The Asian Dust Events of April 1998
R. B. Husar1, D. M. Tratt2, B. A. Schichtel1, S. R. Falke1, F. Li1 D. Jaffe3, S. Gassó3, T. Gill4, N. S. Laulainen5, F. Lu6, M.C. Reheis7, Y. Chun8, D. Westphal9, B. N. Holben10, C. Gueymard 11, I. McKendry12, N. Kuring10, G. C. Feldman10, C. McClain10, R. J. Frouin13, J. Merrill14, D. DuBois15, F. Vignola16, T. Murayama17, S. Nickovic18, W. E. Wilson19, K. Sassen20, N. Sugimoto21, W.C. Malm22
Abstract. On April 15 and 19 1998, two intense dust storms were generated over the Gobi Desert by springtime cold weather systems. The April 15 dust cloud was recirculating and it was removed by a precipitating weather system over East Asia. The dust cloud increased the albedo over the cloudless ocean and land by up to 10-20% but it reduced the cloud reflectance near UV, causing a yellow coloration of all surfaces. The dust was detected and its evolution followed by it’s yellow color on SeaWiFS satellite images, routine surface-based monitoring and through serendipitous observations. The April 19 dust cloud was transported across the Pacific in 5 days in elevated layers (>3 km). Part of the dust continued eastward across North America, a branch turned south along the West Coast at 5-10 km altitude and another significant fraction subsided to the surface between British Columbia and California. Over the West Coast, the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30-40% and doubled the diffuse radiation. This effect was also noticed by the whitish discoloration of the blue sky. On April 29, the average excess Asian dust aerosol concentration over the valleys of the West Coast was about 20-50 mg/m3 with local peaks >100 µg/m3.. The chemical fingerprint of the Asian dust (particle diameter 2-3 mm) was evident throughout the West Coast and extended to Minnesota. According to the chemical aerosol records, the impact of the April 1998 Asian dust event was 2-3 times higher then any other event since 1988. The Asian dust event was observed and interpreted by an ad-hoc international web-based virtual community of researchers. It would be useful to set up Web-based system to monitor the global aerosol pattern for extreme aerosol events, inform the interested communities through alerts and to facilitate the collaborative analysis.
1 Center for Air Pollution Impact and Trend Analysis, Washington University, St. Louis, MO, USA
2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
3 Atmospheric Sciences, University of Washington, Seattle, WA, USA
4 Department of Geosciences, Texas Tech University, Lubbock, TX, USA
5 Pacific Northwest National Laboratory, Richland, WA, USA
6 National Satellite Meteorological Center, China Meteorological Administration, Beijing, P.R. China.
7 U. S. Geological Survey, USA
8 Meteorological Research Institute, Seoul, South Korea.
9 Naval Research Laboratory, Monterey, CA, USA
10 NASA Goddard Space Flight Center, Greenbelt, MD, USA
11 University of Central Florida, Orlando, FL, USA
12 Department of Geography, University of British Columbia, Vancouver, Canada.
13 Scripps Institution of Oceanography, University of San Diego, La Jolla, CA, USA
14 Center for Atmospheric Chemistry Studies, University of Rhode Island, Kingston, RI, USA
15 Air Quality Bureau, New Mexico Environment Department, Santa Fe, NM, USA
16 Solar Monitoring Laboratory, University of Oregon, Eugene, OR, USA
17 Department of Physics, Tokyo University of Mercantile Marine, Tokyo, Japan.
18 Euro Mediterranean Centre on Insular Coastal Dynamics (ICoD), University of Malta, Malta.
19 National Center for Environmental Assessment, US EPA, Research Triangle Park, NC, USA
20 Department of Meteorology, University of Utah, Salt Lake City, UT, USA
21 National Institute for Environmental Studies, Tsububa, Iburaki, Japan
22 NPS Air Resources Division, CIRA, Colorado State University, Ft. Collins, USA
Extreme biogeochemical events such as volcanic eruptions, forest fires, and dust storms provide unique opportunities to examine the inner workings of the atmospheric system. Such events tend to produce large quantities of dust, smoke, or haze, which is then dispersed over regional or global scales. The easily observable atmospheric particles can visualize and quantify the nature of transport, transformation and removal processes along their path. An early example for aerosols as visualizers of global transport was the discovery of the global circulation of the atmosphere after the Krakatoa volcanic eruption in 1883 [Royal Society, 1888]. The volcanic stratospheric aerosol and red sunsets observed all around the globe led to the recognition of the organized global circulation pattern. Clearly, the atmospheric particles from these events are not mere neutral tracers and visualizers, but are also major carriers in the biogeochemical cycle of sulfur, nitrogen, carbon and trace metals, as well as crustal elements. Last but not least, dust, smoke and haze particles produced by these processes influence climate and weather, oceanic and terrestrial fertility, and impact public health.
The eruption of volcanoes, major forest fires and large dust storms are often associated with catastrophic consequences to humans and their environment, but the prediction of their specific occurrence in space and time is virtually impossible. The unpredictability of these events also presents a unique challenge to atmospheric research, since sustained readiness for intensive field campaigns is almost impossible to maintain. Hence, the study of such events generally has to rely on the integration of routine monitoring data and serendipitous observations.
