Tethered balloon measurements of biogenic volatile organic compounds at a Boreal forest site

Measurements of biogenic volatile organic com- pounds (VOCs) were performed at Hyyti¨ al¨ a, a Boreal for- est site in Southern Finland as part of the OSOA (origin and formation of secondary organic aerosol) project in August 2001. At this site, frequent formation of new particles has been observed and the role of biogenic VOCs in this pro- cess 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 be- tween 10 and 170 pptv, with -pinene, limonene and 1 3 - 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 consid- ered, mean afternoon emissions of the sum of terpenes range between 180 and 300µg m 2 h 1 for the period of 2-12 Au- gust 2001. Surface fluxes close to Hyyti ¨ al¨ a 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 photochemi- cal 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. Condens- able 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.

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