ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-14559-2017Long-term profiling of mineral dust and pollution aerosol with multiwavelength
polarization Raman lidar at the Central Asian site of Dushanbe, Tajikistan: case studiesHoferJulianhofer@tropos.dehttps://orcid.org/0000-0001-6657-4072AlthausenDietrichAbdullaevSabur F.https://orcid.org/0000-0003-3468-6939MakhmudovAbduvosit N.NazarovBakhron I.SchettlerGeorgEngelmannRonnyBaarsHolgerhttps://orcid.org/0000-0002-2316-8960FombaK. WadingaMüllerKonradHeinoldBerndKandlerKonradAnsmannAlbertLeibniz Institute for Tropospheric Research, Leipzig, GermanyPhysical Technical Institute of the Academy of Sciences of Tajikistan, Dushanbe, TajikistanHelmholtz Center Potsdam, German Research Center for Geosciences, Potsdam, GermanyInstitut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Darmstadt, GermanyJulian Hofer (hofer@tropos.de)7December20171723145591457715June201719July201724October201725October2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/17/14559/2017/acp-17-14559-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/14559/2017/acp-17-14559-2017.pdf
For the first time, continuous vertically resolved aerosol measurements were
performed by lidar in Tajikistan, Central Asia. Observations with the
multiwavelength polarization Raman lidar PollyXT 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.
Introduction
Atmospheric mineral dust can be transported over tens of thousands of
kilometers away from its arid or semi-arid source regions
. More observations, especially in western and
Central Asia, are needed to describe global and regional dust transport and
to estimate the effect of this dust on air quality , and
climate, via direct and various indirect radiative effects. Extended
investigations into Saharan dust close to its source regions (e.g., SAMUM–1, 2,
Saharan Mineral Dust Experiment; Fennec climate programm; and SHADOW, Study of
SaHAran Dust Over West Africa) as well
as regarding dust long-range transport across the Atlantic ocean (e.g.,
SALTRACE, Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction
Experiment; ) have been conducted to obtain novel data
to reduce uncertainties in the above-mentioned estimations.
However, the global dust belt, which reaches from the Sahara over the Arabian
deserts to the Taklamakan and Gobi deserts, contains a lot more arid and
semi-arid regions which act as sources for atmospheric mineral dust in the
northern hemisphere . This dust has a
sensitive impact on climate, environmental conditions, ecosystems, and
health. Central Asia lies in the middle of this global dust belt and contains
some major dust sources (Fig. ). Therefore, Central Asian
countries are frequently affected by atmospheric mineral dust hazardous to
respiratory health . Even bacteria, fungi, and viruses can
be transported by dust over long distances
. Dust is also
transported across the highly polluted Central Asia
further eastwards
and on its way it is subject to stronger
anthropogenic influence compared to the westward transport of Saharan dust
.
Geographical map with desert regions (black text), dust belt
(bright green dashed line), measurement site in Dushanbe, Tajikistan (red text) highlighted
(http://naturalearth.springercarto.com, adapted).
Dust from different sources have different mineralogical
compositions , different optical properties
, and therefore
different impacts on radiation and climate
.
Central Asia in particular is vulnerable to climate change and is already
affected by it
. For
example, dramatic glacier shrinking took place in the last decades in
Tajikistan , which has an effect on the water
resources of Tajikistan and the whole Central Asian area .
The links between mineral dust occurrence and climate change as well as
possible feedbacks have to be quantified. Therefore, aerosol profiling in
Central Asia is strongly required to better understand regional and global
transport and deposition of mineral dust and its effects on radiation budget,
cloud and precipitation development, and human health.
The sources and emission of dust in Central Asia, interaction with aerosol
pollution, and climate relevant processes are investigated with models and
satellite remote sensing
.
Measurements with sun photometers, despite being rare in inland Central Asia
, can provide column-integrated aerosol
properties . Additionally,
vertically resolved information on aerosol optical properties are needed to
thoroughly investigate radiative effects and dust–cloud interactions.
The widely used satellite-borne lidar CALIPSO/CALIOP (Cloud-Aerosol Lidar and
Infrared Pathfinder Satellite Observations/Cloud-Aerosol Lidar with
Orthogonal Polarization) delivers snapshot-like observations in terms of
backscatter profiles at 532 and 1064 nm wavelength. To estimate
extinction coefficients of dust, the dust lidar ratio is required as an
input, which introduces considerable uncertainty
e.g.,. To obtain
sophisticated sets of optical properties and detailed information on particle
mixture and layering, long-range dust transport, multi-day coherent
structures, vertical mixing between the planetary boundary layer
and lofted aerosol layers, aerosol–cloud interactions, and continuous observations
with ground-based Raman lidars or HSRLs (High-Spectral-Resolution Lidars) are
needed. Up to now, only a few experiments were performed to characterize the
aerosol over Central Asia. In 1989 a Soviet-American research team
conducted a joint experiment on dust in Tajikistan
.
Coordinated ground-based aircraft and satellite measurements took place
during two dust storms on 16 and 20 September 1989. The total area occupied
by the atmospheric dust during the latter dust storm was approximately
105km2. The mass of dust in the air was
estimated as 3.1 t. Chemical analysis of the
collected dust showed a low iron content for
Central Asian dust. A calcareous character and high contents of soluble
salts were also observed . In general, however, a very high variability
lead to considerably different results depending on
sampling location. Analysis of quartz filter samples collected in Dushanbe
showed, besides significant amounts of dust, a strong contribution of
anthropogenic black carbon pollution from fuel combustion and smoke
. Optical and radiative properties have been studied as
well . performed
radiative transfer calculations for the dust storms in 1989 and stated that
the vertical distribution of additional total radiative heating rates can
only be calculated with assumptions on vertically homogeneous optical
properties and on the height of the dust layer. compared
complex refractive indices from the dust collected in Tajikistan with dust
from other sources. They found a seemingly different spectral behavior of the
complex refractive index of Central Asian dust compared to Saharan dust, but
concluded that there are large uncertainties and the
discrepancies could be due to different measurement techniques. The measured
aerosol optical thicknesses (AOTs) during those dust storms were up to 1.3
and 3.3 at 550 nm, respectively. A few vertical profiles of the
scattering coefficient of the dust were measured with a nephelometer on board
an aircraft . These measurements showed that the
majority of the dust was concentrated in the lowermost 3–3.5 km
below a thermal inversion. mentioned dust plumes
reaching through this inversion up to 6 km altitude. The measurements
on 16 September 1989 also revealed a dust layer at a height of about
4 km. used satellite data to obtain the horizontal
distribution of the dust in Central Asia. emphasized
the need for vertical profiles of the dust mass, which can be estimated from
dust extinction coefficient, to determine the movement of the dust.
