Aerosol layers in the free troposphere and their seasonal variations as observed in Wuhan, China

Abstract

Free-tropospheric aerosol layers and their seasonal variation over Wuhan (30.5°N, 114.4°E), China, are presented based on a 532-nm polarization lidar measurements on 162 days from January through December 2013. Using the aerosol layer selection criterions, a total of 402 free-tropospheric aerosol layer events were identified. The bottom height of the aerosol layers below 2 km accounts for 68% of the total, while approximately 76% of the layer's top height ranges from 1 km to 4 km. Out of the 402 events, 269 (67%) are optically-thin layers with aerosol optical depth (AOD) less than 0.1. The free tropospheric AOD2-7 contribute ~13–31% to the AOD0-7 and the free-tropospheric aerosol layers show considerable moderate variation. The aerosol layers have the maximum mean geometrical thickness of 1.2 km in spring, while the minimum mean thickness is 0.7 km in autumn, and the mean thickness is 0.93 km and 1 km in summer and winter, respectively. The mean backscatter coefficient of aerosol layers during spring, summer, autumn and winter were 1.8 ± 1.4 Mm−1sr−1, 2.3 ± 2 Mm−1sr−1, 2.8 ± 2.7 Mm−1sr−1 and 2.3 ± 2.2 Mm−1sr−1, respectively. Aerosol layers in different seasonal are classified by particle depolarization ratio, there are a large amount of non-spherical particles and mixed particles present in spring, autumn and winter, and the mean particle polarization ratio of aerosol layers during spring, summer, autumn and winter were 0.22, 0.06, 0.15 and 0.14, respectively.