FREEZING BEHAVIOR OF STRATOSPHERIC SULFATE AEROSOLS INFERRED FROM TRAJECTORY STUDIES

Temperature histories based on 10-day back trajectories for six ER-2 f lights during AASE I (1989) and AAOE (1987) are presented. These traje ctories along with the properties of the observed PSC (polar stratosph eric cloud) particles are used here to infer the physical state of the preexisting sulfuric acid aerosols. Of all the ER-2 flights described here, only the PSCs observed on the flights of January 24 and 25, 198 9 are consistent with the thermodynamics of liquid ternary solutions o f H2SO4/HNO3H2O (Type Ib PSCs). For these two days, back trajectories indicate that the air mass was exposed to SAT (sulfuric acid tetrahydr ate) melting temperatures about 24 hours prior to being sampled by the ER-2. For the remaining ER-2 flights (January, 16, 19, and 20 for the AASE I campaign and August 17 for the AAOE campaign), the observed PS Cs were probably composed of amorphous solid solutions of HNO3 and H2O (Type Ic PSCs). Formation of such Type Ic PSCs requires the presence of solid H2SO4 aerosols since liquid aerosols yield ternary solutions. The 10-day back trajectories of these flights indicate that the air m ass was not exposed to SAT melting temperatures during the past week a nd had experienced cooling/warming cycles prior to being sampled by th e ER-2. These temperature histories, recent laboratory measurements an d the properties of glassy solids suggest that stratospheric H2SO4 aer osols may undergo a phase transition to SAT upon warming at similar to 198 K after going through a cooling cycle to about 194 K or lower.