The transport of desert dust from Asia to the North Pacific atmosphere is well documented [e.g. Shaw, 1980; Duce et al., 1980; Parrington et al., 1983; Uematsu et al., 1983; Merrill et al., 1989; Bodhaine, 1995; Husar et al., 1997] and results in a maximum in aerosol loading each spring. Over the Pacific, the concentration of species from anthropogenic sources in Asia was also found to be enhanced during spring [Prospero and Savoie, 1989; Jaffe et al., 1997; Talbot et al., 1997] and have been documented to reach North America [Jaffe et al., 1999].
Compelling geological evidence of global scale transport of Asian dust emerged from the chemical analysis of samples in the Greenland ice core [Biscaye et al., 1997] and Hawaiian soil studies [Rex et al., 1969; Dymond et al., 1974; Kennedy et al., 1998; Chadwick et al., 1999]. The chemical and radiological fingerprints of deposited dust at both locations were most consistent with the composition of the Asian dust sources.
This paper reports the formation, transport and other characteristics of two dust storms in April 1998. The focus is on the dust storm that occurred on April 19, 1998 over Mongolia and North-Central China which crossed the Pacific causing aerosol concentrations near the health standard (150 m g/m3) over much of the West Coast of North America.
Several observers in Asia and North America monitored the unusual dust cloud independently. In Japan for example, an integrated lidar monitoring network operated in anticipation of the springtime Kosa season [Murayama et al., this issue]. At CAPITA, Washington University, the dust cloud was detected as a distinct yellow cloud on the daily SeaWiFS color satellite images using the web-based NASA Goddard SeaWiFS Global Browser facility. The dust was also observed over the western Pacific on the GOES 9 geostationary satellite images [Bachmeier, 1998].
When it was evident that the Asian dust cloud was reaching North America, an interactive website was set up on April 26 at CAPITA, Washington University (http://capita.wustl.edu/Asia-FarEast/) to register various observations, exchange opinions and to support general communication and data sharing (cooperation) among the interested observers. By April 29, through broadcast alerts and word of mouth, over 40 scientists and air quality managers from North America and Asia had registered with the ad-hoc workgroup, and began sharing their data and qualitative observations with the spontaneously formed virtual community. Most participants maintained their shared data and preliminary reports on their own web pages but the CAPITA website supported a user-maintained central catalog of web resources along with an open discussion forum. The first phase of the virtual workgroup activity was completed with a preliminary summary of the dust event that was web-published on May 11, 1998. In December 1999, the virtual workgroup was re-activated to produce this research paper. Given the broad and multifaceted nature of this report, the observations are presented chronologically following the evolution of the two dust clouds. This JGR issue contains several other papers prepared independently on various aspects of these dust events.
The dust events were observed through routine satellite sensors, lidar instruments, sun photometers, airborne samplers, and a large array of surface-based aerosol monitors on both sides of the Pacific.
Satellite remote sensing was crucial in detecting the dust clouds, following their evolution as well as characterizing the spatial, temporal, and optical characteristics of the moving dust clouds. Routine quantitative techniques for the retrieval of aerosol properties over land from satellite data are not yet available, but it is an area of active research [King et al., 1999]. For this reason, most of the satellite data are used here as semi-quantitative indices.
Spectral images of the dust cloud from the SeaWiFS sensor [McClain et al., 1998; Barnes et al., 1999] provided detailed spatial and spectral pattern of the dust at about local noon each day. On the SeaWiFS images the dust cloud was recognized by its bright yellow color, partial transparency and smooth spatial texture. The raw (Level 1A) SeaWiFS data were obtained from the SeaWiFS Program [McClain et al., 1998] and processed at Washington University. The resulting spectral reflectance values (fraction of radiation reflected) represented the combined reflectance from the land, clouds and the aerosol. The scattering by air molecules was removed from the total reflectance using the Vermote and Tanre  procedure, which also included nominal corrections for ozone and water vapor absorption.
The on-line, hourly, GMS-5 geostationary satellite data with visible and IR channels were used for near continuous monitoring of the dust plume dynamics throughout the day. Visible images from geostationary satellites (GOES 8, GOES 10, GMS-5) were effective for dust detection during the twilight hours when the sun was in front of the sensor. At dawn or dusk, the reflectance from the ground surface is minimal while the reflectance from the aerosol is still high. This, along with forward scattering from the sun, tends to produce a strong amplification of the aerosol scattering signal.
During the post-analysis of the dust episode, operational retrievals of absorbing aerosol index from the TOMS satellite [Herman et al., 1997] provided useful information on the daily spatial distribution of the dust cloud. The TOMS aerosol absorbing signal is a semi-quantitative index of the columnar absorption by aerosols at 0.340 m m. The signal is derived from the absorption of the upwelling Rayleigh scattering in the lower strata of the atmosphere. The April 1998 dust clouds over Asia were also detected using the AVHRR sensor by the ‘split window’ method, taking the difference between the 11 and 12 m m channels [Murayama et al., this issue].