modeled dust deposition and transport during this 1989 campaign and mentioned the
sensitivity of their model to the dust layer height, especially because of the complicated topography in Tajikistan.
Recently, a lidar station is operated in eastern Kyrgyzstan
. For this station,
report dust transported from the Aral Sea region,
long-range transported dust from the Middle East and dust from the
Taklamakan desert. A lidar is operated in Aksu (northwestern Taklamakan;
), where local dust was measured from ground to a height of
6 km during strong dust outbreaks as well as
lofted long-range transported dust layers at heights of 11 km. Lidar measurements along the Caspian and Aral Sea region and
Lake Balkhash in Kazakhstan were performed by .
showed that dust layers originating in the Caspian and
Aral Sea region can regularly spread over wide areas of Russia and last for
several days.
Nevertheless, the knowledge of the vertical aerosol distribution over Tajikistan and especially
the transport of mineral dust over Central Asia is still insufficient. Therefore, the Central Asian
Dust EXperiment (CADEX) was proposed. CADEX provides long-term data on vertical profiles of particle
optical properties above Tajikistan. A multiwavelength polarization Raman lidar
was run continuously in Tajikistan over the 18-month period from March 2015 to August 2016. These
measurements are part of the PollyNET, which is an attempt to contribute to the
establishment of a ground-based polarization Raman lidar network
e.g.,, with the aim to
support regional and global dust modeling.
This article provides an overview of the observations and focuses mainly on
four exemplary measurement cases, which show typical but differing scenarios
of atmospheric mineral dust in Tajikistan. The vertical profiles of the
measured particle optical properties, calculations of the dust mass
concentration, and backward trajectories for the individual cases are
presented. In a follow-up publication, a summary of the key findings of the
CADEX campaign including a statistical analysis of the entire measurement
period will be provided.
In Sect. of this article, the CADEX campaign, the measurement
site, the lidar system, and the auxiliary data are described.
Section gives an overview of the observations and the
four exemplary measurement cases are presented in detail. The cases are
compared and discussed in Sect. .
The Central Asian Dust EXperiment (CADEX) and instruments
CADEX was a joint German-Tajik project to investigate the atmospheric mineral
dust over Tajikistan. The goal of the project was to provide a not-yet-available
data set of optical and microphysical properties of Central Asian
mineral dust on a longer-term basis. The data are used to characterize and
investigate the particle types, possible aerosol sources, and the aerosol
influence on the radiation budget in Central Asia.
Tajikistan lies in the global dust belt in close proximity of some major dust
sources like the Taklamakan desert, the Aralkum desert of the desiccating
Aral Sea, the Kyzylkum and Karakum deserts east of the Caspian Sea, the
Iranian Dasht-e Kavir and Dasht-e Lut deserts and the deserts in Afghanistan
(Fig. ). Therefore, Tajikistan is frequently affected by severe
dust events and is a net accumulator of dust
. Tajikistan is a country with a dry
continental climate and benefits from its water resources. Melt water
discharge from the high mountain regions ensures a high freshwater
availability. Furthermore, Central Asia and especially Tajikistan are highly
affected by climate change. For example, dramatic glacier shrinking took
place in the last decades
.
This could also have an effect on the water resources of Tajikistan and the
whole Central Asian area in the future since the regionally important
rivers such as the Amu Darya and Syr Darya are fed by glacier and snow melt water
, which are feeding the desiccating Aral Sea,
which now itself became a strong dust source .
Furthermore, deposited dust and anthropogenic black carbon themselves can
accelerate glacier melt by altering the glacier's surface albedo
.
The fieldwork of CADEX included column-integrating sun photometer
measurements, vertically resolved lidar measurements, and meteorological
observations in Dushanbe, Tajikistan. The lidar observations were carried out
in Dushanbe at the Poligon measurement site of the Physical Technical
Institute of the Academy of Sciences of Tajikistan. The measurement site lies
in an urban environment on a hill in the eastern part of Dushanbe. The lidar
is installed in front of a laboratory and office building
(Fig. a). Its coordinates are 38∘33′34′′ N,
68∘51′22′′ E, and its altitude is 864 ma.s.l. The
measurement campaign lasted from 17 March 2015 until 31 August 2016.
On 487 days during that period, lidar data have been acquired for a duration of at
least 3 h. On 308 of these days the lidar ran even longer than
20 h.
In addition, the field experiment is accompanied by 3-D regional modeling
with the regional dust model COSMO-MUSCAT
and the aerosol-climate model ECHAM-HAM . The
modeling provides a perspective on the sources, transport, and the
direct radiative effects of measured mineral dust and associated atmospheric
feedbacks. Furthermore, ground-based in situ aerosol measurements were
conducted during CADEX to investigate the relationship between the settling
of mineral dust along the margin of the Central Asian mountains and the dust
transport at high tropospheric levels recorded by lidar. Collection of
aerosol (March 2015 to April 2016) was carried out on quartz fiber filters
(MK 360, Munktell) using a high-volume filter sampler (DHA-80, DIGITEL) with
an inlet for PM10 (particulate matter with a maximum diameter of
10 µm). Chemical analysis of the collected aerosol was performed
with the techniques described in . The particle number size
distribution was measured with a laser particle counter (GRIMM EDM 180)
. Dust dry deposition was collected (9 days in
August 2016) by means of a flat-plate-type passive particle collector
. Collected particles were subject to electron microscopy
single particle analysis , yielding
information on particle size distribution, chemical and mineralogical
composition, and mixing state for coarse mode particles with diameters
between 0.7 and 60 µm. The additional model and in situ
observational data, however, need further analysis and evaluation and will be
used in a follow-up study.
