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					     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
                                           Comments: rhusar@me.wustl.edu
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
     Irina Sokolik.
     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
     region.
•    Model simulations of dust
     production and the dust
     pattern correspond to the
     observations.
          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
   systems.
                               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
   cloud extended
   1000 km into
   the Pacific.
Over the dark ocean,
   the excess dust
   reflectance
   (inset) was also
   yellow.
                             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
                                                               absorption.
         Dust over the West Coast of North America
a. GOES 10
    geostationary
    satellite image of
    the dust taken on
    the evening of
    April 27.
The dust cloud,
    marked by the
    brighter
    reflectance covers
    the entire
    northwestern US
    and adjacent
    portions of
    Canada.
A dust stream is also
    seen crossing the
    Rocky Mountains
    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
     IMPROVE monitoring
     sites over the 1988-98
     period was well below 1
     mg/m3

  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-
     1998.
                        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
    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.
            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.

				
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