Very fine ash can remain in the air for years carried by the wind if launched high enough into the atmosphere. In cases such as that, we're talking ash so fine that it makes talcum powder look as rough as sandpaper. Being so light, it's quite easy for it to remain in the atmosphere as the force of the wind exerts a much greater force than does gravity.

One would think that large volumes of ash in the atmosphere would be a primary cause of global cooling, but in fact, another by-product of volcanic eruptions, sulfur dioxide, contributes far more to global cooling than ash.

Sulfur dioxide gets converted into sulfate aerosols which persist in the atmosphere and can actually lower global average temps by a degree or two for a couple years. The Tambora eruption in 1815 and Mt. Pinatubo in 1991 have actually caused this global temperature drop. Of course, other atmospheric conditions such as El Niño may exacerbate or inhibit this phenomenon.
Source(s):
http://pubs.usgs.gov/fs/1997/fs113-97/fs113-97.pdf
http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/earthsci/volcano....

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