrations of charged clusters in this size range remained below 50 cm^–3. On average, both neutral and charged clusters seem to have different diurnal cycles on particle formation event and non-event days. We also compared the NAIS-derived concentrations with those obtained from other concurrent air ion and particle instruments and found that the NAIS concentrations are in agreement with concentrations determined with these other instruments.

Ortega, I. K., Suni, T., Grönholm, T., Boy, M., Hakola, H., Hellén, H., Valmari, T., Arvela, H., Vehkamäki, H. & Kulmala, M. 2009: Is eucalyptol the cause of nocturnal events observed in Australia? Boreal Env. Res. 14: 606–615.

Nocturnal nucleation events were observed from July 2005 to October 2006 in Tumbarumba in New South Wales, Australia. These events were observed on one third of the nights and they were often much more intense than normal daytime events. One of the main features found in this environment was the abundance of eucalyptol among the volatile organic compounds (VOC) emitted by the local Eucalypt forest. In contrast, in most other forest environments, such as Hyytiälä in southern Finland, eucalyptol is a minor component of VOC emissions. Therefore, the objective of this study was to evaluate the role of eucalyptol in the nocturnal nucleation events. For this purpose, a series of experiments and quantum mechanical calculations were performed. Both approaches showed that the role of eucalyptol in the nocturnal events can be ruled out.

Paasonen, P., Sihto, S.-L., Nieminen, T., Vuollekoski, H., Riipinen, I., Plaß-Dülmer, C., Berresheim, H., Birmili, W. & Kulmala, M. 2009: Connection between new particle formation and sulphuric acid at Hohenpeissenberg (Germany) including the influence of organic compounds. Boreal Env. Res. 14: 616–629.

We analysed the coupling of atmospheric particle formation and sulphuric acid during the Hohenpeissenberg Aerosol Formation Experiment (HAFEX) carried out in 1998–2000. A clear correlation between the particle formation rate and the sulphuric acid concentration to the power from one to two was identified. For each of the analysed 45 particle formation days, we evaluated the coefficients quantifying this coupling in activation and kinetic nucleation theories, predicting linear and squared dependencies, respectively. The median activation and kinetic coefficients were 1.6 x 10^–7 s^–1 and 3.2 x 10^–14 cm³ s^–1. The daily values of the coefficients varied within two orders of magnitude and, even during one day, the observed formation rates fluctuated around the predicted rates. A squared dependency was almost as frequent as linear, but the kinetic coefficient anticorrelated with sulphuric acid concentration. This anticorrelation and the variation of the coefficients could be related to the observed correlation between the coefficients and monoterpene oxidation product concentrations.

Parshintsev, J., Räsänen, R., Hartonen, K., Kulmala, M. & Riekkola, M.-L. 2009: Analysis of organic compounds in ambient aerosols collected with the particle-into-liquid sampler. Boreal Env. Res. 14: 630–640.

A particle-into-liquid sampler (PILS) was applied to the collection of atmospheric aerosol particles for the analysis of [alpha]-pinene and its oxidation products. A cyclone and four denuders were coupled to the PILS to eliminate large particles and minimize the negative effects of gas-phase compounds. The PILS setup, sampling time, number and quality of denuders, sample pretreatment and analysis were optimized. Pinonaldehyde, [alpha]-pinene, pinic acid and cis-pinonic acid were selected for the investigation. The results showed that the PILS is suitable for the sampling of biogenic aerosols because oxidation, evaporation and adsorption, which are the most important sources for artifacts in filter and impactor sampling, can be minimized. In addition, a short sampling time and capability for on-line analysis of major organic compounds in aerosols make this a promising technique for obtaining reliable information about particle composition with the good time resolution. In addition, the PILS can be on-line coupled to mass spectrometry. Advantages and disadvantages of the PILS in regard to the analysis of organic compounds are discussed.

Portin, H. J., Komppula, M., Leskinen, A. P., Romakkaniemi, S., Laaksonen, A. & Lehtinen, K. E. J. 2009: Observations of aerosol–cloud interactions at the Puijo semi-urban measurement station. Boreal Env. Res. 14: 641–653.

