The Asian Dust Events of April 1998
R. B. Husar, D. M. Tratt, B. A. Schichtel, S. R. Falke, F. Li D. Jaffe, S. Gassó, T. Gill, N. S. Laulainen, F. Lu,
M.C. Reheis, Y. Chun, D. Westphal, B. N. Holben, C. Gueymard, I. McKendry, N. Kuring, G. C. Feldman, C.
McClain, R. J. Frouin, J. Merrill, D. DuBois, F. Vignola, T. Murayama, S. Nickovic, W. E. Wilson, K. Sassen,
N. Sugimoto, W.C. Malm
The full paper to appear the JGR Special Issue on Dust.
Asian Dust Website: http://capita.wustl.edu/Asia-FarEast
Analysis of the 1998 Dust Storms by a ‘Virtual Community’
• On April 15 and 19 1998, dust storms in the Gobi Desert
have produced unusually large dust clouds, some of
which was transported across the Pacific.
• When it was evident that the dust cloud was reaching
North America, an interactive website was set up to share
observations, and ideas. By April 29 the ad-hoc ‘virtual
workgroup’ consisted of over 40 scientists and air quality
managers from North America and Asia.
• This work was produced by the virtual community and
summarizes the formation, transport, dissipation and other
features of the the two dust events.
• The full paper is being published in the Journal of
Geophysical Research, Special Issue on Dust., edited by
Dust Storms in the
Gobi Desert on April
15 and 19, 1998
• Daily measurements of
surface visibility, aerosol
optical depth, TOMS data and
SeaWiFS images for the Gobi
desert, show that major dust
storms occurred on April 15th
and April 19th.
• The April 19th storm had
larger impact on the East Asia
• Model simulations of dust
production and the dust
pattern correspond to the
The April 15th Dust Storm: Dissipation within Asia
Fast surface winds ( > 20 m/s) over the After 1000 km transport from Gobi to Shanghai, the
Gobi desert generated individual yellow dust cloud has retained considerable spatial
dust plumes as seen from the texture. The TOMS absorbing aerosol index data
SeaWiFS reflectance data. (green lines - index=2) and the SeaWiFS image
After about 500 km transport, the plumes show similar pattern over Eastern China.
merged into a dust cloud The April 15th dust was ingested and removed by a
precipitating low pressure system. Yellow muddy
rain was reported from Beijing on April 16-17.
The Cause of Dust Storms:
Low Pressure Systems over Gobi Desert
On both days, April 15 and 19, the high surface wind speeds (>20 m/s) were caused
by extreme pressure gradients between the low and the adjacent high pressure
The April 19th Dust Storm
The surface wind was > 15 m/s and surface visibility The dust layer increases Size distribution data and
reduction was due to dust throughout Mongolia.. by 20-30% the inversions of optical
The GMS-5 animation and the SeaWiFS image show a sharp spectral reflectance data show that the dust
dust front progressing from the the Gobi desert. of soil, particularly volume is in the 1-10
Over the Yellow See and Korea, the TOMS data shows at l>0.6 mm. mm size range with a
another dust cloud while the SeaWiFS does not. volume peak at 2-3 mm
April 20-21: Transport Across East Asia
On April 20 the dust cloud was
stretched along the seaboard
of East Asia
Dust layers over low level white
clouds (inset), turned the
clouds yellow by reducing the
blue (412 nm) reflectance up
to a factor of two.
By April 21 the dust
1000 km into
Over the dark ocean,
the excess dust
(inset) was also
Trans-Pacific Dust Transport
Approximate location of the April 19 dust
cloud over the Pacific Ocean based on daily
SeaWiFS, GMS5/GOES9/GOES10 and
TOMS satellite data.
Over the Pacific Ocean, the dust cloud
followed the path of the springtime East-
Asian aerosol plume shown by the optical
thickness derived from AVHRR data.
Model simulations indicate a wavy
transport pattern at multiple altitudes.
NRL NAAPS Model Animation
ICOD DREAM Model
CAPITA Monte Carlo Model Animation
Throughout the Trans-Pacific transit, the
dust appeared as a yellow dye marking its
own position. Much of the dust was either
in cloud-free regions or over the clouds.
Visual Appearance of the Dust
• The most noticeable impact of the dust was the – Solar radiation data for
discoloration of the sky. Eugene, OR on a clear and
dusty day shows a loss of
• From April 25 onward, the normally blue sky appeared direct radiation and
milky white throughout the non-urban West Coast doubling of the midday
• This effect is due to the redistribution of the direct diffuse radiation due to
solar radiation into diffuse skylight. dust particle scattering and
Dust over the West Coast of North America
a. GOES 10
satellite image of
the dust taken on
the evening of
The dust cloud,
marked by the
A dust stream is also
seen crossing the
toward the east.
b. Contour map of the PM10 concentration on c. Regional average daily PM10
April 29, 1998. Note the coincidence of high concentration over the West Coast.
PM10 and satellite reflectance over The sharp peak on April 27-30 is
Washington due to the Asian dust.
Lidar Dust Profiles of Asian Dust over North America
• Lidar profile at Salt Lake City, UT on • Lidar backscatter profiles at,
April 24 indicates a strongly scattering Pasadena, CA at the peak of the
aerosol layer at 7-9.4 km with event (April 27) show a dust layer
depolarization delta-values up to 18%, between 6 and 10 km.
indicating non-spherical dust particles.
Dust Map over the West Coast
The PM2.5 dust concentration data from the IMPROVE speciated aerosol
network show virtually no dust on April 25th, high values over the West Coast
on April 29th and dust further inland on May 2.
Evidently, on April 25th the dust layer seen by the sun photometers was still
elevated since the surface dust concentration was low.
Hourly PM 10 Concentration in California
• In California, there was a synchronous rise and fall of the hourly PM10
concentration at all sites in the in the Sacramento area.
• During the dust event (April 26-May 1) the excess dust concentration was values
of 30-40 mg/m3.
• The diurnal cycle is attributed to dust removal in the nocturnal BL at night.
The April 98 Asian dust - A unique Event over N. America.
The average PM2.5 dust
concentration at three
sites over the 1988-98
period was well below 1
On April 29, 1998 the sites
show simultaneous sharp
rise to 3-11 mg/m3.
Evidently, the April 1998
Asian dust event caused 2-
3 times higher dust
concentrations then any
other event during 1988-
Abstract - Technical Summary
• 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
• 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.
Conclusions and Discussion
• Currently available space-borne and surface aerosol monitoring allows the
detection and following the evolution of global-scale aerosol events.
• The online data and explanations on the Asian dust have provided ‘just-in-time’
science support to managers responsible for protecting public health.
• The Asian Dust web-based virtual community has shown that ad-hoc collaboration
is a practical way to share observations and to collectively generate the explanatory
knowledge on these major unpredictable atmospheric events.
• Further activities may include (1) organizing the available data into a documented
and shared resource base (2) coordinated global dynamic aerosol model validation
and testing; (3) evaluation of satellite aerosol retrievals using the event data.
• It would be useful to set up a web-based communication, cooperation and
coordination system to monitor the global aerosol pattern for extreme aerosol
events. The system would alert interested communities, so that that the detection
and analysis of such unpredictable 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.