Introduction
Asian mineral dust (also known as dust aerosol) in the atmosphere is deemed
to exert a profound impact on air quality and climate change. It can perturb
the energy budget of the Earth system directly through scattering and
absorption of solar and terrestrial radiation (Huang et al., 2009, 2014; Ge
et al., 2010; Li et al., 2016) and indirectly by altering cloud
microphysical processes and related hydrological cycle (Rosenfeld et al.,
2001; J. Huang et al., 2005, 2006, 2010; Yin and Chen, 2007; W. Wang et al.,
2010; Creamean et al., 2013; Wu et al., 2016), as well as modifying snow and
ice surface albedo (Aoki et al., 2006; Huang et al., 2011; Wang et al.,
2013; Qian et al., 2014). In addition, alkaline mineral dust carries
abundant organic matters and iron ions deposited on the surface of Earth
and hence affects biomass productivity in the North Pacific Ocean and
relevant atmosphere–ocean carbon exchange, which plays a pivotal role in the
global biogeochemical cycle and carbon cycle (Cao et al., 2005; Jickells et
al., 2005; Maher et al., 2010; Shao et al., 2011).
The Taklamakan Desert in northwestern China and Gobi Desert in southern
Mongolia and northern China are widely regarded as two major active centers
of dust storms in East Asia (Sun et al., 2001; Zhao et al., 2006; Wang et
al., 2008; Ge et al., 2016). These extensive arid and desert zones
frequently generate a great deal of tiny soil particles every spring that
are uplifted and entrained into the free atmosphere layer via cold frontal
cyclones (Zhang et al., 1997; Aoki et al., 2005; Kai et al., 2008; J. Huang
et al., 2009, 2010, 2014). Affected by midlatitude prevailing westerlies,
these dust particles can be transported long distances on a subcontinental scale,
even sweep across the remote Pacific Ocean and occasionally arrive at the
west coast of North America during the peak seasons of strong dust storms
(Zhao et al., 2006; Uno et al., 2009, 2011). They then have a far-reaching
influence on climatic and environmental changes both regionally and
globally. Until now, there have been a large number of intensive field
experiments (e.g., ACE-Asia, ADEC, PACDEX, EAST-AIRC) and ground-based
aerosol monitoring networks (e.g., AERONET, SKYNET, CARSNET) for probing the
Asian mineral dust (Holben et al., 1998; Huebert et al., 2003; Nakajima et
al., 2003; Takamura et al., 2004; Eck et al., 2005; Mikami et al., 2006;
Huang et al., 2008a; Che et al., 2009, 2015; Li et al., 2011), which are
crucial for thoroughly understanding the climatic effects of dust
aerosols over East Asian domain. Nevertheless, due to poor sampling of dust aerosols over
desert source areas of northwest China, the light scattering and absorption
properties of mineral dusts in this region are far inadequate and urgently
need to be further surveyed.
The Intergovernmental Panel on Climate Change (IPCC, 2013) reported that the
symbol and magnitude of the radiative forcing of mineral dust is greatly
reliant on the accurate and reliable knowledge of aerosol total loading,
microphysical and chemical characteristics, as well as its spatiotemporal
distribution. The current consensus is that nearly pure dust aerosol in the
globe has relatively low light absorption, with single-scattering albedo (SSA) of
∼ 0.96–0.99 (Dubovik et al., 2002; Anderson et al., 2003;
Uchiyama et al., 2005; Bi et al., 2014, 2016), which depends principally on
the fraction and mixing ways of ferric iron oxides (i.e., hematite and
goethite) in dust (Sokolik and Toon, 1999; Lafon et al., 2004, 2006).
However, the coexistence of both mineral dust and other types of aerosols
originated from diverse human activities (e.g., coal combustion, mobile
source emissions, and biomass burning) is ubiquitous in the real
atmosphere, which increases the complexity and variability of aerosol key
parameters (Arimoto et al., 2004; Xu et al., 2004; Wang et al., 2015). When
the lofted dust plumes in desert source areas travel eastward across
the polluted regions, they commonly mix with anthropogenic pollutants and
enhance heterogeneous chemical reactions with other reactive gas species
and then may markedly alter their chemical and microphysical properties
(Arimoto et al., 2006; Li and Shao, 2009; Nie et al., 2014). It is
well documented that the mineral dust might already mix with polluted
aerosols near dust source regions of northwest China (i.e., Inner
Mongolian Gobi Desert) before the mixing processes on the transport
pathway (K. Huang et al., 2010). Xu et al. (2004) indicated that both dust
aerosol and local pollution sources coexisted in Yulin near the Mu Us
Desert of northwest China during April 2001, which produced a significant
influence on aerosol properties in the region. Likewise, Li et al. (2010)
analyzed trace gases and aerosols observed at Zhangye (39.082∘ N,
100.276∘ E; 1460 m above m.s.l.), a rural site within the Hexi
Corridor in northwest China during spring 2008, and uncovered that the
mixing between mineral dust and anthropogenic air pollutants is
omnipresent in this area, including at nighttime or during severe dust
events. This implies that prior to moving out from the source region, dust
particles were likely related to pollutants. For the sparsely
populated and less anthropogenically affected desert source regions in
northwest China (e.g., the Taklamakan Desert and its adjacent areas), the
interaction between local pollution and mineral dust should be explored
in depth. This is of prime importance to ascertain the relative
contributions of two different aerosol sources in atmospheric chemistry and
regional climate change.
To advance a better understanding of the drought processes and dust-relevant
climatic impacts in northwest China (Huang et al., 2008b; Bi et al., 2011;
G. Wang et al., 2010), the Semi-Arid Climate and Environment Observatory of
Lanzhou University (henceforth referred to as SACOL, http://climate.lzu.edu.cn/english/) carried out a comprehensive field
campaign in Dunhuang during spring of 2012. Dunhuang is situated at the
westernmost fringe of Hexi Corridor in Gansu province, close to the eastern
edge of Kumtag Desert and about 450 km in the downwind zone of Taklamakan
Desert. Dunhuang was an important town on the ancient Silk Road and the
transportation junction to the ancient western region, central Asia and
Europe; it has become a world-famous tourist city with a residential
population of 200 000. Agriculture and tourism are the dominant economic
industries in Dunhuang. An array of ground-based remote sensing and in situ
instruments were set up during the intensive period, which sought to
investigate the key properties of aerosol and its climatic effect on a regional
scale (Bi et al., 2014). This study specifically aims to explore the light
scattering and absorption characteristics of mineral dust and elucidates a
potential anthropogenic influence. In the following, we introduce the
site information and integrated measurements in Sect. 2. The primary
results and discussion are described in Sect. 3. The concluding remarks are
given in Sect. 4, followed by the data availability in Sect. 5.
