Seasonal variation of free tropospheric aerosol particles at Mt. Tateyama, central Japan

Number‐size distribution of atmospheric aerosol particles and O 3 concentration were measured at Murododaira (36.6N, 137.6E, 2450 m above sea level (asl)) on the western flank of Mt. Tateyama in central Japan from January 1999 to November 2002. This study used nighttime data from 2400 to 0500 hours (local time) on the basis of analysis of their diurnal variation to characterize free tropospheric aerosols and O 3 over Japan. The O 3 concentration shows small variability (standard deviation of 4 ppbv) with the mean value of 40 ppbv in winter (October to February), large variability (8 ppbv) with the higher mean value of 51 ppbv in spring (March to May), and large variability (14 ppbv) with the lower mean value of 32 ppbv in summer (June to September). Highest monthly mean volume concentration (2.7 μm 3 /cm 3 ) of accumulation particles (0.3 μm < D < 1.0 μm) was observed in June, while the mean value in winter (October to February) was 0.7 μm 3 /cm 3 . On the basis statistics of backward air trajectory analyses, a stagnant airflow in summer over the coastal areas of the Yellow Sea and near Japan is inferred to be a suitable meteorological condition to form enhanced volume concentration of accumulation particles during transport. Associating with the seasonal changes in the dominant air trajectories, SO 2 emission from Miyakejima volcano since August 2000 is also an important source of the summer enhancement of accumulation particles. Highest monthly mean volume concentration (11.2 μm 3 /cm 3 ) of coarse particles (D > 1.0 μm) was found in April, which was about 10 times higher than the mean value of 1.2 μm 3 /cm 3 from summer to winter. Variability of daily nighttime volume concentrations of the coarse particles was high (standard deviation of 13.6 μm 3 /cm 3 ) in spring and low (about 2 μm 3 /cm 3 ) in the rest of the year. High volume concentration with large variability of the coarse particles in spring is caused by frequent arrival of Kosa (yellow dust) particles from the Asian continent. Rapid enhancement of coarse volume concentration was often observed to increase as much as 30 times within 3 hours during Kosa phenomena. The year 2001 had particularly strong Kosa activity with a prolonged season starting early January and ending early July.