Lidar system
The lidar operated in Dushanbe is a multiwavelength polarization Raman lidar
PollyXT (POrtabLe Lidar sYstem PollyXT, XT stands for
extended version; ) and belongs to PollyNET, a network
of permanent or temporary Polly systems . This specific
PollyXT was already deployed in numerous field campaigns in the
past e.g.,. For the CADEX campaign, this
PollyXT was upgraded with a second depolarization channel at
355 nm wavelength . All PollyXT
systems contain a laser system that emits light at 355,
532, and 1064 nm wavelength by means of frequency doubling
and tripling. The receiver of the system has eight channels and measures
the backscattered light at all three emitted wavelengths. The channels at
387, 607, and 407 nm wavelength allow us to detect Raman scattering at
nighttime. Two other channels detect cross-polarized light at 355 and
532 nm wavelength. Three times a day, the system performs an
automatic Δ90∘ depolarization calibration
. Figure b shows a
sketch of the optical layout of the system. Signals are sampled with a
vertical resolution of 7.5 m and are stored with a temporal
resolution of 30 s. Full overlap is reached at about
1.5 kma.g.l.. The resulting products of the
PollyXT are summarized in Table . The
corresponding uncertainties, discussed in detail in
, , and
, result from uncertain input parameters and signal
noise and are given as error bars in the graphs of Sect. .
The polarization lidar photometer networking method (POLIPHON; ) is applied to separate dust and non-dust optical and related
microphysical properties. This separation is performed with respect to the
particle backscatter coefficient
using the measured total
particle linear depolarization ratio at 532 nm wavelength as well as
typical particle linear depolarization ratio values
for dust (0.31) and non-dust (0.05). For
the conversion of particle extinction coefficients into particle volume
concentration, conversion factors for dust
(0.61 × 10-6m) and non-dust
(0.41 × 10-6m) from extended AERONET data analysis
from are used. Together with the densities
of dust (2.6 gcm-3) and non-dust (1.6 gcm-3), this
allows us to calculate dust mass concentration profiles using characteristic
lidar ratio values of dust (40 sr) and non-dust (80 sr). The
uncertainty of such an estimation is given as 40 % .
compared the POLIPHON method directly to in situ
aircraft measurements and found less than 30 % relative difference. The
measured lidar data are uploaded daily and processed automatically
to be displayed as “quicklooks” on the PollyNET homepage
(http://polly.rsd.tropos.de/).
(a) PollyXT with closed roof, open doors, and removed
protective plates at the Poligon field site in Dushanbe. The cabinet size is
approximately 1.9 m× 1.7 m× 0.9 m. (b) Schematic of the
optical layout of the PollyXT. Left (transmitter part): after the
second harmonic generation (SHG) and the third harmonic generation (THG),
parts of the laser beam are deflected to a power meter (PM) which measures
the UV component to monitor the conversion efficiency. Right (receiver part):
backscattered light is collected with a Newtonian telescope and then passed
towards the receiver unit. The numbers indicate the wavelength in nanometers
of the detection channels and c denotes the cross-polarized channels. A
camera (CAM) is synchronized to the laser trigger and sees the beam at
532 nm wavelength to monitor the overlap. The polarizer mounted in
front of the pinhole is a device for the absolute calibration of the
depolarization measurements for details see.
Products of the PollyXT lidar and their relative
uncertainties. β: particle backscatter coefficient, α: particle
extinction coefficient, S: lidar ratio, αÅ:
extinction-related Ångström exponent, δ: particle linear
depolarization ratio, WV: water vapor, and dust and non-dust mass
concentrations.
Daily means of the AOT (a) and Ångström exponent (b) measured in
Dushanbe from March 2015 until August 2016 by sun photometer (AERONET Level 1.5, cloud-screened).
The red lines indicate the days of the presented example cases.
Auxiliary data
Co-located with the lidar, a sun photometer as part of the AErosol RObotic NETwork (AERONET; ) was operated.
The Dushanbe AERONET station (http://aeronet.gsfc.nasa.gov/cgi-bin/type_one_station_opera_v2_new?site=Dushanbe)
is operational since 2010 . The Cimel sun photometer measures at eight wavelengths
(1020, 870, 675, 500, 490, 440, 380, and 340 nm) and retrieves the AOT and further
column-integrated particle optical and microphysical properties.
To calculate the Rayleigh contribution to the lidar signals in order to
obtain particle optical properties, profiles of temperature and pressure are
needed. Profiles of the GDAS (Global Data Assimilation System) with
1∘ spatial resolution from the National Weather Service's
National Centers for Environmental Prediction (NCEP) at the coordinates
39∘ N, 69∘ E were used , because no
radiosonde launches were performed in Tajikistan for several years.
Publicly available trajectory models were used to assess the possible source
regions and transport routes of the dust. The HYSPLIT model (Hybrid Single
Particle Lagrangian Integrated Trajectory Model;
) and the FLEXPART model version 9 (FLEXible
PARTicle dispersion model; ) were run to calculate
backward trajectories for the presented example cases. The
0.5∘ GDAS meteorology for HYSPLIT and the 1∘
GFS (Global Forecast System) meteorology for FLEXPART were used. The HYSPLIT
and FLEXPART backward trajectories were calculated with a starting time
approximately corresponding to the time of observation of the aerosol layer
by lidar. The arrival heights above the measurement site were set to the
base, center, and top of the observed layer. The FLEXPART backward
trajectories were calculated for the arrival height above the measurement
site in the height range of the aerosol layer measured with the lidar. The
model output is the common logarithm of the accumulated residence time of air
masses below 3 km during the model run time .
