Polar Stratospheric Clouds (PSCs)
						q In winter the polar stratosphere is so cold (-80°C or below) that certain trace atmospheric constituents can condense. q These clouds are called “polar stratospheric clouds” (PSCs). q The particles that form typically consist of a mixture of water and nitric acid (HNO3). q The PSCs alter the chemistry of the lower stratosphere in two ways:       (1) by slow down the coupling between the odd nitrogen and chlorine cycles       (2) by providing surfaces on which heterogeneous reactions can occur.