Properties of the size-resolved and individual cloud droplets collected in western Japan during the Asian dust storm event

Abstract

Abstract With the point of view of the removal mechanism of Asian dust storm particles, in order to study the physiochemical properties of clouds a field campaign was conducted in western Japan during the Asian dust storm event. The polymeric water absorbent film and collodion film replication techniques were employed in the measurements of size-fractionated precipitation cloud and individual cloud droplets, respectively. In addition, to investigate the source profiles of the elements retained in cloud samples, the original desert sand was collected. Particle-induced X-ray emission was applied for the elemental analysis of size-resolved cloud droplets and desert sand. Also for the quantification analysis of the ultra trace elements in residual particles in individual cloud droplets, the X-ray microprobe system equipped at Super Photon ring-8 GeV (SPring-8) BL-37XU was newly applied. Soil derived components like Si, Ca, and Fe show higher mass concentrations in small droplets ( 6.4 μm), while S and Cl dominate at droplet size larger than 20 μm. Three cloud samples have liquid water content ranging from 0.04 to 0.11 g m −3 . The number size distribution of droplets collected at cloud base is monomodal with the maximum level around 15 μm. The size distribution of cloud droplets is widespread (up to 60 μm). The droplet residues mainly consisting of crustal components were successively reconstructed as elemental maps by the X-ray fluorescence (XRF) microprobe analytical technique. From these XRF elemental maps, it can be understood that crustal components are significantly distributed on and/or in the residual particles in individual cloud droplets. The plotting of enrichment factors calculated from the elemental composition of original desert sand in China not only indicates the good correlationship between elemental masses in residual particles of cloud base droplets and those of precipitation cloud, but also classify elements into soil origin and non-soil origin. Mineral components were identified as the main components of dry residual particles in cloud droplets. These soil-origin elements account for up to 68% of the total elemental mass.