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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, D00K04, doi:10.1029/2009JD012684, 2010
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Measurements of Asian dust optical properties over the Yellow Sea
of China by shipboard and ground-based photometers,
along with satellite remote sensing: A case study
of the passage of a frontal system during April 2006
Yi Liu,1 Dongxu Yang,1,2 Wenzhong Chen,3 and Hua Zhang4
Received 17 June 2009; revised 23 September 2009; accepted 6 October 2009; published 17 March 2010.
[1] Aerosol optical properties were measured by a POM-01 MarkII Sun and sky
photometer onboard the Dongfanghong Number 2 Research Ship on the Yellow Sea of
China during the passage of a cold front surrounded by airborne dust that originated in
Mongolia between 21 and 24 April 2006. The aerosol size distributions in clean marine
environment were dominated by an accumulate mode with radius of 0.15 mm and a coarse
mode with radius of 4.5 mm. The mean aerosol optical depth (AOD) and Ångström
exponent were 0.26 and 1.26, respectively. In the frontal zone the aerosol size distribution
was dominated by an accumulate mode with radius of 0.25 mm and two coarse modes with
radii of 1.69 and 7.73 mm, and the AOD and Ångström exponent were 2.46 and 0.84,
respectively. In the nonfrontal dust conditions, the concentration of coarse modes with
radii of 2.5 mm increased to a maximum of 0.3 mm3/mm2, and the mean AOD and
Ångström exponent were 0.70 and 0.30, respectively. Aerosol Robotic Network
(AERONET) observations combined with shipboard measurements reveal the decreasing
concentration of dust aerosol during its transport from continent to Japan. The spatial
distribution of dust aerosol was studied using the Aqua/Moderate Resolution Imaging
Spectroradiometer (MODIS) and Aura/Ozone Monitoring Instrument (OMI) products. On
22 April, for frontal dust, their AOD and UV aerosol index (UVAI) increased with
decreasing distance to the frontal line, peaked with values of 4.36 and 5.21 in the frontal
zone, and decreased rapidly with increasing distance off the frontal line. On 23 April,
nonfrontal dust showed the lower AOD and UVAI with peak values of 2.0 and 2.7,
respectively.
Citation: Liu, Y., D. Yang, W. Chen, and H. Zhang (2010), Measurements of Asian dust optical properties over the Yellow Sea of
China by shipboard and ground-based photometers, along with satellite remote sensing: A case study of the passage of a frontal system
during April 2006, J. Geophys. Res., 115, D00K04, doi:10.1029/2009JD012684.
1. Introduction
[2] Dust is one of the key types of aerosol. Asian dust
from the Taklimakan and Gobi deserts can be transported
from central Asia to the east of China, Korea, and Japan
[Uematsu et al., 2002; Zhang et al., 2003], and even to the
Pacific Ocean and North America [Arimoto et al., 1996;
Takemura et al., 2002; Uno et al., 2001]. Special attention
has therefore been given to Asian dust in several comprehensive experiments, such as ACE-Asian (Aerosol Charac1
Key Laboratory of Middle Atmosphere and Global Environment
Observation, Institute of Atmospheric Physics, Chinese Academy of
Sciences, Beijing, China.
2
Graduate University of Chinese Academy of Science, Beijing, China.
3
College of Information Science and Engineering, Ocean University of
China, Qingdao, China.
4
Laboratory for Climate Studies, National Climate Center, China
Meteorological Administration, Beijing, China.
Copyright 2010 by the American Geophysical Union.
0148-0227/10/2009JD012684
terization Experiments – Asian) [Huebert et al., 2003],
PACDEX (International Pacific Dust Experiment) [Huang
et al., 2008] and APEX (Asian Atmosphere Particle Environment Change Studies) [Sano et al., 2003]. It is well
known that springtime Asian dust storms from Mongolia are
usually created by strong winds associated with cold fronts.
Chemical compositions of dust storms in their transport
pathway over Beijing have been studied; it is found the dust
increased sharply when cold front intruded Beijing, which
neutralized local acidic aerosol [Sun et al., 2005; Yuan et al.,
2008]. At Chuncheon of Korea, a single-particle analysis
method was used during the passage of a dust storm
between 10 and 12 March 2004, which showed that the
mixing of CaCO3 and sea salt particles with dust particles
occurred just at the early stage of the storm [Hwang et al.,
2008].