The vertical optical thickness was detected by sun photometers as part of the AERONET network in Asia and in North America [Holben et al., 1998], by the Solar Radiation Monitoring Laboratory, University of Oregon [Gueymard et. al., 2000] and by a number of other sun photometer networks.
The vertical dust profiles were monitored by lidar instruments on both sides of the Pacific. The passage of the April 15 dust cloud over Japan was monitored by a coordinated network of lidar instruments [Murayama et al., this issue] sited throughout Japan, augmented by sun photometers and surface concentration measurements. The dust cloud was detected over North America as part of the routine long-term monitoring program of the aerosol vertical structure conducted by the CO2 backscatter lidar facility at the Jet Propulsion Laboratory [Tratt and Menzies, 1994]. Timely alerts provided by the ‘virtual community’ through the website allowed normal operations at this facility to be optimally focused on observation of the extreme dust event. Lidar observations of the Asian dust cloud were also reported from the University of Utah as part of their cirrus cloud monitoring program [Sassen, 1997].
The detailed physical, chemical and optical characteristics of the April 1998 dust aerosol were recorded by surface-based in-situ measurements and aircraft samplings. Daily visibility observations around the Gobi Desert provided an indication of reduced visibility due to the dust as well as the cause of the obstruction to vision, i.e. dust [NCDC, url, 1998].
Continuous aerosol size distribution data using an optical counter were reported from Korea [Chun et al., 2000]. More than 200 PM10 samplers were located throughout the West Coast states of North America. These samplers collected particulate mass less than 10 mm in size, some operating hourly, others collecting 24-hour samples [US EPA, 1994]. Full network sampling was conducted every sixth day with limited (25-30 samplers) coverage during the intervening periods. The composition of the dust aerosol for chemical fingerprinting was captured in great detail by over fifty stations of the IMPROVE network that sampled twice a week, Wednesdays and Saturdays [Malm et al., 1994].
Dust clouds are formed when the friction from high surface wind speeds (> 5 m/s) lifts loose dust particles into the atmospheric boundary layer or above [Gillette, 1978]. Wind blown dust originating from the arid deserts of Mongolia and China is a well-known springtime meteorological phenomenon throughout East Asia. In fact, "yellow sand" meteorological conditions are sufficiently common to have acquired local names: Huangsha in China, Whangsa in Korea, and Kosa in Japan. The Asian dust storms have been studied for decades to understand their sources, mechanisms of transport, and aerosol characteristics, including the effects on radiation. [Mizohata and Mamuro, 1978; Zhou et al., 1981; Wang et al., 1982; Iwasaka et al., 1983; Zaizen et al., 1995; Zheng et al., 1998; Zhang and Lu, 1999]. However, quantitative understanding of individual dust events, e.g. the dust emission locations and rates as well as the details of long-range transport and removal, are still incomplete.
In order to gain a broader perspective on the formation of the Gobi dust clouds in April 1998, the daily time series of three measured indices of the dust content were examined over the regions shown in Figure 1a. Figure 1b depicts the time series of (1) The TOMS satellite data averaged over the Gobi Desert as a regional index of total suspended dust. (2) The average surface extinction coefficient based on eight synoptic visibility monitoring stations is a measure of the ground-level dust concentration in the Gobi region. (3) The aerosol optical thickness measured at Dalamzagdad, Mongolia shows quantitatively the total dust extinction in the vertical column, but it is confined to a single location within the Gobi source region. Figure 1c shows TOMS aerosol index over the much larger East Asia region, while Figure 1f presents the concentration of suspended dust particles (diameter < 10 m m) aggregated over East Asia based on the dust model by Westphal .
The two peaks in the daily time series of the aerosol concentration indices over the Gobi Desert region (Figure 1b and d) convey consistently that two major dust storms occurred on April 15 and 19, respectively. Furthermore, the TOMS data (Figure 1c) and the model simulations of suspended dust averaged over East Asia (Figure 1e) indicate that the April 19, 1998 storm was the more intense event causing higher regional average dust concentrations. In what follows, the two dust events are examined in more detail including a comparison of the two storms.
Analysis of meteorological data and modeling results [Nickovic et al., this issue; Westphal, 2000] indicate that the formation of the April 15 was associated with cold weather systems with high surface wind speeds. The April 15 storm was evidently caused by a "cut-off" low-pressure system, which is characterized by strong subsidence within the system and tends to trap the dust near the surface [Murayama et al., this issue].
The most detailed spatial distribution of the dust was obtained from the 1 km resolution SeaWiFS satellite images. Figures 2a-c depict the dust pattern on April 15, 16, and 17, respectively. The spectral reflectance images from SeaWiFS provide a rich visual context, including surface reflectance and the position of cloud systems relative to the dust. For comparison, the contour of the TOMS absorbing aerosol index (green line, aerosol index=2) was superimposed on the spectral SeaWiFS images.
