Number‐size distributions of free tropospheric aerosol particles at Mt. Norikura, Japan: Effects of precipitation and air mass transportation pathways

Abstract

Number‐size distributions of free tropospheric aerosol particles of 9–300‐nm diameter were observed at Mt. Norikura (36.1°N, 137.5°E, 2770 m a.s.l.) in central Japan in August and September 2002. We observed two distinct air masses transported from over the Pacific Ocean and continental Asia. Size distributions of aerosol particles were analyzed in terms of their relation to the air mass transportation pathway, cumulative precipitation amount in the air mass for the last 24 h, and local precipitation rate. Particle concentrations of the accumulation mode range (100–300 nm) decreased by an order of magnitude when cumulative precipitation amount increased from 0 to 20 mm. The cumulative precipitation amount is suggested as an important factor to modify the size distribution by decreasing particle concentration. Considering data for air masses without experience of precipitation scavenging before arrival, concentrations of accumulation mode particles were almost identical (approximately 400 cm−3) in air masses transported from over the Pacific Ocean and continental Asia. Concentration of Aitken mode particles was high (890 cm−3 as a median value) in the air mass subsiding from over the subtropical Pacific Ocean (> 4 km a.s.l.). Based on the relationship with the controlling factors described above, this study provides number‐size distribution parameters of free tropospheric aerosol particles in the west Pacific area.