[3] Over the sea, results from shipboard and island-based
measurements have indicated that the aerosol optical depth
is less than 0.15 and the Ångström exponent is more than
1.0 in clear marine environments unaffected by dust and
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Figure 1. Dots represent geographical locations and time of observation. Gray characters indicate
provinces of China and sea area. Black characters indicate name of the country.
pollution [Smirnov et al., 2003]. Aerosol physical and
chemical properties are particularly complex over the coastal areas of the west Pacific [Bates et al., 2004] due to the
influences of a large number of varied sources, including
biomass and biofuel burning [Bey et al., 2001; Streets et al.,
2001] and volcanic, industrial, and biogenic emissions
[Arndt et al., 1997; Streets et al., 2001]. The shipboard
aerosol samples and analyses were conducted over Yellow
Sea of China from April to June 1988, which showed that
the concentration of dust aerosol (crustal elements) decreased with their distance to the coast in an exponential
decay pattern [Liu and Zhou, 1999]. Numerical studies also
showed that the physical and chemical properties of marine
aerosols mixed with dust changed with time and location
over the China Sea [Bates et al., 2004; Liu and Zhou, 1999].
Shipboard measurements of aerosol optical properties are
scarce over the Sea of China, and hence are extremely
valuable. In addition, the properties of Asian dust over the
East China Sea have not been well studied.
[4] Satellite observation is effective way for spatial distribution and transport of dust. Movement of dust plum has
been observed well by Ozone Monitoring Instrument (OMI)
[Torres et al., 2007] and MODerate Resolution Imaging
Spectroradiometer (MODIS) [Hsu et al., 2006]. The longrange transport and three-dimensional structure of Asian
dust with polluted aerosols has been studied by ground- and
satellite-based instruments in conjunction with model simulations [Hara et al., 2009; Lin et al., 2006]. A two-layer
transport structure between 3 and 9 km has been shown in a
recent observational study by Cloud-Aerosol Lidar with
Orthogonal Polarization (CALIOP) and ground-based lidar
[Huang et al., 2008].
[5] We used multiple observations to investigate the
changes of aerosol optical properties over the Yellow Sea
of China during the passage of a frontal system between 21
and 24 April 2006, which caused the transport of dust
aerosols from Mongolia to the Yellow Sea. In section 2, we
introduce the shipboard POM-01 MarkII Sun and sky
photometers, Aura/OMI, Aqua/MODIS and measurement
method. In section 3, we describe the meteorological
variations during the passage of the front, present the
satellite data and apply a back trajectory analysis. The
shipboard photometer measurements, satellite observations
and ground-based AERONET measurements of the optical
properties of aerosols in different conditions are investigated in section 4. Finally, we summarize the results and
present our conclusions.
2. Instrument and Measurements
2.1. POM-01 MarkII Sun and Sky Photometer
[6] The POM-01 MarkII, built by PREDE, was deployed
to measure aerosol properties onboard the Dongfanghong
Number 2 Research Ship. Shipboard aerosol measurements
were performed between 16 and 30 April 2006 over the
Yellow Sea of China. Data collected between 21 and 24
April 2006 were used to examine the influences of dust
transport associated with the passage of the frontal system.
Figure 1 shows the track of the Dongfanghong Number 2
Research Ship during the experimental period. The ship was
in the north of the Yellow Sea on 21 April 2006 and then
moved to the south after 22 April 2006. All data were
collected in the rectangular region between 120.26°E
124.01°E and 33.31°N38.59°N. Calibration was performed against land-based observations.
[7] The POM-01 MarkII measured direct and diffuse
solar radiance in seven channels with central wavelengths
of 315, 400, 500, 675, 870, 940, and 1020 nm. It was
mounted on a dual axis robot controlled by servomotors and
contained seven interference filters and a collimator. Sun
tracking was achieved using a four-quadrant silicon detector
and a narrow field of view CCD (Charge Coupled Device).
A wide field of view CCD camera was located on top of the
robot to record the position of the sun while the ship moved.
[8] Measurements were taken when the instrument estimated cloud-free conditions. Aerosol properties were calculated using the program Skyrad package (SKYRAD.pack)
developed by the Center for Climate System Research,
University of Tokyo [Nakajima et al., 1996]. This retrieval
process includes two independent programs: MKDTA and
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Figure 2. Composite meteorological factors for (a) 21, (b) 22, (c) 23, and (d) 24 April. Solid and dashed
contours show the sea level pressure and 500 hPa geopotential height, respectively, with ‘‘L’’ indicating
the sea level low-pressure center. The thick line indicates the approximate position of the cloud front at
sea level. Open symbols show the dust reported at the local CMA stations. Squares, rhombuses, and
triangles represent dust storm, floating dust, and blowing dust, respectively.
REDML. The first is for computing simulated data of direct
and diffuse solar radiation, and the second is for retrieving
aerosol properties. Observation condition, such as solar
zenith angle and geometry, number and value of wavelength
and scattering angles, and multimodal aerosol volume radius
distribution are required for retrieval. The pitching and
rolling of ship will affect the precision of AOD observed
from POM-01 MarkII, in order to reduce this uncertain, we
carried out measurement when the ship stopped and sea
surface wind speed was low.