On the satellite images depicting the April 15 cloud (inset in Figure 2a), the sources can be identified as streaks of dust plumes originating from specific patches of land, presumably from sand dunes or loess not covered by vegetation. Dust plumes are evident on both sides of the Mongolia-China border in the Gobi desert. After about 500 km of transport, the plumes merged and the streaky plume structure disappeared.
The April 15, 1998 dust cloud followed a southern route toward central and eastern China and subsequently turned toward Korea to the north. The location of the dust plume on April 15, 16 and 17 is visualized using the superimposed SeaWiFS and TOMS data in Figures 2 a, b and c, respectively. On April 16, the dust plume reached the populated eastern seaboard of China between Beijing and Shanghai (Figure 2b). The inset in Figure 2b also illustrates in more detail that even after about 1000 km of transport from the Gobi desert, the dust plume has retained considerable spatial texture. Also, at the leading edge, the dust cloud is delineated by a sharp front, while the dust level in the tail section is decaying more gradually. By April 17, the dust cloud shows the counterclockwise spiral features of the driving low-pressure weather system. In fact, the highest dust reflectance is adjacent to the center of the system which indicates dust entrainment into the precipitation cloud system over Korea.
The superposition of the TOMS and SeaWiFS data in Figure 2a indicates that on April 15 the dust pattern from TOMS and SeaWiFS did not coincide geographically. This is an indication that the fresh dust layer was near the ground where the TOMS sensor is less sensitive to dust. This is consistent with the lidar data for the April 15 event collected over Eastern China, Korea and Japan [Murayama et al., this issue] indicating a generally well-mixed dust layer from the surface to about 3 km. On April 16 and 17 (Figures 2b and c), the spatial patterns of the SeaWiFS and TOMS signals coincide. On the other hand, the TOMS data indicate that on April 19, (Figure 2d) remnants of the April 15 dust cloud were present over the Yellow Sea and Korea but the SeaWiFS data show virtually no excess reflectance over the same location.
The dust cloud dissipates when the particles are removed from the atmosphere by dry and wet removal processes. Gravitational settling of large particles (>10 m m) occurs near the source within the first day of transport. Wet removal occurs sporadically throughout the 5-10 day lifetime of the remaining smaller size dust particles. The available data do not establish how and where the Asian dust cloud is ultimately dissipated, i.e. removed from the atmosphere. However, various dust aerosol observations allow drawing of a limited number of inferences about the removal processes that affected the dust.
Throughout much of its residence over East Asia, the April 15 dust cloud was either embedded in or near a precipitating low pressure system as illustrated in Figure 2 a, b, and c. As a consequence, the dust cloud appears to have been strongly depleted by wet removal processes throughout much of its residence over East Asia. On April 16, Beijing was under a thick cloud cover but yellow dust was visible just to the south (see inset in Figure 2b). On April 16 and 17, the newspapers in Beijing, China also reported yellow muddy rain [Li, 1998]. Yellow muddy rain with relatively high pH value (6.22) was reported on April 17 in Korea [KMRI, 2000]. However, detailed rain chemical composition data for the April 15 dust events are not available at this time.
The dust size distribution measured on Anmyon Island, Korea on April 19 evidently characterized the aged dust emitted on April 15. The measured volume size distribution function (inset in Figure 2d) shows a sharp peak between 1-5 m m, with a volume-mean diameter of 2 m m, and a logarithmic standard deviation of 1.6 [Chun et al., 2000]. Continuous monitoring of particle concentration in different size ranges exhibited a strong correlation between the particles in the dust peak size range (2-3 m m) and virtually no correlation with particles below 0.8 m m and above 10 m m. Hence, the size ranges below 0.8 and above 10 m m have different origins than the coherent dust size range between 1-10 m m [Chun et al., 2000]. The absence of transported large particles implies that the dust particles above 10 m m were preferentially removed by gravitational settling during the 2-3 day atmospheric transport time from Gobi to Korea. Based on the meteorological conditions and satellite images it is presumed that the April 15, 1998 dust cloud was removed from the atmosphere over East Asia without substantial transport across the Pacific.
Analysis of meteorological data and modeling results [Nickovic et al., this issue; Westphal, 2000] indicate that on April 19 a major storm swept through Mongolia and North-Central China. The dust storm was driven by a low-pressure cold front that entered western Mongolia and swiftly moved eastward. On April 19, the surface wind speeds increased to over 20 m/s as shown in the wind speed contours on Figure 2d. This was well above the generally assumed threshold wind speed (5-6 m/s) for dust suspension [Gillette, 1978]. The region of high wind speeds coincided with the Gobi Desert in southern Mongolia and the loess plateau in the Gansu province of China where on April 15 the dust plumes were seen in the SeaWiFS data (Figure 2a). However, on the April 19 image (Figure 2d), the individual dust plumes are not apparent. Rather, a dense dust front is clearly visible at the leading edge of the dust cloud.
