Aerosol physical and chemical properties and trace gas concentrations were measured during the QUEST field campaign in March–April 2003, in Hyytiälä, Finland. Our aim was to understand the role of oxidation products of VOC's such as mono- and sesquiterpenes in atmospheric nucleation events. Particle chemical compositions were measured using the Aerodyne Aerosol Mass Spectrometer, and chemical compositions of aerosol samples collected with low-pressure impactors and a high volume sampler were analysed using a number of techniques. The results indicate that during and after new particle formation, all particles larger than 50 nm in diameter contained similar organic substances that are likely to be mono- and sesquiterpene oxidation products. The oxidation products identified in the high volume samples were shown to be mostly aldehydes. In order to study the composition of particles in the 10–50 nm range, we made use of Tandem Differential Mobility Analyzer results. We found that during nucleation events, both 10 and 50 nm particle growth factors due to uptake of ethanol vapour correlate strongly with gas-phase monoterpene oxidation product (MTOP) concentrations, indicating that the organic constituents of particles smaller than 50 nm in diameter are at least partly similar to those of larger particles. We furthermore showed that particle growth rates during the nucleation events are correlated with the gas-phase MTOP concentrations. This indicates that VOC oxidation products may have a key role in determining the spatial and temporal features of the nucleation events. This conclusion was supported by our aircraft measurements of new 3–10 nm particle concentrations, which showed that the nucleation event on 28 March 2003, started at the ground layer, i.e. near the VOC source, and evolved together with the mixed layer. Furthermore, no new particle formation was detected upwind away from the forest, above the frozen Gulf of Bothnia.

Long-term measurements of fluxes of sensible heat (H), latent heat (LE) and carbon dioxide (Fc) were made from December 2005 to August 2006 over an urban landscape in Helsinki, Finland using the direct micrometeorological eddy covariance technique. Three distinguished sectors of land-use cover (vegetation, roads and buildings) allowed comparisons of fluxes over different urban surfaces. The normalized standard deviation of wind and scalars as a function of atmospheric stability were typical for rough surfaces, as were turbulence spectra and cospectra. Footprint analysis was performed by a boundary-layer one and half-order closure model allowing for discrimination of surface and canopy sinks/sources and complex topography. Fluxes were analysed as average diurnal courses over winter, spring and summer periods. H exceeded LE reaching 300 W m-2 over urban and road surfaces in the summer and it was close to 100 W m-2 in the winter. LE was highest in the summer over vegetation cover attaining 150 W m-2. The emi...

Long‐term measurements of sensible and latent heat and carbon dioxide fluxes were performed over a boreal lake in southern Finland using the direct micrometeorological eddy covariance (EC) technique. The water column was sampled weekly for dissolved carbon dioxide, and the CO₂ flux was estimated also applying the concentration gradient method. Temperature and oxygen profiles of the lake were measured twice a week. The measurements covered one full open‐water period from April to November 2003, and it is the longest continuous CO₂ record ever measured over a lake by EC. The sensible heat flux H was positive, that is, from the lake to the atmosphere, except in May, when it was >0 W/m² at night and <0 W/m² in daytime. The latent heat flux dominated clearly over H in spring and summer; that is, the Bowen ratio was less than 1. Higher‐moment turbulence statistics proved to be efficient in detection of frequent nonstationary situations. Applying the statistical criteria for CO₂ concentration and vertical wind speed, averaging over a 5‐min period and selecting only the wind direction with longest fetch, we could obtain lake‐representative CO₂ fluxes. Footprint analysis based on a closure model revealed that the source areas were relatively short because of the presence of turbulence generated by the surrounding forest, compared to a larger lake with an extended smooth surface. We observed a net CO₂ source of 0.2–0.4 μmol m⁻² s⁻¹ excluding July, when the flux was closer to zero. The results are consistent with the gradient method, based on more infrequent sampling, and both methods gave the same average flux, 0.2 μmol m⁻² s⁻¹, over the whole open‐water period.