The Puijo measurement station has produced continuous data on aerosol–cloud interactions since June 2006. The station is located on the top floor of an observation tower in a semi-urban environment near the town of Kuopio in central Finland. The top of the tower (306 m a.s.l.) has been detected to be in-cloud approximately 10% of the time. We analysed continuous weather, particle size distribution and cloud droplet size distribution measurements. The effects of local pollutant sources and air mass origin on aerosol–cloud interaction were examined in detail. We were able to find clear evidence of the aerosol indirect effects at the Puijo site. There is a positive correlation between cloud droplet number concentration and particle number concentration. Higher cloud droplet concentration led to a smaller average cloud droplet size. Furthermore, the ratio of cloud droplet number concentration to accumulation mode particle number concentration is smaller when the particle number concentration is higher. Results from our trajectory analysis indicated that at our site marine air masses had higher particle concentrations and the continental aerosols are more effective in acting as cloud condensation nuclei than marine aerosols, probably due to their larger mean size. We could also distinguish the effect of local pollutant sources.

Toivola, M., Napari, I. & Vehkamäki, H. 2009: Structure of water–sulfuric acid clusters from molecular dynamics simulations. Boreal Env. Res. 14: 654–661.

Molecular dynamics method is applied to study the structure of stable sulfuric acid–water clusters at various compositions. Also a planar liquid–vapor interface is studied. Two different potential models were used. In the simpler model, sulfuric acid (H₂SO₄) remains in an undissociated state. A more realistic scheme requires that in the presence of water (H₂O) H₂SO₄ protonates to form bisulfate (HSO₄^–) and hydronium (H₃O⁺) ions. This effect is described by considering a system consisting of HSO₄^– and H₃O⁺ ions, and water. The main focus is on the structure of clusters of hundred molecules at different compositions. The results are compared with those for the planar liquid–vapor interface. In the unprotonated system sulfuric acid lies on the cluster surface, if the total mole fraction of H₂SO₄ is smaller than 0.1, whereas at a planar interface such enhanced surface activity is not seen. In the protonated system the bisulfates are at the center of the cluster and the hydronium ions on the surface when the sulfuric acid concentration is small. The presence of ions is found to destabilize the clusters at higher compositions and the planar interfaces at all compositions.

Vaattovaara, P., Petäjä, T., Joutsensaari, J., Miettinen, P., Zaprudin, B., Kortelainen, A., Heijari, J., Yli-Pirilä, P., Aalto, P., Worsnop, D. R. & Laaksonen, A. 2009: The evolution of nucleation- and Aitken-mode particle compositions in a boreal forest environment during clean and pollution-affected new-particle formation events. Boreal Env. Res. 14: 662–682.

We applied two tandem differential mobility analyzer methods to shed light on the evolution of the nucleation- and Aitken-mode-particle compositions at a virgin boreal-forest site during nucleation events in varying conditions. The overall results show a clear anthropogenic influence on the nucleation- and Aitken-mode-particle compositions during the events. The SO₂/MTOP and NO_x/MTOP (monoterpene oxidation products) ratios best explain the variation in the nucleation mode composition during clean and pollution-affected events, suggesting also the importance of organic sulfur compounds, in addition to other sulfur, nitrogen and organic compounds, in particle formation, composition and properties. During the cleanest events, MTOP explain well the time behaviour of the 10-nm particle composition with an estimated organic fraction of over 95%.

Yli-Juuti, T., Riipinen, I., Aalto, P. P., Nieminen, T., Maenhaut, W., Janssens, I. A., Claeys, M., Salma, I., Ocskay, R., Hoffer, A., Imre, K. & Kulmala, M. 2009: Characteristics of new particle formation events and cluster ions at K-puszta, Hungary. Boreal Env. Res. 14: 683–698.

