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 Raman lidar at Nagoya (35.1°N, 137.0°E) from March 1994 to February 1997 to study the seasonal and altitude characteristics of the aerosol optical properties. We analyzed the relation of these characteristics to relative humidity (RH) and the transport pathways from the source areas. The vertically integrated aerosol backscattering coefficients (IBC) in the altitude ranges of 2–4 and 4–8 km showed an annual maximum in the spring (March–May). A second maximum of IBC was found in 2–4 km in mid-late summer (July–August). Aerosol depolarization ratios as high as 25% were observed frequently in the 2–8 km region in the spring and occasionally in the 4–8 km region 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% were predominantly observed in the winter. The air parcels in these regions had mainly passed over the Asian Continent. Depolarization ratios less than 5% over a wide range of RH and high depolarization ratios (>10%) with relatively low RH values (<50%) were predominantly observed in the 2–8 km region in the summer and in the 2–4 km region in the autumn; most of the air parcels had passed over the Pacific Ocean. Our results suggest that transport pathways from the source areas (the Asian Continent and the Pacific Ocean) and the ambient relative humidity critically control the aerosol depolarization ratio in the free troposphere over Nagoya.