Cleaner air leads to greenhouse effect?

news @ nature.com -Clear skies end global dimming - Earth's air is cleaner, but this may worsen the greenhouse effect. I am so confused. I thought clean air was good.

"It is clear that the greenhouse effect has been partly masked in the past by air pollution," says Andreas Macke, a meteorologist at the Leibniz Institute of Marine Sciences in Kiel, Germany.

Here I thought the greehouse effect was caused by air polution.

When you science guys figure out if air polution is bad (and casues the greenhouse effect ) or good (and keeps the greenhouse effect at bay) call me. This is giving me a headache. [via On the Third Hand]