This 3 km height is chosen because air is likely loaded with dust in
the source regions, where planetary boundary layers of this magnitude occur.
The spatial resolution of the model output is 1∘.
ObservationsOverview
An overview of the daily means of the AOT at 500 nm wavelength and
the Ångström exponent from the 440–870 nm spectral range is
shown in Fig. . The data were measured by sun photometer from
March 2015 until August 2016 (cloud-screened AERONET data, Level 1.5). For the
six 3-month seasons of the measurement campaign, the means of the AOT, the
Ångström exponent, and the fine mode fraction from the 500 nm
measurements are shown in Table . A seasonal transition is obvious
from the winter to the summer months. The AOT increases from spring to summer
and decreases again in autumn. The Ångström exponent and the fine mode
fraction behave reversely. There is a dusty 6-month season from April to September,
and a less dusty season from October to March, but a strong dust event still occurred
in October 2015. Within these seasons, strong variations in the AOT and especially in
the Ångström exponent occur from day to day. This indicates fast changes from
dust-dominated to pollution-dominated aerosol conditions.
(a, b, c) Illustrative measurement examples of aerosol layers
observed with PollyXT at Dushanbe, partly up to the cirrus level
(10 km height). Temporal development of the range-corrected signal
(temporal resolution 30 s, vertical resolution 7.5 m) of the
1064 nm wavelength channel on (a) 20 April 2015, 06:00 UTC – 24 April 2015,
14:46 UTC. (b) 13 May 2015, 12:00 UTC – 14 May 2015, 23:58 UTC. (c) 8 August 2015,
00:02 UTC – 9 August 2015, 23:58 UTC. Blue rectangles denote
periods where no measurements were performed. (d, e, f) Source regions of
the observed aerosol identified based on FLEXPART model runs. The first
lofted layer (a) has Middle Eastern sources (d), the second lofted
layer (b) has North African sources (e) and the third low laying dust (c) has Central
Asian sources (f). The dark red squares in the left panels indicate the
arrival height and time of the calculated backward trajectories in the context of
the lidar measurements. The right panels show maps of the logarithm of
accumulated residence time below 3 km (in seconds) of 144 h
FLEXPART backward trajectories arriving above Dushanbe on (d) 23 April 2015,
20:30–21:30 UTC between 2.7 and 5 km, (e) 13 May 2015, 17:30–18:30 UTC
between 6.6 and 9.9 km height, and (f) 8 August 2015, 21:30–22:30 UTC
between 0.5 and 3.1 km height.
Case 1: a lofted layer of Middle Eastern dust is shown. (a) Same
as Fig. a, b, c, but on 12 April 2015,
18:00 UTC – 13 April 2015, 23:58 UTC. (b) The averaged
lidar profiles were measured on 13 April 2015, 15:10–16:08 UTC.
Lidar signals are smoothed before calculation of the extinction-related
(743 m) and backscatter-related (68 m) optical properties.
Particle backscatter coefficient at 355, 532, and
1064 nm wavelength are shown. Particle extinction coefficient, particle linear
depolarization ratio, and lidar ratio at 355 and 532 nm
wavelength are also shown. Extinction-related Ångström exponent from 355
and 532 nm wavelengths (black), backscatter-related Ångström
exponent from 355 and 532 nm wavelengths (cyan), and
backscatter-related Ångström exponent from 532 and
1064 nm wavelengths (magenta). Dust mass concentration are shown in yellow. (c) 144 h
FLEXPART backward trajectories same as in Fig. d, e, f, but arriving on 13 April 2015, 14:30–15:30 UTC between 2.3
and 4.9 km height. (d) 96 h HYSPLIT backward trajectories
arriving at Dushanbe at 18:00 UTC at 2.3, 3.6, and 4.9 km height.
Case 2: an extreme dust layer with an AOT of 1.5 (at 532 nm
wavelength) and a calculated dust mass concentration of 845 µgm-3 is shown. The source regions of this dust are in Central Asia
(Fig. f). (a–f) Same as Fig. b. The averaged
lidar profiles were measured on 8 August 2015, 22:20–23:57 UTC. Lidar
signals are smoothed before calculation of the extinction-related
(458 m) and backscatter-related (68 m) optical properties.
Case 3: the most extreme dust event with an AOT of above 4 (at
500 nm wavelength) and a calculated dust mass concentration of
2.8 mgm-3 is shown. The dust source regions are in Central Asia.
(a) Same as Fig. b. The averaged lidar profiles were measured on
14 July 2016, 16:00–22:00 UTC. Lidar signals are smoothed before calculation
of the extinction-related (743 m) and backscatter-related
(248 m) optical properties. (b) 120 h FLEXPART backward
trajectories same as in Fig. d, e, f, but arriving on
14 July 2016, 20:30–21:30 UTC between 0.5 and 2.6 km height.
(c) 96 h HYSPLIT backward trajectories arriving at Dushanbe at 16:00 UTC
at 1.25, and 2.5 km height.
Case 4: a contrasting case with local pollution is shown. (a) Same as
Fig. b. Averaged lidar profiles measured on 13 May 2015,
18:10–20:30 UTC. Lidar signals are smoothed before calculation of the
extinction-related (788 m) and backscatter-related (98 m)
optical properties. (b) 48 h HYSPLIT backward trajectories arriving at
Dushanbe at 18:00 UTC at 1, 2, and 3 km height.