Site and instrumentation
Site information
SACOL's Mobile Facility (SMF) was deployed at Dunhuang farmland
(40.492∘ N, 94.955∘ E; 1061 m above m.s.l.) from 1 April to
12 June 2012. The site is a tiny, isolated oasis encompassed by the east–west-oriented Gobi Desert and arid zones in northwest China, with the Mingsha
Shan (Echoing Sand Mountain; elevation of ∼ 1650 m) and Sanwei
Mountain (elevation ∼ 1360 m) to the southwest and the
Beishan Mountain (elevation ∼ 2580 m) to the north (Ma et
al., 2013). The underlying surface is typically covered with Gobi Desert and
saline–alkali land, and the principal vegetation types consist of extremely
sparse Alhagi. Dunhuang farmland is an important agricultural base in Gobi
Desert, mainly growing hami melon and cotton. There are not any significant
manmade pollution sources (e.g., large-scale industries or coal-fired power
plants) around the monitoring station. The southwest–northeast-oriented
National Highway 215 is about 400 m west of the site (Fig. 1a). The nearest Xihu township (with total population of 13 800) is
approximately 7 km to the north of Dunhuang farmland, along with some
scattered villages stretching from west to east. Meanwhile, the station is
located in northeastern Dunhuang (∼ 45 km), to the
west of Guazhou county (∼ 70 km) and to the southwest of
Liuyuan town (∼ 80 km). In general, the major anthropogenic
emission sources at Dunhuang farmland likely include coal combustion from
domestic heating and cooking, mobile sources emissions from vehicle exhaust
gas, and biomass burning from crop residue and traditional ritual
activities, which are ordinarily considered to be a puny contribution to the
mineral dust in present-day climate models. The climate pattern here is
characterized as extreme drought but with a moderate temperature during the
whole sampling period (temperature: 18.3 ± 8.1∘; relative
humidity (RH): 21.9 ± 16.5 %; mean ± standard deviation).
Thereby the dust storms frequently take place in this region from spring to
early summer. Figure 1b shows the overall mean UV aerosol index (AI) from
1 April to 12 June 2012 obtained from the Ozone Monitoring Instrument (OMI)
absorbing aerosol products (Torres et al., 2007). The AI dataset is a very
good indicator for mapping the distribution of absorbing aerosols (mainly
black carbon and dust). The distributions of high AI values (> 0.7) are consistent with the dust-dominated geomorphological features
in arid and semi-arid regions (i.e., Taklamakan Desert and Gobi Desert). It
is very obvious that Dunhuang (marked with a star) is also situated at
the primary dust belt of northwest China, as presented in Fig. 1b.
(a) The Dunhuang farmland site (40.492∘ N,
94.955∘ E; altitude: 1061 m) labeled with a star and its
surrounding region. (b) OMI (Ozone Monitoring Instrument, 2004) mean UV
aerosol index from 1 April to 12 June 2012. The site is located in the
downwind region of the Taklamakan Desert and frequently experiences dust
storms.
Aerosol measurements
An aerosol integrated observing system is installed in the laboratory of SMF
and utilized to continuously measure aerosol optical properties and size
distribution in the field. Prior to the experiment, the in situ aerosol
instruments and broadband radiometers were newly purchased and calibrated by
the manufacturers (Bi et al., 2014). Table 1 summarizes the basic
specification, measured variables, and accuracy of surface-based instruments
deployed at Dunhuang farmland throughout the experiment. These instruments are described below.
The main aerosol observation and other ground-based instruments
deployed at Dunhuang farmland during spring of 2012.
Measured variables
Model, manufacturer
Accuracy
PM10 concentration
Ambient particulate monitor (RP1400a), R&P Corp.
0.1 µg m-3
Aerosol scattering coefficient
Integrating nephelometer (TSI 3563), TSI Inc. 450, 550, and 700 nm
0.44, 0.17, and 0.26 Mm-1
Aerosol absorption coefficient
Multi-angle absorption photometer (MAAP 5012), Thermo Electron Corp.
0.66 Mm-1
Aerosol size distribution
Aerodynamic particle sizer (APS 3321), TSI Inc., 0.5–20 µm
0.001 cm-3
Aerosol-attenuated backscatter profile
Micro-pulse lidar (MPL-4), Sigma Space Corp.
Spatial resolution: ∼ 30 m
Meteorological elements
Weather transmitter (WXT520), Vaisala, Ta, RH, P, u, WD
Ta: ±0.3∘; RH: 0.1 %; P: 0.1 hPa; u: 0.1 ms-1; WD:1∘
Global and diffuse radiation
Pyranometer (PSPa,b), Eppley Lab., 0.285–2.8 µm
Global: 8.46; diffuse: 8.48 µVW-1 m-2
Direct radiation
Pyrheliometer (NIPb), Eppley Lab.,0.285–2.8 µm
8.38 µVW-1 m-2
Downward longwave radiation
Pyrgeometer (PIRa,b), Eppley Lab., 3.5–50 µm
2.98 µVW-1 m-2
24-bit color JPEG image
Total Sky Imager (TSI880), YES Inc., 352 × 288 pixel
Sampling rate: 1 min
a The instrument is equipped with the Eppley ventilation system (VEN).b The instrument is mounted on a two-axis automatic sun tracker (model
2AP, Kipp & Zonen).
An ambient particulate monitor (model RP1400a, Rupprecht and Patashnick
Corp.) can collect the in situ mass concentration of the particulate matter
with an aerodynamic diameter less than 10 µm (PM10) based on
the
tapered element oscillating microbalance (TEOM) technique. The measurement
range and accuracy of PM10 concentration levels are normally 0–5 and 0.1 µg m-3, respectively. The heating temperature
(∼ 50∘) of the sampling tube may cause a partial loss
of volatile and semivolatile aerosol compounds and hence bring about a
negative signal. In this study, we eliminate all the negative values of
PM10 concentrations, which account for less than 1 % of total data
points.
