High-resolution measurement of size distributions of Asian dust using a coulter multisizer

A Coulter Multisizer, which is based on the electrical sensing zone (ESZ) or the Coulter principle, was used to measure the size distribution of Asian dust. Coulter Multisizer analysis provides high-resolution size measurements of water-insoluble aerosol particles (WIPs) and the number concentration at each size bin. Aerosol filter sampling was conducted at four sites in Japan during spring 2003. The measured volume size distributions fit fairly well with a lognormal distribution. The results show that the WIP size distributions of the same Asian dust air mass varied at each sampling site and the volume mode diameter at the sites reduced from west to east. The derived volume mode diameter ranged from 1.4 to 2.2 μm and was comparatively smaller than those in previous studies on Asian dust. This can be explained by the possible internal mixing of Asian dust with other components and by the breaking of particles and dispersion of aggregations by ultrasonification during extraction. The analysis method was improved by washing the aerosol particles collected on a filter using a magnetic stirrer, instead of an ultrasonic cleaner. As a result, the WIP size distribution can be measured in the range of 1-10 μm. The measured mode diameters were 2.6-3.1 and 4.3-5.6 μm in 2 Asian dust phenomena at Kofu, Japan, in 2004. It is demonstrated that the Coulter Multisizer method can furnish detailed information regarding the spatial and temporal variations in the mineral dust size distribution.