Seasonal means of the sun photometer measurements (AERONET Level
1.5, cloud-screened). AOT at 500 nm wavelength, Ångström
exponent from the 440–870 nm spectral range, and the fine mode
fraction from 500 nm wavelength.
and
classified dust events in the Mediterranean region based on the long-term
mean of the AOT measured by satellite and ground stations. According to this
classification, a strong dust event equals or exceeds the mean AOT plus 2 times
the standard deviation, while an extreme dust event equals or exceeds
the mean AOT plus 4 times the standard deviation. This classification can be
pursued for the station in Dushanbe with AERONET data (Level 2.0), which are
available from July 2010 to November 2016 (1413 days). The mean of the daily
mean AOT at 500 nm wavelength is 0.28 with a standard deviation of
0.23. As a comparison, found a long-term mean AOT at
550 nm wavelength of 0.22 ± 0.19 for the Eastern Mediterranean
region. So in Dushanbe, days with an AOT ⩾ 0.74 are therefore
strong dust events, while days with an AOT ⩾ 1.2 at
500 nm wavelength are extreme dust events. Considering only the
measurement period from March 2015 to August 2016, the mean AOT at
500 nm wavelength was 0.28 (with a standard deviation of 0.16) and
therefore equal to the long-term mean. According to this classification,
six episodes of strong dust and six episodes of extreme dust occurred during the
measurement period. In the period July 2010 to November 2016 (6.25 years),
strong dust events occurred 19 times (3 per year), extreme dust events 10
times (1.6 per year), and more than extreme dust events with an AOT above 2 at
500 nm wavelength occurred 4 times (0.6 per year).
Illustrative measurement examples
To illustrate the frequently observed variety and complexity of dust layers
above Tajikistan, examples of dust layers of various origins occurring at all
heights from the surface to the cirrus level are presented in
Fig. . Figure a shows the temporal development of
the range-corrected signal of the 1064 nm wavelength channel from
20 to 24 April 2015. During these 4 days, multiple dust layers arrived above
Tajikistan. On 20 April 2015 there were aerosol layers up to 4 km
height. At the same time, descending dust layers crossed the lidar station at
heights of about 10 km. On 22 April 2015 a second dust layer arrived
at 10 km height, which again descended. According to FLEXPART
trajectories, the Arabian Peninsula, Iran, and parts of Uzbekistan and
Turkmenistan were also source regions for the detected dust
(Fig. d). A high-altitude aerosol layer was measured on 13 May
2015, reaching roughly from 6.5 to 10 km altitude
(Fig. b). FLEXPART back-trajectory analysis shows that this high
layer was long-range transported towards the measurement site from the
Mediterranean and North African region (Fig. e). The third scenario
is a near-ground dust layer (Fig. c; described in detail in
Sect. ), which was measured on 8 and 9 August 2015 and had
Central Asian sources (Fig. f).
Case studies
In the following sections we discuss four strongly contrasting cases in more
detail: (1) A lofted layer of Middle Eastern dust that occurred on 13 April
2015 (Case 1; Fig. ; Sect. ); (2) an extreme dust
event with Central Asian dust, which was recorded on 8 and 9 August 2015
(Case 2; Fig. ; Sect. ); (3) the most extreme
dust event during the CADEX campaign with dust from Central Asian sources,
which was observed on 14 July 2016 (Case 3; Fig. ;
Sect. ); and (4) a contrasting case with a pollution layer of
local origin, which was measured on 13 May 2015 (Case 4; Fig. ;
Sect. ).
Case 1: 13 April 2015, lofted dust layer
Figure a shows the temporal development of the range-corrected
signal of the 1064 nm wavelength channel on 12–13 April 2015. The dust
layer arrived over Dushanbe on 12 April 2015 at an altitude between 5
and 6 km. The slowly descending dust layer (red) contained dense clouds (white).
On 13 April 2015 the dust layer extended at about 2.5–5 km height. Its base
was very sharp while on top of the layer a thinner aerosol layer was measured up to
7 km. At the end of that day, a down-mixing of dust into the lowermost 2 km occurred.
The HYSPLIT backward trajectories arriving on 13 April 2015 show that the air
parcels are coming from a southwestern direction (Fig. d). The
upper and the lower trajectories come from the Arabian Peninsula and travel
over central Iran and along the Afghanistan-Turkmenistan border. The upper
trajectory is always more than 3 km above ground, while the lower
trajectory reached below 2 km once above the Arabian Peninsula. The
center trajectory is coming from a more southern direction, traveling over
southern Iran, where it reaches down to below 1 km above the
Iranian Dasht-e Lut desert.
Similar to HYSPLIT, the FLEXPART backward trajectories indicate Iran
including its southern parts and large parts of the Arabian Peninsula as
source regions (Fig. c). In contrast to HYSPLIT, the FLEXPART
backward trajectories resided partly above Afghanistan.
Vertical profiles of the optical properties of this dust layer are shown in
Fig. b. The particle linear depolarization ratio at
532 nm wavelength is on average 0.31 and 0.34 at its maximum. This
shows that this lofted dust layer consists of almost unpolluted dust.
Depolarization ratios of pure Middle Eastern dust close to its source regions
have been found as 0.3–0.35 at 532 nm wavelength
. The mean lidar ratio is
35.7 ± 1.7 sr at 532 nm wavelength, which is comparable
to measurements for Middle Eastern dust .
The lofted dust layer has a maximum dust mass concentration of 196 µgm-3 at the bottom of the layer at 2.7 km. In the middle of the
layer at 3.5 km the dust mass concentration reaches 171 µgm-3. The integrated dust mass up to 5 km altitude is
0.51 gm-2. The non-dust components in this layer are negligible,
except in the upper parts of the layer, where the particle linear
depolarization ratio partly drops below 0.31 at 532 nm wavelength.
Case 2: 8 and 9 August 2015, extreme dust event
An extreme dust event was measured on 8 and 9 August 2015.
Figure c shows the temporal development of the range-corrected
signal of the 1064 nm wavelength channel on 8–9 August 2015. The
dust got lifted up with the evolution of the convective boundary layer. This
led to a very strong signal in the lowest 1.5 km with dust extending
up to about 3 km height.
