Coarse and accumulation mode particles associated with Asian dust in southwestern Japan

Number and mass concentrations of atmospheric particles were measured at a surface site on the southwestern Japan coast from March to May 2002. Particles were collected when Asian dust appeared at the site in this period, and later characterized with their morphology and elemental composition from electron microscopic analysis. The mass concentration of suspended particulate matters with diameter smaller than 10 μm (SPM10) during dust episodes was predominated by particles larger than 1.0 μm (coarse particles), which were mainly mineral dust with a small fraction of sea salt. As dust-loading low-pressure systems were approaching, SPM10 and the number concentration of coarse particles decreased gradually until the arrival of cold fronts. After that, they increased due to dust arrival except during rainfall. Time series of the number concentration of coarse particles during dust plume passages was not in parallel with that of accumulation mode particles in the range of 0.1–0.3 μm and they were even inverse in some episodes, reflecting a horizontal structure with multiple intervals of dust and secondary particles. The electron microscopic analysis confirmed the frequent mixture of sea salt in dust particles and suggested that the probability for dust particles to become mixtures with sea salt was likely dependent on the vertical thermodynamic structure of the marine boundary layer, through which aloft dust particles descended to the ground. More mineral dust particles mixed with sea salt in cases with deep mixing layers than with shallow mixing layers. No correlation between the mixture degree and the transport time for dust particles to travel in the marine atmosphere from the Asian continent to southwestern Japan was found.