Abstract. During a 4-week run in October-November 2006, a pilot experiment was performed at the CERN Proton Synchrotron in preparation for the Cosmics Leaving OUtdoor Droplets (CLOUD) experiment, whose aim is to study the possible influence of cosmic rays on clouds. The purpose of the pilot experiment was firstly to carry out exploratory measurements of the effect of ionising particle radiation on aerosol formation from trace H 2 SO 4 vapour and secondly to provide technical input for the CLOUD design. A total of 44 nucleation bursts were produced and recorded, with formation rates of particles above the 3 nm detection threshold of between 0.1 and 100 cm −3 s −1 , and growth rates between 2 and 37 nm h −1 . The corresponding H 2 SO 4 conCorrespondence to: J. Duplissy (jonathan.duplissy@cern.ch) centrations were typically around 10 6 cm −3 or less. The experimentally-measured formation rates and H 2 SO 4 concentrations are comparable to those found in the atmosphere, supporting the idea that sulphuric acid is involved in the nucleation of atmospheric aerosols. However, sulphuric acid alone is not able to explain the observed rapid growth rates, which suggests the presence of additional trace vapours in the aerosol chamber, whose identity is unknown. By analysing the charged fraction, a few of the aerosol bursts appear to have a contribution from ion-induced nucleation and ion-ion recombination to form neutral clusters. Some indications were also found for the accelerator beam timing and intensity to influence the aerosol particle formation rate at the highest experimental SO 2 concentrations of 6 ppb, although none was found at lower concentrations. Overall, the exploratory measurements provide suggestive evidence for ion-induced nucleation or ion-ion recombination as sources of aerosol Published by Copernicus Publications on behalf of the European Geosciences Union. particles. However in order to quantify the conditions under which ion processes become significant, improvements are needed in controlling the experimental variables and in the reproducibility of the experiments. Finally, concerning technical aspects, the most important lessons for the CLOUD design include the stringent requirement of internal cleanliness of the aerosol chamber, as well as maintenance of extremely stable temperatures (variations below 0.1 • C).

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The x-ray diffraction crystal structures 18,19 of [(CH 3 ) 3 SnF] tt and [(CH 3 ) 3 SnOH] n show that these compounds are one-dimensional linearpolymeric chains held together by weak van der Waals forces between methyl groups in neighboring chains, whereas (C6H 5 ) 3 SnCl is a weakly bonded threedimensional monomeric unit. The structure of [(CH 3 ) s SnF] n is shown in Fig. 2, where a set of latticedynamic principal axes are indicated. In an intermediate temperature range T% such linear polymers have ((u r 2 )ztt(ur 2 )y)> {{u r 2 ) z ), the eccentricity of the vibrational ellipsoid being maximized. 20 ' 21 At low and high temperatures 7\ and T$ {u r 2 ) x~{ u r 2 )ytt{u?) Zi the thermal motion being nearly a vibrational sphere. For a monomeric material, {u r 2 )o^{u 2 )y^i{u 2 ) z at almost . all temperatures. From Fig. 1, it is seen that the linearchain compounds display a strong temperature-dependent Karyagin effect due to the significant anisotropic thermal behavior of the linear polymers in T 2 . For [(CH 3 ) 3 SnF] n , d{I 2 /h)/dT<^ whereas d(I % /I 1 )/dT>0 for QCH 3 ) 3 SnOH] w in the range T 2 . If X ± is constant in T 2 and ^1, then a difference in the sign of d(I 2 /Ii)/dR indicates that 5E z >/5z f in (CH 3 ) 3 SnOH and (CH 3 ) 3 SnF probably have opposite signs, and hence the (=bf, |+) and the (dbj, |+) levels are energetically reversed in these two compounds. At low temperatures, i.e., in the range R h (I 2 /I x )ttl and d(I 2 /Ii)/dR->0, and the Karyagin effect vanishes in these chain compounds. That {I 2 /h)ttl and d(I 2 /Ii)/dRttO for (C 6 H 5 ) 3 SnCl reflects the almost isotropic thermal behavior of the Sn 119 atom in this compound and a negligible Karyagin effect is observed. This result is in agreement with the evidence found in Ref. 8 but substantially disagrees with that of Goldanskii et aLThe stopping cross section S=dE/NdR in thin carbon foils has been measured for ions with 21

During a 4-week run in October-November 2006, a pilot experiment was performed at the CERN Proton Synchrotron in preparation for the Cosmics Leaving OUtdoor Droplets (CLOUD) experiment, whose aim is to study the possible influence of cosmic rays on clouds. The purpose of the pilot experiment was firstly to carry out exploratory measurements of the effect of ionising particle radiation on aerosol formation from trace H 2 SO 4 vapour and secondly to provide technical input for the CLOUD design. A total of 44 nucleation bursts were produced and recorded, with formation rates of particles above the 3 nm detection threshold of between 0.1 and 100 cm −3 s −1 , and growth rates between 2 and 37 nm h −1. The corresponding H 2 SO 4 con-Correspondence to: J. Duplissy (jonathan.duplissy@cern.ch) centrations were typically around 10 6 cm −3 or less. The experimentally-measured formation rates and H 2 SO 4 concentrations are comparable to those found in the atmosphere, supporting the idea that sulphuric acid is involved in the nu-cleation of atmospheric aerosols. However, sulphuric acid alone is not able to explain the observed rapid growth rates, which suggests the presence of additional trace vapours in the aerosol chamber, whose identity is unknown. By analysing the charged fraction, a few of the aerosol bursts appear to have a contribution from ion-induced nucleation and ion-ion recombination to form neutral clusters. Some indications were also found for the accelerator beam timing and intensity to influence the aerosol particle formation rate at the highest experimental SO 2 concentrations of 6 ppb, although none was found at lower concentrations. Overall, the exploratory measurements provide suggestive evidence for ion-induced nucleation or ion-ion recombination as sources of aerosol Published by Copernicus Publications on behalf of the European Geosciences Union. 1636 J. Duplissy et al.: Results from the CERN pilot CLOUD experiment particles. However in order to quantify the conditions under which ion processes become significant, improvements are needed in controlling the experimental variables and in the reproducibility of the experiments. Finally, concerning technical aspects, the most important lessons for the CLOUD design include the stringent requirement of internal cleanliness of the aerosol chamber, as well as maintenance of extremely stable temperatures (variations below 0.1 • C).

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