The FLEXPART trajectories arriving on 8 August 2015 (Fig. f)
exhibit a large area northwest of Tajikistan with high accumulated residence
times, even west of the Caspian Sea.
Vertical profiles of the optical properties of this dust layer are shown in
Fig. . The particle extinction coefficients at 355
and 532 nm wavelength are about 550 Mm-1 at 1 km
height. Apparently, the layer is divided in two parts. There is a maximum in
the first 1 km above ground and then again at about 2.5 km
height. However, looking at the intensive properties, especially the particle
linear depolarization ratio and the extinction-related Ångström
exponent, continuous features are recognizable. The twofold layer probably
contains the same aerosol and originates from the same source region since it
has almost identical optical properties. The apparent layer boundary at
1.6 km is probably formed by the diurnal cycle of the boundary layer.
The development of the convective boundary layer after about 06:00 UTC on 8
August 2015 is visible in Fig. c. The mean particle linear
depolarization ratios are higher than in the lofted dust layer before
(Case 1), on average 0.35 at 532 nm wavelength. The maximum dust mass
concentration of that extreme near-ground dust layer is 845 µgm-3 at 0.65 km height, in the middle of the layer the minimum
is 475 µgm-3 at 1.7 km. In the upper part at
2.4 km the dust mass concentration is again 663 µgm-3. The integrated dust mass up to 4 km altitude is
3.2 gm-2.
Case 3: 14 July 2016, most extreme dust event
The most extreme dust event during the CADEX campaign occurred on 14 July
2016. The dust persisted during the next 4 days (Fig. ). The
FLEXPART trajectories arriving on 14 July 2016 (Fig. b) show
large accumulated residence times as far away as western Iran, but the
highest values are in Uzbekistan. The accumulated residence times above
Kazakhstan are high, but they are also reaching eastwards towards Lake
Balkhash, differently than on 8 August 2015 (Fig. f). The
HYSPLIT backward trajectories arriving on 14 July 2016 are shown in
Fig. c. The higher trajectory arrived from the Aral Sea and the
lower one from central Kazakhstan through Turkmenistan and Afghanistan. The
vertical profiles of the measured optical properties of this dust layer are
presented in Fig. a. The particle extinction coefficients at
355 and 532 nm wavelength are about 1.7 km-1 at
1 km height. The resulting AOT is 3.89 at 532 nm wavelength,
which is much higher than the long-term mean plus 10 times the standard
deviation (see Sect. ). The 14 July 2016 has the highest
daily mean AOT since the beginning of the record in Dushanbe with the AERONET
sun photometer in 2010. Therefore, this dust event could well be called
record-breaking similar to how called an extreme dust event in
September 2015 in the Mediterranean. The mean particle linear depolarization
ratio at 355 nm wavelength is 0.29 and thus higher than in Case 2,
while at 532 nm wavelength it is equally 0.35. The calculated dust
mass concentrations for that most extreme dust is largest at 1 km
altitude with 2.8 mgm-3. The integrated dust mass up to
2.7 km altitude is 6.5 gm-2.
Case 4: 13 May 2015, contrasting case with local pollution
Figure b shows the temporal development of the range-corrected
signal of the 1064 nm wavelength channel on 13–14 May 2015. There
were several aerosol layers distributed up to 10 km height. The
HYSPLIT backward trajectories arriving on 13 May 2015 are shown in
Fig. b. The upper trajectory is coming from northwest of
Tajikistan and the lower trajectories reach down to the ground the same day
inside Tajikistan, indicating the contribution of local pollution to the
lower altitude aerosol layers.
The vertical profiles of the measured optical properties are presented in
Fig. a. The particle extinction coefficients in the layer at
about 2–4 km are about 40 Mm-1 at 355 nm and
20 Mm-1 at 532 nm wavelength. This aerosol layer is barely
depolarizing and the mean extinction-related Ångström exponent is
2.07 ± 0.72. The dust mass concentration reaches 6.9 µgm-3 at 2.5 km height, the non-dust mass concentration is at
the same time dominating with 30.7 µgm-3. The integrated dust
mass up to 10.4 km altitude is 0.05 gm-2 and the
integrated non-dust mass is 0.13 gm-2. Finally, an overview of
the individual cases is given in Table , where the
intensive optical properties and AOTs of the presented example cases are
summarized.
Overview of the selected example cases. S355,S532: lidar
ratios at 355 and 532 nm wavelength. δ355,δ532: particle linear depolarization ratios at 355 and
532 nm wavelength. τlidar: aerosol optical thickness measured
with lidar at 532 nm wavelength. τSPM: aerosol optical
thickness measurement of the co-located sun photometer at 500 nm
wavelength (time of measurements in the footnote).
αÅ355/532: extinction-related Ångström exponent
based on the particle extinction coefficients at 355 and
532 nm wavelength.
βÅ355/532,βÅ532/1064: backscatter-related
Ångström exponents based on the particle backscatter coefficients at
355, 532, and 1064 nm wavelength. The height
range to average the optical properties of the dust layers is placed within
the core of each layer. The provided uncertainties are the standard
deviations of the averaged values.
UTC times of sun photometer measurements:
1 13 April 2015, 12:54:46,
2 8 August 2015, 13:04:24,
3 9 August 2015, 02:08:05,
4 14 July 2016, 09:16:02,
5 15 July 2016, 03:29:3, and
6 13 May 2015, 13:21:50.
Discussion
Four exemplary measurement cases are described in this article. Two of them
are from spring and one is from summer 2015 and one is from summer 2016. In
early spring, the AOT was predominantly low (≤ 0.3; Fig. )
but a lot of dust layers occurred, e.g., 13 April 2015 (Case 1;
Fig. ) and 20–24 April 2015 (Fig. a). In summer, the
AOT in general was higher (≥ 0.35) and some dust events overtopped that
clearly, e.g., 8–9 August 2015 (Case 2; Fig. ) and 14 July 2016
(Case 3; Fig. ). However, a statistical analysis of the lidar
profiles of the whole measurement period has yet to follow.
