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During the ASTAR (Arctic Study of Tropospheric Aerosol and Radiation)campaign nucleation mode particles (4 to 13 nm) were quitefrequently observed at altitudes below 4000 m. However, in theupper free troposphere, nucleation mode particles were only observedonce, namely during the flight on 24 May 2004 (7000 m).  Toinvestigate if vertical motion were the reason for this difference thaton one particular day nucleation mode particles were observed but noton the other days we employ a microphysical box model. The box modelsimulations were performed along air parcel trajectories calculated6-d backwards based on European Center for Medium-Range WeatherForecasts (ECMWF) meteorological analyses using state parameters suchas pressure and temperature in combination with additional parameterssuch as vertical stability. Box model simulations were performed forthe 24 May where nucleation mode particles were observed (nucleationevent) as well as for the days with measurements before and after (22and 26 May) which are representative for no nucleation (non-nucleationevent). A nucleation burst was simulated along alltrajectories, however, in the majority of the simulations thenucleation rate was either too low or too high so that no nucleationmode particles were left at the time when the measurements wereperformed. Further, the simulation results could be divided into threecases. Thereby, we found that for case 1 the temperature was the onlydriving mechanism for the formation of new particles while forcase 2 and 3 vertical motion haveinfluenced the formation of new particles. The reason why nucleationmode particles were observed on 24 May, but not on the other days, canbe explained by the conditions under which particle formationoccurred. On 24 May the particle formation was caused by a slowupdraft, while on the other two days the particle formation was causedby a fast updraft

atmospheric chemistry and physics

Particle formation in the Arctic free troposphere during the ASTAR 2004 campaign: a case study on the influence of vertical motion on the binary homogeneous nucleation of H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;SO&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;/H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O