Overview of the international project on biogenic aerosol formation in theboreal forest (BIOFOR)

Aerosol formation and subsequent particle growth in ambient air have been f requently observed at a boreal forest site (SMEAR II station) in Southern F inland. The EU funded project BIOFOR (Biogenic aerosol formation in the bor eal forest) has focused on: (a) determination of formation mechanisms of ae rosol particles in the boreal forest site; (b) verification of emissions of secondary organic aerosols from the boreal forest site; and (c) quantifica tion of the amount of condensable vapours produced in photochemical reactio ns of biogenic volatile organic compounds (BVOC) leading to aerosol formati on. The approach of the project was to combine the continuous measurements with a number of intensive field studies. These field Studies were organise d in three periods, two of which were during the most intense particle prod uction season and one during a non-event season. Although the exact formati on route for 3 nm particles remains unclear, the results can be summarised as follows: Nucleation was always connected to Arctic or Polar air advectin g over the site, giving conditions for a stable nocturnal boundary layer fo llowed by a rapid formation and growth of a turbulent convective mixed laye r closely followed by formation of new particles. The nucleation seems to o ccur in the mixed layer or entrainment zone. However two more prerequisites seem to be necessary, A certain threshold of high enough sulphuric acid an d ammonia concentrations is probably needed as the number of newly formed p articles was correlated with the product of the sulphuric acid production a nd the ammonia concentrations. No such correlation was found with the oxida tion products of terpenes. The condensation sink, i.e., effective particle area, is probably of importance as no nucleation was observed at high value s of the condensation sink. From measurement of the hygroscopic properties of the nucleation particles it was found that inorganic compounds and hygro scopic organic compounds contributed both to the particle growth during day time while at night time organic compounds dominated. Emissions rates for s everal gaseous compounds was determined. Using four independent ways to est imate the amount of the condensable vapour needed for observed growth of ae rosol particles we get an estimate of 2-10 x 10(7) vapour molecules cm(-3). The estimations for source rate give 7.5-11 x 10(4) cm(-3) s(-1). These re sults lead to the following conclusions: The most probable formation mechan ism is ternary nucleation (water-sulphuric acid-ammonia). After nucleation, growth into observable sizes (greater than or equal to3 nm) is required be fore new particles appear. The major part of this growth is probably due to condensation of organic vapours. However, there is lack of direct proof of this phenomenon because the composition of 1-5 nm size particles is extrem ely difficult to determine using the present state-of-art instrumentation.