Free tropospheric aerosol backscatter, depolarization ratio, and relative humidity measured with the Raman lidar at Nagoya in 1994-1997: contributions of aerosols from the Asian Continent and the Pacific Ocean

Vertical profiles of free tropospheric humidity, aerosol backscattering and the depolarization ratio (particle nonsphericity) were measured with a Ram an lidar at Nagoya (35.1 degrees N, 137.0 degrees E) from March 1994 to Feb ruary 1997 to study the seasonal and altitude characteristics of the aeroso l optical properties. We analyzed the relation of these characteristics to relative humidity (RH) and the transport pathways from the source areas. Th e vertically integrated aerosol backscattering coefficients (IBC) in the al titude ranges of 2-4 and 4-8 km showed an annual maximum in the spring (Mar ch-May). A second maximum of IBC was found in 2-4 km in mid-late summer (Ju ly-August). Aerosol depolarization ratios as high as 25% were observed freq uently in the 2-8 km region in the spring and occasionally in the 4-8 km re gion in the autumn (September-November) and winter (December-February). In the 4-8 km regions these high values were observed over a wide range of RH values. Depolarization ratios less than 5% with RH values less than 20% wer e predominantly observed in the winter. The air parcels in these regions ha d mainly passed over the Asian Continent. Depolarization ratios less than 5 % over a wide range of RH and high depolarization ratios ( > 10%) with rela tively low RH values ( < 50%) were predominantly observed in the 2-8 km reg ion in the summer and in the 2-4 km region in the autumn; most of the air p arcels had passed over the Pacific Ocean. Our results suggest that transpor t pathways from the source areas (the Asian Continent and the Pacific Ocean ) and the ambient relative humidity critically control the aerosol depolari zation ratio in the free troposphere over Nagoya. (C) 1999 Elsevier Science Ltd. All rights reserved.