An integrating nephelometer (model 3563, TSI Inc.) is designed to
simultaneously measure the total scattering coefficients (σsp)
and hemispheric backscattering coefficients (σbsp) of aerosol
particles at three wavelengths of 450, 550, and 700 nm, with the σsp detection limits of 0.44, 0.17, and 0.26 Mm-1 (1 Mm-1=10-6 m-1), respectively (signal-to-noise ratio of 2)
(Anderson et al., 1996). To quantify the instrument drift and improve
accuracy, we periodically perform the routine calibration using air and
high-purity CO2 gases. Furthermore, the truncation errors of
near-forward scattering (i.e., non-ideal angular effects) are corrected
according to the method of Anderson and Ogren (1998). The observed ambient
RH values are mostly smaller than 40 % throughout the entire period. It is
well documented that RH induced the variations in aerosol light scattering
coefficients are minimized under a low sampling stream RH of 10–40 %
(Covert et al., 1972). In this paper, we computed the scattering
Ångström exponent at 450–700 nm (SAE 450/700 nm) from σsp at 450 nm and σsp at 700 nm by utilizing a log-linear
fitting algorithm. Thus σsp at 670 nm (σsp,670) was logarithmic interpolated between σsp,450
and σsp,700.
A multi-angle absorption photometer (MAAP model 5012, Thermo Electron Corp.)
is capable of observing the aerosol light absorption coefficient at 670 nm
(σap,670) by filter-based methods without requiring of
post-measurement data correction or parallel-measured aerosol
light scattering coefficients (Petzold et al., 2002). The instrument detects
an emitted light at 670 nm in the forward and back hemisphere of airborne
aerosols deposited on a fiber filter, which is used to improve multiple
scattering effects in the aerosol optical properties via a radiative
transfer scheme (Petzold et al., 2002, 2005). The sample flow rate is 1000 L h-1, with flow error of < 1 %. We made use of a specific
absorption efficiency at 670 nm of 6.5 ± 0.5 m2 g-1 to
estimate black carbon concentration from σap,670 as
recommended by Petzold et al. (2002).
An Aerodynamic Particle Sizer (APS) spectrometer (model 3321, TSI Inc.) can
continuously provide the real-time, high-resolution aerosol size
distribution with aerodynamic diameters from 0.5 to 20 µm range (52
channels). When extreme dust episodes outbreak, an aerosol diluter (model
3302A, TSI Inc.) is operated in series with APS to reduce particle
concentrations in high-concentration aerosols, which offers a representative
sampling that meets the input requirements of the APS spectrometer. All the
abovementioned aerosol datasets were acquired at 5 min and hourly
averages and reported for sampling volumes under standard air conditions
(i.e., 1013.25 hPa and 20∘).
Other ground-based measurements
A micro-pulse lidar (model MPL-4, Sigma Space Corp., USA) is a compact
and unattended apparatus for providing continuous data information of
extinction coefficient and depolarization ratio profiles of aerosols and
clouds (Welton et al., 2000). The MPL-4 emits a laser beam at a wavelength of 527 nm
from a Nd:YLF pulsed laser diode and receives the attenuated
backscattering intensity and depolarized signals from aerosol particles or
cloud droplets with a 30 m vertical resolution and a 1 min average
interval. We can acquire the accurate backscattering profile by means of
a series of corrections (e.g., dead time, background signal, afterpulse,
overlap, and range-corrected) according to the standard methods (Campbell et
al., 2002). The detailed data acquisition and retrieval algorithms of the
lidar system can be found in the publications of Campbell et al. (2002)
and Z. Huang et al. (2010).
A weather transmitter (model WXT520, Vaisala, Finland) was set up on
top of the SMF trailer to record the air temperature (T in ∘), RH, ambient pressure (P, unit: hPa), wind speed, and wind
direction at 10 s intervals. In this article, we calculated the
5 min and hourly averages from the raw data.
A dozen state-of-the-art broadband radiometers were installed in a row on
a standard horizontal platform (∼ 4 m above the surface) where
the field of view was unobstructed in all directions (Bi et al., 2014). The
direct normal irradiance and diffuse irradiance were independently measured
by an incident pyrheliometer (model NIP, Eppley Lab.) and by a ventilated
and shaded pyranometer (model PSP, Eppley Lab.), which were mounted on a
two-axis automatic sun tracker (model 2AP, Kipp&Zonen). The global
irradiance (0.285–2.8 µm) and downward longwave irradiance
(3.5–50 µm) were, respectively, gathered from a ventilated PSP pyranometer and a
ventilated and shaded pyrgeometer (model PIR, Eppley Lab.). All irradiance
quantities were stored in a Campbell data logger with 1 min resolution.
Additionally, a Total Sky Imager (model TSI–880, YES Inc.) provides
high-resolution sky pictures every minute during the daytime, which can
detect and identify important weather conditions, such as dust storms,
smoky pollution, and rainy, cloudy, or cloudless days.
MERRA reanalysis products
The MERRA (Modern-Era Retrospective Analysis for Research and Applications)
reanalysis assimilates a variety of conventional observations (i.e.,
temperature, pressure, height, wind components) from surface weather
stations, balloons, aircraft, ships, buoys, and satellites from 1980 to the
present and is primarily committed to improving the hydrologic cycle
and energy budget for the science community (Rienecker et al., 2011). In
this paper, we utilized the 6-hourly average wind fields at 500
and 850 hPa levels from the MERRA reanalysis products.
Results and discussion
Aerosol optical properties
The aerosol SSA at 670 nm is defined as the ratio
of the light scattering coefficient (σsp,670) to the total
extinction coefficient (the sum of σsp,670 and σap,670). The SSA reflects the absorptive ability of aerosol particle
and is a key quantity in determining the sign (warming or cooling) of
aerosol radiative forcing for a certain underlying surface (Hansen et al.,
1997; Ramanathan et al., 2001).
Statistical summary of hourly-averaged aerosol optical properties
measured during an intensive observation perioda.
Variable
Mean
SDb
Median
10th percentile
25th percentile
75th percentile
90th percentile
PM10 (µg m-3)
113
169
54
17
29
111
300
σsp (Mm-1)
53.3
74.8
28.3
11.2
16.0
55.8
123.5
σap (Mm-1)
3.20
2.40
2.50
1.27
1.69
3.90
5.94
SSA (670 nm)
0.913
0.055
0.923
0.850
0.892
0.949
0.967
SAE (450/700 nm)
0.45
0.45
0.42
-0.1
0.1
0.73
0.99
a All aerosol data reported for volumes under 1013.25 hPa and 20∘.b SD denotes the standard deviation.
