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This paper describes the background, instrumentation, goals, and the regional influences on the HUMPPA-COPEC intensive field measurement campaign, conducted at the Boreal forest research station SMEAR II (Station for Measuring Ecosystem-Atmosphere Relation) in Hyytiälä, Finland from 12 July–12 August 2010. The prevailing meteorological conditions during the campaign are examined and contrasted with those of the past six years. Back trajectory analyses show that meteorological conditions at the site in 2010 were characterized by a higher proportion of southerly flow than in the other years studied. As a result the summer of 2010 was anomalously warm and high in ozone making the campaign relevant for the analysis of possible future climates. A comprehensive land use analysis, provided on both 5 and 50 km scales, shows that the main vegetation types surrounding the site on both the regional and local scales are: coniferous forest (Scots pine and/or Norway spruce); mixed forest (Birch and conifers); and woodland scrub (e.g. Willows, Aspen); indicating that the campaign results can be taken as representative of the Boreal forest ecosystem. In addition to the influence of biogenic emissions, the measurement site was occasionally impacted by sources other than vegetation. Specific tracers have been used here to identify the time periods when such sources have impacted the site namely: biomass burning (acetonitrile and CO), urban anthropogenic pollution (pentane and SO2) and the nearby Korkeakoski sawmill (enantiomeric ratio of chiral monoterpenes). None of these sources dominated the study period, allowing the Boreal forest summertime emissions to be assessed and contrasted with various other source signatures.

In terrestrial ecosystems, the amount and availability of water is one of the key factors affecting the net primary productivity and other biological processes of the system. At the SMEAR-II station, we have monitored the water balance of two adjacent micro-catchments since 1997. In this study, we report the long-term measurements of precipitation, throughfall, snow depth, soil water content, runoff and evapotranspiration and the annual water balances based on these measurements and discuss the uncertainties related to different measurements. The proportion of throughfall, evapotranspiration and runoff was 67%, 43% and 32% of the annual precipitation, respectively. The measured amounts of evapotranspiration and runoff were so small that the aim of closing the water balance of the studied system was not fully reached. The largest uncertainties are related to the evapotranspiration measurements and the determination of the actual surface area of the catchments used in the calculation of the runoff.

We conducted airborne observations of aerosol physical properties over the southern Finland boreal forest environment. The aim was to investigate the lower tropospheric aerosol (up to 4-km altitude) over an area of 250 by 200 km, in particular during new particle formation (NPF) events, and to address the spatial variability of aerosol number concentration and number size distribution. The regional NPF events, detected both airborne and at the ground, with air masses originating from the Arctic or northern Atlantic Ocean were studied throughout the boundary layer and throughout the area covered. Three suitable case studies are presented in more detail. In two of these studies, the concentrations of nucleation mode particles (3-10 nm in diameter) were found considerably higher (up to a factor of 30) in the upper parts of the planetary boundary layer compared to ground-based measurements during the nucleation events. The observed vertical variation can be connected to boundary layer dynamics and interactions between the boundary layer and the lower free troposphere, likely yielding high concentrations of newly formed aerosol particles. Our results suggest that nucleation does not necessarily occur close to the surface. In one presented case we found evidence of NPF occurring in a limited area above cloud, in the complete absence of a regional NPF event.

Biomass burning emissions from intensive wildfires in eastern Europe were observed in Finland in the spring of 2006 and in the late of summers 2006 and 2010. The smoke plumes were detected at three ground-measurement stations around Finland and in the lower troposphere after long-range transport from fire areas. The vertical extent of the smoke was estimated by flight measurements over southern Finland and the measurements were compared with CALIPSO satellite data from 29 July 2010. The history of the arriving air masses was analysed by using backward trajectories and MODIS fire detections. The smoke plumes had elevated concentrations of aerosol particle number, black carbon, CO, C[O.sub.2], S[O.sub.2], [O.sub.3] and N[O.sub.x], and the differences as compared with the background air were clear. The smoke was observed to be highly scattering, with a single-scattering albedo of 0.96 ± 0.01. The median particle size was 60%-250% larger during the plume days than during July-August on average, and the growth of smoke particles was observed even after long-range transport of several hundreds of kilometres.

