Seasonal variations in SO 2 plume transport over Japan: Observations at the summit of Mt. Fuji from winter to summer

Abstract

We undertook continuous measurements of SO 2 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 SO 2 time series for the springtime middle troposphere over Japan. The time series shows trends in high-SO 2 long-range transport events from winter to spring. Elevated SO 2 concentrations are always accompanied by elevated CO and 222Rn 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-SO 2 events decline in strength and frequency during spring. The observed seasonal variation of SO 2 events may in fact have nothing to do with seasonality in SO 2 emissions from the Asian continent, as seasonal patterns in SO 2 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 SO 2 events. A major factor appears to be the removal of SO 2 from the continental pollution plume during long-range transport. The SO 2-bearing polluted air is dry during the depths of winter; accordingly, little SO 2 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 SO 2 by cloud processes. RH data along the air mass travel path also support the importance of cloud processes in reducing SO 2 concentrations within the pollution plume.