Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the G obi desert by springtime low-pressure systems descending from the northwest . The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and th rough serendipitous observations. The April 15 dust cloud was recirculating , and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the su rface along the mountain ranges between British Columbia and California, an d impacted severely the optical and the concentration environments of the r egion. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10-20%, but it reduced the near-UNI cloud reflecta nce, causing a yellow coloration of all surfaces. The yellow colored backsc attering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has inc reased the spectrally uniform optical depth to about 0.4, reduced the direc t solar radiation by 30-40%, doubled the diffuse radiation, and caused a wh itish discoloration of the blue sky. On April 29 the average excess surface -level dust aerosol concentration over the valleys of the West Coast was ab out 20-50 mug/m(3) with local peaks > 100 mug/m(3). The dust mass mean diam eter was 2-3 mum, and the dust chemical fingerprints were evident throughou t the West Coast and extended to Minnesota. The April 1998 dust event has i mpacted the surface aerosol concentration 2-4 times more than any other dus t event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aeros ol pattern for such extreme aerosol events, to alert and to inform the inte rested communities, and to facilitate collaborative analysis for improved a ir quality and disaster management.