Stratospheric ozone depletion: Its impact on tropospheric chemistry and on climate
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- CICERO Reports 
Observations show a long term decrease in stratospheric ozone on a global scale during the last two decades. Over the southern polar region the reductions are large and clearly connected to man made emissions of CFCs. There are mounting evidences that also Northern Hemispheric ozone reductions observed since 1980 are connected to man made emissions of CFCs. Observations give global average reductions of the order 4 0ver the last 10–15 years. The reductions have been particularly large at mid and high northern latitudes after 1991 and occur in the lower stratosphere, during winter and spring. Model studies give strong indications that a font size="-4"stantial fraction of the reduction is due to enhanced chemical loss through chemical reactions involving chlorine compounds (ClO and Cl). The enhanced ozone loss which has occurred since 1991 coincides with enhanced particle formation in the stratosphere from the Mt. Pinatubo volcanic eruption in 1991, and enhanced formation of Polar Stratospheric Clouds. Observations also show that solar cycle variation affects stratospheric ozone on a short time scale. Ozone has increased during the last two decades in the northern free troposphere, although there seem to have been a levelling off in the increase since the mid 80s. Enhanced emissions of NOx, CO and hydrocarbons are the main cause for the ozone growth. Tropospheric ozone is also affected by the enhanced UV fluxes caused by reductions in stratospheric ozone. Models give reduced ozone and methane levels in the free troposphere due to increased UV fluxes. Since both ozone and methane are greenhouse gases, reductions in stratospheric ozone also affect climate through chemical changes in the troposphere.