Origins and Aging of Calcium-rich Mineral Particles in Asian Dust Arriving in Southwestern Japan: A Comparison of Slow- and Fast-moving Events

Calcium (Ca) is a principal element of mineral dust in the atmosphere, and its presence varies in time and space, making its distribution in widespread aerosol particles and environmental significance poorly understood. Using a scanning electron microscope, we analyzed and compared Ca-rich particles collected during slow-moving and fast-moving dust events in southwestern Japan. The abundance and occurrence of the particles differed significantly between the two types of events. In slow-moving events, their number fraction among the total mineral particles ranged from 51.2 to 55.6%, while in fast-moving events, the fraction was much lower, ranging from 3.6 to 16.7%. The Ca-rich particles in slow-moving events were larger (with a peak size of about 2.0 μm) and rounder (with a roundness peak of about 0.8) than those in fast-moving events (about 1.4 μm and 0.7, respectively), suggesting a more aged state of the particles in slow-moving dust events. The Ca-rich particles were attributed to anthropogenic emissions based on their distinctive characteristics and likely entrained into the dusty air when the events passed populated areas in eastern China. These results indicate substantial anthropogenic Ca-rich particles within slow-moving Asian dust plumes, and their active involvement in atmospheric physical and chemical processes.