Dust AOT and mass concentration
The lofted dust layer in Case 1 contributed significantly to the total AOT of
about 0.4 at 532 nm wavelength. Case 2 had an AOT of 1.5 at
532 nm wavelength. The most extreme dust case during the measurement
campaign was Case 3 with an extraordinary AOT of above 3.5 at 532 nm
wavelength as measured by lidar and above 4 as measured by sun photometer. The AOT
of Case 2 is comparable to a dust event during the Soviet-American campaign
in 1989 (AOT of 1.3 at 550 nm wavelength;
). The most extreme dust event (Case 3) is
comparable to the second dust event during the Soviet-American campaign (AOT
of 3.3 at 550 nm wavelength; ).
estimated the maximum AOT of that dust event a day
before was greater than 10, based on horizontal visibility, which was
reported to be 50–200 m. As a comparison,
reported a visibility of 500–600 m for an
extraordinary dust event in the Mediterranean area, inferring an AOT of 4.8
to 9 depending on the layer height. This again led to an estimate of the
column dust load of 8–15 gm-2. These values are higher than for
Case 3, where the AOT was 3.89 at 532 nm wavelength, and the
integrated dust mass was 6.7 gm-2. The visibility calculated
from the measured particle extinction coefficient of 1.7 km-1
(measured at 0.87 km altitude) was 2.3 km. The AOT and the
dust mass concentrations for this Case 3 were calculated based on the lidar
data only, but the sun photometer measured an even higher AOT of 4.45 at
500 nm wavelength.
Dust layer height
The multiple high-altitude dust layers preceding the lofted dust layer on 24
April 2015 (Fig. a) and the high aerosol layer on 13 May
2015 (Fig. b) reached heights of about 10 km a.g.l.
This is higher than the highest point of the Pamir mountains
(7.6 kma.s.l.), which means that these layers can cross the Pamir
or the Tien Shan mountains (7.4 kma.s.l.) and can be transported
further eastwards. A similar observation was made in spring 2003 when layers
of elevated depolarization at 9–11 km height have been measured by
lidar in Aksu (northwestern Taklamakan; ). Within a week
time, lidar stations in Japan measured dust at altitudes between 2
and 6 km, without having major dust outbreaks in the Taklamakan and
Gobi deserts. Model simulations indicated that the dust was transported to
Japan via north of the Tien Shan mountains . For that case,
estimated 50 % of the dust particles arriving in
Japan came from the Sahara, 30 % from the Middle East, and only 10 %
from China.
Lidar ratio and dust source region
The observed lidar ratios in the presented dust cases (Cases 1, 2, 3) range
from 40.3 to 46.9 sr at 355 nm and from 35.7 to
42.9 sr at 532 nm wavelength. The lofted dust layer (Case 1)
has lidar ratios of 42.2 sr at 355 nm and 35.7 sr at
532 nm wavelength. The near-ground dust layers (Cases 2, 3) have lidar
ratios of 46.9 and 40.3 sr at 355 nm and 42.9 and
38.7 sr at 532 nm wavelength. The only weakly depolarizing
pollution aerosol in Case 4 has lidar ratios of 55.8 sr at
355 nm and 32.8 sr at 532 nm wavelength, but with
high variability.
Saharan dust is found to have lidar ratios of 50–60 sr at
532 nm wavelength e.g.,.
used AERONET data sets to retrieve pure dust lidar
ratios and found lower values for west-Asian dust than for North African
dust. Similar results were found from Raman lidar measurements in Cyprus with
lidar ratios of 35–45 sr for Middle Eastern dust
. In east Asia, lidar ratios of Asian dust
of, for example, 47 ± 18 sr and
42–73 sr at 532 nm wavelength were
reported. In China, lidar ratios of 40 ± 5 sr and 35 ± 5 sr at
532 nm wavelength for dust from the Gobi desert were measured. The
situation in Central Asia is even more unclear, as almost no measurements
exist. Although direct measurements of dust lidar ratios inside of the
Taklakaman are not yet available, used constrains to
retrieve a dust lidar ratio of 42 ± 3 sr at 532 nm
wavelength in Aksu (northwestern Taklamakan). measured very
low lidar ratios of 8–29 sr at 532 nm wavelength in weakly
depolarizing dust layers in Kyrgyzstan. For dust from the Caspian and Aral
sea region, a lidar ratio of 43 ± 3 sr at 532 nm was
measured .
The lidar ratios of the example dust cases (Cases 1, 2, 3) agree well with
these values. There is no clear difference between the lofted dust layer
(Case 1), which is long-range transported Middle Eastern dust and the
near-ground fresh dust layers (Cases 2, 3) from Central Asian sources. The
lofted dust layer in Case 1 has a very low lidar ratio of 35.7 ± 1.7 at
532 nm wavelength, indeed comparable to the ones (33.7 ± 6.7 to
39.1 ± 5.1 sr at 532 nm) found by
for dust from the Middle East. However, the most extreme dust
event (Case 3) also has a lidar ratio below 40 sr at 532 nm
wavelength. Then again, slightly larger lidar ratios above 40 sr were
measured in the extreme near-ground dust layer (Case 2). The lidar ratios of
the pollution layer (Case 4) differ significantly between the two
wavelengths.
Dust particle linear depolarization ratio
The observed depolarization ratios in the presented dust cases (Cases 1, 2,
3) range from 0.18 to 0.29 at 355 nm and from 0.31 to 0.35 at
532 nm wavelength. The lofted dust layer (Case 1) has a
depolarization ratio of 0.18 at 355 nm and 0.31 at 532 nm
wavelength. The two near-ground dust cases (Cases 2, 3) have depolarization
ratios of 0.23 and 0.29 at 355 nm and both are 0.35 at 532 nm
wavelength. In contrast to the dust cases, the local pollution aerosol
(Case 4) has low depolarization ratios of 0.03 at 355 nm and 0.08 at
532 nm wavelength.
measured a particle linear depolarization ratio of
0.23 ± 0.02 at 532 nm wavelength for lofted dust from the
Caspian and Aral sea region. From the Kyrgyzstan station depolarization ratio
values of 0.1–0.15 for lofted dust layers and around 0.2
at 532 nm wavelength for near-ground dust layers were reported
. However, it has to be considered that this station is already
located at 1.9 kma.s.l.
