Dehydration in the Northern Hemisphere midlatitude tropopause region observed during STREAM 1998

• Measurements in the vicinity of the polar jet stream during the STREAM 1998 campaign in Timmins, Canada, show that during the flight on 15 July, a deep intrusion of stratospheric air into the troposphere occurred. At the edge of the deep intrusion dehydration was observed. The dehydration can be identified in tracer–tracer correlations of H2O and O3 and by the comparison of these correlations with correlations of H2O and O3 derived from two other flights of the STREAM 1998 campaign. Trajectories, calculated backwards for 10 days starting at each point of the measurement for the flight on 15 July, show that the saturation ratios required for homogeneous freezing are reached. However, boxmodel simulations along the trajectories indicate no substantial growth of H2SO4/H2O particles due to H2O uptake. Since ice nuclei were not measured during the campaign, it cannot be precisely determined which freezing process, heterogeneous or homogeneous, is responsible for the formation of the ice particles. Most likely, both processes were involved in the formation of ice particles that led to the observed dehydration on 15 July 1998.

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