1. The Asian Dust Events of April 1998 Prepared by: R. B. Husar, D. Tratt, B. A. Schichtel, S. R. Falke, F. Li D. Jaffe, S. Gassó, T. Gill, N. S. Laulainen, F. Lu. M Reheis, Y. Chun, D. Westpha, B. N. Holben, C. Geymard, 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 Paper to be submitted to the JGR issue on dustJGR issue on dust December 27, 1999 http://capita.wustl.edu/Asia-FarEast/reports/JGR/AsianDustEpisodeApril1998Draft5.htm

2. April 1998 Asian Dust Events - Synopsis In April 1998, several unusually intense dust storms were generated over the Gobi Desert by springtime cold weather systems with over 20 m/s surface wind speed. The dust cloud from the April 19 storm was swiftly transported across the Pacific reaching North America within 5 days. Part of the cloud subsided to the surface between British Columbia and CA while another part was observed aloft in layers up to 10 km. During the peak on April 29 the dust increased PM concentrations 20-50  g/m 3 over the West Coast and the daily PM10 concentration approached the health standard.

3. Asian Dust Cloud Source Region The largest dust storms of the season occurred on April 15 and 19. TOMS data indicate that the April 19, 1998 storm was the most intense dust event in the 1997-99 period.

4. April 15 Dust Cloud Over Asia SeaWiFS data with TOMS overlays (green lines) SeaWiFS satellite data indicate that the dust storms on April 15 and 19 originated from the same region of Gobi Desert. The dust sources are streaks of dust plumes originating from specific patches of land.

5. April 19 Dust Cloud Over Asia SeaWiFS data with TOMS overlays (green lines) After about 500 km of transport, the plumes tend to merge and the streaky plume structure disappears. On April 19, the visibility was reduced due to dust throughout central and eastern Mongolia, and the optical thickness increased from  2 on April 19. April 19, ‘98

6. Dust transport over the Pacific Ocean Figure 3. Dust transport over the Pacific Ocean between April 21-25. In the SeaWiFS images [Kuring, 1998], the dust appears as a yellow dye marking its own position at noon each day.

8. Dust Cloud Over North America GOES 10 S GOES 10 Geostationary satellite image By April 27 th, the dust cloud rolled into North America and split with one branch heading southward along the CA coast and the another branch continuing eastward across the Canadian Rockies.

9. West Coast PM10 Concentration Regional average PM10 levels reached 65 µg/m 3 compared to typical values of 10-25 µg/m 3 On April 29, the PM10 exceeded 100 µg/m 3 over parts of Washington and Oregon

10. Diurnal pattern of dust of Northern California. a) Hourly PM10 concentration averaged over 12 stations in Northern California. b) Diurnal pattern of PM10 on April 29, 1998. c) Location of the hourly PM monitoring sites in northern California. West Coast PM10 Concentration

11. IMPROVE Fine Particle Dust Concentrations April 25, 1998 April 29, 1998 May 2, 1998 On April 25, the western U.S. was virtually dust-free, but reached high concentrations by April 29. On May 2, the elevated dust concentrations moved over the Rocky Mountains and the Colorado Plateau

12. Fine Particle Dust Ten Year Trends Figure 8. Ten-year trend of fine particle dust concentration at three IMPROVE monitoring sites.

13. Vertical Profile of Dust Cloud LIDAR aerosol data at JPL, Pasadena, CA shows the dust layer was between 6-10 km on April 27.

14. Vertical Profile of Dust Cloud Over North America The height of the dust layer on April 27 was between 6-10 km Lidar backscatter at JPL, Pasadena.Lidar profiles at Salt Lake City

15. Asian Dust Size Distribution In Korea, the size distribution function shows a sharp peak between 1-5 micron, with a volume diameter of 2  m, and a logarithmic standard deviation of 1.6. In S. CA, the peak is shifted toward larger sizes and centered at 3  m.

16. Impact of Dust on Spectral Reflectance Excess spectral reflectance of dust over the ocean Change of the spectral reflectance of soil with increasing dust. Note, the yellow coloration of white clouds viewed through a dust layer.

17. Solar Radiation Data for Eugene, OR

18. Discussion and Recommendations