In Aksu (northwestern Taklamakan),
measured depolarization ratios at 532 nm wavelength
of 0.09*–0.11* in a lofted dust layer and 0.18*–0.33*
in a near-ground dust layer. measured a depolarization
ratio of 0.27* at 532 nm wavelength in a lofted dust layer in
Dunhuang (northern Taklamakan). Note that the values denoted with *
are published as aerosol depolarization potentials and are converted to
particle linear depolarization ratios see.
The measured depolarization ratios for the presented dust cases (Cases 1, 2, 3)
are mostly higher than these literature values for dust measured in or close
to Central Asia. This suggests that those studies might also have described
observations of polluted or mixed dust. The range of the presented particle
depolarization ratios (0.31–0.35) at 532 nm wavelength is comparable
with the values of fresh Saharan dust (0.27–0.35;
) or Middle Eastern dust (0.25–0.32; ). The spectral
difference between the wavelengths is considerable. The range of the
presented particle depolarization ratios at 355 nm wavelength
(0.18–0.29) is large, with the exceptionally high value during the most
extreme dust event (Case 3). In Saharan dust, particle depolarization ratios
at 355 nm wavelength in the range of 0.22–0.31 were measured at the
source region and 0.21–0.27 after long-range
transport . The
near-ground dust cases (Cases 2, 3) had higher particle depolarization ratios
at both wavelengths than the lofted dust layer of Case 1. Nevertheless, the
lofted dust layer in Case 1 had a depolarization ratio of 0.31 at
532 nm wavelength, which indicates only minor changes in Middle
Eastern dust depolarization characteristics during its long-range transport
towards Central Asia. However, the lofted dust layer in Case 1 had
a lower depolarization ratio at 355 nm wavelength than long-range
transported Saharan dust.
Conclusions
For the first time, multiwavelength polarization Raman lidar observations
have been carried out in Tajikistan in the framework of the CADEX campaign.
The continuous 18-month measurements provide a unique data set of vertically
resolved aerosol optical properties in Central Asia. Additional ground-based
in situ measurements and modeling studies accompanied the CADEX campaign (not
presented here). During the campaign, dust layers were observed frequently
from the surface to tropopause heights, having source regions in Africa,
the Middle East, and Central Asia. Four case studies, which are typical for the
conditions at the location, have been presented including lofted dust layers
and extreme dust events. As a contrast to the dust cases, an example of a
pollution aerosol layer of local origin illustrates the non-negligible
anthropogenic influence on the aerosol in Tajikistan. The observed particle
linear depolarization ratios for the dust cases range from 0.18 to 0.29 at
355 nm and 0.31 to 0.35 at 532 nm wavelength. The presented
examples of near-ground layers of Central Asian dust had high depolarization
ratios at both wavelengths indicating pure dust conditions. The presented
lofted dust layer of long-range transported Middle Eastern dust had slightly
lower depolarization ratios, especially at 355 nm wavelength.
Nevertheless, the values still indicate almost unpolluted dust conditions
after long-range transport to Central Asia. The observed lidar ratios in the
presented dust cases range from 40.3 to 46.9 sr at 355 nm and
from 35.7 to 42.9 sr at 532 nm wavelength. These
lidar ratio values are lower than typical lidar ratios of Saharan dust
(50–60 sr) and comparable to lidar ratios of dust from the Middle
East or west-Asia (35–45 sr). Further analyses of the data set, to be
published in a follow-up publication, include a statistical analysis of the
whole measurement period regarding the optical properties, dust mass
concentrations, dust layer heights, seasons, and source regions.
Nevertheless, more measurements in Central Asia are needed to analyze
long-term trends, especially with respect to climate change. Such
measurements also offer new satellite comparison possibilities. Moreover, the
assimilation of these data into dust models (regional and global transport,
aerosol optical properties, and radiative transfer) will help to reduce
uncertainties. Therefore, a permanent lidar station in Tajikistan will be
established starting most likely from 2019.
HYSPLIT backward trajectories are calculated via the
available online tools (Stein et al., 2015; Rolph et al., 2017; http://ready.arl.noaa.gov/HYSPLIT.php). AERONET
sun photometer data are available from the AERONET web page
(http://aeronet.gsfc.nasa.gov/cgi-bin/type_one_station_opera_v2_new?site=Dushanbe).
GDAS data is available at https://www.ready.noaa.gov/gdas1.php (GDAS, 2017).
Data of the FLEXPART backward trajectory calculations and the CADEX
PollyXT lidar data are available at the Leibniz Institute for
Tropospheric Research, Leipzig.
The authors declare that they have no conflict of interest.
Acknowledgements
The CADEX project was funded by the German Federal Ministry of Education and
Research (BMBF) in the context of “Partnerships for sustainable problem
solving in emerging and developing countries” under the grant number
01DK14014. This project has also received funding from the European Union's
Horizon 2020 research and innovation programme under grant agreement no. 654109.
We would like to thank the Academy of Sciences of Tajikistan for the
helpful support of any kind during all stages of the CADEX campaign. We would like
to thank Lars Klüser from the German Aerospace Center, Emmanouil Proestakis
from the National Observatory of Athens, and Alexandra Chudnovsky from the University of
Tel Aviv for the opportunity to compare the ground-based measurements to
satellite measurements. Konrad Kandler acknowledges support from the Deutsche
Forschungsgemeinschaft (grant KA2280/2). The
publication of this article was funded by the open-access
fund of Leibniz Universität Hannover.
Edited by: Matthias Tesche
Reviewed by: three anonymous referees
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