Chemistry and microphysics of polar stratospheric clouds and cirrus clouds

Ice particles found within polar stratospheric clouds (PSCs) and upper trop ospheric cirrus clouds can dramatically impact the chemistry and climate of the Earth's atmosphere. The formation of PSCs and the subsequent chemical reactions that occur on their surfaces are key components of the massive oz one hole observed each spring over Antarctica. Cirrus clouds also provide s urfaces for heterogeneous reactions and significantly modify the Earth's cl imate by changing the visible and infrared radiation fluxes. Although the r ole of ice particles in climate and chemistry is well recognized, the exact mechanisms of cloud formation are still unknown, and thus it is difficult to predict how anthropogenic activities will change cloud abundances in the future. This article focuses on the nucleation, chemistry, and microphysic al properties of ice particles composing PSCs and cirrus clouds. A general overview of the current state of research is presented along with some unre solved issues facing scientists in the future.