Figure 2 delineates the time series of hourly-averaged PM10 mass
concentration, aerosol optical properties, and size distribution at Dunhuang
farmland during the whole period. The overall mean, standard deviation,
median, and different percentiles of aerosol optical properties are also
tabulated in Table 2. Aerosol optical features exhibit dramatic day-to-day
variations at Dunhuang. It is apparent that aerosol loadings in April and
early May are systematically higher than those in late May and June, which
agrees well with the results of columnar aerosol optical depths (AODs) derived from
sky radiometer (Bi et al., 2014). This is chiefly attributed to the invading
mineral particulates from the frequent occurrences of intense dust storms in
spring. The highly unstable synoptic cyclones (i.e., Mongolian
cyclone) are regularly hovering about northern China and Mongolia in
springtime, which trigger high-frequency strong surface winds (Sun et al.,
2001; Shao et al., 2011). The rising temperature in this season leads to the
melting of frozen soil and snow cover, leaving behind a loose land surface
and abundant bare soil sources, therefore creating favorable conditions for
dust storms. In addition, the contributions of local dust emissions cannot
be ignored. We have clearly recorded that there were numerous agricultural
cultivation operations (e.g., land planning, plowing, and disking) throughout
the Dunhuang farmland district from 1 April to 10 May, which produced a
large amount of agricultural soil particles under strong winds and thus had
a significant superimposed effect on elevated dust loading in the source and
downwind regions prior to the growing season. Figure S1 in the Supplement also presents
photographs of a variety of agricultural cultivations in Dunhuang farmland before
the planting period, which supplies direct and powerful evidence to
support our results. Those dust aerosols originated from disturbed soils
induced by human activities and are interpreted as anthropogenic dust (Tegen and
Fung, 1995). Recently, some investigators estimated that anthropogenic dust
could account for approximately 25 % of the global dust load (Ginoux et
al., 2012; Huang et al., 2015), and more than 53 % of the anthropogenic
sources came from semi-arid and semi-wet zones (Huang et al., 2015;
Guan et al., 2016). Nonetheless, it still remains a challenging task to
distinguish between the natural and anthropogenic fractions of mineral dust
by employing a single technology such as laboratory analysis, in
situ measurements, model simulations, active, and passive remote sensing
methods (e.g., multichannel lidar, sun–sky radiometer), which should be
combined together (Bi et al., 2016). The overall mean PM10
concentration was 113 ± 169 µg m-3 (mean ± standard
deviation), which is ∼ 39 % lower than the 184.1 ± 212 µg m-3 average level in Dunhuang (40.1∘ N,
94.6∘ E, 1139 m) during the spring of 2004 (Yan, 2007) and
∼ 26 % smaller than the value of 153 ± 230 µg m-3 measured at Zhangye (39.082∘ N, 100.276∘ E,
1460 m) during spring of 2008 (Li et al., 2010). Wang et al. (2015) obtained
a total average PM10 concentration of 172 ± 180 µg m-3 at
SACOL during late spring of 2007 (from 25 April to 25 June). The mean
PM10 levels at Hunshan Dake sandland in northern China during spring of
2001 varied between 226 and 522 µg m-3 (Cheng et al., 2005).
Time series of hourly-averaged (a) PM10 mass concentration in
µg m-3, (b) aerosol absorption coefficient at 670 nm, (c) aerosol
scattering coefficient at 670 nm, and (d) aerosol size distribution in
cm-3 at the Dunhuang farmland during the whole sampling period.
Time evolutions of the MPL normalized relative backscatter
intensity (top panel) and depolarization ratio (bottom panel) at Dunhuang
farmland from 1 April to 12 June 2012.
The hourly-averaged aerosol σsp,670 was 53.3 ± 74.8 Mm-1. The large standard deviations of
PM10 and σsp are possibly associated with the injection of
dust particles during the intense dust storms. Our result was about a factor
of 3 lower than the σsp at 500 nm in the other
sites over northern China (i.e., 126 ± 90 Mm-1 for Dunhuang,
159 ± 191 Mm-1 for Zhangye, 164 ± 89 Mm-1 for SACOL).
Despite a relatively small magnitude, σap,670 also presented pronounced variations, with
an average value and a maximum of 3.2 ± 2.4 and 25.0 Mm-1, respectively. This result was a factor of 2 smaller than Yulin
(6 ± 11 Mm-1; Xu et al.,
2004) and a factor of 5–7 smaller than that at the Shangdianzi
rural site (17.5 ± 13.4 Mm-1) in northern China (Yan et al., 2008)
and Lin'an site (∼ 23 Mm-1) in southern China (Xu et al.,
2002). The mean light scattering and absorption coefficients in this study
are comparable to the background levels (∼ 46.9 ± 16.9
and 2.5 ± 1.1 Mm-1) in the US Southern Great Plains (Delene and
Ogren, 2002). This suggests that extremely low levels of light absorption
and scattering substances are widely distributed throughout the Dunhuang
region during the spring of 2012. Therefore, a little perturbation stemmed
from human activities (e.g., agricultural cultivation, coal combustion from
domestic heating and cooking, and biomass burning) would undoubtedly exert a
considerable impact on the light absorption property.
A few strong dust episodes (4, 21–22, and 30 April, 1–3, 8–11, and 20
May, 4 and 10 June, corresponding to DOY 95, 112–113, 121, 122–125,
129–132, 141, 156, and 162) could remarkably elevate the hourly-averaged
values of PM10, σsp, σap, and aerosol size
distribution (see Fig. 2). During these dust events, the hourly PM10
concentrations generally exceeded 1000 µg m-3 and even approached
2000 µg m-3, which were 10-fold greater than the overall mean
level. The hourly σsp was more than 400 Mm-1 or even
close to 800 Mm-1, and the corresponding σap varied
between 10 and 25 Mm-1. Moreover, the peak values of aerosol number size distribution appear at the particle diameters of 1–3 µm, which was consistent with the result from remote sensing (Bi et al.,
2014, 2016).
Figure 3 depicts the time evolutions of MPL normalized relative backscatter
and depolarization ratio at Dunhuang farmland from 1 April to 12 June 2012.
The depolarization ratio (δ) is a useful indication to discriminate
between spherical particles (δ of ∼ 0–0.1) and
nonspherical particles (mainly dust aerosol, δ > 0.2),
since it is very sensitive to the nonsphericity of scattering particles
(Kobayashi et al., 1985; Murayama et al., 1999; Shimizu et al., 2004; Huang
et al., 2015). From Fig. 3 we can distinctly see that there was a
dense dust layer below 4 km during the entire duration of the
experiment, with the peak value centered on 1.0–1.5 km, which was above the
planetary boundary layer (PBL). The δ values commonly reached
above 0.3 (> ∼ 0.3–0.5) during the heavy dust
events and varied between 0 and 0.1 under clear-sky conditions (e.g., 6–7
April, 14–15 and 29 May, 9 June).
