1. 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.full paper JGR Special Issue Asian Dust Website: http://capita.wustl.edu/Asia-FarEasthttp://capita.wustl.edu/Asia-FarEast Comments: rhusar@me.wustl.edurhusar@me.wustl.edu

2. 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.full paper Special Issue on Dust

3. 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.

4. The April 15th Dust Storm: Dissipation within Asia Fast surface winds ( > 20 m/s) over the Gobi desert generated individual dust plumes as seen from the SeaWiFS reflectance data. After about 500 km transport, the plumes merged into a dust cloud After 1000 km transport from Gobi to Shanghai, the yellow dust cloud has retained considerable spatial texture. The TOMS absorbing aerosol index data (green lines - index=2) and the SeaWiFS image show similar pattern over Eastern China. 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.

5. 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.

6. The April 19th Dust Storm The surface wind was > 15 m/s and surface visibility reduction was due to dust throughout Mongolia.. The GMS-5 animation and the SeaWiFS image show a sharp dust front progressing from the the Gobi desert.GMS-5 animation Over the Yellow See and Korea, the TOMS data shows another dust cloud while the SeaWiFS does not. The dust layer increases by 20-30% the spectral reflectance of soil, particularly at >0.6  m. Size distribution data and inversions of optical data show that the dust volume is in the 1-10  m size range with a volume peak at 2-3  m

7. 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.

8. Trans-Pacific Dust Transport Model simulations indicate a wavy transport pattern at multiple altitudes. NRL NAAPS ModelNRL NAAPS Model AnimationAnimation ICOD DREAM Model CAPITA Monte Carlo ModelCAPITA Monte Carlo Model AnimationAnimation 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. 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.

9. Visual Appearance of the Dust The most noticeable impact of the dust was the discoloration of the sky. 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. –Solar radiation data for Eugene, OR on a clear and dusty day shows a loss of direct radiation and doubling of the midday diffuse radiation due to dust particle scattering and absorption.

10. 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 April 29, 1998. Note the coincidence of high PM10 and satellite reflectance over Washington c. Regional average daily PM10 concentration over the West Coast. The sharp peak on April 27-30 is due to the Asian dust.

11. Lidar Dust Profiles of Asian Dust over North America Lidar profile at Salt Lake City, UT on April 24 indicates a strongly scattering aerosol layer at 7-9.4 km with depolarization delta-values up to 18%, indicating non-spherical dust particles. Lidar backscatter profiles at, Pasadena, CA at the peak of the event (April 27) show a dust layer between 6 and 10 km.

12. 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 29 th 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.

13. 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  g/m 3. The diurnal cycle is attributed to dust removal in the nocturnal BL at night.

14. 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  g/m 3 On April 29, 1998 the sites show simultaneous sharp rise to 3-11  g/m 3. Evidently, the April 1998 Asian dust event caused 2- 3 times higher dust concentrations then any other event during 1988- 1998.

15. 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  g/m 3 with local peaks >100 µg/m 3.. The chemical fingerprint of the Asian dust (particle diameter 2-3  m) 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.

16. 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.