Summary
Ozone in the troposphere has four important roles. Tropospheric ozone is a greenhouse gas, ranked third in anthropogenic radiative forcing after CO2 and CH4; ozone in the troposphere makes up an important fraction of the atmospheric shield against ultraviolet radiation particularly when ozone depletion events occur and when the total column abundance of ozone drops below 200 DU; tropospheric ozone drives tropospheric chemistry including the sink for gases such a methane, and increases in background ozone have impacts also on the occurrence of urban and regional photochemical smog, related health effects and crop damage. There is very limited long term monitoring and few process studies of tropospheric ozone in the Southern Hemisphere. Due to the very different mixes of anthropogenic and natural sources, different regimes of tropospheric ozone may occur in each hemisphere. Long term trends in tropospheric ozone, calculated from deseasonalised data, for the periods 1980 – 2009, 1990-1999, 1990 - 2009 and 2000 – 2009, where data permits, will be presented for four sites spanning the Southern Ocean. The sites and relevant details are: Cape Grim Tasmania, 40.38ºS, 144.69ºE, 104 m asl, where surface ozone measurements from 1982 to 2010 will be presented including baseline ozone concentrations representative of the Southern Ocean marine boundary layer, and non-baseline representing regional influences. Broadmeadows Victoria, Australia, 37.69ºS 144.95ºE where the ozone sonde data from 1999- 2010 will be presented and trends calculated for the standard heights 0-1 km, 1-3 km, 3-8 km, and upper troposphere/lower stratosphere. Macquarie Island, 54.50ºS 158.94ºE, where the ozone sonde data from 1994-2010 will be presented and trends calculated for the standard heights 0-1 km, 1-3 km, 3-8 km, and upper troposphere/lower stratosphere. Davis, Antarctica 68.58ºS, 77.97ºE, where the ozone sonde data from 2003-2010 will be presented and trends calculated for the standard heights 0-1 km, 1-3 km, 3-7 km, and upper troposphere/lower stratosphere. A lower height of 7 km is used at this site for the upper bound of the middle troposphere layer because of the lower tropopause in winter. Analyses of the contributions of various processes to the observed southern hemisphere tropospheric ozone ternds will be discussed. These include (a) dynamical effects including ENSO, SAM and the strength of the stratospheric/tropospheric exchange and (c) chemical processes including changes in biomass burning and the trends in methane in the southern hemisphere.