The diurnal variations of (first row, left to right) wind vector
(ms-1), air temperature (T in ∘), relative humidity (RH in
%), (second row, left to right) PM10 concentration (µg m-3), aerosol scattering coefficient at 670 nm (σsp,670
in Mm-1), aerosol absorption coefficient at 670 nm (σap,670 in Mm-1), (third row, left to right) scattering
Ångström exponent at 450–700 nm (SAE 450/700 nm), aerosol
single-scattering albedo at 670 nm (SSA670), and aerosol size
distribution (dN / dlogD in cm-3) at the Dunhuang site from 1 April to 12
June 2012 (30 May to 12 June for aerosol size distribution). Median values
(red square) are shown to give a more apparent diurnal feature than mean
values, which could be affected by several strong dust episodes. The
25th (blue diamond) and 75th (green triangle) percentiles for each
hour of the day are also displayed.
Diurnal variations
Figure 4 illustrates the diurnal variations of wind vector (ms-1), air
temperature (T in ∘), RH (%), PM10 (µg m-3),
σsp,670 (Mm-1), σap,670 (Mm-1), aerosol
number size distribution (dN / dlogD in cm-3), SAE at 450–700 nm, and
SSA at 670 nm in Dunhuang farmland from 1 April to 12 June 2012. Note that
the APS spectrometer was operated from 30 May to 12 June. A discernible wind
vector was shown in the diurnal variation; in other words, strong southwest
and south winds dominated in the daytime, from 11:00 to 24:00 LT (local
time), and transformed into the weak northeast wind prevailed from the
midnight to the following morning of 10:00 LT. The prominent phenomenon can
be roughly interpreted by classical mountain-valley wind circulation, which
was primarily generated by the diurnal differences of temperature between
the mountain slope and the valley floor. During the daytime, the huge
Beishan Mountain slope heats up by solar radiation more rapidly than the
valley floor, which causes convection above the mountain slope. The
compensating airflow is consequently directed toward the mountain slope,
inducing upslope southerly wind or the valley wind, which usually peaks
near midday and gradually disappears after sunset. Conversely, at night,
radiative cooling of the mountain slope occurs more quickly than on the valley
floor, inducing the mountain wind, which generally reaches maximum strength
just before sunrise (Arya, 1999). Throughout the experiment, air temperature
displayed a large diurnal variation (with the diurnal difference of δT∼26∘) and RH always kept below 40 % for the
whole day. It is very clear that the minimal T and maximal RH arose at
around 06:00–07:00 LT, and the maximal T and minimal RH occurred at about
16:00 LT, which represented an energetic vertical turbulent motion in
daytime and a stable radiative temperature inversion during nighttime.
The aerosol optical parameters also exhibited striking diurnal variations,
which were closely related to the local meteorological elements. During the
daytime (10:00–18:00 LT), the PM10 concentration remained high level
(∼ 57–65 µg m-3) and increased sharply from 19:00 LT
and reached a maximum of 84.2 µg m-3 at 20:00 LT. The PM10
began to decrease from 21:00 LT until the next morning. A low level
(∼ 40–46 µg m-3) was maintained at midnight (00:00–05:00 LT) and rose gradually from 06:00 LT and attained a secondary peak value of
55.7 µg m-3 at 07:00 LT. The aerosol light scattering (σsp,670) presented a similar pattern with PM10, but the maximal
value (∼ 42 Mm-1) appeared at 13:00 LT, with the other
two secondary peak values occurring at 20:00 (∼ 34.1 Mm-1)
and 07:00 LT (∼ 27.3 Mm-1). The high levels of PM10
and σsp during the daytime were primarily attributable to
a strong south wind from Gobi and local dust emissions. By contrast,
σap,670 showed a more
pronounced diurnal feature, which proved to be mostly controlled
by anthropogenic emissions (Li et al., 2010). The diurnal σap
always stayed at a low level (∼ 2.0 Mm-1) from
13:00 to 18:00 LT and reached a maximum of 3.3 Mm-1 at 20:00 LT.
Subsequently, σap dramatically reduced from midnight and
preserved at a low value of about 2.2 Mm-1 from 02:00 to 04:00 LT and
remained a steadily high level of ∼ 2.7–2.9 Mm-1 from
05:00 to 10:00 LT. It can probably be explained as follows. The influences of
local anthropogenic pollutants were commonly small in the afternoon, because
the strong southerly wind from Gobi Desert and powerful daytime vertical
convection mixing efficiently dilute local air pollutants. Because weak
northeast wind and stable temperature inversion at night facilitate the
accumulation of pollutants within the PBL, nighttime levels were
normally larger. Increasing human activities (e.g., domestic cooking,
traffic emissions for transportation and agriculture) in the early morning
might also be responsible for the morning peaks in the aerosol absorption
coefficient. The σap maximum at 20:00 LT was presumably
influenced by the mixture of mineral dust and anthropogenic pollutants. This
conclusion could be partly supported by the diurnal variation of SAE at
450–700 nm (Fig. 4), which showed high SAE values (∼ 0.5–0.6) at 02:00–10:00 LT and low SAE values (∼ 0.2–0.3)
at 13:00–22:00 LT. Generally, a large SAE around 0.6 represents
small particles (e.g., urban-polluted aerosol or soot) and a small SAE less than
0.3 or a negative value corresponds to coarse-dominated large particles
(e.g., dust or sea salt) (Anderson et al., 2003).
Wind rose plots for (a) morning (06:00–09:00 LT), (b)
evening (19:00–22:00 LT), and (c) all hours; shade represents wind
speed (ms-1). Wind roses for all hours, with shade representing levels
of (d) PM10 concentration (µg m-3), (e) aerosol scattering
coefficient at 670 nm (σsp in Mm-1), and (f) aerosol
absorption coefficient at 670 nm (σap in Mm-1).
Time series of (a) wind vector (ms-1); (b) PM10
concentration (µg m-3); (c) aerosol scattering coefficient (σsp in Mm-1) at 450 nm (red), 550 nm (green), and 700 nm
(blue);
(d) scattering Ångström exponent (SAE) at 450–550 nm (red),
550–700 nm (green), and 450–700 nm (blue); (e) aerosol absorption
coefficient at 670 nm (σap in Mm-1); and (f)
single-scattering
albedo at 670 nm (SSA670) during a typical Tomb-Sweeping Day on 4 April
2012, which implies a potential anthropogenic influence on aerosol optical
properties. All data points are obtained from 5 min average values.