Atmospheric new particle formation events were analyzed based on particle size distributions measured with a Differential Mobility Particle Sizer (DMPS) and an Air Ion Spectrometer (AIS) during the BIOSOL (Formation mechanisms, marker compounds, and source apportionment for biogenic atmospheric aerosols) campaign on 22 May–29 June 2006 at the K-puszta measurement site in Hungary. The particle size distribution data were classified into different new particle event classes and growth and formation rates of the particles were calculated. New particle formation was observed on almost all days and the median diameter growth rates of nucleation mode particles increased with increasing particle size. The observed formation rate of 10 nm particles was typically somewhat larger than 1 cm^–3 s^–3 (median 1.2), and the growth rate for sub 3 nm particles was 1.7 nm h^–1 and for nucleation mode 6 nm h^–1. The ambient concentrations of gases or meteorological data were not able to explain the differences in the growth and formation rates or in the particle formation between the days. However, 0.3–1.8 nm cluster ion concentrations correlated negatively with wind speed.

Aurela, M., Lohila, A., Tuovinen, J.-P., Hatakka, J., Riutta, T. & Laurila, T. 2009: Carbon dioxide exchange on a northern boreal fen. Boreal Env. Res. 14: 699–710.

Long-term net ecosystem CO₂ exchange measurements were conducted on a nutrient-rich fen in northern Finland using the eddy covariance method. During the three measurement years (2006–2008), the mean daytime CO₂ flux in July was –0.40 mg CO₂ m^–2 s^–1, while in mid-winter (January–March) the mean efflux was 0.008 mg CO₂ m^–2 s^–1. Annual balances of –12, –123 and –216 g CO₂ m^–2 a^–1 were observed in 2006, 2007 and 2008, respectively. It is suggested that the low uptake in 2006 was related to the warm and dry conditions during the growing season and the consequent reduction in vegetation activity. When compared with two other fens in Finland, there was a clear correspondence between the nutrient status, mean pH value, maximum LAI and the mid-summer CO₂ uptake. A similar pattern was not seen in the annual CO₂ balance, which correlates more with the growing season length.

Dal Maso, M., Hari, P. & Kulmala, M. 2009: Spring recovery of photosynthesis and atmospheric particle formation. Boreal Env. Res. 14: 711–721.

We investigated springtime atmospheric particle formation in a Finnish boreal forest with respect to the spring recovery of photosynthetic capacity by using particle size distribution data together with a temperature-dependent state of photosynthetically active substances parameter, S. Spring is a period of exceptionally high particle formation event frequency and the events also produce a high number of aerosol particles. We found that the onset of the period of a high particle formation event frequency coincides with start of the spring recovery of the photosynthetic activity in the boreal coniferous forest. The highest particle formation frequency was found to occur with values of S close to 0 deg.C, which corresponds to a relative photosynthetic efficiency of about 30% of the maximum efficiency. The nucleation mode particle number concentration was highest with values of S between –3 deg.C and 0 deg.C, higher S resulting in lower average particle number concentrations. The increase in particle formation efficiency was also clearly reflected in the particle size distribution. Investigations in other atmospheric parameters important for nucleation revealed elevated ozone concentrations, but no clear other reason for the elevated particle formation activity. Therefore, we suggest that the particle formation peak is caused by biogenic volatile compound emissions from the boreal ecosystem that is undergoing spring recovery.

Hakola, H., Hellén, H., Tarvainen, V., Bäck, J., Patokoski, J. & Rinne, J. 2009: Annual variations of atmospheric VOC concentrations in a boreal forest. Boreal Env. Res. 14: 722–730.

Ambient atmospheric concentrations of monoterpene compounds were measured above a boreal forest in Hyytiälä, Finland during 2000–2007. For most of the time, two samples per week were collected, although there are some gaps in the data due to analytical or other issues. The monoterpene concentrations reached their maximum in summer, although they were found to be quite high also during winter. The main compounds found during winter were [alpha]-pinene, [delta]³-carene, [beta]-pinene and camphene. In summer 1,8-cineol and sabinene were also present in the samples. The concentrations of [alpha]-pinene, [beta]-pinene/myrcene, camphene, [delta]³-carene increased during the measurement period both in winter and in summer. This increase cannot be explained by meteorological conditions. The possible explanations could be human activities in the vicinity of the sampling site in addition to forest growth. The seasonal cycles of daytime concentrations were found to follow emission fluxes modeled using a simple temperature dependent parameterisation. The measured monoterpene concentrations were used, together with emission rate measurements, for estimating ambient atmospheric [beta]-caryophyllene concentration which cannot be directly measured due to its high reactivity against ozone.

