Summary
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 stratospheric ozone depletion events occur. Tropospheric ozone drives tropospheric chemistry being the major precursor of the OH radical which is the sink for gases such as methane. In the boundary layer increases in background ozone have other impacts through urban and regional photochemical smog. These increases affect human health, crop productivity and the sequestration of carbon into the biosphere. There are few long]term monitoring stations and few process studies of tropospheric ozone in the Southern Hemisphere. Since 1976 there have been measurements of ozone in surface air at Cape Grim and one of the early purposes identified for these measurements was to detect long term trends in tropospheric ozone. In fact it took until 1982 to establish a reliable monitoring system that has continued into the present. Part of this record will be presented here along with observations from elsewhere in the extratropical southern hemisphere. Some of our previous work on these issues includes Elsworth & Galbally (1984), Elsworth, Douglas & Galbally (1985), Galbally et al. (2011) and Oltmans et al. (1998, 2006, 2012). In an international collaboration, analyses are presented of two decades of observations from seven background surface ozone stations spanning 34‹ S to 90‹ S and four long]term ozonesonde stations spanning 38‹ S to 69‹ S. Special consideration is taken to consider only those parts of the ozone record that are free of local influences of ozone production and destruction. Halogen based ozone destruction and local ozone production are evident in lower tropospheric air at some Antarctic stations. The ozone distribution measurements reflect a NOx depleted environment where there is net ozone loss at most oceanic boundary layer and free troposphere locations. There is little destruction of ozone at the water and snow surfaces. The seasonal signature of stratospheric]tropospheric exchange is evident in upper tropospheric ozone records in the mid]latitudes but not in the polar regions. Over the last two decades there has been a growth in ozone abundance in the lower troposphere. This growth is stronger over Antarctica compared with the mid]latitudes and regional differences are evident. As well, there is inter]annual variability that appears to be correlated over long distances. Possible drivers of both the growth and inter]annual variability will be discussed.
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