| Summary: | During the POLSTAR-1 winter campaign conducted from Kiruna (Sweden) in 1998 (20 January - 6 February), measurements were made of gas-phase and total water vapour, gas-phase nitric acid, reactive nitrogen, trace gases like carbon monoxide and ozone, meteorological data and particles along a quasi-Lagrangian flight track in the tropopause region. In this paper we give criteria to investigate on the validity of comparisons between quasi-Lagrangian in situ measurements and box model calculations. These criteria are sued to evaluate the flight on 24 January. This flight was performed northwest of Kiruna across a cold trough of air with temperatures around 196 K cutting through the tropopause. These measurements show ice particle formation in the moist tropospheric region and suggest that ternary HNO3-H2SO4-H2O droplets or NAT (with currently no means for distinction) formed in the dry stratospheric air up and downstream of the ice cloud. Ice particles seem to be in thermodynamic equilibrium with water vapour. Uptake of HNO3 into cirrus cloud particles seems to be consistent with the coexistence of HNO3-H2SO4-H2O liquid and ice in the same air mass.
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