2.2. MODIS and OMI
[9] Satellite remote sensing can observe dust aerosols
over large areas efficiently. However, polar orbiting satellite
can observe a large area, and usually does so once a day for
fixed area. In this study, Aqua/MODIS and Aura/OMI
aerosol data products were used to focus on the spatial
distribution of aerosol optical properties during the passage
of the cold front over the Yellow Sea of China.
[10] MODIS level 2 aerosol products, such as AOD
(550 nm) and Ångström exponent (550860 nm), were
employed in this study, which were retrieved by aerosol
algorithm over ocean [Remer et al., 2005]. Thousands of
MODIS AOD collocated with AERONET measurements
confirmed that one standard deviation of MODIS optical
depth retrievals fell within the predicted uncertainty of Dt =
±0.03 ± 0.05t over ocean [Remer et al., 2005]. Similar
results which compared by MODIS retrieval and shipboard
measurements showed that 73% data fell within the predicted uncertainty over China Sea [Yang et al., 2009].
[11] Dust particles absorb light significantly in the blue
and ultraviolet wavelengths due to iron oxide (rust) impurities [Claquin et al., 1999; Sokolik and Toon, 1996]. Such
high absorption properties at short wavelengths can be
measured by the Aura/OMI in the 354 and 388 nm channels
through the UVAI (Ultra Violet Aerosol Index) [Torres et
al., 2002]. A near-zero value of UAVI means that the
atmosphere is free of aerosols or contains only large nonabsorbing aerosol particles and clouds with near-zero Ångström exponents. Nonabsorbing small particles have a small
negative UVAI, and absorbing aerosol is the most important
source of positive UVAI [Torres et al., 1998, 2007].OMI
level 2 and level 3 UVAI data products were used in this
study. Level 3 global 1° 1° grid data were used to show
the transport of high-absorption dust, and level 2 data were
used to show the variation of UVAI with different meteorological conditions. Retrieval conditions such as cloud
fraction and sun glint were recorded in the ‘‘algorithm
flags’’ of level 2 data. We used only data with flags equal
to 1 or 2, which meant that the retrieval results of the
aerosol absorption properties were confident.
3. Dust Transport
3.1. Meteorological Conditions
[ 12 ] National Centers for Environmental Prediction
(NCEP) reanalysis data (1400 Beijing Time Coordinate
(BTC)) and local station data from the China Meteorology
Administration (CMA) records were used to analyze the
meteorological conditions. As indicated in Figure 2, a
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Figure 3. Contour plots of Aura/OMI UVAI for (a) 21, (b) 22, (c) 23, and (d) 24 April 2006. The lightgray regions indicate a lack of retrieval data; the vector is the 850 hPa wind field; the solid lines are the
24 h (Figure 3b) and 48 h (Figure 3c) forward trajectories, and the dotted line in Figure 3d is the 24 h
back trajectory. Circles and squares indicate the air mass position at time of observation. The up and
down triangles show the position of Xianghe and Shirahama stations, respectively.
Mongolian cyclone accompanied by a cold front crossed
northern China between 21 and 24 April 2006, surrounded
by a wide distribution of airborne dust. The cold front
associated with this cyclone arrived in Beijing on 21 April
and moved to the northern Yellow Sea on 22 April. During
22– 23 April, a narrow, long cloud belt mixed with airborne
dust reached the observation ship, with little precipitation
recorded by local stations on the Shandong and Korean
Peninsulas. In the 500 hPa geopotential height field, the
low-pressure center of the cyclone fell behind that at sea
level. The two low-pressure centers coincided with each
other on 23 April. On 24 April, the full low-pressure system
moved to the Sea of Japan.
3.2. Spatial Distributions of Dust
[13] Figure 3 shows the OMI level 3 UVAI and air mass
trajectory between 21 and 24 April during the passage of the
frontal system to the China Sea. Compared to Aqua/MODIS
RGB images (Figure 4), which show the cloud and dust
plume in visible, both of the satellite observations showed
similar frontal and nonfrontal dust systems. Air mass
trajectories, including backward and forward traces, were
calculated online using the NOAA/Air Resources Laboratory HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Dust from the Gobi desert
generally has a high-concentration transport layer around
3 km above sea level (ASL) [Huang et al., 2008; Sun et al.,
2001]. We calculated the 24 and 48 h forward trajectories at
2.5 km ASL for inner Mongolia on 21 April (blue lines in
Figures 3b and 3c). We also calculated the 24 h back
trajectory at 2.5 km ASL over the Yellow Sea on 24 April
(green line in Figure 3d).
