Saturday, March 27, 2010
Springtime Dust storms
During the past week there has been a major dust storm over east Asia, with dust spreading over eastern China, Japan, and over the Pacific. This is not an academic issue for us here in Northwest because occasionally this dust is carried across the Pacific, influencing visibility and air quality over the Northwest.
Take a look at a picture in Beijing before and during the current dust storm above. Pretty amazing change. Visibility can drop to 1/8 mile or less in major events. And such dust storms are very evident in satellite imageryover east Asia a few days ago, as show below. The dust is the tan looking stuff.
East Asian dust storms tend to occur in spring and generally start over the deserts of Mongolia, northern China and Kazakhstan when strong springtime winds lift fine, dry soil particles into the air. Overgrazing, deforestation and drought have contributed to an increase in this phenomenon.
Unfortunately, here in the Pacific Northwest we can feel the effects of major dust storms. The dust can mix high into the troposphere (the lowest layer of the atmosphere which tops out around 30,000 ft) and the strong winds there (the "jet stream") can blow the dust across the Pacific in a few days. Spring is a good time for the dust to make it over since the jet stream is still fairly strong and at the right latitudes for transport from Asia to our region. There have been a number of such events with ones in 1998 and 2001 being particularly memorable. Visibility can plummet here in the Northwest as the Asian dust overspreads the region; take a look at an example below:
One study found that ∼50% of the interannual variability in springtime average visibility here in the NW can be explained by changes in Asian dust emissions.
But the situation is worse than just dust. Not only dust makes it across the Pacific, but so does some nasty pollutants, both on the dust and in the free air. Professor Dan Jaffe, of UW Bothell, is an atmospheric chemist who has extensively studied cross-Pacific transport of pollutants. He has run two Northwest observatories to collect Asian pollutants--one at Cheeka Peak on the northwest corner of the Olympic Peninsula and at Mt. Bachelor, Oregon. He has a nice web page on the Asian pollution and dust transport at http://faculty.washington.edu/djaffe/FAQs.htm
The bottom line of all this is that the atmosphere is highly interconnected and pollution and dust storms half a world away can affect us here in the Northwest.
Posted by Cliff Mass at 9:02 AM