A superposition of the TOMS and SeaWiFS data in Figure 2d indicates that on April 19 the dust pattern from TOMS and SeaWiFS coincides geographically. This implies that the dust layer was higher than on April 15, since in this case the TOMS sensor detected it. This is supportive of the notion that a low-pressure front produces a deeper dust layer than a subsiding "cut-off" low-pressure system [Murayama et al., this issue].
The transport of the April 19 dust cloud was due east over eastern Mongolia, toward the Pacific. Figures 2 d, e and f show the position of the dust cloud on April 19, 20 and 21, respectively. By April 20 (Figure 2e), the leading edge of the dust cloud reached the Pacific and by April 21 (Figure 2f) the yellow dust cloud was stretching over 1000 km into the Pacific Ocean.
The location of the dust cloud was also observed in surface visibility observations reported by the global synoptic network. On April 19, the visibility was reduced throughout central and eastern Mongolia as indicated by the blue circles in Figure 2d. Over 30 visibility stations reported dust as the cause for the obstruction to vision.
The vertical optical thickness of the suspended dust measured in Dalanzagdad, Mongolia [Holben, 1999] indicated that the turbidity increased from t < 0.5 on April 18 to t > 2 on April 19 as the dust cloud passed by. Also, the measured spectral optical thickness was virtually constant between 0.4 and 1.02 m m (a ~ 0) [Holben, 1999], which indicates a characteristic size in excess of 2 m m.
In the SeaWiFS reflectance data (Figure 2) the dust is recognized by its distinctly yellow color. The inset in Figure 2d shows the spectral reflectance function over soil with and without the dust cloud. Evidently, the presence of the thick atmospheric dust increases the soil reflectance more in the red (from 0.25 to 0.55) than in the blue (from 0.05 to 0.3). As a consequence, the dust appears brighter and more yellow then the underlying soil.
Dust layers were also frequently observed above low-lying white clouds, imparting a yellow hue to the clouds. It is presumed that the yellow coloration is due the scattering and absorption of the superimposed dust layer. The inset in Figure 2d shows the spectral reflectance of clouds with and without the dust layer. The 'white' cloud had a spectrally flat reflectance of about 0.65 while the 'yellow' clouds show a similar reflectance at 0.67 m m but only 0.35 at 0.412 m m. The resulting spectral reflectance curve for the 'yellow' clouds is similar but somewhat higher than the reflectance of dust over soil.
The dust also appears yellow in color over the dark surface of the cloud-free ocean as shown in the inset of Figure 2f. Near the dust source over the China Sea and Western Pacific the excess dust reflectance over the ocean is in the range 0.1-0.2 above 0.5 m m wavelength but it drops sharply below that wavelength. In fact, even a thick dust layer reflects only about 7% of the incoming solar radiation at 0.412 m m. This sharp drop in dust reflectance and the other dust-induced optical phenomena elude plausible explanation, but served well for dust visualization. The spherical Mie scattering assumption with constant imaginary refractive index is clearly inappropriate, but the relative roles of particle spectral absorption in the near UV and of irregular particle shape are not clear.
Once the dust cloud reached the Pacific Ocean, it was carried by the westerly winds that are typical for the northern mid-latitudes (30-60 ° N) in the springtime. The sinuous dust transport path across the Pacific was visualized on the SeaWiFS images by the dust itself, which appears as a streaky yellow dye (Figure 3). It is evident that during the trans-Pacific passage, the dust cloud was stretched longitudinally. Also, large segments of the dust cloud were peeled off and transported northward into the Arctic reducing the amount of dust reaching North America. Model simulations [Westphal, 2000; Nickovic et al., this issue] also show such piece-wise disintegration of the dust cloud. It is remarkable that the dust cloud reached the West Coast of North America within 5-6 days following the emissions, corresponding to 12 m/s average transport speed.
As a reference, Figure 3 also shows the contours of the springtime seasonal average (March-May) aerosol optical depth over the Pacific Ocean derived from the AVHRR sensor (Husar et al., 1997). The superposition shows that the April 98 dust cloud trajectory roughly coincided with the seasonal average seasonal aerosol plume emanating from East Asia.
The height at which the dust traversed the Pacific is not well documented. The yellow discoloration of the clouds near the coast of China on April 21, Figure 2 f, indicates that at least part of the dust layer is above the low-lying white clouds. Similar qualitative observations over the Pacific also indicate that some of the dust was above the cloud layers. The fast (>12 m/s) trans-Pacific transport indicates that the dust layer must have been well above the marine boundary layer. Unfortunately, the April 19 dust storm passed to the north of the East Asian lidar network, and lidar profiles are not available.
The earliest record of the arrival of the April 19 Asian dust to North America was late on April 24 by the lidar at the Facility for Atmospheric Remote Sensing in Salt Lake City, UT. The dust layer detected at Salt Lake City occurred somewhat before the arrival of the main dust cloud on April 25. The leading edge of the dust cloud observed at Salt Lake City was optically too thin to be detected in the satellite imagery, which was seen on April 25 just to the west. The vertical aerosol profile detected by the ruby (0.694 m m) polarization lidar [Sassen, 1997] shows a distinct aerosol layer at about 7.5 km altitude (Figure 4). The maximum depolarization of ~0.2 in the elevated aerosol layer is unusually high for this location and indicates the presence of super-micron sized non-spherical particles of at least 1-2 m m in size [Mishchenko and Sassen, 1998]. The elevated dust layer was noticeable from the ground, produced a solar aureole, and at times resembled very thin cirrus.
