LARGE AEROSOL-PARTICLES IN CIRRUS TYPE CLOUDS

Research flights in November 1990 over the central parts of the United States, Wyoming and Colorado, were aimed to the investigation of the properties and microstructure of cirrus clouds (mainly cirrocumulus le nticularis). Among the other parameters measured on board the NCAR Sab erliner were the concentration and size distribution of submicron part icles and, in some cases, the particle deliquescence. For coarse insol uble particles found inside and outside of cloud elements, size distri butions and morphology information were obtained by evaluating inertia l impactor samples with an optical microscope and scanning electron mi croscope. In addition, the coarse particle composition was determined by x-ray energy spectrum analysis. The following conclusions from thes e measurements are: The large and coarse particle size distribution ca n be roughly simulated by a log-normal function with the modus around r = 0.5 mu m. Particle concentrations are Very variable between severa l tenths and several particles per cm(3). Particle volume distribution features a distinct maximum around 0.75 mu m without a broad plateau which was observed in the case of sampling at lower altitude. Aerosol composition heterogeneity at cirrus cloud level is well documented by the evaluation of the fine particle sampling taken with the UMR sampli ng system. This heterogeneity can be partly explained by the interacti on between aerosol and cloud elements, which is documented by the meas ured particle size distribution curves inside and outside of cloud ele ments. Assuming that particle deliquescence is caused by H2SO4 and/or by (NH4)(2)SO4, particle soluble mass fractions were found to be aroun d 30% in the first case and about 40% in the second. The most frequent ly occurring elements in large and coarse particles at cirrus cloud le vel were Si, Cl, Ba, S, Ca and C.