Abstract. Measurements of biogenic volatile organic compounds (VOCs) were performed at Hyytiälä, a Boreal forest site in Southern Finland as part of the project OSOA (origin and formation of secondary organic aerosol) in August 2001. At this site, frequent formation of new particles has been observed and the role of biogenic VOCs in this process is still unclear. Tethered balloons served as platforms to collect VOC samples within the planetary boundary layer at heights up to 1.2 km above ground during daytime. Mean mixed layer concentrations of total monoterpenes varied between 10 and 170 pptv, with α-pinene, limonene and Δ3-carene as major compounds, isoprene was detected at levels of 2–35 pptv. A mixed layer gradient technique and a budget approach are applied to derive surface fluxes representative for areas of tens to hundreds of square kilometres. Effects of spatial heterogeneity in surface emissions are examined with a footprint analysis. Depending on the source area considered, mean afternoon emissions of the sum of terpenes range between 180 and 300 μg m−2 h−1 for the period of 2 to 12 August 2001. Surface fluxes close to Hyytiälä were higher than the regional average, and agree well with mean emissions predicted by a biogenic VOC emission model. Total rates of monoterpene oxidation were calculated with a photochemical model. The rates did not correlate with the occurrence of new particle formation, but the ozone pathway was of more importance on days with particle formation. Condensable vapour production from the oxidation of monoterpenes throughout the mixed layer can only account for a fraction of the increase in aerosol mass observed at the surface.

Abstract. As one part of the OSOA (Origin and formation of Secondary Organic Aerosols) project, two intensive field campaigns were conducted in Melpitz, Germany and Hyytiälä, Finland. This paper gives an overview of the measurements made during the Hyytiälä campaign, which was held between the 1st and 16th of August 2001. Various instrumental techniques were used to achieve physical and chemical characterisation of aerosols and to investigate possible precursor gases. During the OSOA campaign in Hyytiälä, particle formation was observed on three consecutive days at the beginning of the campaign (1 to 3 August 2001) and on three days later on. The investigation of the meteorological situation divided the campaign into two parts. During the first three days of August, relatively cold and clean air masses from northwest passed over the station (condensation sink – CS: <0.002 s−1, NOx: < 0.5 ppb). Daily particle bursts of one fraction of the nucleation mode aerosols (3–10\\,nm) with number concentrations between 600–1200 particles cm-3 were observed. After this period, warmer and more polluted air from south-west to south-east arrived at the station (CS: 0.002-0.01 s−1, NOx: 0.5–4 ppb) and during these 13 days only three events were observed. These events were not as apparent as those that occurred during the earlier period of the campaign. The chemical analyses from different institutes of PM2, PM2.5 and PM10 particles confirmed the assumption that organic matter from the oxidation of various terpenes contributed to the formation of secondary organic aerosols (SOA). Concerning these conclusions among others, the ratio between formic (oxidation product of isoprene and monoterpenes by ozone) and acetic acid (increased by anthropogenic emissions) (ratio=1 to 1.5) and concentration of different carboxylic acids (up to 62 ng m−3) were investigated. Gas/particle partitioning of five photo-oxidation products from α- and β-pinene resulted in higher concentrations for pinonic, nor pinonic and pinic acids in the particle phase than in the gas phase, which indicates preference to the particle phase for these compounds. The average growth factors (GF) from 100 nm particles in water vapour gave a diurnal pattern with a maximum during daytime and values between 1.2 and 1.7. On average, the amount of secondary organic carbon reached values around of 19% of the sampled aerosols and the results indicate that formation of SOA with the influence of photo-oxidation products from terpenes was the reason for the observed particle bursts during the campaign. However, correlations between the precursor gases or the favourable condensing species with the monitored nucleation mode particles were not found. For the investigated time period other factors like the condensation sink of newly formed particles to the pre-existing aerosols, temperature and solar irradiance seem to be more important steering parameters for the production of new aerosols. Another open question concerns the vertical distribution of the formation of SOA. For this reason measurements were conducted at different altitudes using a tethered balloon platform with particle sampling and particle counting equipment. They were incorporated with eddy covariance (EC) flux measurements made at 23 m above ground level. The results give first indications that the process of the production of new aerosols happens throughout the planetary boundary layer (PBL), whereby different parameters e.g. temperature, CS, solar irradiance or concentration of monoterpenes are responsible for the location of the vertical maximum.