Experimental investigation of ice nucleation by different types of aerosols in the aerosol chamber AIDA: implications to microphysics of cirrus clouds

The aerosol chamber AIDA was used as a moderate expansion cloud chamber with cooling rates at the onset of ice nucleation between –1.3 and –3.0 K min−1 to investigate the nucleation and growth of ice crystals in sulphuric acid, ammonium sulphate, and mineral dust aerosols at temperatures between 196 and 224 K. Supercooled sulphuric acid droplets with mean diameters of about 0.2 to 0.3 μm nucleated ice by homogeneous freezing at RHice increasing from 144 to 166 % with temperatures from 220 and 196 K. This is in good agreement both with previous results of AIDA experiments and literature data. In contrast, ammonium sulphate particles of similar size nucleated ice at the significantly lower RHice of 120 to 127 % in the same temperature range. Fourier-Transform infrared (FTIR) extinction spectra of the aerosol revealed that the ammonium sulphate particles, mainly consisted of the liquid phase. The number concentration of ice crystals formed during the homogeneous freezing experiments agree well with model results from the literature. Higher ice crystal number concentrations formed during the ammonium sulphate, compared to the sulphuric acid experiments, can be explained by the also somewhat higher cooling rates at ice nucleation. Deposition ice nucleation on mineral dust particles turned out to be the most efficient ice nucleation mechanism both with respect to RHice at the onset of ice nucleation (102 to 105 % in the temperature range 209 to 224 K) and the ice crystal number concentration. Almost all mineral dust particles nucleated ice at the lower temperatures.