Two extreme types of mixing of dust with urban aerosols observed in Kosa particles: ‘After’ mixing and ‘on-the-way’ mixing

Abstract Besides well-known episodic Kosa during spring, high concentrations of Ca 2+ in aerosols were observed early in summer as well as in the semi-continuous data of the aerosols at the summit of Mt. Fuji. We further analysed the data to study the chemical characteristics of the high calcium event during early summer. The back trajectory analyses of the event indicated that Ca was transported from arid and semi-arid regions (e.g. the Taklamakan desert) through the westerly-dominated troposphere higher than the height of the summit of Fuji. The amount of SO 4 2− was always equivalent to that of NH 4 + unlike the case of the normal Kosa period where SO 4 2− is in excess with respect to NH 4 + . This shows the ‘after’ mixing of unreacted CaCO 3 of Kosa origin with (NH 4 ) 2 SO 4 , which was only realized by the downward injection of Kosa particles from higher altitudes to the air masses of different origin. In the case of normal Kosa, the air bearing Kosa particles passed through the polluted area to absorb unneutralized acids (‘on-the-way’ mixing), whereas in the case of the Kosa-like phenomena in summer, the acids from the polluted area have been neutralized by NH 4 + and become inactive before mixing with CaCO 3 (“after” mixing). We have simplified the chemistry of aerosols using their three major components, Ca 2+ , SO 4 2− and NH 4 + , and introduced a new triangle diagram with the three assumed end-members of CaCO 3 , CaSO 4 and (NH 4 ) 2 SO 4 to quantify the contribution of the ‘after’ mixing to the aerosols (AMI; ‘after’ mixing index). Based on the back trajectories of some high AMI cases, CaCO 3 in Kosa particles was transported through the middle troposphere (5000–7000 m) and descended to meet another air mass where SO 4 2− had been already neutralized by NH 3 .