[14] Dust aerosol associated with a cold front from inner
Mongolia in the region of 110°E114°E and 40°N44°N
on 21 April. The highly absorbing dust plume was transported directly to the Bohai Sea, crossing Beijing and Hebei
province in the process. On 22 April, because the frontal belt
had strong winds and uplift current, a highly absorbing
narrow long dust belt formed in this region, arriving in
Shandong province, Shandong peninsula, the northern Yellow Sea, and the Korean peninsula (Figure 3b). On 23 April,
after 24 h of transport, the high-concentration band reached
the southern Yellow Sea, and the airflow turned eastward
and passed the Korean peninsula. Because the dust concentration decreased, the absorbing properties of the dust region
weakened. At the same time, a thin nonfrontal dust layer
arrived in Hebei province and the eastern peninsula of
Liaoning province behind the frontal system (Figures 3c
and Figure 4c). On 24 April, A large region of weakly
absorbing nonfrontal dust appeared over the Yellow Sea, and
influenced Shandong and Shanxi provinces and the southwest of Japan on the same day.
4. Results and Discussion
4.1. Shipboard Measurements
[15] As described in section 3, the shipboard measurements were taken under varying meteorological conditions.
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Figure 4. Aqua/MODIS RGB images from (a) 21, (b) 22, (c) 23, and (d) 24 April 2006. Red and
magenta circled dots show the Xianghe and Shirahama sites, respectively. The cyan anchors show the
approximate positions of the ship. The red line indicates the frontal line (see section 4.3) on 22 and
23 April.
We summarize the clear marine, prefrontal, frontal, postfrontal, and nonfrontal dust situation during 21 –24 April
2006.
[16] On 21 April 2006, measurement was carried out in a
clean marine environment. Aerosol optical data were measured over the northern Yellow Sea (123.71°E124.00°E,
37.05°N38.59°N). No obvious cloud or dust was visible
in the Terra/MODIS true color images (not shown here) at
1030 BTC. In the Aqua/MODIS image (Figure 4a), a small
amount of cloud was visible at midday (at 1210 BTC). In
five measurements taken from 0709 to 1643 BTC, no
precipitation was recorded in coastal local meteorological
stations, either in China or in Korea.
[17] On 22 April 2006, measurement was carried out in
the prefrontal zone. The ship took samples in the southern
Yellow Sea (122.84°E123.00°E, 35.50°N35.66°N) in
front of the cloud-dust belt, where a slight influence from
the frontal system was apparent. According to coastal local
station records, floating dust over the Shandong and Korean
peninsula was observed at this time.
[18] On 23 April 2006, measurement was carried out in
the frontal and postfrontal zone. The front reached the ship
on 23 April. The ship measured aerosol properties over the
southern Yellow Sea (123.25°E, 35°N). Measurements were
taken in the frontal zone at 1033 BTC and in the postfrontal
zone at 1356 BTC. As the frontal zone passed over the ship,
precipitation was recorded in local station meteorological
data.
[19] On 24 April 2006, measurement was carried out in
nonfrontal dust conditions. After the frontal system passed
over the southern Yellow Sea, a large region of nonfrontal
dust from Mongolia was observed by the ship-based Sun
and sky photometer and local stations in the northeast of
China and Korea (Figure 2, yellow symbols). According to
meteorological records on the ship, no clouds were observed during 24 April, and another thick dust layer
appeared in the afternoon.
4.1.1. Aerosol Optical Depth
[20] Aerosol Optical Depths (AOD) at 500 nm are given
in Figure 5 for 4 days of observations. The AOD values
were between 0.2 and 0.4 during the whole day on 21 April
in the clean marine environment over the northern Yellow
Sea. A low AOD mean value of 0.26 and Standard
Deviation (SD) of 0.03 indicated the relatively weak influence of aerosols from the Asian continent, and our results
agree with measurements taken over the Yellow Sea during
the spring of 2003 and 2006 by handheld Sun photometers
[Shen et al., 2008; Zhao et al., 2005]. The value of 0.26
should therefore be recognized as the background AOD
during this period. In the studies based on five island
AERONET sites over the Atlantic and Pacific oceans, the
AOD values of less than 0.15 can be attributed to maritime
aerosols [Smirnov et al., 2003]. Our AOD result was higher
than the results over other regions, which is mainly because
the aerosols composition over the Yellow Sea is influenced
by air mass from Asian continent under western winds.
High concentrations of natural and anthropologic continent
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Figure 5. The daily variations of shipboard measured aerosol optical depth (solid circles) and Ångström
exponent (open circles) over the Yellow Sea.
sources make the AOD higher than that under relatively
clean ocean conditions.
[21] On 22 April, the dust layer was approaching the
Yellow Sea. The mean value (SD) of the AOD increased to
0.36 (0.02), 70% higher than that on 21 April. Similarly,
AOD increased by a factor of 1.7 under the influence of
continental air mass, was reported by Smirnov during
shipboard observation between New York and Bermuda
[Smirnov et al., 2000]. On 23 April, at 1033 BTC, the AOD
increased to 2.46, the maximum during the 4 days of
observations. At 1330 BTC, the AOD decreased sharply
to 0.42 after the dust-cloud belt passed.
