Long-term profiling of mineral dust and pollution aerosol with multiwavelength polarization Raman lidar at the Central Asian site of Dushanbe, Tajikistan: case studies

摘要

For the first time, continuous vertically resolved aerosol measurements wereperformed by lidar in Tajikistan, Central Asia. Observations with themultiwavelength polarization Raman lidar Polly were conductedduring CADEX (Central Asian Dust EXperiment) in Dushanbe, Tajikistan, fromMarch 2015 to August 2016. Co-located with the lidar, a sun photometer wasalso operated. The goal of CADEX is to provide an unprecedented data set onvertically resolved aerosol optical properties in Central Asia, an areahighly affected by climate change but largely missing vertically resolvedaerosol measurements. During the 18-month measurement campaign, mineral dustwas detected frequently from ground to the cirrus level height. In this study, anoverview of the measurement period is given and four typical but differentexample measurement cases are discussed in detail. Three of them are dustcases and one is a contrasting pollution aerosol case. Vertical profiles ofthe measured optical properties and the calculated dust and non-dust massconcentrations are presented. Dust source regions were identified by means ofbackward trajectory analyses. A lofted layer of Middle Eastern dust with anaerosol optical thickness (AOT) of 0.4 and an extinction-relatedÅngström exponent of 0.41 was measured. In comparison, two near-grounddust cases have Central Asian sources. One is an extreme dust event with anAOT of 1.5 and Ångström exponent of 0.12 and the other one is a mostextreme dust event with an AOT of above 4 (measured by sun photometer) and anÅngström exponent of −0.08. The observed lidar ratios (and particlelinear depolarization ratios) in the presented dust cases range from 40.3 to46.9 sr (and 0.18–0.29) at 355 nm and from 35.7 to42.9 sr (0.31–0.35) at 532 nm wavelength. The particlelinear depolarization ratios indicate almost unpolluted dust in the case of alofted dust layer and pure dust in the near-ground dust cases. The lidarratio values are lower than typical lidar ratio values for Saharan dust(50–60 sr) and comparable to Middle Eastern or west-Asian dust lidarratios (35–45 sr). In contrast, the presented case of pollutionaerosol of local origin has an Ångström exponent of 2.07 and a lidarratio (particle linear depolarization ratio) of 55.8 sr (0.03) at355 nm and 32.8 sr (0.08) at 532 nm wavelength.