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Studies on the connections between ... - Helda - Helsinki.fi

particle growth rates after nucleation) to be measured with different counting efficiencies,

resulting in an apparent increase of the nucleation rate with H 2 SO 4 concentration

and a high slope of the log(J) vs log([H 2 SO 4 ]) curve. Thus, in addition to the importance

of a suitable detector, also the growth rate (determined by H 2 SO 4 concentration

and residence time in the flow reactor) affects the results. Many earlier experiments

were performed with rather short residence times, resulting in a small growth rate, and

a large fraction of particles remaining so small that they were not counted.

The difference between liquid source and production of H 2 SO 4 from SO 2 +OH reaction

(”the sulphuric acid mystery”) was explained by different concentration profiles (as

a function of time) between these two cases: with a liquid, point-like instantaneous

source the concentration of H 2 SO 4 decreased steeply after nucleation, whereas in-situ

production yielded quite constant H 2 SO 4 concentration as long as the OH source (UVlight)

was on. In the former case, the growth rates were smaller than in the latter

case, resulting in a considerable fraction of nucleated particles not reaching a size big

enough to be detected.

In summary, proper instrumentation and high enough growth rates are required in

order to obtain correct results in nucleation experiments. Especially the Particle Size

Magnifier (PSM), which has close-to-unity counting efficiency for small particles, has

made it possible measure nucleation rates with good accuracy. It is possible that all

earlier laboratory experiments of nucleation are affected by the errors sources pointed

out by Sipilä et al. (2010), and therefore earlier laboratory results on sulphuric acidwater

nucleation should be interpreted with care.

Current thinking on atmospheric particle formation is as follows (Kulmala et al., 2013).

A more or less constant neutral cluster pool at 1–2 nm is observed to exist in the

atmosphere (Kulmala et al., 2007). Under certain, partly yet unidentified, conditions

these pre-critical clusters start to grow to bigger sizes. This happens normally during

daytime, with the participation of condensable vapours produced by photo-oxidation.

The most important vapour forinitiation ofthe growthof pre-nucleation clusters seems

to be sulphuric acid. Simultaneously with sulphuric acid, organic vapours start to

condense, possibly with the nano-Köhler mechanism, and speed up the particle growth

rate.

2.3 Formation and loss processes of sulphuric acid in the

atmosphere

Gas-phase sulphuric acid is produced in the atmosphere mainly through oxidation of

sulphur dioxide (SO 2 ) by OH radicals. The main sources of sulphur dioxide in today’s

atmosphere are emissions from fossil fuel burning and industry, global estimate for

emissions being 70 Tg(S)/year (Seinfeld and Pandis, 2006). Naturally SO 2 is emitted

from volcanic eruptions (global estimate 7–8 Tg(S)/year) and forest fires (global estimate

for emissions from biomass burning 2.8 Tg(S)/year) (Seinfeld and Pandis, 2006).

Over oceans, which lack extensive anthropogenic sulphur emissions, dimethylsulphide

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