Furthermore, aerosol number size distribution exhibited a noticeably
dominant
supermicron particles the entire day, probably linked to the
predominant dust aerosol in daytime and local anthropogenic emissions at
nighttime. In this study, we postulated that the aerosol light extinction at
the
shortwave waveband is completely caused by those particles with aerodynamic
diameters of 10 µm or less. The mass scattering efficiency is
designated as the ratio of σsp to PM10 concentration.
Therefore, the mass scattering efficiency for PM10 aerosols was about
0.67 m2 g-1 in the afternoon and ∼ 0.77 m2 g-1 in the morning (∼ 0.25 for heavy dust events
and ∼ 0.70 for the whole period). Our results were slightly
less than ∼ 1.05 m2 g-1 in Dunhuang during spring of
2004 (Yan, 2007). Similarly, the mass absorption efficiency was
∼ 0.017 m2 g-1 under heavy dust episodes and
∼ 0.08 m2 g-1 in the morning, which was coincident
with the laboratory analytical result of natural desert aerosol at 660 nm
(∼ 0.01–0.02 m2 g-1) in the Ulan Buh Desert
(39∘26′ N, 105∘40′ E) of northern China
(Alfaro et al., 2004). These diurnal variations of the mass scattering and
absorption efficiencies likely reflect the changes in aerosol chemical
composition. The SSA at 670 nm displayed distinct differences between
daytime and nighttime (Fig. 4), and the two minimal values at 07:00 LT
(∼ 0.90) and 20:00 LT (∼ 0.921) were consistent
with the aforementioned σap,670 diurnal feature. The peak
values of SSA (0.945 ± 0.04) for dominant dust particles in the
afternoon agreed well with other field campaigns in Zhangye (0.95 ± 0.02; Li et al., 2010) and Yulin (0.95 ± 0.04; Xu et al., 2004). The
daily low SSA (0.90–0.92) or overall mean of 0.913 ± 0.055 at Dunhuang
was still bigger than that in both urban (0.81; Bergin et al., 2001) and
rural (0.81–0.85; Li et al., 2007) regions adjacent to Beijing, presumably
ascribed to dust particles at night. Yan et al. (2008) conducted 2-year
long field measurements at Shangdianzi Global Atmosphere Watch (GAW) rural
site in northern China (∼ 150 km from Beijing) and estimated a
mean SSA of 0.88 ± 0.05, but their data contained summer when aerosol
scattering coefficients may be strengthened by hygroscopic growth and
secondary chemical process.
The wind fields (black arrows) at 500 hPa (left panel) and 850 hPa
(right panel) levels during three heavy dust events on 30 April (top), 1 May
(middle), and 10 June (bottom) 2012, based on MERRA reanalysis data. Note
that the Dunhuang farmland is marked with a red star and the white
regions at 850 hPa are on behalf of the missing values.
The same as Fig. 6, except for (a) wind speed (ms-1) and
wind direction (∘) during three heavy dust events on 30 April, 1 May,
and 10 June 2012. There were no measurements of aerosol scattering
coefficient (σsp in Mm-1) on 10 June due to equipment
failure.
The wind rose plots give a further insight into the linkages between the
meteorological factors and pollutants, as described in Fig. 5. In the
morning (06:00–09:00 LT), a marked northeast wind was prevalent and wind
speed was mostly less than 4 ms-1, which revealed that emissions were
primarily descended from nearby farmlands and rural residences (Fig. 5a).
Although a prominent northwest wind mainly occurred in the evening hours
(19:00–22:00 LT), the east wind and southwest wind also appeared, which
indicated that anthropogenic pollution came from both local sources and a
relatively large region along the valley (Fig. 5b). Additionally, Fig. 5c showed
the predominant winds were northeast and southwest winds in Dunhuang area,
with the maximal hourly-averaged wind speed exceeding 10 ms-1. It was
very clear that the southwest and northwest winds created higher levels
of PM10 mass concentration (> 250 µg m-3), aerosol
light scattering coefficient
(σsp > 150 Mm-1),
and absorption coefficient (σap > 8 Mm-1),
whereas northeast wind generated slightly smaller concentrations of
PM10 (∼ 50–100 µg m-3), σsp
(∼ 30–60 Mm-1), and σap (∼ 2–4 Mm-1). This possibly implies that southwest and northwest
winds may bring about dust particles and northeast wind may transport the
air pollutants.
Time evolutions of aerosol optical depth (AOD) at five wavelengths
(400, 500, 675, 870, and 1018 nm) versus Ångström exponent (α) at 400–870 nm on (a) 14 May, (b) 9 June, (c) 30 April, and (d) 10 June
2012. Note that (a)–(b) are adopted from Bi et al. (2014) with an addition
of the Ångström exponent plot in the original publication.
Diurnal variations of ground-based measurements of 1 min
average (a) direct, (b) diffuse, and (c) global irradiances, and (d)
downward longwave irradiance under completely clear-sky conditions (14
May, 29 May, and 9 June) and dust events (30 April and 10 June).
Local anthropogenic emission sources
As mentioned previously, crop residue burning and agricultural cultivated
operations before the growing season could produce local emission source
proximity to the study area. Also, sporadic straw burning occurred
throughout the Dunhuang farmland from 1 April to 10 May 2012, which was the
major source of black carbon surrounding the site. To clarify the potential
anthropogenic influence on aerosol optical properties in desert region, we
investigated a typical biomass burning event.
