DELIQUESCENCE AND FREEZING OF STRATOSPHERIC AEROSOL OBSERVED BY BALLOONBORNE BACKSCATTERSONDES

Stratospheric sulfate aerosols, originating from the Mt. Pinatubo volc anic eruption, have been observed during three winters in the Arctic b y balloonborne backscattersondes. A measured color index, defined as t he quotient between the aerosol backscatter ratios at wavelengths 940 and 480 nm, provides information of the size of the observed particles . The effects of liquid particle growth, by water vapor uptake, clearl y show upas changes in the color index, whereas measurements on other days indicate the particles to be frozen. Air parcel trajectories have been calculated, providing the temperature history of the observed pa rticles. Evidences appear of a temperature hysteresis in the freezing and melting cycle of the aerosol, indicating melting temperatures arou nd 215-220 K in good agreement with labor;;tory measurements, and free zing of the particles within less than 5 K above the ice frost point. The changes in color index of the liquid particles are in good agreeme nt with predictions from theoretical model calculations of growth by w ater vapor uptake.