Uploaded: Wednesday, March 25, 2009, 9:29 AM
Asian dust cloud reaches Bay Area today
Springtime cloud of acidic particles, desert dust can impact water reserves
by Sue Dremann
Palo Alto Online Staff
A giant high-altitude cloud of desert dust and urban pollutants from China will reach the Bay Area today.
The cloud -- which is part of a phenomenon that carries dust from China's Gobi Desert on spring winds -- picks up soot, metals and other pollutants from urban industries. The pollutants mix and can chemically change to create calcium nitrate or nitric acid and sulfuric acid, according to scientists.
Some scientists believe that particles precipitating out of the clouds have a small impact on climate change.
Mark Jacobson, professor of civil and environmental engineering at Stanford University, said most of the dust will probably not affect public health, since the pollutants are high in the atmosphere. But some scientific models show it reduces precipitation by 2 to 5 percent
-- the equivalent of one large reservoir of water -- because smaller raindrops that form around the dust particles may evaporate before hitting the ground.
Water supplies are also affected because soot and dust precipitate out onto the snowpack when the cloud hits mountain peaks. The particles darken the snow and cause snow to melt more quickly, he said.
Federal meteorologists said the cloud would begin to pass over the Bay Area around sunrise Wednesday morning and will pass through California between then and Friday.