Size dependent single particle and chemical bulk analysis of droplets and interstitial particles in an orographic cloud

Abstract

In order to determine trace substances in a multiphase atmospheric system, their distribution and the extent of homogeneity of both the droplets and interstitial particles with respect to chemical composition, an approach was used analyzing bulk samples as well as single particles of both phases of an orographic cloud. Besides standard techniques (ion chromatography, total-reflectance X-ray fluorescence spectroscopy, and others) laser microprobe mass spectrometry was used as an analytical tool. Particles and droplets of different sizes exhibited different chemical composition. All trace substances analysed have been found to be more concentrated in those particles (and droplets) exceeding 2 μm of size (aerodynamic equivalent diameter) than in droplets exceeding 15 μm of size. Mineral particles in particular were incorporated into large droplets. The ions ammonium, sulphate and nitrate have partially been found in internal mixture: Ammonium and sulphate ions dominated the nonactivated particle mode (interstitial aerosol) in the size range from 1 to 5 μm and have been mixed with carbon internally in the fine particle mode, whereas nitrate was more localized in large droplets. It is suggested that the existence of a haze droplet mode with a chemical composition distinguishly different from the bulk cloud-water may explain strong discrepancies between solute mass size distribution and the mass size distributions of individual constituents. These are governed by the degree of supersaturation achieved and by chemical composition and the degree of mixing of the underlying aerosol.