The main dust cloud arrived at North America on April 25, 1998. The most noticeable impact of the dust was the discoloration of the sky. Human observer reports and digital photographs indicate that from April 25 onward, the normally blue sky appeared milky white throughout the non-urban West Coast. This effect is due to the redistribution of the direct solar radiation into diffuse skylight. The redistribution effect was well documented through numerous direct/diffuse solar radiation measurements. For example, during the five-day dust event in Oregon, there was a 25-35% decrease in direct normal solar radiation (Figure 5), although most of the loss from the direct solar beam was still reaching the ground as diffuse skylight. It is quite remarkable, however, that the total broadband radiation reaching a horizontal surface during the dust event was reduced only by about 2%, leaving only 2% for aerosol absorption and backscattering to space. This is consistent with SeaWiFS reflectance data, which indicate that the increase of the surface albedo near Eugene, OR from the dust-free day on April 20 to the dusty day April 27 was below 2%. More extensive analysis [Gueymard et. al., 2000] shows that on April 27 the noon direct irradiance at Eugene, OR, Burns, OR and Boise, ID was reduced by 28, 31 and 31 percent respectively, substantially diminishing the solar energy available for concentrating solar collectors. The noon global horizontal irradiance was also reduced by 34 W/m2 (3.8%), 39 W/m2 (4.1%) and 45 W/m2 (5.1%) respectively at these sites, compared to the dust-free conditions.
The arrival of the main dust cloud to North America was evidenced by the sun photometer data at Reno, NV and San Nicolas Island in Southern California. At Reno, the aerosol optical thickness (0.525 m m) rose sharply on April 25 and remained high (0.3 < t <0.5) until April 29, compared to t < 0.1 on the preceding days [Du Bois, 2000]. The optical depth also increased at San Nicolas Island on April 25 reaching a peak value of t = 0.5 at 0.500 m m. [Tratt et al., this issue]. During the same period over the Pacific Northwest the aerosol optical depth was in the range 0.4< t <0.5 [Laulainen et al., 2000].
During the dust event, the slope of the spectral optical depth, i.e. the Angstrom exponent, at the San Nicolas Island and the Pacific Northwest was below 0.5, which is indicative of larger characteristic particle size. Based on the inversion of additional sun and sky radiance measurements at San Nicolas Island, Tratt et al. [this issue] reported coarse mode diameter of 2-4 µm for the dust. The entire retrieved dust volume distribution function is show in Figure 2d along with the data from Korea. Aircraft measurements through the dust layer near Richland, WA on May 1, exhibited a similar volume distribution [Laulainen et al., 2000]. Size segregated dust samples at numerous remote locations over the Northwest and adjacent Canada showed that about 30-50% of the dust mass was below 2.5 m m [McKendry et al., this issue]. These size estimates all indicate that the aged Asian dust arriving at the West Coast had a mass median diameter of about 2-3 m m.
A variety of observations over North America indicated a layered structure of the Asian dust cloud. Lidar backscatter data from the Jet Propulsion Laboratory in Pasadena, CA (Figure 4b) show the vertical aerosol profiles during the dust event on April 27 and after the event on May 1 [Tratt et. al., this issue]. On April 27, the dust layer was observed between 6 and 10 km altitude and the lidar backscattering was more than two orders of magnitude above the nominal prevailing background values.
Serendipitous aircraft aerosol sampling on April 27 near Seattle, WA showed a distinct dust layer at about 2-3 km altitude [Gassó, 1999] and virtually no dust below. On the other hand, a subsequent aircraft sounding over eastern Washington State on May 1 indicated a surface-based dust layer up to about 2 km [Laulainen et al., 2000]. Aerosol Optical Depth (AOD) measurements at two elevations (1088m and 100 m) are consistent with an aerosol scale height of about 2 km.
In southern British Columbia, the PM10 levels increased dramatically to ~100 µg/m3 in the southern interior of the province on April 28. To the west, concentrations peaked in the Lower Fraser Valley (Vancouver region) on April 29 and then further west on Vancouver Island on April 30. This pattern is consistent with mesoscale modeling of the event [McKendry et al, this issue] showing strong subsidence and downward mixing of the dust layers over the mountainous interior and then westward surface transport of dust in "outflow" winds. Monitoring of aerosol light scattering at the Cheeka Peak Observatory in Washington State also showed the arrival of the dust cloud on April 28 with easterly flow [Jaffe, 1999]. Based on analysis of elemental composition and meteorological analogues, it was estimated that Asian dust contributed 40-50% to peak observed PM10 levels in the Vancouver area [McKendry et al, this issue].
