The preceding discussion suggests that ion induced nucleation INU is a frequent process in the cold upper troposphere. However, homogeneous nucleation HONU is also taking place. According to the model simulations reported by Arnold et al. (2006), upper troposphere INU and HONU rates are relatively high even for typical SO2 mole fractions. Therefore the bottleneck of upper troposphere CR mediated formation of CCN sized particles seems to be new particle growth and not new particle formation by INU and HONU. In fact, the gaseous [H2SO4] required to allow new particle growth to become efficient is mostly more than sufficient for INU.

Particle growth by gaseous H2SO4 condensation is to a large part controlled by SO2. In fact spatial and temporal variations of upper troposphere SO2 should have a much stronger effect on N30 than the small variations of the CR ionization rate.

Therefore more information on upper troposphere SO2 and its spatial and temporal variability is needed. Furthermore efforts should be made to detect additional condensable gases Y which may eventually be present in the upper troposphere. The upper troposphere seems to be a source region of new sulphuric acid aerosols which may experience transport to the lower troposphere as well as the stratosphere. Their transport to the marine lower stratosphere would be potentially relevant for climate since marine clouds distribute markedly to the earths albedo.

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