[22] Under the nonfrontal dust conditions on 24 April, the
AOD mean value (SD) was 0.70 (0.08), with maxima and
minima of 0.81 and 0.56, respectively. Associated with the
thicker dust layer over the Yellow Sea in the afternoon, the
AOD increased throughout the whole day. The AOD in
nonfrontal dust conditions was 2 times greater than that in
the clean marine environment.
4.1.2. Ångström Exponent
[23] The wavelength dependence of AOD can be represented in a simple way using the Ångström exponent
[Ångström, 1964]. The method of least squares was applied
to calculate Ångström exponents at five wavelengths (400,
500, 675, 870, and 1020 nm).
[24] Different types of aerosol have different AOD dependencies on wavelength, in part because the fractions
between accumulation and coarse modes have extinctions
that vary with wavelength. In the atmosphere, Ångström
exponents are sensitive to the volume fraction of aerosols
with radii less than 0.6 mm [Schuster et al., 2006]. Therefore, Ångström exponents are often used as a qualitative
indicator of aerosol particle size, with values greater than
2.0 indicating small particles (e.g., combustion byproducts),
and values less than 1.0 indicating large particles (e.g., sea
salt and dust) [Schuster et al., 2006].
[25] Ångström exponents during this observation are
shown in Figure 5. Ångström exponents decreased continuously from 21 to 24 April 2006. One interpretation of this
phenomenon is that the ratio of large to small particles
increased continuously during the dust aerosol transporta-
tion process. On 21 April, in the clean marine environment,
the mean Ångström exponent was 1.26, with a standard
deviation of 0.09. This high value indicted a polluted and
stable atmospheric environment [Smirnov et al., 2003].
[26] When the frontal system was approaching, a lower
Ångström exponent (SD) of 1.20 (0.07) was measured on
22 April. The weak influence of dust aerosols made the
Ångström exponents decreased slightly. A negative relationship between the AOD and Ångström exponents was
observed in later measurements on this day.
[27] When the frontal system reached the observation
ship, the Ångström exponent decreased to 0.84, which
was similar to the values of 0.76, 0.80, and 0.66 in the
Amami, Noto, and Shirahama measurements [Sano et al.,
2003]. The exponents reduced continually after the front
passed, and their values decreased from 0.84 to 0.49 after
3 h, with a small amount of precipitation. On 24 April, the
exponents had a mean value (SD) of 0.30 (0.04).
[28] Under high dust aerosol conditions, a negative relationship was seen between the AOD and the Ångström
exponents. From Figure 5, all data were clearly separated
into three clusters corresponding to prefrontal, frontal, and
postfrontal conditions. A similar phenomenon has been
observed in measurements over Seoul [Chun et al., 2001].
Large dust particles caused the AOD to increase with
decreasing Ångström exponents. A special cluster appeared
under the frontal conditions, with the high AOD corresponding to medium Ångström exponents. This was mainly
because the volume concentration of particles of all radii
increased in the frontal zone, whereas the ratio of large to
small dust particles increased a little. Further discussion of the
volume size distributions is given in section 4.1.3.
4.1.3. Volume Size Distributions
[29] The volume size distributions of marine aerosols
have been summarized in the study of Smirnov et al.
[2002]. The accumulation mode has a typical radius of
0.10.2 mm, depending on wind speed [O’Dowd et al.,
1997]. Similar results were obtained from different seas
over the world [Smirnov et al., 2002]. On 21 April, in the
clean marine environment (Figure 6a), two modes appeared
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Figure 6. The daily variations of shipboard-measured (a) volume size distribution and (b) single
scattering albedo versus wavelength over the Yellow Sea of China. The solid triangles on the left and
right sides indicate each measurement.
in the size distribution: the accumulation mode (radius
0.10.2 mm) and the coarse mode (radius 45 mm).
[30] On 22 April, when the frontal system was approaching, the concentration of nucleation modes with a radius of
0.010.04 mm increased, probably because the continental
air mass moved to the Yellow Sea with the northwest
current. This would have enhanced the conversion of
continental gas pollutants to aerosols by gas-to-particle
conversion processes because of high relative humidity over
sea surface. The accumulation mode radius was between 0.1
and 0.2 mm. The coarse mode radius changed from 1.69 to
3.62 mm between 1352 and 1431 BTC. After this time, the
coarse mode radius did not change, whereas the peak
volume concentration decreased.
[31] On 23 April, the ship was just within the frontal
system and three modes were present. One was an accumulation mode with radius of 0.25 mm, and two were coarse
modes with radii of 1.69 and 7.73 mm. The second coarse
mode volume concentration increased to 1.03 mm3/mm2, the
highest value observed in this study. Unlike on 22 April, in
this case, mode radius of the large particles is 7.73 mm.
After the passage of the frontal system, accumulation and
coarse modes appeared at 0.08 and 3.62 mm, respectively.
The concentrations were lower than 0.2 mm3/mm2 in both
modes.
