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

Abstract

Abstract. For the first time, continuous vertically resolved aerosol measurements were performed by lidar in Tajikistan, Central Asia. Observations with the multiwavelength polarization Raman lidar Polly XT were conducted during CADEX (Central Asian Dust EXperiment) in Dushanbe, Tajikistan, from March 2015 to August 2016. Co-located with the lidar, a sun photometer was also operated. The goal of CADEX is to provide an unprecedented data set on vertically resolved aerosol optical properties in Central Asia, an area highly affected by climate change but largely missing vertically resolved aerosol measurements. During the 18-month measurement campaign, mineral dust was detected frequently from ground to the cirrus level height. In this study, an overview of the measurement period is given and four typical but different example measurement cases are discussed in detail. Three of them are dust cases and one is a contrasting pollution aerosol case. Vertical profiles of the measured optical properties and the calculated dust and non-dust mass concentrations are presented. Dust source regions were identified by means of backward trajectory analyses. A lofted layer of Middle Eastern dust with an aerosol optical thickness (AOT) of 0.4 and an extinction-related Ångström exponent of 0.41 was measured. In comparison, two near-ground dust cases have Central Asian sources. One is an extreme dust event with an AOT of 1.5 and Ångström exponent of 0.12 and the other one is a most extreme dust event with an AOT of above 4 (measured by sun photometer) and an Ångström exponent of −0.08. The observed lidar ratios (and particle linear depolarization ratios) in the presented dust cases range from 40.3 to 46.9 sr (and 0.18-0.29) at 355 nm and from 35.7 to 42.9 sr (0.31-0.35) at 532 nm wavelength. The particle linear depolarization ratios indicate almost unpolluted dust in the case of a lofted dust layer and pure dust in the near-ground dust cases. The lidar ratio values are lower than typical lidar ratio values for Saharan dust (50-60 sr) and comparable to Middle Eastern or west-Asian dust lidar ratios (35-45 sr). In contrast, the presented case of pollution aerosol of local origin has an Ångström exponent of 2.07 and a lidar ratio (particle linear depolarization ratio) of 55.8 sr (0.03) at 355 nm and 32.8 sr (0.08) at 532 nm wavelength.

Lidar Measurements of Dust Aerosols during Three Field Campaigns in 2010, 2011 and 2012 over Northwestern China

“…From April to June of 2010, 2011 and 2012 at Minqin, SACOL and Dunhuang, the occurrence frequencies of dust events retrieved from lidar observations were all higher than 88%. Dust events occurred quite often in April and Comparing our results with those from the literature at sites in adjacent areas, PDRs of 0.31-0.35 were measured at Dushanbe, Tajikistan [26]. PDRs of 0.09-0.11 in a lofted dust layer and of 0.18-0.33 in a near-ground dust layer were reported in Aksu [30].…”

“…Polarization-function lidar instruments are highly sensitive to the particle shape, produce a signal that increases with the degree of particle nonsphericity and easily detect the faintest traces of dust [25]. Therefore, dust aerosol profiling using sophisticated and continuous observations by ground-based lidar instruments are required in order to better understand dust storm motions, the temporal-spatial distribution and their effects on the radiation budget, as well as cloud and precipitation development [26].Until now, to the best of our knowledge, only a few campaigns have been performed in order to characterize dust aerosols using ground-based lidar measurements over northwestern China. In the summer of 2002, dust particles exhibiting a high depolarization ratio in the free troposphere were measured using lidar equipment at Dunhuang [27,28].…”

“…Polarization-function lidar instruments are highly sensitive to the particle shape, produce a signal that increases with the degree of particle nonsphericity and easily detect the faintest traces of dust [25]. Therefore, dust aerosol profiling using sophisticated and continuous observations by ground-based lidar instruments are required in order to better understand dust storm motions, the temporal-spatial distribution and their effects on the radiation budget, as well as cloud and precipitation development [26].…”

A 3-D evaluation of the MACC reanalysis dust product over Europe, northern Africa and Middle East using CALIOP/CALIPSO dust satellite observations

“…This product is corrected for the dust LR value which is specific for the region on which we focus in this study, based on multiyear measurements performed by the ground-based lidar stations of the EARLINET (European Aerosol Research Lidar Network). These regionspecific LRs are equal to 55 ± 5 sr for the northern Africa-Europe domain (Tesche et al, 2009Groß et al, 2011Groß et al, , 2015 and equal to 40 ± 5 sr for the Middle East and central Asia at longitudes further east than 30 • E (Mamouri et al, 2013;Nisantzi et al, 2015;Hofer et al, 2017). The correction leads to an AOD 532 absolute bias of ∼ −0.03 compared to spatially and temporally collocated AERONET observations above Europe and northern Africa while the corresponding biases for the standard CALIPSO product are much higher (∼ −0.10) (Amiridis et al, 2013).…”

Long-term profiling of aerosol light extinction, particle mass, cloud condensation nuclei, and ice-nucleating particle concentration over Dushanbe, Tajikistan, in Central Asia

“…The separated backscatter profiles are converted into dust and aerosol-pollution extinction profiles by using typical lidar ratio values for Central Asian, Middle Eastern, and eastern Saharan dust of (40 sr) and for Central Asian aerosol pollution (50 sr) (Hofer et al, 2017). The lidar ratio observations are presented in Hofer et al (2020).…”