Patokoski, J., Ruuskanen, T. M., Hellén, H., Taipale, R., Grönholm, T., Kajos, M. K., Petäjä, T., Hakola, H., Kulmala, M. & Rinne, J. 2014: Winter to spring transition and diurnal variation of VOCs in Finland at an urban background site and a rural site. Boreal Env. Res. 19: 79–103.
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Voutilainen, A., Huttula, T., Juntunen, J., Rahkola-Sorsa, M., Rasmus, K. & Viljanen, M. 2014: Diverging site-specific trends in the water temperature of a large boreal lake in winter and summer due to mixed effects of local features and climate change. Boreal Env. Res. 19: 104–114.
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Tammet, H. & Kulmala, M. 2014: Empiric equations of coagulation sink of fine nanoparticles on background aerosol optimized for boreal zone. Boreal Env. Res. 19: 115–126.
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Lehtonen, I., Ruosteenoja, K., Venäläinen, A. & Gregow, H. 2014: The projected 21st century forest-fire risk in Finland under different greenhouse gas scenarios. Boreal Env. Res. 19: 127–139.
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Lehmann, A., Hinrichsen, H.-H. & Getzlaff, K. 2014: Identifying potentially high risk areas for environmental pollution in the Baltic Sea. Boreal Env. Res. 19: 140–152.
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Patokoski, J., Ruuskanen, T. M., Hellén, H., Taipale, R., Grönholm, T., Kajos, M. K., Petäjä, T., Hakola, H., Kulmala, M. & Rinne, J. 2014: Winter to spring transition and diurnal variation of VOCs in Finland at an urban background site and a rural site. Boreal Env. Res. 19: 79–103.

We measured volatile organic compound (VOC) volume mixing ratios (VMRs) using a quadrupole proton-transfer-reaction mass spectrometer, and investigated the differences between winter and spring VMRs and sources at an urban background site in Helsinki (2006) and a rural site in Hyytiälä (2007) utilizing a receptor model EPA Unmix. In Helsinki, VMRs of VOCs were typically higher,and their diurnal variations were more pronounced than at the rural site. At both sites, monoterpenes have anthropogenic influence in winter, while in spring biogenic influence is dominating. At the urban background site, the main aromatic hydrocarbon source was traffic, which also explained most of the oxidated VOCs during the urban winter. At other times and at the rural location most of oxidated VOCs originated mainly from distant sources. At the rural site, traffic and distant sources contributed equally to the aromatics in winter but in spring the distant source dominated.

Voutilainen, A., Huttula, T., Juntunen, J., Rahkola-Sorsa, M., Rasmus, K. & Viljanen, M. 2014: Diverging site-specific trends in the water temperature of a large boreal lake in winter and summer due to mixed effects of local features and climate change. Boreal Env. Res. 19: 104–114.

We present diverging long-term trends in the water temperature of a large boreal lake, Lake Pyhäselkä (263 km²), located in eastern Finland. The dataset was constructed from a half century of monitoring (1962–2010). The direction of the temperature trend depended on the water layer and the season, in that the yearly average temperature in the top layer (1–10 m) as recorded in summer (June–August) increased by 2.5 °C over the monitoring period, whereas that recorded when the lake was covered by ice (January–April) decreased from 0.6 to 0.2 °C. The water temperature in the bottommost layers showed no trends. We suggest that the water temperature under the ice is decreasing in this case as a consequence of mixed effects of lake-specific physical features and climate change, which cause variations in the heat content of the inflowing water of the Pielisjoki. The epilimnetic water temperature in summer appeared in turn to follow general trends in air temperatures. Our results stress the need for taking local and site-specific phenomena into account when drawing conclusions about the effects of climate change on lakes.

Tammet, H. & Kulmala, M. 2014: Empiric equations of coagulation sink of fine nanoparticles on background aerosol optimized for boreal zone. Boreal Env. Res. 19: 115–126.

Fundamental models of aerosol nanoparticle coagulation sink in the atmosphere are sophisticated and require detailed knowledge about the size distribution of background aerosol particles. Here, we present empiric equations which help to make quantitative relations graspable and support quick and rough estimation of the coagulation sink according to background aerosol particle concentration and vice versa when browsing limited data. Accuracy of an empiric equation is described by a deviation between the results obtained according to two methods: fundamental models and empiric equations. Optimizing empiric equations is discussed considering a set of aerosol size distributions measured at the SMEAR II station, Finland, during years 2008–2010. Fundamental models include the particle-particle coagulation model by Dahneke and the air ion attachment model by Hoppel and Frick. Small air ions are considered a special kind of nanoparticles. The background aerosol is characterized with a power-weighted integral concentration in a restricted range of particle diameters. The power exponent and borders of the diameter range are adjusted when optimizing the equation. Nanoparticles are represented with a power function of the diameter or with the electric mobility. Compromises between the simplicity of the equation and the accuracy of the approximation are considered and different versions of empiric equations are proposed.

Lehtonen, I., Ruosteenoja, K., Venäläinen, A. & Gregow, H. 2014: The projected 21st century forest-fire risk in Finland under different greenhouse gas scenarios. Boreal Env. Res. 19: 127–139.

We evaluated forest fire potential at four locations in Finland in the current climate and in projected future climates under the B1, A1B and A2 greenhouse-gas (GHG) emission scenarios. In evaluating the forest fire danger potential, the Canadian fire weather index (FWI) system was used. Using the results of the earlier experimental ignition studies, we further estimated the number of fire danger days in different forest stands typical to the northern boreal zone. By the end of the current century, the annual median number of days with elevated forest fire risk is projected to increase by 10%–40%, depending on the GHG scenario. In different forest stands, approximately 5–10 additional fire risk days were found annually based on the A1B and A2 scenarios. Substantially smaller changes are projected under the low-emission B1 scenario. However, there is great inter-annual variability in the forest fire potential which, in the nearest future, largely overwhelms the projected change.

Lehmann, A., Hinrichsen, H.-H. & Getzlaff, K. 2014: Identifying potentially high risk areas for environmental pollution in the Baltic Sea. Boreal Env. Res. 19: 140–152.

The study aims at the identification of areas in the Baltic Sea from where potential pollution is transported to vulnerable regions. Generally, there is higher risk of ship accidents along the shipping routes and along the approaching routes to the harbors. The spreading of harmful substances is mainly controlled by prevailing atmospheric conditions and wind-induced local sea surface currents. Especially, spawning, nursery and tourist areas are considered high-vulnerable areas. With sophisticated high resolution numerical models, the complex current system of the Baltic Sea has been simulated, and with subsequent drift modeling areas of reduced risk or high-risk areas for environmental pollution could be identified. In a further step, optimum fairways of reduced risk could be obtained by following probability minima of coastal hits or maxima for the time it takes to reach the coast. The results could be useful for environmental management for the maritime industry to minimize the risk of environmental pollution in case of ship accidents.