Ilvesniemi, H., Levula, J., Ojansuu, R., Kolari, P., Kulmala, L., Pumpanen, J., Launiainen, S., Vesala, T. & Nikinmaa, E. 2009: Long-term measurements of the carbon balance of a boreal Scots pine dominated forest ecosystem. Boreal Env. Res. 14: 731–753.

We measured on annual basis the magnitude and relative importance of different components of the carbon balance of a boreal Scots pine dominated forest ecosystem. The continuous 10-year-long measurement period of this study and the miscellaneous measurements of the components of the ecosystem carbon balance carried out at the same site are almost unique. The ecosystem was shown to be a carbon sink in all measured years. The average net ecosystem exchange (NEE) estimated with the eddy covariance (EC) method was ^–206 g C m^–2 a^–1 and the average annual accumulation of carbon into trees was 242 g C m^–2 a^–1. The above-ground litter production was 186 g C m^–2 a^–1 of which 92 g consisted of needles and leaves. The average respiration rates of the soil, canopy and stems were 646, 316 and 62 g C m^–2 a^–1, respectively and the TER deduced from EC measurements was 826 g C m^–2 a^–1. The average rate of the tree and ground vegetation photosynthesis was 982 and 114 g C m^–2 a^–1, respectively. In forest ecosystems the application of biomass equations to measurements of tree dimensions and increment cores can give a reliable and unbiased estimate of carbon accumulation into trees. The chamber based flux measurements are useful in showing short term response to changes in light, temperature and moisture conditions, but the generalization of the results over time and space is difficult.

Kulmala, L., Grönholm, T., Laakso, L., Keronen, P., Pumpanen, J., Vesala, T. & Hari, P. 2009: Pressure responses of portable CO₂ concentration sensors. Boreal Env. Res. 14: 754–760.

Here, we investigated the pressure response of five different models of portable CO₂ infrared sensors: GMP343 (Vaisala Oyj, Finland), ADC LCA-2 (The ADC Bioscientific Ltd., UK), and three different EGMs (PP Systems, UK). In the pressure range of 750–1000 hPa, we found that the analysers function systematically in an individual manner and the original pressure corrections that the manufacturers have provided are inaccurate from a few to tens of ppm. Therefore, we determined new empirical pressure correction functions for each sensor. The resulting corrections perform better than the original ones in the pressure, CO₂ concentration (370–490 ppm), temperature (5–21 deg.C) and water-vapour (4–11 g m^–3) range of this study. However, since the individual analysers had different pressure responses, we recommend that each individual instrument be tested in cases where changes in pressure may affect the results.

Kolari, P., Kulmala, L., Pumpanen, J., Launiainen, S., Ilvesniemi, H., Hari, P. & Nikinmaa, E. 2009: CO₂ exchange and component CO₂ fluxes of a boreal Scots pine forest. Boreal Env. Res. 14: 761–783.

We studied CO₂ fluxes derived from eddy covariance (EC), modelled with a stand photosynthesis model, and upscaled from continuous measurements with chambers in a Scots pine stand. The annual photosynthesis (GPP_EC), ecosystem respiration (R_e,EC) and net CO₂ exchange (NEE_EC) derived from EC were correlated with each other. Soil CO₂ efflux dominated R_e for the whole year, most clearly in winter. The relative contributions of the aboveground respiration components were largest in spring and early summer. The respiration components generally followed the seasonal patterns of temperature although temperature-normalised respiration was higher in the growing season than in winter. The respiration components showed parallel decline during drought. Interannual variability in the annual chamber-based CO₂ budgets was twice as large as in the EC-based fluxes, the uncertainty in the chamber fluxes was also larger. Using different environmental drivers for estimating R_e from NEE_EC affected the annual R_e,EC and GPP_EC ±4%.

