Figure 1a shows that modelled annual mean surface O3 for 2000 varies between 40-50 ppbv over large parts of Europe, Asia and North America as a direct result of emissions of O3 precursors. The background O3 levels are from 15 ppbv to 25 ppbv with the year-round low values in Central Pacific Ocean. Tropospheric O3 is not only produced in the source regions of its precursors but it is also transported to remote oceanic regions under favourable meteorological conditions. For example, the elevated O3 levels in the northern Pacific Ocean are the result of transport of pollutants from Asia.
Figure 1b shows changes in surface O3 between 2000 and 2100 assuming only changes in anthropogenic emissions, i.e., with unchanged climate. The background O3 levels are calculated to increase 10-15 ppbv by the end of this century based on the A2 emission scenario. In the Northern Hemisphere (NH), over 30 ppbv of O3 increase are calculated over polluted continents. The largest annual averaged O3 increases (over 40 ppbv) occur in Asia. The peak ozone concentration is calculated to reaching 50 ppbv in summer months. For these regions, rapid economic growth and a population increase are predicted and can lead to an unacceptable air quality.
Increased surface emissions of O3 precursors not only contribute to O3 formation in source regions but also increase O3 levels in remote regions through long-range transport. Vertical profiles of tropospheric O3 in the year 2000 and the ΔO3 (2100-2000) are shown in Figures 2a & 2b. The Figures show the propagation of chemically produced O3 from the lower troposphere to the upper troposphere. The largest absolute increase of O3 is in the upper troposphere between 20-40οN where O3 has a longer chemical lifetime and there is no surface deposition. Note that O3 in this region has a high climate sensitivity, i.e. a larger contribution to radiative forcing to lower temperatures in the upper troposphere. There is a 75% increase in total tropospheric O3 burden as a result of increased anthropogenic emissions.
Impact of climate change
Changes in meteorology can affect the O3 distribution by modifying both chemistry and transport. Figure 2c shows O3 changes due only to changes in meteorology. Feedbacks from climate change may have both positive or negative signs. Indeed, increased water vapour promotes chemical destruction especially in the tropical lower and middle troposphere, following photolysis of O3 and the subsequent reaction of O(1D) with water vapour.