[32] Under the cloud-free and nonfrontal dust conditions
on 24 April, there were complex size distributions, with
coarse mode radii varying from 1 to 5 mm. The concentration of the coarse mode was higher than 0.3 mm3/mm2 with
no apparent accumulation mode, in contrast to the frontal
dust. It is interesting that the nucleation mode appeared
again at this time, with its concentration increasing to
0.03 mm3/mm2 in the afternoon.
4.1.4. Single Scattering Albedo
[33] Daily variations of single scattering albedo (SSA)
with wavelengths are shown in Figure 6b. On 21 April,
relativity low absorption with high SSA occurred at five
wavelengths. The mean value of SSA (SD) at 500 nm
wavelength was 0.99 (0.02). Similar measurement results
were obtained at Lanai/Hawaii, referred to as an oceanic
environment [Dubovik et al., 2002]. Stronger absorption
appeared on 22 April. The SSA was lower at all wavelengths at 1431 BTC, and absorption was stronger at shorter
wavelengths at 1516 BTC. The mean value (SD) of SSA
was 0.986 (0.021) at 500 nm on this day. On 23 April,
frontal measurements showed high SSA as weak absorption, After 1300 BTC, the SSA was less than 0.90 at 500 nm
under postfrontal conditions.
[34] Under nonfrontal dust conditions on 24 April 2006,
there was a typical wavelength dependence of dust SSA
compared to the measurements at Solar Village/Saudi Arabia and Cape Verde [Dubovik et al., 2002]. The stronger
absorption at short wavelengths, with stronger scattering at
long wavelength, was attributed to the high ratio of large
dust particles, as could be seen from the volume size
distributions (Figure 6a). The SSA at long wavelengths
decreased at midday, whereas the nucleation mode volume
concentration increased.
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Figure 7. (top) Xianghe site on 20– 25 April 2006. (a) AOD and Ångström exponents. Open circles
indicate single data values, and solid circles are the daily means; the abscissa is the measurement
date. (b) Mean value of aerosol volume size distributions. (bottom) Shirahama site on 24– 25 April
2006. (c) AOD and Ångström exponents. The abscissa is the measurement date. (d) Mean values of
aerosol volume size distributions; values were sorted and calculated on the basis of the AOD. Graphic
symbols in the top left of the plot indicate the sorting and calculation conditions, which correspond to
A, B, C, and D in Figure 7c.
4.2. AERONET Observations
[35] According to satellite observations and the air mass
trajectory analysis, the China/Xianghe AERONET site was
located in the middle of the dust transport path. We can
therefore use its observations to describe the transport of
dust associated with the cold frontal system. The Xianghe
site (39.75°N, 116.38°E, red symbols on Figure 4) is located
70 km east of Beijing. A CIMEL CE-318 Sun and sky
photometer is installed there [Holben et al., 1998]. The
AOD and Ångström exponents were calculated from direct
solar radiation measurements at eight wavelengths (340,
380, 440, 500, 675, 870, 940, and 1020 nm). Volume size
distributions were derived from direct and diffuse solar
radiation. The AERONET level 2.0 data were used in this
study.
[36] Figure 7a shows the variations of AOD and Ångström exponents from 20 to 24 April 2006. From the
MODIS and OMI observations, the frontal system
approached the Xianghe station on April 21. At this time,
the AOD increased from 0.31 (on 20 April) to 1.03, whereas
Ångström exponents decreased from 1.29 to 0.80. Meanwhile, the volume concentrations of both accumulate and
coarse modes were higher than were those on 20 April
(Figure 7b). Accumulation and coarse mode radii were 0.11
and 3 mm, respectively. These results were similar to
shipboard measurements on 22 April 2006, when the dust
was approaching the ship.
[37] On 23 April 2006, under nonfrontal dust conditions,
a higher AOD of 0.98 and lower Ångström exponent of
0.006 were observed. In the volume size distribution, the
concentration of the coarse mode (radius 3.0 mm) increased
from 0.1 to 0.97 mm3/mm2 in 24 h, but the concentration
of the accumulation mode decreased a little. This result
was similar to nonfrontal dust results over Yellow Sea on
24 April 2006. Figure 7a also shows daily variations of the
AOD on 23 April, which indicate the inhomogeneous
variation of dust concentration in time and space. Interestingly, on 23 April the accumulation and coarse mode
volume concentrations were lower and higher, respectively,
than on 21 April.
