SEARCH

SEARCH BY CITATION

Keywords:

  • aerosol;
  • dust;
  • cold front

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Instrument and Measurements
  5. 3. Dust Transport
  6. 4. Results and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[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 μm and a coarse mode with radius of 4.5 μm. 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 μm and two coarse modes with radii of 1.69 and 7.73 μm, 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 μm increased to a maximum of 0.3 μm3/μm2, 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.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Instrument and Measurements
  5. 3. Dust Transport
  6. 4. Results and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[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 Characterization 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 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 long-range 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

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Instrument and Measurements
  5. 3. Dust Transport
  6. 4. Results and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

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°N∼38.59°N. Calibration was performed against land-based observations.

image

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.

Download figure to PowerPoint

[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 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 (550∼860 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 Δτ = ±0.03 ± 0.05τ 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

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Instrument and Measurements
  5. 3. Dust Transport
  6. 4. Results and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

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 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.

image

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.

Download figure to PowerPoint

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).

image

Figure 3. Contour plots of Aura/OMI UVAI for (a) 21, (b) 22, (c) 23, and (d) 24 April 2006. The light-gray 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.

Download figure to PowerPoint

image

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.

Download figure to PowerPoint

[14] Dust aerosol associated with a cold front from inner Mongolia in the region of 110°E∼114°E and 40°N∼44°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

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Instrument and Measurements
  5. 3. Dust Transport
  6. 4. Results and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

4.1. Shipboard Measurements

[15] As described in section 3, the shipboard measurements were taken under varying meteorological conditions. 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°E∼124.00°E, 37.05°N∼38.59°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°E∼123.00°E, 35.50°N∼35.66°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 sources make the AOD higher than that under relatively clean ocean conditions.

image

Figure 5. The daily variations of shipboard measured aerosol optical depth (solid circles) and Ångström exponent (open circles) over the Yellow Sea.

Download figure to PowerPoint

[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 μm [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 transportation 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 0.1∼0.2 μm, 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 in the size distribution: the accumulation mode (radius 0.1∼0.2 μm) and the coarse mode (radius 4∼5 μm).

image

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.

Download figure to PowerPoint

[30] On 22 April, when the frontal system was approaching, the concentration of nucleation modes with a radius of 0.01∼0.04 μm 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 μm. The coarse mode radius changed from 1.69 to 3.62 μm 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 μm, and two were coarse modes with radii of 1.69 and 7.73 μm. The second coarse mode volume concentration increased to 1.03 μm3/μm2, the highest value observed in this study. Unlike on 22 April, in this case, mode radius of the large particles is 7.73 μm. After the passage of the frontal system, accumulation and coarse modes appeared at 0.08 and 3.62 μm, respectively. The concentrations were lower than 0.2 μm3/μm2 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 μm. The concentration of the coarse mode was higher than 0.3 μm3/μm2 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 μm3/μm2 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.

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 μm, respectively. These results were similar to shipboard measurements on 22 April 2006, when the dust was approaching the ship.

image

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.

Download figure to PowerPoint

[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 μm) increased from 0.1 to 0.97 μm3/μm2 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 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.

image

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.

Download figure to PowerPoint

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 (−100∼200 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 (200∼300 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 (−200∼200 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.

[47] In the postfrontal zone (200∼300 km), the AOD decreased to lower values (0.3∼0.5), 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 400∼600 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

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Instrument and Measurements
  5. 3. Dust Transport
  6. 4. Results and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[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 μm and coarse mode with radius of 4.5 μm. 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 μm 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 μm and two coarse modes with radii of 1.69 and 7.73 μm. 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 μm increased to a maximum of 0.3 μm3/μm2, and the mean AOD (SD) and Ångström exponent (SD) were 0.70 (0.08) and 0.30 (0.04), respectively.

[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 μm. 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.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Instrument and Measurements
  5. 3. Dust Transport
  6. 4. Results and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

[53] 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.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Instrument and Measurements
  5. 3. Dust Transport
  6. 4. Results and Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References