Figure 6 outlines the time series of 5 min average wind vector
(ms-1), PM10 (µg m-3), σsp at 450, 550, and
700 nm (Mm-1), SAE (450–550, 550–700, and 450–700 nm), σap,670 (Mm-1), and SSA at 670 nm during a typical Tomb-Sweeping
Day on 4 April 2012. Tomb-Sweeping Day is a traditional Chinese festival for
sacrificial rites, in commemoration of dead ancestors. To pay homage to
loved ones, people burn joss sticks, candles, and paper
offerings and set off firecrackers on that day throughout China, which
emits a large amount of air pollutants, such as biomass burning
aerosol, sulfur dioxide, organic matter, and fugitive dust. From Fig. 6a we
see that
slight and variable winds (with wind speed < 4 ms-1) mainly
came from the northeast from 00:00 to 12:00 LT and abruptly changed into
weak southeast wind and south wind; finally, gradually intensified southwest
winds (> 10 ms-1) became dominant and triggered a severe
dust storm from 15:00 LT to the midnight. Prior to the occurrence of dust
episode, the aerosol optical characteristics stayed stable, but a moderate
increase was evident from 08:00 to 10:00 LT. For instance, PM10
concentration gradually increased from background level ∼ 30 µg m-3 to a maximum of 62.5 µg m-3 at about 09:00 LT,
σsp,550 from ∼ 15 to 49.6 Mm-1,
and σap,670 from ∼ 2.0 to 4.75 Mm-1. This is ascribed to the contribution of biomass burning from
ritual activities during Tomb-Sweeping Day. The SAE value at
450–700 nm remained invariant (∼ 0.50) before 08:00 LT and
sharply rose to a maximal value of 0.87 at 09:00 LT, afterwards gently
reducing to around 0.4, which indicated that the fine-mode particles (i.e.,
black carbon or soot) were dominated from 08:00 to 10:00 LT. The SAEs at
various wavelengths systematically decreased from 0.4 at 15:00 LT to -0.25
at midnight, suggesting the dust-dominant coarse-mode particles were
prevailed. Meanwhile, the lidar depolarization ratio (δ) also
further verified the existence of small size soot particle. The δ
value remained steady at 0.15–0.20 during 08:00 to 10:00 LT, rapidly
increased to above 0.3 from 15:00 LT onwards, and even approached 0.50 during intense dust
storm (see Fig. 3). The diurnal variation of SSA670 showed a more
prominent feature, as illustrated in Fig. 6f. The SSA670 values remained
between 0.88 and 0.92 during 00:00 to 07:00 LT and dramatically reduced to
a minimum of ∼ 0.83 at 08:30–09:00 LT, then rose to 0.925,
confirming the very striking impacts by light absorbing substances. After
15:00 LT, the SSA670 gradually increased and reached up to about 0.96
during dust storms occurred. Bi et al. (2014) demonstrated that dust
aerosols shortwave radiative forcing at the top of the atmosphere
was a warming effect when SSA500 was less than 0.85 but was a
cooling effect when SSA500 was greater than 0.85 for the Dunhuang/Gobi
Desert area with high surface albedo. Therefore such significant anthropogenic
influence would clearly modify the microphysical and chemical properties of
dust aerosols and eventually exert remarkable impacts on environmental
quality and climatic forcing of dust particle on both local and regional
scales.
Dust case study
In this section, we explored the absorptive and optical
characteristics of mineral dust during several typical dust cases and
discussed its influence on Earth's radiation balance. Figure 7 provides the
wind fields at 500 and 850 hPa levels during three heavy dust events,
based on MERRA reanalysis products. Note that Dunhuang farmland is marked
with a red star and the white areas at 850 hPa represent the missing
values. It is evident that the East Asian region was governed by the powerful
and stable westerlies at a height of 500 hPa on 30 April and 1 May 2012, whereas
two very strong synoptic cyclones at a height of 500 hPa hovered about
the Mongolia and Kazakhstan on 10 June 2012, matching up with
corresponding cyclone systems appearing at the 850 hPa level. Although there
were missing data in most of northwest China, extremely intense northeast wind
and east wind (> 10 ms-1) at 850 hPa level prevailed
over the northern territory of Xinjiang Uyghur Autonomous Region during the
selected dust storms, which was close to the Dunhuang site. This could be
confirmed by the simultaneous observations of wind speed and wind
direction near the surface at Dunhuang farmland, as delineated in Fig. 8a.
The measured strong northeast and east winds always dominated in
Dunhuang and 5 min average wind speed were attained above 10 ms-1 during
intense dust episodes. The selected three dust processes regularly lasted
for several hours during daytime (e.g., from 10:00 to 18:00 LT) and the dust
event on 1 April persisted until midnight, which contributed
massive dust particles into the atmosphere.
There were no measurements of aerosol scattering coefficient (σsp) on 10 June due to equipment failure. From Fig. 8, we see
that PM10 concentrations usually exceeded 400 µg m-3 and even
reached up to 1000 µg m-3 during the heavy dust storms, and
corresponding σsp,550 and σap,670 values were
generally more than 100 and 5 Mm-1, respectively, or
approached 350 and 15 Mm-1 in our case. It is worth noting
that even though pure dust aerosol possesses relatively low light-absorption
ability (with mass absorption efficiency at 660 nm of ∼ 0.01–0.02 m2 g-1), the injection of plentiful mineral particles
from dust episodes led to considerably high values of σap,670.
The SAEs at diverse wavelengths commonly kept at 0.50 or more during
non-dust conditions, while corresponding values dramatically reduced to
-0.25–0 under heavy dust cases, which is taken for granted.
The SSA670 also exhibited apparent diurnal variations in Fig. 8f.
The SSA670 values regularly preserved between 0.88 and 0.92 at
nighttime or non-dust weather and gradually increased to a maximum of
∼ 0.96–0.98 during strong dust processes, which were close to
the measured value of ∼ 0.97–0.99 for nearly pure Asian dust
particles (Anderson et al., 2003; Bi et al., 2016). These abundant mineral
particles in desert source regions were very likely mixed with local air
pollutants, especially at night, when the anthropogenic pollution favorably
built up within the PBL. Moreover, airborne dust particles ordinarily
traveled long distances to downstream areas via synoptic cyclones, which
would deteriorate the ambient air quality and affected atmospheric chemistry
and climate change on a regional scale.
Figure 9 describes the column-integrated AOD at five
wavelengths (400, 500, 675, 870, and 1018 nm) versus Ångström
exponent (α) at 400–870 nm on two completely clear-sky days (14 May
and 9 June) and two typical dusty days (30 April and 10 June), which were
acquired from a sky radiometer (model POM-01, PREDE Co. Ltd.). The sky
radiometer can measure the direct solar irradiances and sky diffuse
radiances at narrow spectral wavebands during daytime with 10 min
interval. The columnar aerosol optical properties under cloudless
conditions were retrieved from sophisticated inversion algorithms (Nakajima
et al., 1996). Note that the cloud-contaminated datasets have been
eliminated by means of a series of cloud-screening procedures developed by
Khatri and Takamura (2009). From Fig. 9, all AOD values under clear-sky
days kept very stable variations throughout the day and ranged from 0.02 to
0.12, which was comparable to the clean background levels in the central
Tibetan Plateau (Xia et al., 2011) and Badain Jaran Desert (Bi et al.,
2013). The corresponding Ångström exponent α on 14 May
and 9 June were greater than 0.6, indicating extremely low aerosol loading.