Lallo, M., Aalto, T., Hatakka, J. & Laurila, T. 2009: Hydrogen soil deposition in northern boreal zone. Boreal Env. Res. 14: 784–793.

Deposition velocities (v_d) of molecular hydrogen were measured in the northern boreal zone in Pallas. Three separate methods were used to estimate the soil uptake rate. For soil chamber measurements, three sites (forest, wetland and above tree line) were selected according to different soil properties. The deposition velocity was calculated with radon tracer method taking advantage of nocturnal radon build-up and hydrogen decrease in the shallow mixing layer above ground. A two dimensional model was used to estimate corresponding night time values for v_d and radon exhalation rate. The radon tracer and two-dimensional model results were in the range of 0.18–0.52 mm s^–1. All results were obtained for unfrozen soils, and there was no clear temperature dependence related to v_d. Soil chamber results were in the range of 0.06–0.52 mm s^–1 excluding the wetland site which had negligibly small v_d due to water saturation.

Porcar-Castell, A., Peñuelas, J., Owen, S. M., Llusià, J., Munné-Bosch, S. & Bäck, J. 2009: Leaf carotenoid concentrations and monoterpene emission capacity under acclimation of the light reactions of photosynthesis. Boreal Env. Res. 14: 794–806.

Factors controlling the seasonal dynamics in leaf monoterpene emission capacity are not yet well understood. In particular, temperature and light algorithms alone cannot explain the pattern of volatile organic compound (VOC) emissions during spring recovery of photosynthesis in boreal forests, suggesting further physiological controls. Higher isoprenoids, such as carotenoids, and VOCs share the initial steps of their synthesis pathways. Therefore, it could be expected that the pool size of leaf carotenoids and its acclimation interact with leaf monoterpene emission capacity. We examine this interaction in evergreen foliage using the non-storing Quercus ilex as a model species. We modified the light environment of a number of potted trees in order to induce acclimation in leaf carotenoids, and studied the effect on monoterpene emission capacity. The results indicate that monoterpene emission capacity and photosynthetic pigment metabolism are coupled in Q. ilex seedlings growing at or acclimating to different light levels.

Rinne, J., Bäck, J. & Hakola, H. 2009: Biogenic volatile organic compound emissions from the Eurasian taiga: current knowledge and future directions. Boreal Env. Res. 14: 807–826.

In this paper, the research conducted on the emissions of the biogenic volatile organic compounds (BVOCs) from the European boreal zone, or taiga, is reviewed. We highlight the main findings and the key gaps in our knowledge. Ecosystem scale BVOC emissions from the Eurasian taiga are observed to be relatively low as compared with those from some forest ecosystems in warmer climates. One of the distinctive features of the Eurasian taiga is the predominance of monoterpene emitting coniferous trees. Recent research indicates that in addition to evaporation from storage structures, part of the monoterpene emission of conifers originates directly from synthesis. Monoterpene emission from boreal deciduous trees originates mainly directly from synthesis. The boreal trees exhibit distinct intra-species variation in the monoterpene mixtures they emit. Important sources of isoprene in the Eurasian taiga include Norway spruce, open wetland ecosystems and some non-dominant woody species, such as European aspen and willows. Many boreal tree species also emit non-terpenoid compounds and highly reactive sesquiterpenes. The future challenges in the research on BVOC emissions from the Eurasian taiga include (i) quantification and understanding the non-terpenoid VOC emissions from the taiga ecosystems, (ii) bringing ecosystems in the eastern Eurasian taiga into the sphere of BVOC emission studies, (iii) establishing long-term ecosystem flux studies combined with plant physiological measurements, and (iv) integrating knowledge and research skills on BVOC synthesis, strorages and emissions, land cover changes and atmospheric processes in different spatial and temporal scales in order to better understand the impact of biosphere on atmospheric chemistry and composition in changing climate.