[38] Nonfrontal dust moving from the Yellow Sea to
Japan was measured by the Sun and sky photometer at
the Japan/Shirahama AERONET site (33.69°N, 135.36°E,
magenta symbol in Figure 4) between 24 and 25 April 2006
(Figure 7c). The results indicated that the AOD increased
from 0.3 to 0.5 on 24 April, and it increased to 0.7 then
decreased to 0.25 on 25 April. The Ångström exponent
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Figure 8. Satellite-observed aerosol physics and optical properties on (a) 22 April and (b) 23 April
2006. Open circles, solid circles, and open squares represent OMI/UVAI, MODIS/AOD, and MODIS/
Ångström exponents, respectively. Gray lines define different regions: A, clean marine; B, prefrontal
zone; C, frontal zone; D, postfrontal zone; and E, nonfrontal zone.
decreased from 0.7 to 0.5 on 24 April and increased from
0.3 to 0.8 after the passage of the dust on 25 April. In the
volume size distributions (Figure 7d), when dust reached
the observation site, the coarse mode volume concentration
increased rapidly, while the accumulation mode increased a
little. This meant that the increase of large particles contributed to an increase in AOD.
[39] During nonfrontal dust transportation, AOD decreased from 0.98 to 0.70, and then to 0.46, whereas
Ångström exponent increased from 0.006 to 0.30, and then
to 0.51, at Xianghe, Yellow Sea and Shirahama respectively.
These characters reveal the decreasing concentration and
particle radius of dust aerosol during it is transported from
continent (Xianghe) to Yellow Sea and Japan (Shirahama).
4.3. Satellite Observations
[40] From MODIS images (Figure 3b), we can easily
identify a long dust belt mixed with a frontal cloud belt. To
study the dust aerosol distribution around the frontal zone,
we defined a line within the cloud belt over the frontal zone.
It was served as a boundary between cloud and dust (we
refer to it as the frontal line in sections 4.3.1 and 4.3.2). In
this study, we classify the region into clean marine
environment, prefrontal, frontal, postfrontal, and nonfrontal
dust bands based on their distances from the frontal line
(Figure 8). The frontal line on 22 and 23 April is indicated
in Figures 4b and 4c, respectively.
4.3.1. On 22 April 2006
[41] During the evolution of the cold front, the dust
aerosol was transported with the frontal zone. A strong
upward current was present there, with a high concentration
plume behind the frontal zone (Figure 4b). In the prefrontal
areas (more than 200 km from the frontal line), the mean
AOD (SD) was 0.21 (0.06), the Ångström exponent (SD)
was 1.21 (0.61), and the UVAI was between 1.0 and 1.0
(Figure 8a). These results agreed closely with shipboard
measurements under clean marine conditions on 21 April.
[42] In the prefrontal zone ( 200 100 km), the AOD
and UVAI increased with decreasing distance to the frontal
line. The higher AOD of 0.33 at 150 km observed before
the frontal line was similar to shipboard results on 22 April.
The AOD of 0.76 at 100 km before the frontal line was the
highest value in the prefrontal zone.
[43] In the frontal zone ( 100200 km), the AOD had a
peak value of 4.36, and the UVAI was 5.21. The UVAI
decreased rapidly with increasing distance from the frontal
line, to around 2.0 between 140 and 200 km. In the
postfrontal region (200300 km), a lower mean (SD) UVAI
of 1.08 (0.30) was observed, which was higher than clean
marine conditions but lower than prefrontal conditions.
4.3.2. On 23 April 2006
[44] On 23 April, the frontal system became weak and
mixed with the high concentration of dust aerosols. We
defined the frontal line in the same way as on 22 April.
Because the frontal system over the Yellow Sea could not be
covered in the track of the MODIS (shown in Figure 4c), we
combined two tracks of MODIS data within 1.5 h to cover
the whole Yellow Sea. As a result, a small difference in the
direction of the frontal line is observed in the two swaths
(shown as the two red lines in Figure 4c). Because of the
differences in satellite observation time and location, we
focus on the statistical characteristics of the dust aerosol in
different regions, sorted by distance from the frontal line.
[45] In the prefrontal zone ( 400 200), the AOD
values (shown in Figure 8b) were stable, but larger than
the results on 22 April at a similar distance from the frontal
line. Statistics results of AOD indicated that more than 90%
of the results occurred between 0.5 and 1.0, with the
maximum value lower than 1.5 in the prefrontal region.
[46] In the frontal zone ( 200200 km), the AOD and
UVAI decreased sharply with increasing distance from the
frontal line, and their maximum values occurred near the
line. Both the AOD and UVAI had lower peak values on
23 April than on 22 April.
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[47] In the postfrontal zone (200300 km), the AOD
decreased to lower values (0.30.5), which are shown as a
trough in Figure 8. All AOD values were between 0.25 and
0.75, with more than 70% of values lower than 0.5. This
result is similar to the shipboard measurements on 23 April.
The Ångström exponents were lower than 1.0, with more
than 70% of values lower than 0.25. The UVAI was lower
than 2.0, with more than 90% of values in the range 0.5 to
1.5. This meant that the atmosphere in the postfrontal region
was cleaner, and the ratio of large dust particles was higher
than in the prefrontal region. Such phenomena were attributed to wet deposit by precipitation.