In contrast, the AODs under dust events (30 April and 10 June) displayed
pronounced diurnal variations and all AOD values were larger than 0.30 (with
maximum of 0.60), and α varied between 0.10 and 0.25, representing
high dust concentration levels. These elevated dust particles in the
atmosphere would readjust the energy distributions of solar radiative fluxes
at the surface.
Based on aforementioned measurements of the Total Sky Imager, micro-pulse
lidar,
and sky radiometer, we identified three completely clear-sky days (14 May,
29 May, and 9 June) and two “clean” dusty days (30 April and 10 June). The
“clean” dusty days in this study were denoted as the dust storms weather
without the influence of clouds. This afforded us a good opportunity to
elucidate the potential impacts of dust events on radiation balance at the
ground. Figure 10 draws the 1 min average solar direct normal radiation,
sky diffuse radiation, total shortwave radiation, and downward longwave
radiation fluxes under the selected 5 days, which were derived from the
high-precision broadband radiometers as described in Sect. 2.3. All
radiative quantities presented smooth diurnal variations under clear-sky
cases (14 May, 29 May, and 9 June). The airborne dust particles impeded the
sunlight to the ground through scattering and absorbing solar radiation; for
instance, they significantly reduced the surface direct radiative
fluxes in daytime to about 200–350 Wm-2 (Fig. 10a) but
considerably increased the surface diffuse radiative fluxes up to
∼ 150–300 Wm-2 (Fig. 10b). As a result, the overall
attenuation effect on total shortwave radiative fluxes varied between -150
and -50 Wm-2. The incoming solar energy absorbed by dust particles
would heat the atmospheric dust layer (Bi et al., 2014) that likely played a
profound role in the structure of atmospheric boundary layer and cloud
microphysical process (J. Huang et al., 2006, 2010; Li et al., 2016). The
downward longwave radiation (DLW) at the surface was majorly reliant on the
clouds, water vapor, CO2, and other greenhouse gases (Wang and
Dickinson, 2013). In general, the presence of clouds in the atmosphere would
drastically affect the diurnal variation of DLW. The smooth changes
of DLW under both clear-sky and dusty days in Fig. 10d revealed the
robustness of the cloud screening method used in this paper. Figure 10d
shows that the DLW values in dusty cases were always greater than
in clear-sky cases, with the total average differences of
+40–+60 Wm-2. The warming dust layer could enhance
the surface DLW; hence the dust particles should contribute a large
percentage to the increased DLW, but not all. This is because the potential
water vapor in the atmosphere could substantially affect the DLW variations.
For instance, the DLW on 9 June was distinctly greater than in other
cloudless cases (i.e., 14 and 29 May) and the dusty case of 30 April. It is
partly attributable to the higher RH values on 9 June than on other
days, as shown in Fig. S2.
Concluding remarks
In this article, we surveyed the optical features and size distribution of
dust aerosol in a Gobi farmland region of northwest China from 1 April to 12
June 2012 and uncovered a potential anthropogenic influence. The overall
average PM10 mass concentration, light scattering coefficient (σsp,670), absorption coefficient (σap,670), and
single-scattering albedo (SSA670) throughout the experiment were
113 ± 169 µg m-3, 53.3 ± 74.8 Mm-1,
3.2 ± 2.4 Mm-1, and 0.913 ± 0.05, which were comparable to the background
levels in the southern United States but lower than those in eastern and
other northwestern Chinese cities. Frequent dust storms could markedly elevate dust
loading and dominated the temporal evolution of airborne aerosol in the Dunhuang
region. The hourly-averaged PM10, σsp,670, and σap,670 reached up to 2000 µg m-3, 800 Mm-1, and
25 Mm-1 during the severe dust events that were 10-fold greater than the
total mean values, along with the peak values of aerosol number size distribution appearing at the particle diameters
1–3 µm. Meanwhile, the correspondingly high SSA670
(∼ 0.96–0.98) and depolarization ratio (δ of
∼ 0.3–0.5) and low SAE (-0.25–0) values
adequately verified the presence of coarse-mode mineral dust, resulting in
significantly reduced solar direct radiation (∼ 200–350 Wm-2) and increased diffuse radiation
(∼ 150–300 Wm-2) at the surface, thus affecting the regional climate.
Due to relatively low aerosol levels observed in Dunhuang, any slightly
anthropogenic perturbation would induce a substantial influence on the
aerosol physicochemical property. The so-called anthropogenic dust produced
by agricultural cultivating operations (e.g., land planning, plowing, and
disking) brought a significant superimposed effect on high dust
concentrations in Dunhuang farmland prior to the growing season, when the
underlying surface was primarily covered with bare soils. This could to some
extent be interpreted as the drastic changes of aerosol loadings in April
and early May. In contrast, the local pollutant emissions mainly affected
the absorptive characteristics of dust aerosol especially at night, when the
anthropogenic pollutants favorably accumulated within the PBL and likely
mixed with abundant mineral dust in the atmosphere. Therefore, the diurnal
variations of σap,670 and SSA670 exhibited prominent
features, both of which have two peak values at night and in the early
morning. For instance, ∼ 3.3 Mm-1 at 20:00 LT and
∼ 2.9 Mm-1 at 08:00 LT for σap,670 were much
higher than the low level of ∼ 2.0 Mm-1 in the afternoon,
which was attributed to the influence of anthropogenic emissions. The
mean SSA670 of predominant dust particles in the afternoon
(13:00–18:00 LT) was 0.945 ± 0.04, which was evidently greater than the
mixed dust-pollutant-dominated SSA670 of ∼ 0.90 at 07:00 LT and ∼ 0.92 at 20:00 LT.
The findings of this study directly demonstrated that mineral dust in
Dunhuang farmland was substantially affected by anthropogenic pollutants,
which may provide further insight into the interaction of
dust aerosol, atmospheric chemistry, and regional climate in desert source
regions. However, the potentially anthropogenic influences on dust aerosol in
Dunhuang were much smaller than that measured in eastern China, which was
expected for remote desert areas with sparse populations and fewer
human activities. Recently, Huang et al. (2016) indicated that most of the
drylands in the world were fragile and susceptible to climate change and
human activities and would be subject to the acceleration of drought
expansion by the end of 21st century. Under the possible scenario,
it is critical to make clear the relative contributions of natural and
anthropogenic forcing factors on global climate change, such as natural
dust and anthropogenic dust, which calls for further investigation through a
lot more observations and technological development.