http://www.atmos-chem-phys.net/10/1635/2010/acp-10-1635-2010.html
pdf link at the bottom:
5 Conclusions
Initial measurements have been made with a pilot CLOUD
experiment at the CERN Proton Synchrotron. The accelerator
beam generated equilibrium ion-pair concentrations in
the aerosol chamber of between one and ten times the atmospheric
values at ground level, which corresponds to between
one and almost a hundred times the intensity of galactic cosmic
rays. Experimental measurements in the presence of low
aerosol backgrounds confirmed a dependence of equilibrium
ion-pair concentrations on the square root of the beam intensity,
as expected when the dominant loss mechanism is
ion-ion recombination.
During the 4-week run, around 50 nucleation bursts were
produced and recorded, with typical formation rates of
particles above the 3 nm detection threshold of about 1–
10 cm−3 s−1, and growth rates of 5–20 nm h−1. Concentrations
of H2SO4 were experimentally measured with a chemical
ionisation mass spectrometer to be around 106 cm−3 or
less. The large observed growth rates indicate the presence
of additional trace vapours in the aerosol chamber, whose
identity is unknown but for which there is indirect evidence
of background organic vapours. The presence of background
vapours is also inferred from the observation that small (of
order 0.1 C) increases of temperature invariably trigger nucleation
bursts, which is attributed to the release of unknown
vapours from the chamber walls.
Interestingly we were able to observe different kinds of
new particle formation events. A few of the events appear
to be related to ion-induced nucleation or ion-ion recombination
to form stable neutral clusters. In these cases, a small
but significant fraction of new particle formation could be explained
by ion processes. However, during most nucleation
events, the contribution of ion processes appeared to be minor.
The accelerator beam was also used to search for timecorrelated
nucleation bursts in the chambers. These revealed
some evidence for a dependence of particle formation on
beam intensity at the highest SO2 concentrations of 6 ppb,
although no evidence was found at lower concentrations.
In summary, the exploratory measurements made with a
pilot CLOUD experiment at the CERN Proton Synchrotron
have validated the basic concept of the experiment, provided
valuable technical input for the CLOUD design and instrumentation,
and provided, in some of the experiments, suggestive
evidence for ion-induced nucleation or ion-ion recombination
as sources of aerosol particles from trace sulphuric
acid vapour at typical atmospheric concentrations.
pdf link at the bottom:
5 Conclusions
Initial measurements have been made with a pilot CLOUD
experiment at the CERN Proton Synchrotron. The accelerator
beam generated equilibrium ion-pair concentrations in
the aerosol chamber of between one and ten times the atmospheric
values at ground level, which corresponds to between
one and almost a hundred times the intensity of galactic cosmic
rays. Experimental measurements in the presence of low
aerosol backgrounds confirmed a dependence of equilibrium
ion-pair concentrations on the square root of the beam intensity,
as expected when the dominant loss mechanism is
ion-ion recombination.
During the 4-week run, around 50 nucleation bursts were
produced and recorded, with typical formation rates of
particles above the 3 nm detection threshold of about 1–
10 cm−3 s−1, and growth rates of 5–20 nm h−1. Concentrations
of H2SO4 were experimentally measured with a chemical
ionisation mass spectrometer to be around 106 cm−3 or
less. The large observed growth rates indicate the presence
of additional trace vapours in the aerosol chamber, whose
identity is unknown but for which there is indirect evidence
of background organic vapours. The presence of background
vapours is also inferred from the observation that small (of
order 0.1 C) increases of temperature invariably trigger nucleation
bursts, which is attributed to the release of unknown
vapours from the chamber walls.
Interestingly we were able to observe different kinds of
new particle formation events. A few of the events appear
to be related to ion-induced nucleation or ion-ion recombination
to form stable neutral clusters. In these cases, a small
but significant fraction of new particle formation could be explained
by ion processes. However, during most nucleation
events, the contribution of ion processes appeared to be minor.
The accelerator beam was also used to search for timecorrelated
nucleation bursts in the chambers. These revealed
some evidence for a dependence of particle formation on
beam intensity at the highest SO2 concentrations of 6 ppb,
although no evidence was found at lower concentrations.
In summary, the exploratory measurements made with a
pilot CLOUD experiment at the CERN Proton Synchrotron
have validated the basic concept of the experiment, provided
valuable technical input for the CLOUD design and instrumentation,
and provided, in some of the experiments, suggestive
evidence for ion-induced nucleation or ion-ion recombination
as sources of aerosol particles from trace sulphuric
acid vapour at typical atmospheric concentrations.