[kee-muh-sfeer, kem-uh-] 
| the region of the atmosphere most characterized by chemical, esp. photochemical, activity, starting in the stratosphere and including the mesosphere and perhaps part of the thermosphere. |
| che·mo·sphere (kē'mə-sfîr', kěm'ə-) n. The region of the atmosphere from 30 to 190 kilometers (20 to 120 miles) above the earth's surface, in which photochemical reactions initiated by solar radiation occur. |
məˌsfɪər