THE NUCLEATION RATE CONSTANTS AND FREEZING MECHANISM OF NITRIC-ACID TRIHYDRATE AEROSOL UNDER STRATOSPHERIC CONDITIONS

The nucleation rates for submicron-diameter nitric acid trihydrate (NA T) aerosol particles were measured under stratospheric conditions usin g a temperature-programed flow tube and Fourier transform infrared ext inction spectroscopy to detect the phase change from liquid to solid. The temperature range from 175 to 155 K was examined. The measured nuc leation rate constants (in units of cm(-3) s(-1)) are between 3.8 +/- 1.8 x 10(10) and 9.7 +/- 6.3 x 10(12) for the temperature range betwee n 167.2 and 163.5 Ii. The experiments show directly that critical nucl ei form rapidly in NAT droplets at these temperatures, but subsequent crystallization is very slow, so the nucleated droplets can persist as liquids for long times unless their temperature is raised to increase the crystal growth rate. In these experiments, complete crystallizati on of the nucleated droplets was achieved in the observation time (abo ut 10 s) by raising their temperature to near 180 K. Although demonstr able in the laboratory, the freezing of NAT by homogeneous nucleation at low temperature and subsequent warming are unlikely to be important for type 1 polar stratospheric cloud formation because the temperatur es required for nucleation are lower than those commonly found in the lower stratosphere.