[48] The nonfrontal dust was transported from Mongolia
without the aid of the frontal system. The nonfrontal dust
was 400600 km behind the cold front, and appeared
over Bohai Sea on 23 April. It moved to the Yellow Sea
on 24 April (Figures 4c and 4d). In both the nonfrontal and
frontal dust, there were high AOD and UVAI values and
low Ångström exponents. However, for the nonfrontal dust,
the AOD peak value was lower (2.0), with a wider peak
range; the UVAI peak value was larger (2.7) and had a wider
peak range compared to the frontal dust (Figure 8b) in the
same day.
5. Conclusions
[49] We studied the passage of a cold front that transported dust from Mongolia to the Yellow Sea of China
between 21 and 24 April 2006. During this period, the
aerosol optical properties were measured by a shipboard
POM-01 MarkII Sun and sky photometer, AERONET Sun
photometers, and satellite observations. The measurements
focused on the evolution and spatial distribution of dust
aerosols associated with the frontal system, as well as the
nonfrontal dust.
[50] In the shipboard measurement results, the aerosol
size distributions in the clean marine environment on
21 June 2006 were dominated by an accumulate mode with
radius of 0.15 mm and coarse mode with radius of 4.5 mm.
The mean values of the AOD (SD) and Ångström exponent
(SD) were 0.26 (0.03) and 1.26 (0.09), respectively. As the
frontal system approached on 22 April 2006, the mean
AOD (SD) and Ångström exponent (SD) changed to 0.36
(0.02) and 1.20 (0.07). The concentration of the nucleation
mode with radius of 0.03 mm increased, which was attributed to the prefrontal continental air mass moving to the
Yellow Sea with the frontal system. In the frontal zone on
23 April 2006, the aerosol size distributions were dominated
by an accumulate mode with radius of 0.25 mm and two
coarse modes with radii of 1.69 and 7.73 mm. The AOD and
Ångström exponents were 2.46 and 0.84, respectively. This
was mainly caused by the high concentrations of water
vapor and dust aerosols there. After the long cloud-dust belt
passed over the ship on the afternoon of 23 April, the AOD
and Ångström exponent decreased to 0.42 and 0.49, respectively, due to dry and wet depositions of dust aerosol. Under
nonfrontal dust conditions on 24 April 2006 the concentration of the coarse mode with radius of 2.5 mm increased to a
maximum of 0.3 mm3/mm2, and the mean AOD (SD) and
Ångström exponent (SD) were 0.70 (0.08) and 0.30 (0.04),
respectively.
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[51] In AERONET observations over China/Xianghe,
when frontal dust approached, AOD increased from 0.31
to 1.03, whereas Ångström exponents decreased from 1.29
to 0.80; when nonfrontal reached, AOD and Ångström
exponents was 0.98 and 0.006, and size distribution was
dominated by an coarse mode with radius of 3.0 mm. In
Japan/Shirahama, when nonfrontal reached, AOD and Ångström exponent were 0.46 and 0.51. AERONET and shipboard measurements indicated decreasing concentration and
particle radius of dust aerosol during it is transported from
continent (Xianghe) to Yellow Sea and Japan (Shirahama).
[52] OMI UVAI, MODIS RGB image and air mass back
trajectory analyses indicated that a long frontal cloud belt
surrounded by a wide distribution of airborne dust moved to
the East of China and the Yellow Sea during 22– 23 April
2006. The long cloud-dust belt was represented as a frontal
line, and the spatial distributions relative to this line were
characterized as prefrontal, frontal, and postfrontal. Statistical results showed that the aerosol optical properties varied
with distance from the frontal line in the following ways.
The AOD and UVAI peaked with values of 4.36 and 5.21 in
the frontal zone and then decreased rapidly with increasing
distance off the frontal line. After the cold front moved to
the southern Yellow Sea and became weak, the prefrontal
region expanded. The dust influence then became weaker in
the frontal zone and stronger in the prefrontal zone, because
the dust aerosols diffused in a large area around the frontal
system. Nonfrontal dust was identified by satellite observations during 23– 24 April 2006, lower AOD and UVAI with
peak values of 2.0 and 2.7 occurred with wider peak ranges,
compared to the frontal dust on 22 April.
[53] Acknowledgments. This work was funded by the National Basic
Research Program of China under grant 2006CB403702, the National
Science Foundation of China under grant 60638020, and the Public
Meteorology Special Foundation of MOST (grant GYHY200706036).
The authors would like to thank the PI of AERONET Xianghe and
Shirahama Station for managing aerosol data; we also thank the OMI and
MODIS team for providing satellite data sets. The in situ shipboard aerosol
measurements are highly appreciated.
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W. Chen, College of Information Science and Engineering, Ocean
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Y. Liu and D. Yang, Key Laboratory of Middle Atmosphere and Global
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China. ([email protected])
H. Zhang, Laboratory for Climate Studies, National Climate Center,
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