The visible channel of the GOES 10 geostationary satellite provided a view of the dust spatial distribution on the evening of April 27 at about 6 pm PST (Figure 6). Evidently, the subsided dust cloud covered the entire west coast of North America from California to British Columbia and a wedge shaped region of the northwestern US and adjacent Canada extending well into the center of the continent.
Extensive PM10 monitoring data [US EPA, 1994] over the U.S. West Coast provide a relatively detailed temporal pattern and a spatial map of the surface dust concentrations (Figure 7). Daily PM10 concentrations were averaged over 230 AIRS stations throughout the West Coast every sixth day, and over 20-30 stations for the in-between days. The time series show a strong peak between April 26 and May 1 (Figure 6c). During the dust incursion, the regional average PM10 concentration reached 65 µg/m3 compared to 10-25 µg/m3 during the remainder of the April-May 1998 period. This suggests that the excess dust concentration over the West Coast reached about 40 µg/m3.
On April 29, the contour map shows that the PM10 concentration over the low-lying areas of California, Nevada and Idaho also experienced concentrations well above 50 µg/m3. The PM10 levels over parts of Washington and Oregon exceeded 100 µg/m3 (Figure 6b). This patch of high dust concentration coincided with the bright aerosol reflectance in the GOES satellite image for April 27. The spatial coincidence of satellite and surface data confirms that by April 29 the dust layer had subsided to the surface.
The chemical composition of the sampled dust over North America shows that the dust is composed of crustal elements with constant elemental ratios throughout the episode [McKendry et al., this issue]. Aerosol samples collected by aircraft over Washington State on May 1 were non-volatile and refractory in nature [Laulainen et al., 2000], which is consistent with crustal particle composition. The chemical composition of the Asian dust was also established through the IMPROVE monitoring network data (Figure 7). Based on specific dust elemental signatures [Malm et al., 1994], the speciated aerosol data revealed the pattern of dust without interference from other local aerosol contributions, such as sulfates, organics, soot, and nitrates.
On April 25, the western U.S. was virtually dust-free as confirmed by the AIRS PM10 (Figure 6) and the IMPROVE data (Figure 7a). This means that the rise in the optical depth detected by the sun photometers at Reno and San Nicolas Island was exclusively due to elevated dust layers well separated from the surface.
The dust concentration on the surface reached high levels on April 29, particularly in the Northwest (Figure 7b). By May 2, the dust levels along the coast declined and the elevated dust concentrations were highest over the Rocky Mountains and the Colorado Plateau (Figure 7c). By May 6, the dust levels had declined throughout most of the western U.S. Photographs and observations from southeastern Utah taken on May 3-4 also recorded the progress of "strange haze" across the Colorado Plateau, moving from northern Arizona into western Colorado [Reheiss, 1998]. However, this dust transport to the interior of North America has not been confirmed by detailed meteorological analysis.
The IMPROVE network data over North America show that the Asian dust chemical components are not correlated with particulate sulfur during the dust event. In fact, the samples with the highest dust concentrations have virtually no sulfur content. Aircraft sampling of the dust near Richland, WA on May 1 showed that the aerosol had no volatile components up to 300şC, which is consistent with a purely soil derived chemical composition of the dust [Laulainen et al., 2000]. Thus, based on the above crude observations there is no evidence that the April 19 dust cloud had significant chemical transformations due to interaction with pollutant species, such as sulfur and nitrogen.
A unique aspect of the April 19 dust cloud is that there was no evidence of significant wet removal neither during its residence over the Asian continent, or during the trans-Pacific transport. This is inferred from the lack of major precipitating cloud systems over East Asia and the Pacific during the passage.
The partial dissipation of the dust cloud over the West Coast of North America was facilitated by the subsidence of the dust layer from the mid-troposphere to the surface, between April 27 and 29 [McKendry et al., this issue]. In British Columbia, a zonally-oriented jet core to the north of Vancouver helped generate substantial mountain waves and strong subsidence (~0.05m/s) in the lee of the coast mountain range and the Rocky Mountains. Rapid downward transport permitted interception of dust layers by surface based mixing and removal processes and then coastward transport by easterly surface winds. As a result of such processes, high surface level dust concentrations, lasting from April 27 until May 1, were observed over much of the northwestern U.S. and adjacent Canada.
Hourly PM10 concentration data averaged over 12 stations in Northern California show a strong and consistent diurnal cycle throughout the Asian dust episode between April 26 and May 2 (Figure 8a). Following the sharp concentration rise on April 26, there was a daily modulation with a peak of about 55 m g/m3 during the daytime hours (7AM-7PM) and a decline to 35 m g/m3 at night. The diurnal modulation is evidence for the presence of a stable dust reservoir that remained aloft in the boundary layer for six days and was well mixed to the surface during the daytime. Possibly, the surface concentrations declined at night, since particles were removed by dry deposition within the shallow nocturnal layer slowly depleting the dust reservoir aloft. [Wilson and Stockburger, 1990].
