Large depolarization ratio of free tropospheric aerosols over the Taklamakan Desert revealed by lidar measurements: Possible diffusion and transport of dust particles

Lidar measurements were made at Dunhuang (40degrees00'N, 94degrees30'E), China, to understand the vertical distribution of aerosols in the free troposphere over the Taklamakan desert in summer of 2002. The vertical distributions of the scattering ratio suggested that particulate matter distributed from near the ground to about 6 km in the range of values of about 2 to 5 and rapidly decreased to about 1 at about 6 km. The depolarization ratio indicated that dust particles distributed in a aerosol layer, and the dust particle layer distributed to about 6 km. A very clear boundary was also identified at 6 km in the distribution of the depolarization ratio. Particulate materials were directly collected with a balloonborne particle impactor in the free troposphere over the Taklamakan desert, and an electron microscopic experiment on the particles suggested that the large depolarization ratio was certainly due to irregularly shaped dust particles. Vertical profiles of the scattering ratio and depolarization ratio suggested that nonspherically shaped dust particles floated from near the surface to about 6 km and this corresponded well with the wind system suggested by Sun et al. [ 2001] and Sun [ 2002], who reported that the typical surface wind was easterly and/or northerly in the Tarimu Basin, and westerly wind dominated above about 5 km. The trajectory of the balloon also showed that westerly wind appeared at about 4 km, and the wind speed largely increased above about 5 km. This suggests the possible long-range transport of dust particles entrained at an elevation of >5 km.