A prescribed fire experiment was conducted on 26 June 2009 in Hyytiälä, Finland, to study aerosol and trace gas emissions from prescribed fires of slash fuels and the effects of fire on soil properties in a controlled environment. A 0.8 ha forest near the SMEAR II measurement station (Station for Measuring Ecosystem-Atmosphere Relations) was cut clear; some tree trunks, all tree tops and branches were left on the ground and burned. The amount of burned organic material was ~46.8 tons (i.e., ~60 tons ha−1). The flaming phase lasted 2 h 15 min, the smoldering phase 3 h. Measurements were conducted on the ground with both fixed and mobile instrumentation, and in the air from a research aircraft. In the middle of the burning area, CO2 concentration peaked around 2000–3000 ppm above the baseline, and peak vertical flow velocities were ~9 m s−1, as measured with a 10 Hz 3-D sonic anemometer placed within the burn area. In the mobile measurements the peak particle number concentrations were approximately 1–2 × 106 cm−3 in the plume at a distance of 100–200 m from the burn area. On the ground at the SMEAR II station the geometric mean diameter of the mode with the highest concentration was 80 ± 1 nm during the flaming phase and in the middle of the smoldering phase, but, at the end of the smoldering phase, the largest mode was 122 nm. In the volume size distributions, geometric mean diameter of the largest volume mode was 153 nm during the flaming phase and 300 nm during the smoldering phase. The lowest single-scattering albedo of the ground-level measurements was 0.7 in the flaming-phase plume and ~0.9 in the smoldering phase. Elevated concentrations of several volatile organic compounds (VOC) (including acetonitrile, a biomass burning marker) were observed in the smoke plume at ground level. Measurements at the forest floor (i.e., a richly organic layer of soil and debris, characteristic of forested land) showed that VOC fluxes were generally low and consisted mainly of monoterpenes, and VOC flux peaked after the burning. After one year, the fluxes had nearly stabilized close to the level before the burning. The clear-cutting and burning of slash increased the total long-term CO2 release from the soil, and altered the physical, chemical and biological properties of the soil, such as increased the available nitrogen contents of the soil, which in turn, affected the long-term fluxes of greenhouse gases.

During a prescribed fire experiment, C[O.sub.2] and particle number concentrations, light scattering and absorption coefficients were measured from a Cessna 172 airplane. Peak number concentrations were (3 [+ or -] 1) x [10.sup.6] [cm.sup.-3] and they decreased faster than what can be explained by coagulation alone. The single-scattering albedo of particles grew from the values of 0.4 ± 0.1 closest to the emissions to the values of 0.8 ± 0.1 at the distance of 400 m from the emissions. The mean Angstrom exponent of absorption, 1.70 ± 0.24, is in line with the published spectral absorption values of wood-smoke aerosol. The estimated emission factors were 1600 ± 1020, 5.9 ± 6.3 and 1.4 ± 1.0 g [kg.sup.-1] (dry biomass), for C[O.sub.2], particulate organic matter and black carbon (BC), respectively, and (4.8 ± 2.9) x [10.sup.15] particles per kg (dry biomass) for the particle number. The BC emission factor may be overestimated by a factor of about 1.6 ± 0.2 due to condensation of organics on the filter of the absorption photometer. During the smoldering phase, there were clear indications of new particle formation.

A novel relaxed eddy accumulation (REA) system for aerosol particle flux measurement has been developed and tested. The system consisted of a fast-response sonic anemometer, a flow system, and software for operating the valves and the concentration analysis system. The prototype was used during September-October 2001 at the SMEAR II station of the University of Helsinki. The REA system was operated with a varying threshold for valve switching determined by the running mean standard deviation of the vertical wind speed. Such a varying threshold made the flux proportionality coefficient [beta] independent of observation conditions. Using temperature as a tracer, [beta] was determined to be 0.392 ± 0.002. The system was validated by comparing the carbon dioxide fluxes estimated by REA with the ones measured by the eddy covariance technique. The system was used subsequently for flux measurements of 50-nm aerosol particles and deposition velocity estimation. Observed deposition velocities over a pine forest during the autumn season were on the average 0.43 ± 0.06 (standard error) cm s^sup -1^, which is higher than the earlier model estimates for forest canopies. Deposition velocity was dependent on the turbulence level and stability. To the authors' knowledge, no direct experimental data of deposition velocities on this size range is available in the literature. [PUBLICATION ABSTRACT]

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.

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.

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.