秋梦无痕

一场秋雨无梦痕,春夜清风冻煞人。冬来冷水寒似铁,夏至京北蟑满城。

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PM 2.5

from: http://www.nasa.gov/topics/earth/features/health-sapping.html


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Global satellite-derived map of PM2.5 averaged over 2001-2006.
Credit: Dalhousie University, Aaron van Donkelaar