Seasonal variations in SO2 plume transport over Japan:: Observations at the summit of Mt. Fuji from winter to summer

We undertook continuous measurements of SO2 Concentrations at the summit of Mt. Fuji, Japan (3776 m above sea level), during the period January to July, 2004. These data include the first data set that fills the previous gap in SO2 time series for the springtime middle troposphere over Japan. The time series shows trends in high-SO2 long-range transport events from winter to spring. Elevated SO2 concentrations are always accompanied by elevated CO and Rn-222 concentrations, and backward trajectories for such events indicate a continental origin. These data display clear sporadic signatures of Asian outflow over Japan to the North Pacific. Such high-SO2 events decline in strength and frequency during spring. The observed seasonal variation of SO2 events may in fact have nothing to do with seasonality in SO2 emissions from the Asian continent, as seasonal patterns in SO2 events show a similar phase shift to that of the prevailing wind direction and relative humidity (RH) observed on-site. The changes in air mass indicated by daily backward trajectory analysis may also explain the observed temporal changes in SO2 events. A major factor appears to be the removal of SO2 from the continental pollution plume during long-range transport. The SO2-bearing polluted air is dry during the depths of winter; accordingly, little SO2 is removed by cloud processes at this time of year. The nature of the air mass changes during spring when increasing humidity levels lead to the increased removal of SO2 by cloud processes. RH data along the air mass travel path also support the importance of cloud processes in reducing SO2 concentrations within the pollution plume. (c) 2006 Elsevier Ltd. All rights reserved.