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Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model that explains this apparent dichotomy.

We present nearly six years of measurements of surface fluxes of momentum, sensible heat, water vapour, carbon dioxide and aerosol particles, along with a three-month time series of ozone flux, measured by eddy covariance above a Scots pine forest in southern Finland from April 1996 to December 2001. The results show marked seasonal and diurnal variation and a significant effect of local and remote anthropogenic pollution on CO2 and particle data. The flux of inert CO2 followed environmental factors through plant and soil-microbe metabolism very closely. The flux of reactive O3 depended on the overlapping influences of environmental parameters driving photosynthesis and of the available amount of reaction partners. The flux of multireactive aerosol particles showed no clear connection with any environmental variable. On new-particle-formation days, the average deposition velocity of particles was, however, greater than usual because small nucleation-mode particles have greater deposition velocities than do larger accumulation-mode particles.

Spring temperatures were reconstructed by multiproxy database for south-west Finland since 1750. Proxy records used here were ice break-up in the Aurajoki River, the Baltic Sea ice extent, the plant phenological index and the annual varve thickness in the Pyhäjärvi Lake. Records were integrated into one palaeoclimate model using time-scale dependent calibration techniques. Reconstruction was verified with statistics showing a high degree of validation between the reconstructed and observed temperatures in Turku, south-west Finland. Reconstruction demonstrates that the springs have become warmer and reveals a warming trend since 1850s. Except for the period from 1750 to around 1850, the springs have been characterized as having a larger low-frequency variability, as well as by having a smaller range of annual temperature variations. Analyses of decadal variations revealed that the coldest springtimes occurred in the 1840s and 1850s and the first decade of the 19th century. Reconstruction was compared with the available meteorological series of central England, Stockholm, St. Petersburg, Uppsala and the spring-temperature reconstruction from western Norway. The effect of global solar, volcanic, greenhouse gases and aerosol forcings were examined together with the North Atlantic Oscillation (NAO) indices at local scale over the reconstructed period. Reconstructed spring-temperature changes have been related to changes in the atmospheric circulation, as indicated by the NAO (February-June).

We aimed to assess the feasibility of an affordable instrumentation, based on a non-dispersive infrared analyser, to obtain atmospheric CO2 mole fraction data for background CO2 measurements from a flux tower site in southern Finland. The measurement period was November 2006–December 2011. We describe the instrumentation, calibration, measurements and data processing and a comparison between two analysers, inter-comparisons with a flask sampling system and with reference gas cylinders and a comparison with an independent inversion model. The obtained accuracy was better than 0.5 ppm. The inter-comparisons showed discrepancies ranging from –0.3 ppm to 0.06 ppm between the measured and reference data. The comparison between the analyzers showed a 0.1 ± 0.4 ppm difference. The trend and phase of the measured and simulated data agreed generally well and the bias of the simulation was 0.2 ± 3.3 ppm. The study highlighted the importance of quantifying all sources of measurement uncertainty.

Long-term, continuous observations are needed for Earth system investigations and evaluation of simulations. The atmospheric and ecological communities have independently established field sites that have been running for many decades and are integrated into global networks. In the past decade, the importance of long-term observational networks focused on land ecosystem–atmosphere exchange, and the processes controlling land–atmosphere coupling, had been increasingly recognized and has led to the building of a global network of water, carbon and energy flux sites. This is an important step but further enhancements are necessary in order to quantify all of the land–atmosphere processes that need to be included in Earth system models. This paper describes the current land ecosystem–atmosphere measurement capabilities and presents the status and needs for global observational networks.

We aimed to assess the feasibility of an affordable instrumentation, based on a non-disper-sive infrared analyser, to obtain atmospheric CO 2 mole fraction data for background CO 2 measurements from a flux tower site in southern Finland. The measurement period was November 2006-December 2011. We describe the instrumentation, calibration, measurements and data processing and a comparison between two analysers, inter-comparisons with a flask sampling system and with reference gas cylinders and a comparison with an independent inversion model. The obtained accuracy was better than 0.5 ppm. The inter-comparisons showed discrepancies ranging from-0.3 ppm to 0.06 ppm between the measured and reference data. The comparison between the analyzers showed a 0.1 ± 0.4 ppm difference. The trend and phase of the measured and simulated data agreed generally well and the bias of the simulation was 0.2 ± 3.3 ppm. The study highlighted the importance of quantifying all sources of measurement uncertainty.

On-line measurements of atmospheric VOC concentrations in the European boreal zone with a proton transfer reaction mass spectrometer were conducted at SMEAR II station in Hyytiälä, south-western Finland on 2–22 July 2004. The measurements showed a strong diurnal variation of several compounds. A factor analysis performed for the concentration data was used to classify the measured VOC masses into three classes based on the behavior of their concentrations. The masses in the first class had a high diurnal variation with maximum values in the afternoon. Compounds contributing to masses in this category were e.g. methanol, acetone, methyl-vinyl-ketone and hexanal. The concentrations of masses in the second class had also a high diurnal variation, but with maxima during the night when the mixing of the atmospheric surface layer was weak. Monoterpenes and phenol are compounds contributing to the masses in this category. The masses in the third class did not have a marked diurnal cycle and were not dependent on the local meteorological parameters. The masses having a strong positive loading on this factor were those associated with anthropogenic compounds with relatively long atmospheric life-times, such as benzene. Considering the difference in the measurement height, the total monoterpene concentration measured by the PTR-MS was consistent with the concentration measured by gas chromatography-mass spectrometer with adsorbent sampling.

Ozone fluxes were measured over a forest in southern Finland between August 2001 and July 2002 using the eddy covariance method. Systematic errors due to the imperfect frequency response of the instrumentation and random errors due to the stochastic nature of turbulence were estimated. Flux underestimation correction factors for unstable stratification were 1.03-1.19. Random errors of the flux estimates were most frequently about 20% of the flux value. Fluxes were highest during the summer, declining to near zero during the winter. In summer, fluxes were higher during daytime than at night coinciding with higher turbulence and higher rate of stomatal uptake. Maximum summertime deposition velocities were 6-7 mm s super(-1). During winter, the diurnal pattern in ozone flux was weak and the deposition velocity was 0.5-1.5 mm s super(-1). Comparison between eddy covariance and profile flux measurement results generally showed good agreement during daytime.

The effect of boreal forest on the Earth's energy budget was estimated in field conditions in Hyytiälä, southern Finland. The indirect aerosol effect due to new particle formation, the effect of forest carbon sequestration and the effect of forestation- or deforestation-related albedo change were investigated. The young, fast-growing boreal forest was estimated to be a relatively significant carbon sink and an important aerosol source, and thus the effect on the Earth's energy budget was seen to be negative. The estimated values (reported as annually averaged energy budget perturbations per square meter of forest) were –36 W m–2 due to the carbon sink, +10 W m–2 due to the albedo effect and between –5 and –14 W m–2 due to the aerosol effect. However, the values are uncertain.