Case study of Raman lidar measurements of Asian dust events in 2000 and 2001 at Nagoya and Tsukuba, Japan

Vertical distributions of aerosol optical properties and relative humidity were measured with a Raman lidar at Nagoya and Tsukuba, Japan, during the Asian dust events in 2000 and 2001. The data obtained on 23 April 2001 showed a maximum of aerosol backscattering ratio (R) of 3.5–4.0 at 532 nm at an altitude of 5 km over both measurement sites. Around that height the aerosol depolarization ratios (δa), which indicate the aerosol nonsphericity, were higher than 20% and the relative humidities (RH) were lower than 30%. The aerosol optical thickness between 4 and 7 km was 0.18±0.02 and the average aerosol extinction-to-backscatter ratio (lidar ratio) was 46±5 sr at Tsukuba. This aerosol layer was present for over 6 h and finally showed the highest δa value of about 33% and the lidar ratio of 10±3 sr at the uppermost region, where the RH was almost saturated with respect to ice. The data obtained on 4 May 2000 at Nagoya showed relatively high R values ( 2.5) below 5 km. The values of δa were higher than 15% between 3.5 and 8 km in which the maximum RH was about 60%. The values of δa and RH showed a weak negative correlation below 3.5 km, where the RH varied between 30% and 70%. In the 5–8 km region, the δa values were correlated negatively with the wavelength exponent of the aerosol backscattering coefficient (α), whereas they showed the lowest value ( 7%) at smaller α values (<0) in the 2–5 km region. We hypothesized that these relations of δa to α and RH are due to the mixing state of mineral dust, sea-salt, and sulfate-containing particles that are major aerosol constituents in the free troposphere during the Asian dust period and to the dependence of their shape and size on RH.