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We have investigated the formation and early growth of atmospheric secondaryaerosol particles building on atmospheric measurements. The measurements werepart of the QUEST 2 campaign which took place in spring 2003 inHyytiälä (Finland). During the campaign numerous aerosol particleformation events occurred of which 15 were accompanied by gaseous sulphuricacid measurements. Our detailed analysis of these 15 events is focussed onnucleation and early growth (to a diameter of 3 nm) of fresh particles. Itrevealed that new particle formation seems to be a function of the gaseoussulphuric acid concentration to the power from one to two when the time delaybetween the sulphuric acid and particle number concentration is taken intoaccount. From the time delay the growth rates of freshly nucleated particlesfrom 1 nm to 3 nm were determined. The mean growth rate was 1.2 nm/h and itwas clearly correlated with the gaseous sulphuric acid concentration. Wetested two nucleation mechanisms – recently proposed cluster activation andkinetic type nucleation – as possible candidates to explain the observeddependences, and determined experimental nucleation coefficients. We foundthat some events are dominated by the activation mechanism and some by thekinetic mechanism. Inferred coefficients for the two nucleation mechanismsare the same order of magnitude as chemical reaction coefficients in the gasphase and they correlate with the product of gaseous sulphuric acid andammonia concentrations. This indicates that besides gaseous sulphuric acidalso ammonia has a role in nucleation

Atmospheric sulphuric acid and aerosol formation: implications from atmospheric measurements for nucleation and early growth mechanisms