Several simulations were carried out, based on emission scenarios from IPCC for preindustrial, present-day and the future (2100). Calculated changes of tropospheric ozone and the oxidation capacity of the Earth's atmosphere, from the past to the present, and from the present to the future were assessed. Significant increases of ozone between preindustrial and present-day atmospheres are calculated following increasing surface emissions of human induced chemical compounds over the last century. This trend is predicted to continue in the future. The changes of ozone from the present to the future depend on climate change (see Fig. 1). When doubled CO2 is included in the background atmosphere, the increase of ozone is different compared to the scenario which includes only emission changes; an increased stratosphere to troposphere exchange in a changed climate is crucial to our results. The changes of OH can be of either sign and depend critically on changes of NOx and hydrocarbons. When climate changes are included in the scenario, a substantial increase of OH is calculated due to an increase of water vapour in a warmer and wetter climate. The oxidation capacity of the atmosphere is evidently affected by a changing climate.
Interannual variability of ozone due to the natural variability of meterological conditions was examined by a 12-year simulation covering 1990-2001, driven by the observed SSTs. The El Nino Southern Oscillation (ENSO) is a major factor for interannual variability of ozone. It is found that there are close correlations between the ENSO signals and the stratosphere-troposphere exchange (STE). Different circulation patterns in El Nino years, especially in 1997-1998, lead to significantly increased ozone transport from the stratosphere to the troposphere and the increase of lower stratospheric ozone abundance. In contrast, La Nina events result in a decrease in STE, which is most evident in 1999. STE is a driving force in determining chemical lifetimes of ozone in the troposphere and for ozone budgets in both the stratosphere and troposphere.
Figure 1: Changes in surface ozone (ppbv) in July between 2000 and 2100
Figure 2: Simulated (a) Annual total STE in Tg and (b) mean Southern Oscilation Index (SOI) in hPa. SOI is defined as deseasonalized monthly mean tropical Tahiti (18S,150W) minus tropical Darwin (13S,131E) surface pressure to indicate El Nino (negative value) and La Nina (positive value) events in the Pacific.