The ultimate fate of the entire dust cloud reaching North America is not known but there is evidence that part of the dust was transported eastward across the continent. Near the US-Canadian border, streaks of dust indicate swift transport toward the upper Midwest and Ontario (Figure 6). A second stream turned toward the south, blanketing the West Coast from British Columbia to California. The stream of Asian dust was visually and chemically detectable to Minnesota, and it is probable that its eastward transport was unhindered until the dust reached the 3000 m plateau of Greenland, where much of the high latitude precipitation takes place. These observations of dust transport support the Asian origin of the dust deposited in the Greenland ice sheet over geological times [Biscaye et al., 1997].
The remarkable high dust concentrations over North America reported here raise the question of frequency at which such trans-Pacific dust transport events take place. Evidently, the April 1998 Asian dust transport to North America was a rare event. Figure 9 shows the daily PM2.5 dust concentration at three IMPROVE monitoring sites for which long-term records exist: Mt. Rainier, WA, Crater Lake, OR, and Boundary Water, MN. The total dust concentration was not measured but based on the characteristic dust size of 2-3 m m, it can be estimated that the total dust mass concentration was 2-3- times higher then the PM2.5 dust concentration.
At each site, the fine particle dust concentration was variable but generally below 5 m g/m3 throughout the decade of 1988-1998. The exception is the sharp dust peak at each of these sites reaching 5-10 m g/m3 on April 29, 1998. Hence, the April 1998 Asian dust event was more intense than any other dust incursion to the western U.S. over the past decade.
The Asian dust storms of April 1998 had major consequences to human health and welfare. According to CNN, on April 15, twelve people perished in the dust storm in the Xinjiang Autonomous Region of China alone. It is likely that this natural disaster has caused additional casualties along its path. Yellow muddy rain, reported over eastern China and Korea, produced an economic loss of unknown magnitude.
When the dust cloud reached British Columbia, Washington, Idaho and Oregon, the respective state health agencies issued air pollution advisory warnings to the general public with a corresponding ban on open prescribed burning. In making their decisions and informing the public, the state air pollution regulatory agencies actively participated in the virtual community by supplying and using information on the Asian Dust website.
Such dust events have both research and regulatory implications. With a measured mass mean diameter of 2-3 : m, the dust will influence the fine PM2.5 concentration as well as PM10 concentrations. This is important since there is increasing information to suggest that, at least for some health effects categories, fine particle mode particles are more likely to cause acute health effects than coarse-mode soil particles [Schwartz et. al., 1997; Schwartz et. al., 1999]. Therefore, episodic intrusions of soil dust need to be identified not only to allow waivers for possible exceedances of the PM2.5 and PM10 standards but also to correct PM2.5 time series for use in correlation of fine-mode particles with health effects.
The available routine and the limited serendipitous dust observations used in this paper (e.g., mass loading, particle size and chemical composition, aerosol optical thickness, dust layer height) have been useful for elucidating some features of the two Asian dust events in April 1998. However, a firm and full quantitative characterization of these events, particularly the physico-chemical processes, was not possible since many of the necessary measurements were not made. Also, it is not clear how representative the April 1998 events were. The planned ACE-Asia studies (http://saga.pmel.noaa.gov/aceasia) beginning in 2000 are expected to provide a rich characterization of east-Asian aerosol properties over time and above the surface, and to better quantify the transport, transformations and fate of these aerosols.
The Asian dust event has demonstrated that the currently available spaceborne and surface aerosol monitoring can enable virtual communities of scientists and regulatory bodies to detect and follow such major aerosol events, whether resulting from fires, volcanoes, pollution or dust storms. It has also been shown that ad-hoc collaboration of scientists is a practical way to share observations and to collectively generate the explanatory knowledge about such unpredictable events. The experience from this event could also help in more effectively planning disaster mitigation efforts, such as an early detection, warning and analysis system.
Additional work on the April 98 dust events could include (1) organizing the available data into a documented and shared resource base (2) a coordinated global dynamic aerosol model validation and testing program; (4) additional collaborative data integration and fusion and analysis. A web-based communication, cooperation and coordination system would be useful to monitor the global aerosol pattern for unusual or extreme aerosol events. The system would inform the interested communities through alerts, so that the detection and analysis of such events is not left to serendipity. It is envisioned that such a community-supported global aerosol information network a) be open to a broad international participation; b) complement and synergize with other monitoring programs and field campaigns and c) support the scientific as well as the air quality and disaster management communities.
Acknowledgements. The authors thank the virtual community that assembled on the dust website for sharing their observations and ideas. In particular, the assistance of Larry Altose, Air Quality Program, Washington State DNR, Steven Sakiyama, British Columbia Ministry of Environment, Lands and Parks, ARB and State of Idaho DEQ are gratefully acknowledged. During the dust event, Maja Husar maintained the CAPITA Asian Dust website and Janja Husar contributed to the preparation of the manuscript. We also thank one of the reviewers for suggesting a chronological organization of the paper. Portions of this work were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The CAPITA part of this research was funded in part by the US Environmental Protection Agency (EPA) through CX-825834 (OAR-OAQPS). Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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